Network technologies: Local area network and Ethernet

A local area network (LAN) is a group of computers that are connected together in a localized area to communicate with one another and share resources such as printers. Data is sent in the form of packets and to regulate the transmission of the packets, different technologies can be used. The most widely used LAN technology is the Ethernet and it is specified in a standard called IEEE 802.3. (Other types of LAN networking technologies include token ring and FDDI.)

Ethernet uses a star topology in which the individual nodes (devices) are networked with one another via active networking equipment such as switches. The number of networked devices in a LAN can range from two to several thousand.

The physical transmission medium for a wired LAN involves cables, mainly twisted pair or fiber optics. A twisted pair cable consists of eight wires, forming four pairs of twisted copper wires and is used with RJ-45 plugs and sockets. The maximum cable length of a twisted pair is 100 m (328 ft.) while for fiber, the maximum length ranges from 10 km to 70 km, depending on the type of fiber. Depending on the type of twisted pair or fiber optic cables used, data rates today can range from 100 Mbit/s to 10,000 Mbit/s.

Twisted pair cabling includes four pairs of twisted wires, normally connected to a RJ-45 plug at the end.

A rule of thumb is to always build a network with greater capacity than is currently required. To future-proof a network, it is a good idea to design a network such that only 30% of its capacity is used. Since more and more applications are running over networks today, higher and higher network performance is required. While network switches (discussed below) are easy to upgrade after a few years, cabling is normally much more difficult to replace.

Types of Ethernet networks

Fast Ethernet

Fast Ethernet refers to an Ethernet network that can transfer data at a rate of 100 Mbit/s. It can be based on a twisted pair or fiber optic cable. (The older 10 Mbit/s Ethernet is still installed and used, but such networks do not provide the necessary bandwidth for some network video applications.)

Most devices that are connected to a network, such as a laptop or a network camera, are equipped with a 100BASE-TX/10BASE-T Ethernet interface, most commonly called a 10/100 interface, which supports both 10 Mbit/s and Fast Ethernet. The type of twisted pair cable that supports Fast Ethernet is called a Cat-5 cable.

Gigabit Ethernet

Gigabit Ethernet, which can also be based on a twisted pair or fiber optic cable, delivers a data rate of 1,000 Mbit/s (1 Gbit/s) and is becoming very popular. It is expected to soon replace Fast Ethernet as the de facto standard.

The type of twisted pair cable that supports Gigabit Ethernet is a Cat-5e cable, where all four pairs of twisted wires in the cable are used to achieve the high data rates. Cat-5e or higher cable categories are recommended for network video systems. Most interfaces are backwards compatible with 10 and 100 Mbit/s Ethernet and are commonly called 10/100/1000 interfaces.

For transmission over longer distances, fiber cables such as 1000BASE-SX (up to 550 m/1,639 ft.) and 1000BASE-LX (up to 550 m with multimode optical fibers and 5,000 m with single-mode fibers) can be used.

Longer distances can be bridged using fiber optic cables. Fiber is typically used in the backbone of a network and not in nodes such as a network camera.

10 Gigabit Ethernet

10 Gigabit Ethernet is the latest generation and delivers a data rate of 10 Gbit/s (10,000 Mbit/s), and a fiber optic or twisted pair cable can be used. 10GBASE-LX4, 10GBASE-ER and 10GBASE-SR based on an optical fiber cable can be used to bridge distances of up to 10,000 m (6.2 miles). With a twisted pair solution, a very high quality cable (Cat-6a or Cat-7) is required. 10 Gbit/s Ethernet is mainly used for backbones in high-end applications that require high data rates.

Switch

When only two devices need to communicate directly with one another via a twisted pair cable, a so-called crossover cable can be used. The crossover cable simply crosses the transmission pair on one end of the cable with the receiving pair on the other end and vice versa.

To network multiple devices in a LAN, however, network equipment such as a network switch is required. When using a network switch, a regular network cable is used instead of a crossover cable.

The main function of a network switch is to forward data from one device to another on the same network. It does it in an efficient manner since data can be directed from one device to another without affecting other devices on the same network.

How it works is that a switch registers the MAC (Media Access Control) addresses of all devices that are connected to it. (Each networking device has a unique MAC address, which is made up of a series of numbers and letters that is set by the manufacturer and the address is often found on the product label.) When a switch receives data, it forwards it only to the port that is connected to a device with the appropriate destination MAC address.

Switches typically indicate their performance in per port rates and in backplane or internal rates (both in bit rates and in packets per second). The port rates indicate the maximum rates on specific ports. This means that the speed of a switch, for example 100 Mbit/s, is often the performance of each port.

With a network switch, data transfer is managed very efficiently as data traffic can be directed from one device to another without affecting any other ports on the switch.

A network switch normally supports different data rates simultaneously. The most common rates used to be 10/100, supporting 10 Mbit/s as well as Fast Ethernet. However, 10/100/1000 are quickly taking over as the standard switch, thus supporting 10 Mbit/s, Fast Ethernet and Gigabit Ethernet simultaneously. The transfer rate and mode between a port on a switch and a connected device are normally determined through auto-negotiation, whereby the highest common data rate and best transfer mode are used. A switch also allows a connected device to function in full-duplex mode, i.e. send and receive data at the same time, resulting in increased performance.

Switches may come with different features or functions. Some switches include the function of a router. A switch may also support Power over Ethernet or Quality of Service, which controls how much bandwidth is used by different applications.

Power over Ethernet

Power over Ethernet (PoE) provides the option of supplying devices connected to an Ethernet network with power using the same cable as for data communication. Power over Ethernet is widely used to power IP phones, wireless access points and network cameras in a LAN.

The main benefit of PoE is the inherent cost savings. Hiring a certified electrician and installing a separate power line are not needed. This is advantageous, particularly in difficult-to-reach areas. The fact that no power cable has to be installed can save, depending on the camera location, up to a few hundred dollars per camera. Having PoE also makes it easier to move a camera to a new location, or add cameras to a video surveillance system.

Additionally, PoE can make a video system more secure. A video surveillance system with PoE can be powered from the server room, which is often backed up with a UPS (Uninterruptible Power Supply). This means that the video surveillance system can be operational even during a power outage.

Due to the benefits of PoE, it is recommended for use with as many devices as possible. The power available from the PoE-enabled switch or midspan should be sufficient for the connected devices and the devices should support power classification. These are explained in more detail in the sections below.

