CMOS Sensor Operation

Until recently the industrial digital vision sensor market was dominated by the CCD array. However technological advances in CMOS production techniques have led to a gradual increase in the popularity of this sensor type. Like CCD arrays, CMOS sensors are also formed on a silicon substrate but the structure is more akin to that of other CMOS technology such as RAM and ROM memory devices.
The diagram below is that of an actual CMOS sensor showing the active pixel area in green and the area occupied by the on chip circuitry in yellow, which replaces that of the shuttered area on a CCD based sensor. The on chip circuitry actually converts the charge into voltage on each pixel whereas the CCD sensor shifts the charge vertically row by row, and then horizontally pixel by pixel to be converted to voltage when it reaches one or more output nodes. This gives CMOS sensors an advantage when it comes to windowing or a region of interest as the pixels can be read out randomly. CCD sensors can only limit its region of interest vertically with the resulting image always containing the data for the full image width.

The on chip active amplifier and the sampling capacitor give CMOS sensors advantages in terms of speed, full well capacities and much improved response characteristics yet introduce dark current level noise and higher black pixel content. CMOS sensors can also produce higher levels of fixed pattern noise than that of CCD, but this type of noise can be easily removed with a software filter.

The development of CMOS sensor technology has been a rapid and varied process. The initial aim of CMOS sensors was to match the imaging performance of CCD technology, with lower power requirements and at less cost. To achieve this performance it was discovered that a much greater level of manufacturing process adaptation and deeper submicron lithography were required than initially expected. This led to the desired CMOS performance but increased development costs more than anticipated.

At first the low power feature of the CMOS imaging sensors was set to be one of their distinct advantages, however the improved development of CCD sensors means that while CMOS has the advantage in this area, the margin is now much smaller.

The integration of on chip control circuitry with the CMOS imager provides the sensor with greater flexibility and integration, the downside has been the introduction of greater noise levels. Both CMOS and CCD imaging sensors still require support chips to process the image, however CMOS imagers can be produced with more functionality on the sensor chip, as shown below.

The spectral response of a CMOS sensor differs from that of the CCD sensors in that the peak response is sited at around 700Nm. Both sensors operate over the same range, typically 200Nm to 1100Nm.

The main advantages of CMOS imaging sensors still remain as faster response, increased integration flexibility and lower on-chip power demands. However the image quality has yet to match that of the CCD and the supporting chips required to increase the CMOS image quality goes some way to squander its previous advantages. Yet neither sensor is categorically superior to the other. They both have their own advantages and disadvantages and with CMOS developers working on the image quality, and CCD developers aiming to reduce power demands and increase flexibility, the existing margins in place to decide which sensor is most suitable for an application look to narrow further.

CCD : Charge Coupled Devices

It was a historic moment for digital cameras when their shutters opened for the fraction of a second to capture the committee, award the coveted Nobel Prize to the ones who invented the electronic eyes. Had it not been the duo Willard Boyle and George Smith, those cameras might still have been shuttering light on a photosensitive film and then drying them on rails in the dark room. By inventing the Charge Coupled Device and predicting its applications way back in 1969, Boyle and Smith opened up the domain of solid state devices for imaging and memory applications.

Though Boyle and Smith invented the method to convert light into an electrical signal, the entire research would not have been possible had it not been Albert Einstein’s groundbreaking explanation of the Photoelectric effect. Einstein explained how materials could absorb incident radiation and the eventual knocking out of the electrons from the surface. Boyle and Smith studied how this incidence of light caused disturbances in electrons and how it could be utilized. They succeeded in doing this by grouping a number of capacitors into an array, something which makes up the pixels of a digital image.

Initially named as Charge Bubble Devices, the device’s operation as a shift register and a linear imaging device was recorded. It was based on a principle similar to Bucket-Brigade Devices, where charge is transferred from one capacitor to another along the semiconductor surface. Bell labs succeeded in building the first solid state video camera in 1970. By 1971, Michael F. Tompsett and other researchers at Bell Labs succeeded in capturing images with linear CCD arrays. After that, many semiconductor majors like Texas Instruments, Fairchild Semiconductors, Sony etc. started investing in CCD technologies. One of the first commercial CCD devices was built by Fairchild in 1974 which was a 100x 100 pixel device having about 500 CCDs array. The first CCD based reconnaissance satellite KH-11 KENNAN was launched in 1976. By 1983, CCDs had started to replace photographic plates in astronomical telescopes too. Companies like Kodak had been manufacturing CCD based professional cameras since 1985, but by 1995, cost effective high resolution CCD cameras started flooding the markets.

A CCD can be thought of as a subset of Charge Transfer Devices. These are based upon Metal Oxide Semiconductor (MOS) capacitors. Two types, viz surface channel and buried channel MOS capacitors have been used in CCD, but primarily buried channel capacitors have been used for manufacturing since these do not have problems caused by surface irregularities at oxide-semiconductor interface. A thin n-type buried channel is formed on a p-type substrate through ion implantation. The Silicon Dioxide insulator layer is grown on top of the n-region, and to complete the capacitor, gates of metal or heavily doped polycrystalline silicon are placed on top of the insulating SiO₂ using CVD process. To isolate the charge packets of one column from other, thermally grown oxide ‘channel stops’ are placed parallel to the channels.