802.3af standard and High PoE

Most PoE devices today conform to the IEEE 802.3af standard, which was published in 2003. The IEEE 802.3af standard uses standard Cat-5 or higher cables, and ensures that data transfer is not affected. In the standard, the device that supplies the power is referred to as the power sourcing equipment (PSE). This can be a PoE-enabled switch or midspan. The device that receives the power is referred to as a powered device (PD). The functionality is normally built into a network device like a network camera, or provided in a standalone splitter.

Backward compatibility to non PoE-compatible network devices is guaranteed. The standard includes a method for automatically identifying if a device supports PoE, and only when that is confirmed will power be supplied to the device. This also means that the Ethernet cable that is connected to a PoE switch will not supply any power if it is not connected to a PoE-enabled device. This eliminates the risk of getting an electrical shock when installing or rewiring a network.

In a twisted pair cable, there are four pairs of twisted wires. PoE can use either the two ‘spare’ wire pairs, or overlay the current on the wire pairs used for data transmission. Switches with built-in PoE often supply electricity through the two pairs of wires used for transferring data, while midspans normally use the two spare pairs. A PD supports both options.

According to IEEE 802.3af, a PSE provides a voltage of 48 V DC with a maximum power of 15.4 W per port. Considering that power loss takes place on a twisted pair cable, only 12.95 W is guaranteed for a PD. The IEEE 802.3af standard specifies various performance categories for PDs.

PSE such as switches and midspans normally supply a certain amount of power, typically 300 W to 500 W. On a 48-port switch, that would mean 6 W to 10 W per port if all ports are connected to devices that use PoE. Unless the PDs support power classification, the full 15.4 W must be reserved for each port that uses PoE, which means a switch with 300 W can only supply power on 20 of the 48 ports. However, if all devices let the switch know that they are Class 1 devices, the 300 W will be enough to supply power to all 48 ports.

Class Minimum power level at PSE Maximum power level used by PD Usage 0 15.4 W 0.44 W - 12.95 W default 1 4.0 W 0.44 W - 3.84 W optional 2 7.0 W 3.84 W - 6.49 W optional 3 15.4 W 6.49 W - 12.95 W optional 4 treat as Class 0 reserved for future use

Power classifications according to IEEE 802.3af.

Most fixed network cameras can receive power via PoE using the IEEE 802.3af standard and are normally identified as Class 1 or 2 devices.

With IEEE 802.3at pre-standard or PoE+, the power limit will be raised to at least 30 W via two pairs of wires from a PSE. The final specifications are still to be determined and the standard is expected to be ratified in mid-2009.

In the meantime, IEEE 802.3at pre-standard (High PoE) midspans and splitters can be used for devices such as PTZ cameras and PTZ dome cameras with motor control, as well as cameras with heaters and fans, which require more power than can be delivered by the IEEE 802.3af standard.

Midspans and splitters

Midspans and splitters (also known as active splitters) are equipment that enable an existing network to support Power over Ethernet.

An existing system can be upgraded with PoE functionality using a midspan and splitter.

The midspan, which adds power to an Ethernet cable, is placed between the network switch and the powered devices. To ensure that data transfer is not affected, it is important to keep in mind that the maximum distance between the source of the data (e.g., switch) and the network video products is not more than 100 m (328 ft.). This means that the midspan and active splitter(s) must be placed within the distance of 100 m.

A splitter is used to split the power and data in an Ethernet cable into two separate cables, which can then be connected to a device that has no built-in support for PoE. Since PoE or High PoE only supplies 48 V DC, another function of the splitter is to step down the voltage to the appropriate level for the device; for example, 12 V or 5 V.

Internet communication

To send data between a device on one local area network to another device on another LAN, a standard way of communicating is required since local area networks may use different types of technologies. This need led to the development of IP addressing and the many IP-based protocols for communicating over the Internet, which is a global system of interconnected computer networks. (LANs may also use IP addressing and IP protocols for communicating within a local area network, although using MAC addresses is sufficient for internal communication.) Before IP addressing is discussed, some of the basic elements of Internet communication such as routers, firewalls and Internet service providers are covered below.

Routers

To forward data packages from one LAN to another LAN via the Internet, a networking equipment called a network router must be used. A router routes information from one network to another based on IP addresses. It forwards only data packages that are to be sent to another network. A router is most commonly used for connecting a local network to the Internet. Traditionally, routers were referred to as gateways.

Firewalls

A firewall is designed to prevent unauthorized access to or from a private network. Firewalls can be implemented in both hardware and software, or a combination of both. Firewalls are frequently used to prevent unauthorized Internet users from accessing private networks that are connected to the Internet. Messages entering or leaving the Internet pass through the firewall, which examines each message, and blocks those that do not meet the specified security criteria.

Internet connections

In order to connect a LAN to the Internet, a network connection via an Internet service provider (ISP) must be established. When connecting to the Internet, terms such as upstream and downstream are used. Upstream describes the transfer rate with which data can be uploaded from the device to the Internet; for instance, when video is sent from a network camera. Downstream is the transfer speed for downloading files; for instance, when video is received by a monitoring PC.

In most scenarios — for example, a laptop that is connected to the Internet — downloading information from the Internet is the most important speed to consider. In a network video application with a network camera at a remote site, the upstream speed is more relevant since data (video) from the network camera will be uploaded to the Internet.

IP addressing

Any device that wants to communicate with other devices via the Internet must have a unique and appropriate IP address. IP addresses are used to identify the sending and receiving devices. There are currently two IP versions: IP version 4 (IPv4) and IP version 6 (IPv6). The main difference between the two is that the length of an IPv6 address is longer (128 bits compared with 32 bits for an IPv4 address). IPv4 addresses are most commonly used today.

IPv4 addresses

IPv4 addresses are grouped into four blocks, and each block is separated by a dot. Each block represents a number between 0 and 255; for example, 192.168.12.23.

Certain blocks of IPv4 addresses have been reserved exclusively for private use. These private IP addresses are 10.0.0.0 to 10.255.255.255, 172.16.0.0 to 172.31.255.255 and 192.168.0.0 to 192.168.255.255. Such addresses can only be used on private networks and are not allowed to be forwarded through a router to the Internet. All devices that want to communicate over the Internet must have its own individual, public IP address. A public IP address is an address allocated by an Internet service provider. An ISP can allocate either a dynamic IP address, which can change during a session, or a static address, which normally comes with a monthly fee.

Ports

A port number defines a particular service or application so that the receiving server (e.g., network camera) will know how to process the incoming data. When a computer sends data tied to a specific application, it usually automatically adds the port number to an IP address without the user’s knowledge.

Port numbers can range from 0 to 65535. Certain applications use port numbers that are pre-assigned to them by the Internet Assigned Numbers Authority (IANA). For example, a web service via HTTP is typically mapped to port 80 on a network camera.

Setting IPv4 addresses

In order for a network camera or video encoder to work in an IP network, an IP address must be assigned to it. Setting an IPv4 address for an Axis network video product can be done mainly in two ways: 1) automatically using DHCP (Dynamic Host Configuration Protocol), and 2) manually by either entering into the network video product’s interface a static IP address, a subnet mask and the IP address of the default router, or using a management software tool such as AXIS Camera Management.

DHCP manages a pool of IP addresses, which it can assign dynamically to a network camera/ video encoder. The DHCP function is often performed by a broadband router, which in turn gets its IP addresses from an Internet service provider. Using a dynamic IP address means that the IP address for a network device may change from day to day. With dynamic IP addresses, it is recommended that users register a domain name (e.g., www.mycamera.com) for the network video product at a dynamic DNS (Domain Name System) server, which can always tie the domain name for the product to any IP address that is currently assigned to it.

Using DHCP to set an IPv4 address works as follows. When a network camera/video encoder comes online, it sends a query requesting configuration from a DHCP server. The DHCP server replies with an IP address and subnet mask. The network video product can then update a dynamic DNS server with its current IP address so that users can access the product using a domain name.

With AXIS Camera Management, the software can automatically find and set IP addresses and show the connection status. The software can also be used to assign static, private IP addresses for Axis network video products. This is recommended when using video management software to access network video products. In a network video system with potentially hundreds of cameras, a software program such as AXIS Camera Management is necessary in order to effectively manage the system.

NAT (Network address translation)

When a network device with a private IP address wants to send information via the Internet, it must do so using a router that supports NAT. Using this technique, the router can translate a private IP address into a public IP address without the sending host’s knowledge.

Port forwarding

To access cameras that are located on a private LAN via the Internet, the public IP address of the router should be used together with the corresponding port number for the network camera/video encoder on the private network.

Since a web service via HTTP is typically mapped to port 80, what happens then when there are several network cameras/video encoders using port 80 for HTTP in a private network? Instead of changing the default HTTP port number for each network video product, a router can be configured to associate a unique HTTP port number to a particular network video product’s IP address and default HTTP port. This is a process called port forwarding.

Port forwarding works as follows. Incoming data packets reach the router via the router’s public (external) IP address and a specific port number. The router is configured to forward any data coming into a predefined port number to a specific device on the private network side of the router. The router then replaces the sender’s address with its own private (internal) IP address. To a receiving client, it looks like the packets originated from the router. The reverse happens with outgoing data packets. The router replaces the private IP address of the source device with the router’s public IP address before the data is sent out over the Internet.

Thanks to port forwarding in the router, network cameras with private IP addresses on a local network can be accessed over the Internet. In this illustration, the router knows to forward data (request) coming into port 8032 to a network camera with a private IP address of 192.168.10.13 port 80. The network camera can then begin to send video.

Port forwarding is traditionally done by first configuring the router. Different routers have different ways of doing port forwarding and there are web sites such as www.portfoward.com that offer step-by-step instruction for different routers. Usually port forwarding involves bringing up the router’s interface using an Internet browser, and entering the public (external) IP address of the router and a unique port number that is then mapped to the internal IP address of the specific network video product and its port number for the application.

To make the task of port forwarding easier, Axis offers the NAT traversal feature in many of its network video products. NAT traversal will automatically attempt to configure port mapping in a NAT router on the network using UPnP™. In the network video product interface, users can manually enter the IP address of the NAT router. If a router is not manually specified, then the network video product will automatically search for NAT routers on the network and select the default router. In addition, the service will automatically select an HTTP port if none is manually entered.

IPv6 addresses

An IPv6 address is written in hexadecimal notation with colons subdividing the address into eight blocks of 16 bits each; for example, 2001:0da8:65b4:05d3:1315:7c1f:0461:7847.

The major advantages of IPv6, apart from the availability of a huge number of IP addresses, include enabling a device to automatically configure its IP address using its MAC address. For communication over the Internet, the host requests and receives from the router the necessary prefix of the public address block and additional information. The prefix and host’s suffix is then used, so DHCP for IP address allocation and manual setting of IP addresses are no longer required with IPv6. Port forwarding is also no longer needed. Other benefits of IPv6 include renumbering to simplify switching entire corporate networks between providers, faster routing, point-to-point encryption according to IPSec, and connectivity using the same address in changing networks (Mobile IPv6).

An IPv6 address is enclosed in square brackets in a URL and a specific port can be addressed in the following way: http://[2001:0da8:65b4:05d3:1315:7c1f:0461:7847]:8081/

Setting an IPv6 address for an Axis network video product is as simple as checking a box to enable IPv6 in the product. The product will then receive an IPv6 address according to the configuration in the network router.

Data transport protocols for network video

The Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) are the IP-based protocols used for sending data. These transport protocols act as carriers for many other protocols. For example, HTTP (Hyper Text Transfer Protocol), which is used to browse web pages on servers around the world using the Internet, is carried by TCP.

TCP provides a reliable, connection-based transmission channel. It handles the process of breaking large chunks of data into smaller packets and ensures that data sent from one end is received on the other. TCP’s reliability through retransmission may introduce significant delays. In general, TCP is used when reliable communication is preferred over transport latency.

UDP is a connectionless protocol and does not guarantee the delivery of data sent, thus leaving the whole control mechanism and error-checking to the application itself. UDP provides no transmissions of lost data and, therefore, does not introduce further delays.

Protocol Transport protocol Port Common usage Network video usage FTP (File Transfer Protocol) TCP 21 Transfer of files over the Internet/intranets Transfer of images or video from a network camera/video encoder to an FTP server or to an application SMTP (Send Mail Transfer Protocol) TCP 25 Protocol for sending e-mail messages A network camera/video encoder can send images or alarm notifications using its built-in e-mail client. HTTP (Hyper Text Transfer Protocol) TCP 80 Used to browse the web, i.e. to retrieve web pages from web servers The most common way to transfer video from a network camera/video encoder where the network video device essentially works as a web server making the video available for the requesting user or application server. HTTPS (Hypertext Transfer Protocol over Secure Socket Layer) TCP 443 Used to access web pages securely using encryption technology Secure transmission of video from network cameras/video encoders. RTP (Real Time Protocol) UDP/TCP Not defined RTP standardized packet format for delivering audio and video over the Internet— often used in streaming media systems or video conferencing A common way of transmitting H.264/MPEG-based network video, and for synchronizing video and audio since RTP provides sequential numbering and timestamping of data packets, which enable the data packets to be reassembled in the correct sequence. Transmission can be either unicast or multicast. RTSP (Real Time Streaming Protocol) TCP 554 Used to set up and control multimedia sessions over RTP

Common TCP/IP protocols and ports used for network video.