 

Buried Channel MOS

But if a CCD cell were to pass along charges, it would just be any other MOS capacitor. The additional property that it is sensitive to light makes a CCD unique. The response to photons is through the epitaxial layers of doped silicon grown on the substrate. When photons are incident on the semiconductor surface, they dislodge electrons which create charge that is proportional to the light falling on the surface. A single CCD cell performs four functions:

1.      Receive charge from the cell above it in the array.

2.      Hold that charge for sometime without much loss.

3.      Pass that charge to the cell below it in array.

4.      Respond to outside stimulus like light and generate its own charge.

IP Camera: Next generation surveillance. part 2

Network camera types

Network cameras can be classified in terms of whether they are designed for indoor use only or for indoor and outdoor use. Outdoor network cameras often have an auto iris lens to regulate the amount of light the image sensor is exposed to. An outdoor camera will also require an external, protective housing unless the camera design already incorporates a protective enclosure. Housings are also available for indoor cameras that require protection from harsh environments such as dust and humidity, and from vandalism or tampering. In some camera designs, vandal and tamper-proof features are already built-in and no external housing is required. Network cameras, whether for indoor or outdoor use, can be further categorized into fixed, fixed dome, PTZ, and PTZ dome network cameras.

Fixed network cameras

A fixed network camera, which may come with a fixed or varifocal lens, is a camera that has a fixed field of view (normal/telephoto/wide-angle) once it is mounted. A fixed camera is the traditional camera type where the camera and the direction in which it is pointing are clearly visible. This type of camera represents the best choice in applications where it is advantageous to make the camera very visible. A fixed camera usually enables its lens to be changed. Fixed cameras can be installed in housings designed for indoor or outdoor installation.

Fixed dome network cameras

A fixed dome network camera, also called a mini dome, essentially involves a fixed camera that is pre-installed in a small dome housing. The camera can be directed to point in any direction. Its main benefit lies in its discreet, non-obtrusive design, as well as in the fact that it is hard to see in which direction the camera is pointing. The camera is also tamper resistant. One of the limitations of a fixed dome camera is that it rarely comes with an exchangeable lens, and even if it is exchangeable, the choice of lenses is limited by the space inside the dome housing. To compensate for this, a varifocal lens is often provided to enable the camera’s field of view to be adjusted.

PTZ cameras and PTZ dome cameras

A PTZ camera or a PTZ dome camera can manually or automatically pan, tilt and zoom in and out of an area or object. All PTZ commands are sent over the same network cable as for video transmission; no RS-485 wires need to be installed as is the case with an analog PTZ camera. Some of the features that can be incorporated in a PTZ camera or a PTZ dome camera include:

• Electronic image stabilization (EIS). In outdoor installations, PTZ dome cameras with zoom factors above 20x are sensitive to vibrations and motion caused by traffic or wind. EIS helps reduce the affects of vibration in a video. In addition to getting more useful video, EIS will reduce the file size of the compressed image, thereby saving valuable storage space.

• 3D privacy masking. 3D privacy masking is supported in most Axis PTZ dome cameras and enables selected areas of a scene to be blocked or masked from viewing and recording. It allows masking to be maintained even as the camera’s field of view changes through panning, tilting and zooming as the masking moves with the camera’s coordinate system.

• Preset positions. Many PTZ cameras and PTZ dome cameras enable a number of preset positions, normally between 20 and 100, to be programmed. Once the preset positions have been set in the camera, it is very quick for the operator to go from one position to the next.

• E-flip. When a PTZ dome camera is mounted on a ceiling and is used to follow a person in, for example, a retail store, there will be situations when a person will pass just under the camera. When following through on the person, images would be seen upside down without the E-flip functionality. E-flip electronically rotates images 180 degrees in such cases. It is performed automatically and will not be noticed by an operator.

• Auto-flip. PTZ cameras, unlike PTZ dome cameras, do not normally have a full 360-degree continuous pan due to a mechanical stop that prevents the cameras from making a continuous circular movement. However, with the Auto-flip functionality, a PTZ network camera can instantly flip the camera head 180 degrees and continue to pan beyond its zero point. The camera can then continue to follow a passing person or object in any direction.

• Auto-tracking. Auto-tracking is an intelligent video functionality that will automatically detect a moving person or vehicle and follow it within the camera’s area of coverage. Autotracking is particularly beneficial in unmanned video surveillance situations where the occasional presence of people or vehicles requires special attention. The functionality cuts down substantially the cost of a surveillance system since fewer cameras are needed to cover a scene. It also increases the effectiveness of the solution since it allows a PTZ camera or PTZ dome camera to record areas of a scene with activity.