Digital Imaging web resource

In recent years, optical microscopy has slowly migrated from a dependence on traditional photomicrography using emulsion-based film and has become increasingly reliant on technology that produces electronic images. Indeed, the choice of an imaging device is a critical decision for modern microscopists, but the range of light detection methods and the tremendous variety of imaging devices available can make the selection process difficult. This collection of area array detector resources is designed to simplify this process, providing links to many of the best sites on the Internet that offer CCD and CMOS detectors, as well as other imaging solutions to microscopists.
Andor-Technology - A leader in developing CCDs with the low light sensitivity from the near infrared to the X-ray region, Andor has built a ten-year corporate history of innovation making them a top choice among researchers in Physics, Chemistry, Biology, and Engineering.
Apogee Instruments - A distributor of high-performance CCD digital imaging cameras, Apogee focuses on solutions for microscopy, spectroscopy, and astronomy. The website features CCD University, a series of tutorials on CCD technology and digital imaging.
CCD Direct - CCD Direct is a new concept in Internet marketing featuring order fulfillment for the industrial and scientific digital vision markets. The site offers a great on-line source for industrial and scientific imaging solutions.
Cooke Corporation - Cooke is a manufacturing, sales and marketing organization specializing in the areas of optical non-contact measuring and monitoring instrumentation, NIST traceable light measurement instrumentation, high speed electronic imaging and light systems and high performance CCD imaging systems. The website also features technical articles describing various aspects of CCD imaging in the biomedical sciences and the photonics industry.
Dage-MTI Inc. - Specializing in electronic digital imaging, Dage-MTI features a complete line of high resolution CCD and tube cameras, monitors, intensifiers, processors/integrators and frame grabbers.
Dalsa - Concentrating on the design and manufacture of CCD (Charge-Coupled Device) and CMOS image capture technology, Dalsa is a leader in this rapidly growing industry that is revolutionizing such applications as optical and electron microscopy, semiconductor and electronics inspection, document scanning, postal sorting and medical applications. Dalsa's website offers extensive technical support, technical papers, primers, FAQs, application notes, and third party links.
Diagnostic Instruments - Diagnostic Instruments manufactures the Spot family of color digital cameras for microscopes as well as a complete line of microscope stands, couplers and transmitted light bases.
Digital Video Camera, Inc. - The DVC company manufactures world-class digital and analog cameras for scientific imaging applications. They also package a complete lineup of imaging systems including cameras, frame grabbers, cables and software for one-stop-shopping.
Fairchild Imaging - Located in Milpitas, California, Fairchild Imaging develops and manufactures solid-state electronic imaging components, including CCD and CMOS image sensors, cameras, and systems for a wide variety of applications including optical and electron microscopy.
FujiFilm Digital Products - Fuji's Digital Products site covers their product lineup of CCD's, digital cameras, comparisons of digital products, software, printers, storage solutions and related digital video imaging products.
Hamamatsu - Hamamatsu CCD's are currently used for spectroscopy, semiconductor wafer inspection and process control, airborne sensing systems, research and astronomy, as well as optical mcirsocopy. The company continues to develop new devices based on their leading edge back-side thinning technology. Featured on the site are technical and application notes, a frequently asked questions section, glossary of terms, software support, and extensive product information.
Hitachi - The Industrial Video Systems Division of Hitachi provides color and monochrome cameras, monitors and accessories for all types of imaging applications including machine vision, robotics, microscopy, distance learning and video conferencing.
Indigo Systems - Indigo is a primary source of advanced infrared solutions for commercial, industrial, scientific, and military applications. Products and service offerings include a wide variety of infrared cameras, OEM IR sensor engines, focal plane arrays, and readout integrated circuits. The website also features on-line publications focusing on technical aspects of IR digital imaging.
Intracellular Imaging, Inc. - I³ designs, manufactures and markets low-cost, high performance digital fluorescence imaging and photometric instrumentation for the global research community. Digital fluorescence imaging, microphotometry and epifluorescence microscopes are a few of the products offered. The Intracellular website offers application notes to help researchers identify and correct problems with fluorescence digital imaging.
Keyence - Keyence manufactures advanced sensors, digital video microscopes and measuring instruments for science and industry. Their broad product line includes laser displacement meters, plc's, laser through-beam photoelectric and fiber optic sensors.
Kodak Digital Imaging - Kodak has developed an extensive website to support the solid state image sensor division, which has in-depth coverage of product information, application support, and news releases on advanced technologies and products.
Media Cybernetics - Manufacturer of the popular Image-Pro Plus scientific image analysis software package, Media Cybernetics provides a wide spectrum of solutions for digital imaging and analysis. Also included on the website are a number of tutorials, technical support articles, and software downloads.
Micron Imaging Products - Centering primarily around complementary metal-oxide semiconductor (CMOS) imaging devices, this captivating site provides great content and information covering the latest advances in CMOS technology in considerable depth. The website includes an online tutorial describing the basics of CMOS imaging.
Optronics - A leader in the microscopy digital imaging arena, Optronics produces high quality video and digital imaging systems for a variety of scientific and technical applications. The Optronics website offers a technical support section with support documents and technical white papers.
Panasonic Industrial and Medical Cameras - Panasonic offers a wide spectrum of products geared to digital imaging applications in the industrial and medical arenas. This website features detailed information about Panasonic's product lineup.
PCO AG - PCO distributes high performance Digital CCD camera systems targeted at the scientific community. The company hosts a nice site with product manuals and drivers in addition to technical information on CCDs, image intensifiers, and general optics.
Photonics - One of the most comprehensive and respected websites in the industry, the Photonics index page features daily headline news, business and technology articles, product reviews, an industry directory, a calendar of courses and events, links to trade publications, and an employment center.
QImaging - The QImaging Corporation specializes in a wide variety of high-performance CCD cameras and imaging systems for the industrial and scientific imaging markets. Their digital cameras are used around the world in a broad spectrum of applications including microscopy, cancer imaging, DNA analysis, industrial inspection, materials analysis, fluorescence imaging, academic research, and medical imaging.
Roper Scientific - Princeton Instruments, Inc., Photometrics, Ltd., and Acton Research Corporation have joined forces to become Roper Scientific, Inc., a new leader in high-performance digital imaging and spectroscopy systems. The conglomerate offers solutions in high-performance CCD imaging, motion analysis, industrial imaging, spectroscopy, and optics. Also included on the website is an extensive on-line library containing tutorials, application notes and briefs, technical notes, an encyclopedia, glossary, and white papers.
Sony Semiconductor - Sony offers a wide selection of both area and linear CCD chips for OEM manufacturers. Visit this website for data sheets and specifications on particular chips and other information concerning their digital imaging product lineup.
Sony Broadcast and Professional Visual Imaging - Sony Visual Imaging Products offers a full line of industrial CCD video cameras that are reliable even in the most demanding applications. From machine vision, factory automation, microscopy and inspection to security and process control, Sony's emphasis is on providing value, choice and flexibility.
Digital Imaging Web Magazines
Biophotonics International - Published bimonthly, Biophotonics International presents the latest global developments and techniques in the photonics industry to those in the medical and biotechnology fields. Each issue contains special applications features plus topical columns such as "Biophotonics in Practice", "Technology Solutions", or "Biophotonics in Research", in addition to the latest industry news, government news, information on courses, conferences, literature, and new product listings.
Megapixel.net - Visit one of the premier Webzine monthly Internet magazines covering all aspects of digital photography. Featured are digital camera reviews and the latest information on a wide variety of topics in this rapidly growing arena. Tutorials and technical information are included on a wide spectrum of subjects as well as quick tips and general information on digital photography.
Photonics Spectra - Published by Laurin Publishing Company, this monthly trade magazine covers news, events, and feature articles in the photonics industry. Photonics, the science of light, has a history of success in solving clinical and research problems in diverse applications through such products and techniques as spectroscopy, lasers, microscopy, imaging and fiber optics. Also featured each month is a section dedicated to analysis of new products ranging from detectors to fiber optic components, positioning equipment, testing instruments, and lasers.
Solutions! Digital Microscopy Internet Magazine - Addressing a variety of topics in video microscopy, the Digital Microscopy Internet Magazine provides detailed information on specific areas of interest to those working with imaging solutions in all areas of microscopy.

Sequential Three-Pass Color CCD Imaging

Three-pass sequential color CCD imaging systems employ a rotating color wheel to capture three successive exposures in order to obtain the desired RGB (red, green, and blue) color characteristics of a digital image. The major advantage of this technique is the ability to fully utilize the entire pixel array of a CCD imaging chip, by using one pass for each color.
Silicon based charge-coupled devices lack the ability to distinguish color information presented to the pixel elements by incoming photons. Even though electromagnetic radiation of varying energy passes through the devices to a depth determined by the wavelength, the interaction that produces free elections and holes is not color sensitive. A typical sequential color imaging system design is illustrated in Figure 1, which shows the red filter being used to pass illuminating light waves from the microscope optics to the CCD surface. The primary advantage of this technique is the ability to achieve the highest resolution capable of the device, which equals the size of the CCD array.
After all of the image information has been captured in three individual passes, it is recombined off-chip and processed in a manner similar to that of other CCD architectures. The major disadvantage of this system is the relatively long exposure times necessary to accumulate three individual color arrays, which requires an almost stationary subject and vibration-free operation of the rotating color wheel mechanical components. This technique is being slowly phased out as single-shot CCD cameras with higher resolutions become commonplace. However, a number of applications now incorporate a rapidly switchable liquid crystal array screen that can be used to capture the three colors in milliseconds, thus speeding the throughput of the device and reducing the risk of mechanically-induced vibration.
Contributing Authors
Mortimer Abramowitz - Olympus America, Inc., Two Corporate Center Drive., Melville, New York, 11747.
Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/threepass.html

Electron Multiplying Charge-Coupled Devices (EMCCDs)

The inherent advantages of scientific charge-coupled device (CCD) sensors for digital imaging in optical microscopy have made them ubiquitous in a wide variety of applications. One of the few significant shortcomings of conventional high-performance CCD cameras is that very low signal levels typically fall beneath the read noise floor of the sensor, limiting the imaging capabilities in a number of currently productive research areas that demand rapid frame-rate capture at extremely low light levels. An innovative method of amplifying low-light-level signals above the CCD read noise is employed in electron multiplying CCD technology. By incorporating on-chip multiplication gain (see Figure 1), the EMCCD achieves, in an all solid-state sensor, the single-photon detection sensitivity typical of intensified or electron-bombarded CCDs at much lower cost and without compromising the quantum efficiency and resolution characteristics of the conventional CCD structure.
The primary feature that distinguishes this novel new technology is the inclusion of a specialized extended serial register on the CCD chip that produces multiplication gain through the process of impact ionization in silicon. By elevating photon-generated charge above the read noise of the device, even at high frame rates, the EMCCD has the capability of meeting the needs of ultra-low-light imaging applications without the use of external image intensifiers. Consequently, the approach is applicable to any of the current CCD sensor architectures, including back-illuminated devices, and sensors employing electron multiplying registers are considerably less expensive to manufacture due to the signal amplification stage being incorporated directly into the CCD structure.
Several major areas of current research focus in the biomedical sciences rely on specific targeting of subcellular structures or single molecules with appropriate fluorophores in order to follow the dynamics of biological processes. The rapid kinetics combined with extremely small specimen volumes and low fluorophore concentrations utilized in such experiments require both high sensitivity and rapid frame-rate data acquisition. In the evaluation of transient, low-intensity signals, such as those encountered in single-molecule investigations, total internal reflection fluorescence (TIRFM; see Figure 1), spinning disk confocal in live-cell imaging, flux determinations of calcium or other ions, and time-resolved three-dimensional microscopy (4-D techniques), the electron multiplying CCD offers significant advantages over other sensors designed for low signal levels. Additionally, when employed with the higher signal levels of conventional fluorescence imaging techniques, the extreme sensitivity of the EMCCD system allows the use of lower fluorophore concentrations and/or lower power levels from the excitation source, thereby reducing both the potential toxicity to living cells and photobleaching of the fluorescent probe.
The performance of all CCD-based detectors has improved dramatically in recent years, and increased sensitivity has lowered detection limits in high-performance low-light imaging systems significantly. Quantum efficiency now exceeds 90 percent and read noise is limited to less than 4 electrons rms (root-mean-square) in some high performance back-illuminated CCD camera systems. This low level of read noise performance is attainable in traditional CCD sensors only at moderate readout rates, however. In addition, because the charge packet from a single pixel is often only a few electrons in challenging microscopy investigations, the signal is too frequently lost in the read noise even at slow readout rates. Furthermore, when imaging is performed at video frame rates and faster, the read noise increases to an unacceptable level, relative to the signal, in low-light conditions.
One proven solution to the read noise limitation when higher frame rates are required has traditionally been to employ an image intensifier to multiply the number of emitted specimen photons prior to detection and readout by a conventional CCD. In this approach, which is based on an operation principle similar to that of photomultiplier tubes, the signal is amplified to a level that exceeds the read noise generated at the desired frame rate. The intensified CCD (ICCD) camera system is presently among the most commonly employed imaging methods for low-light techniques such as time-resolved fluorescence experiments, ratio imaging of ion-sensitive fluorochromes, single molecule fluorescence, and other dynamic studies in living cells. These systems are sometimes referred to as proximity-focused image intensifiers and utilize a photocathode closely coupled to a micro-channel plate (MCP) electron multiplier.
The amplified electron output from the MCP is accelerated by a high potential difference onto a phosphorescent screen that converts the electrons to photons, which are subsequently relayed to the CCD surface through an optical relay lens or direct fiber optic coupling. Because potential differences ranging from 2500-5000 volts are maintained to accelerate electrons across the gaps separating the components of the ICCD, a high internal vacuum is necessary, requiring the device to be precisely assembled and completely free of contaminants. The manufacturing costs are consequently relatively high, and the intensifiers present certain other disadvantages as well, among them reduced spatial resolution compared to an equivalent conventional (non-intensified) CCD, high background noise, relatively low quantum efficiency (Figure 2), and susceptibility to irreversible damage from exposure to high light levels.
The resolution of the ICCD is ultimately limited by the resolution of the photocathode, the micro-channel plate, and the output phosphor. Continued improvements in the phosphor composition and microchannel plate architecture of modern devices has resulted in resolutions of 64 line pairs per millimeter, or better, which corresponds to a full-width at half maximum (FWHM) spot size of approximately 25 micrometers. Unfortunately, for single electron recording events (necessary in single molecule imaging) about 50 percent of the signal is spread into neighboring pixels with an ICCD, which results in considerable spatial averaging compared to an EMCCD. This effect must be carefully scrutinized when examining data collected during single molecule experiments using ICCD detector systems.
Electron-bombarded CCDs (EBCCD) are a less widely used detector variation for low-light camera systems, and in similarity to intensified CCDs, incorporate a photocathode for photon-to-electron conversion, followed by acceleration across a high-voltage gradient. The energetic electrons impinge directly on a back-thinned CCD, where they generate multiple charges, resulting in a modest signal gain. The devices can be operated at video frame rates, but have limited gain adjustment range, and exhibit similar disadvantages to the intensified CCD, including reduced quantum efficiency and resolution, and the potential for damage to the external image-intensifying components if exposed to high light levels. The development of the electron multiplying CCD employing on-chip multiplication gain provides the basis for cameras that achieve the signal gain benefit of systems using external intensifiers, while maintaining the customary CCD advantages of high, spectrally broad quantum efficiency (Figure 2), full native pixel resolution, and immunity to damage from high light levels.
Diagrammed in Figure 2 are the quantum efficiencies and spectral profiles of front and back illuminated electron multiplying CCDs, as well as the photocathodes in popular Gen II and Gen III intensifiers. The back-thinned CCD has a quantum efficiency of 90-percent or greater over the wavelength region of 500 to 700 nanometers and exhibits the highest values of any device in the near-infrared. In contrast, the front-illuminated CCD features a much lower quantum efficiency (approximately 60 percent) over a narrower wavelength range of 550 to 700 nanometers. Both of the CCD devices depicted in Figure 2 have significantly greater quantum efficiencies than the Gen intensifiers, which range between 35 and 45 percent in the visible spectral region. Intensifier photocathodes designed to operate more efficiently in the ultraviolet and infrared region are available.
On-Chip Multiplication Gain
Conventional cooled CCD cameras achieve relatively high sensitivity through the process of integrating signal within each pixel prior to readout in order to overcome read noise, which is incurred only once for each frame. At low light levels, long exposures are required in order to accumulate sufficient signal and achieve the detector's maximum read-noise performance. Consequently, frame rate speeds are limited to a relatively slow fraction of a frame up to a few frames per second. In applications suitable for "slow-scan" signal acquisition, for which the detector can be operated in the photon shot noise-limited regime, traditional back-illuminated CCD systems provide superior overall performance, including maximum quantum efficiency (as illustrated in Figure 2), which takes into account noise factors associated with electron multiplication. However, when it is necessary to capture temporal data requiring video frame rates or faster, at very low light levels, the conventional CCD camera is fundamentally limited by read noise.
The electron multiplying CCD incorporates a structural enhancement to amplify the captured signal before the charge is transferred to the on-chip amplifier, which has the effect of reducing the read noise, relative to signal, by the value of the multiplication gain factor. Because very weak specimen signal levels may produce a charge packet from a single pixel of only a few electrons, even with slow readout from a high-performance CCD, the signal is lost in read noise. The primary advantage of the EMCCD is to provide a mechanism to improve signal-to-noise ratio for signal levels below the CCD read-noise floor. In applications that require extremely fast gating (on the nanosecond level), the EMCCD is not appropriate, and intensified CCDs maintain an advantage in rapid kinetic data collection of this type.
Electron multiplying CCD sensors are produced utilizing conventional CCD fabrication techniques by making relatively simple structural modifications. The unique feature of the EMCCD is an electron multiplying structure (in effect, a charge amplifier) positioned between the end of the shift register and the output node, which is often referred to as the multiplication register or gain register (see Figure 3). This special extended serial register provides multiplicative gain following detection of photons in the device's active pixel array, and therefore, the technology can be adapted to any current CCD architecture and format. The most widely used sensors produced by the two companies that pioneered the technology employ frame-transfer architecture, and camera manufacturers have also introduced systems based on back-illuminated versions of the electron multiplying CCDs.
The functional layout of a frame-transfer electron multiplying CCD is illustrated in Figure 3, in which the gain register is added to the charge transfer path following the frame-transfer area of the chip and the conventional serial register, and preceding the on-chip charge-to-voltage conversion circuitry. The structure of the additional register differs from the regular shift register in that the full-well capacity is increased and electrons are accelerated from element to element in the multiplication register by application of much higher clock voltages at selected transfer electrodes. When charge is transferred by applying a higher-than-normal voltage, secondary electrons are generated in the silicon by the process of impact ionization. In the gain multiplying register, each stage comprises four gates, three of which are clocked as in the conventional 3-phase structure, with the fourth (between phases 1 and 2) being held at a low fixed direct current (DC) potential.
Figure 4 illustrates the transfer of charge through the gates. Note that the gates for phases 1 and 3 (R1 and R3) are clocked with drive pulses of normal potential, which is typically on the order of 5 to 15 volts (the R3 gates have zero potential for the clocking phase illustrated in Figure 4). The clock pulses used in the same phases of the regular readout register can be employed for these gates. Phase 2 (R2 in Figure 3) is clocked at higher voltage (35-50 volts) preceded by a gate held at a low DC level (denoted by the Low DC gate in Figure 4). The potential difference between the fixed-level gate and the high-voltage clocked gate results in sufficient field intensity to sustain the impact ionization process as electrons are transferred from phase 1 to phase 2 in the normal clocking sequence. Although the charge multiplication per transfer is only on the order of 1.01 to 1.016, the gain accumulated over the large number of pixels in the multiplication register (dependent on the horizontal pixel array size) is substantial, and can be hundreds or thousands. The multiplication gain is exponentially proportional to the applied high phase-2 voltage, and can be increased or decreased by varying the clock voltages.
Figure 5(a) illustrates the exponential increase in gain that accompanies increasing amplitude of the clocking voltage applied to the phase 2 electrode. It is obvious that relatively small adjustments to the voltage, beyond a certain value, result in large changes in the on-chip multiplication gain. In commercial EMCCD camera systems, this voltage adjustment is commonly mapped to a high-resolution digital-to-analog converter that can be precisely controlled through computer software. In spite of the very low probability of impact ionization occurring and the low mean gain per stage, the overall gain factor in the multiplication register can easily be in excess of 1000x due to the large number of pixels over which the electron charge packet grows in cascading fashion. The probability of secondary electron generation is dependent on the serial clock voltage levels and the CCD temperature, and as indicated above, typically ranges from 1 to 1.6 percent. While the probability of secondary electron generation is described by a complex function, the total gain (M) of the cascaded elements in the multiplication register is given by the following equation:
M = (1 + g)^N where g is the probability of generating a secondary electron and N is the number of pixels in the multiplication register. A CCD having 512 elements in the gain register and an impact ionization probability of 1.3 percent (0.013) would, therefore, generate a total charge multiplication gain of over 744.
Because of the (exponential) relationship of the multiplication gain to clock voltage, a wide adjustment range is available, allowing setting the gain to a sufficiently high level to effectively reduce readout noise to insignificant levels under most imaging conditions. Since the multiplication gain is independent of readout speed, setting a gain level equivalent to the read noise in electrons, at the desired readout frequency, produces an effective noise level of 1 electron rms. Increasing gain beyond this range will reduce noise to sub-electron levels. Significantly, by utilizing higher gain settings at faster frame rates, this noise performance can be achieved at any speed. As an example, a current high-performance back-illuminated electron multiplying CCD, with a read noise specification of 60 electrons rms at 10 megahertz, can achieve a sub-electron effective noise level with any on-chip multiplication gain value of 60 or greater.
As discussed, electron multiplication gain can be used to overcome any readout noise, although it is desirable to minimize this factor because at some level, increasing gain results in a limitation of sensor dynamic range (as illustrated in Figure 5(b)). Although the bit depth of the analog-to-digital converter of the camera system determines the maximum dynamic range, at gain levels beyond that required to overcome read noise, dynamic range will decrease due to the multiplied signal exceeding the pixel full well capacity and/or the magnitude of the output amplifier. For example, if a register designed to contain a normal full-well charge of 200,000 photoelectrons is used at a gain level of 250x, then pixels at the end of the gain register will become saturated whenever the original charge packet is greater than 800 photoelectrons. By taking specific design steps to maximize full well depth and amplifier throughput, camera manufacturers are able to provide for high bit-depth imaging with moderate gain and at high frame rates. Because this requires the readout amplifier to be optimized for larger pixels at high speeds, the read noise specification is necessarily increased. In addition, the ultimate size of a register pixel is limited by the fact that a triplet of transfer electrodes can only control photoelectrons over a maximum silicon band size of approximately 18 micrometers.
Increasing the EMCCD multiplication gain factor overcomes higher levels of read noise, but the dynamic range of the camera system suffers, limiting the use of the camera with brighter signals amenable to slower readout. To maintain full dynamic range, some electron multiplying camera systems are being equipped with dual amplifiers (see Figure 6), including a conventional unit for slow-scan wide dynamic range applications such as brightfield or fluorescence imaging, as well as a high-speed amplifier for high-sensitivity operation requiring the use of on-chip gain. Such a combination provides a camera system with the traditional CCD advantages of high resolution, high quantum efficiency, and wide dynamic range coupled to the highest sensitivity achievable.
Additional Noise and Performance Variables
Several additional factors are significant with regard to the performance of electron multiplying CCDs, including the relationship between on-chip gain and dynamic range (discussed above), other gain-related noise sources, evaluation of quantum efficiency, a phenomenon known as gain ageing, and considerations regarding cooling requirements of the image sensors. The efficiency of the impact ionization process, which produces charge gain during electron transfer in the specialized serial register, is inversely dependent on temperature. The probability of secondary electron generation increases as temperature is decreased, and consequently a camera equipped with a well designed cooling system is able to achieve higher gain values at lower clock voltage settings.
The optimum level of cooling depends on the camera system and application, but the variation of multiplication gain with temperature illustrates the importance of maintaining precise temperature stability in order to avoid adding noise to the measured signal. Dark noise arising from thermal dark current generation in the electron multiplying CCD is identical to that in conventional CCDs, and is similarly reduced by cooling the sensor. With conventional high-performance detectors, the sensor is usually cooled to a temperature at which dark current shot noise arising during the expected integration (exposure) interval is negligible. Once the dark noise is substantially lower than the noise associated with signal readout, further cooling does not provide any additional practical benefit.
Electron multiplying CCD cameras are able to detect even single-photon events when the on-chip multiplication is utilized to elevate the signal above the read noise level, and it must be recognized that any level of unsuppressed dark current is significant since it is subject to being multiplied along with the signal. Ideally, therefore, the dark current should be completely eliminated in the EMCCD, and cooling systems designed to reduce CCD temperature to -75 degrees Celsius or lower are incorporated in the most advanced camera systems.
Note that different noise components are relevant in intensified CCD systems. While signal is amplified above both dark current and read noise in the ICCD, making increased cooling less beneficial, another source of noise arising in the intensifier photocathode, referred to as equivalent background illuminance (EBI), occurs in intensified systems. The electron multiplying CCD does not exhibit EBI, and overall, dark current is a less significant limitation for the EMCCD with effective cooling than EBI is for intensified CCD cameras. Although increased cooling can reduce EBI in the photocathode, effective cooling systems for the more complicated multi-component structure of intensified CCDs, which usually include fiber optic couplings, are much less practical.
Due to the probabilistic nature of the impact ionization process utilized in the EMCCD, a statistical variation occurs in the on-chip multiplication gain. The uncertainty in the gain produced introduces an additional system noise component, which is evaluated quantitatively as the excess noise factor (or simply noise factor, abbreviated F), and which acts as a multiplying factor for both dark and photon-generated signal in the camera system. Excess noise factors vary for the different low-signal detector types, and are attributable to a combination of various loss mechanisms (if they exist) and to statistical variation in the electron multiplication process arising either in the silicon crystal lattice of the EMCCD or the micro-channel plate of the ICCD.
A conventional CCD that does not have any significant loss mechanisms or additional noise from amplification processes exhibits a noise factor of unity, as does an EMCCD utilizing normal clock voltages and producing no multiplication gain. With increased gain settings, the statistical variations begin to add additional noise, the magnitude of which depends upon both the gain and the signal level. According to theory, the excess noise factor for the electron multiplication process is approximately 1.4 (square root of 2) over a wide range of gain levels. Experimental measurements are typically lower, and range between 1.0 and 1.4 for multiplication gain factors up to 1000x. A value of 1.3 is a commonly stated average for EMCCDs, in comparison to noise factors of 1.6 to 2 for intensified CCDs employing Gen II and Gen III filmed and filmless photocathodes. Filmed image intensifiers generally have higher noise factors because of the additional loss mechanism imposed on electrons by the film.
One noise phenomenon that exists in the EMCCD, and which has no equivalent in intensified CCDs, is referred to as spurious charge or clocking induced charge (CIC). When electrons are being transferred through the multiplication register under the influence of clocking pulses, the sharp clock waveform inflections produce impact ionization in a small proportion of transfers even with normal clocking voltages. Furthermore, the clock pulses may produce a secondary electron even when no primary electron is present for transfer. By careful manipulation of clock waveform amplitudes and edges, manufacturers can minimize CIC, which is normally estimated to produce only one electron in approximately 100 transfers. Even in high-performance low noise conventional CCDs, clocking induced charge is totally lost in readout noise; however in the EMCCD at high gain settings, additional CIC is generated, and is generally treated as an additional component of dark-related signal.
Clocking induced charge is independent of exposure time, but because it is attributed to impact ionization, it is usually considered to increase with decreasing temperature, just as electron multiplication does. When EMCCDs are utilized at high gain, single electron events are recorded as spikes in the image, and any contribution from CIC would seemingly be visible. Under typical operating conditions of the EMCCD, background events causing such spikes, rather than readout noise, determine the detection limit of the camera. Recent dark image tests performed at various cooling temperatures by one manufacturer indicate that whatever the CIC contribution is to dark current, it does not appear to set a cooling limit as temperature is reduced to as low as -95 degrees Celsius. In those tests background spikes appearing above readout noise are attributed to dark current, and are dramatically reduced as temperature is lowered.
Evaluation of the signal-to-noise ratio (SNR) of an electron multiplying CCD requires that the conventional expression applied in the calculation for CCD sensors be modified to reflect the effect of on-chip multiplication gain and the excess noise factor. In effect the SNR is equivalent to the total number of photons detected per pixel during the integration interval divided by the combined noise from all sources, as follows:
SNR = (S � Q_e) / N_total where S represents the number of incident photons per pixel, and Q(e) is the quantum efficiency, or proportion of total photons actually detected as signal. The total noise in the system is represented by N(total), which combines several variables according to the following relationship:
N_total = [(S � Q_e � F²) + (D � F²) + (N_r / M)²] ^1/2 where F represents the excess noise factor, D is the total dark signal, N(r) is the camera read noise, and M is the on-chip multiplication gain. The noise terms in the denominator of the EMCCD noise equation represent the familiar CCD noise components, photon shot noise, dark noise, and read noise, respectively, with appropriate modifications to account for loss mechanisms and statistical noise sources specific to the process of on-chip multiplication gain. This is accomplished by applying the excess noise factor (F) to the first two terms, and the multiplication gain factor (M) to the read noise term. The effective shot noise and dark noise are increased by the excess noise factor, while read noise is reduced by the multiplication gain achieved in the gain register.
Both electron multiplying and intensified CCDs suffer from a gain degradation artifact known as gain ageing, which occurs in the gain register or microchannel plate of the devices. In EMCCDs, gain ageing is manifested by a slow decrease in gain over time and is quantitatively based on the total electric charge that has passed through the multiplication register. Although the exact nature of gain ageing has not been fully elucidated, CCD designers speculate that the high voltages used in the multiplication process (30 to 50 volts) trap accelerated electrons in the silicon-silicon dioxide interface region beneath the transfer electrode. The trapped electrons effective alter the electric field gradient at this point and thus create the gain ageing phenomenon. Gain ageing occurs exponentially over time and is most prominent during the first hours of use in the EMCCD gain register.
In order to compensate for gain ageing, commercial camera manufacturers often pre-age cameras at high gain settings for several hundred hours or more before readjusting the circuitry. Additionally, several manufacturers are now using computer algorithms to compensate for gain ageing and protect the gain register. Gain ageing can be controlled by reducing the gain and blocking illumination when the camera is not being used. In general, the gain should be adjusted to a level that just offers sufficient gain to overcome the readout noise. No further increase in the signal-to-noise ratio is achieved once the readout noise becomes less than one, and continuing to add gain only enhances the rate of gain degradation. Finally, the investigator can periodically monitor gain characteristics with a standardized specimen to ensure maximum performance from EMCCD cameras.
The solid-state on-chip electron multiplication of the EMCCD gives it a number of decided advantages over intensified CCDs, including preservation of the spatial resolution of the CCD, and superior quantum efficiency performance due to not being constrained by limitations of the intensifier phosphor. In comparing quantum efficiencies of different detector types, the effect of all loss mechanisms and statistical noise sources must be considered. In terms of the resulting effective quantum efficiencies, electron multiplying CCDs, particularly back illuminated versions, exhibit substantially broader and higher quantum efficiency values than any other low-light detector.
Contributing Authors
Thomas J. Fellers and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
 http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/emccds.html