Although PTZ cameras and PTZ dome cameras may share similar functionalities, there are differences between them:

• PTZ network cameras do not have a full 360-degree continuous pan due to a mechanical stop. It means that the camera cannot follow a person walking continuously in a full circle around the camera. Exceptions are PTZ cameras that have the Auto-flip functionality.

• PTZ network cameras are not made for continuous automatic operation or so-called guard tours where the camera automatically moves from one preset position to the next.

Mechanical PTZ network cameras

Mechanical PTZ cameras are mainly used indoors and in applications where an operator is employed. The optical zoom on PTZ cameras typically ranges from 10x to 26x. A PTZ camera can be mounted on a ceiling or wall.

Non-mechanical PTZ network cameras

A non-mechanical PTZ network camera offers instant pan, tilt, zoom capabilities with no moving parts, so there is no wear and tear. Using a wide-angle lens, it offers a wider field of view than a mechanical PTZ network camera.

A non-mechanical PTZ camera uses a megapixel image sensor and allows an operator to instantly zoom in on any part of a scene without any loss in image resolution. This is achieved by presenting an overview image in VGA resolution (640×480 pixels) even though the camera captures a much higher resolution image. When the camera is instructed to zoom in on any part of the overview image, the camera uses the original megapixel resolution to provide a full 1:1 ratio in VGA resolution. The resulting close-up image offers good details with maintained sharpness. With a normal digital zoom, the zoomed-in image often loses detail and sharpness. A non-mechanical PTZ camera is ideal for discreet, wall-mounted installations.

PTZ dome network cameras

PTZ dome network cameras can cover a wide area by enabling greater flexibility in pan, tilt and zoom functions. They enable a 360-degree, continuous pan, and a tilt of usually 180 degrees. PTZ dome cameras are ideal for use in discreet installations due to their design, mounting (particularly in drop-ceiling mounts), and difficulty in seeing the camera’s viewing angle (dome coverings can be clear or smoked).

A PTZ dome network camera also provides mechanical robustness for continuous operation in guard tour mode, whereby the camera automatically moves from one preset position to the next in a per-determined order or at random. Normally up to 20 guard tours can be set up and activated during different times of the day. In guard tour mode, one PTZ dome network camera can cover an area where 10 fixed network cameras would be needed. The main drawback is that only one location can be monitored at any given time, leaving the other nine positions unmonitored.

The optical zoom of a PTZ dome typically ranges between 10x and 35x. A PTZ dome is often used in situations where an operator is employed. This type of camera is usually mounted on a ceiling if used indoors, or on a pole or wall of a building in outdoor installations.

Day & Night network cameras

All types of network cameras—fixed, fixed dome, PTZ, and PTZ dome—can offer day and night functionality. A day and night camera is designed to be used in outdoor installations or in indoor environments with poor lighting.

A day and night, color network camera delivers color images during the day. As light diminishes below a certain level, the camera can automatically switch to night mode to make use of near infrared (IR) light to deliver high-quality, black and white images.

Near-infrared light, which spans from 700 nanometers (nm) up to about 1000 nm, is beyond what the human eye can see, but most camera sensors can detect it and make use of it. During the day, a day and night camera uses an IR-cut filter. IR light is filtered out so that it does not distort the colors of images as the human eye sees them. When the camera is in night (black and white) mode, the IR-cut filter is removed, allowing the camera’s light sensitivity to reach down to 0.001 lux or lower.

Day and night cameras are useful in environments that restrict the use of artificial light. They include low-light video surveillance situations, covert surveillance and discreet applications, for example, in a traffic surveillance situation where bright lights would disturb drivers at night.

An IR illuminator that provides near-infrared light can also be used in conjunction with a day and night camera to further enhance the camera’s ability to produce high-quality video in low-light or nighttime conditions.

Megapixel network cameras

Megapixel network cameras, available in Axis’ fixed cameras and fixed dome cameras, incorporate a megapixel image sensor to deliver images with one million or more pixels. This is at least two times better pixel resolution than what can be provided by analog cameras.

A megapixel, fixed network camera can be used in one of two ways: it can enable viewers to see greater details in a higher resolution image, which would be helpful in identifying people and objects, or it can be used to cover a larger part of a scene if the image resolution is kept the same as a non-megapixel camera.

Megapixel cameras today are normally less light sensitive than a non-megapixel network camera. The higher-resolution video streams generated by a megapixel camera also put higher demands on the network bandwidth and storage space for recordings, although this can be mitigated by using the H.264 video compression standard.

IP Camera: Next generation surveillance

Overview of a network video system

 
Network video, often also called IP-based video surveillance or IP- Surveillance as it is applied in the security industry, uses a wired or wireless IP network as the backbone for transporting digital video, audio and other data. When Power over Ethernet (PoE) technology is applied, the network can also be used to carry power to network video products.
A network video system allows video to be monitored and recorded from anywhere on the network, whether it is, for instance, on a local area network (LAN) or a wide area network (WAN) such as the Internet.
The core components of a network video system consist of the network camera, the video encoder (used to connect to analog cameras), the network, the server and storage, and video management software. As the network camera and the video encoder are computer-based equipment, they have capabilities that cannot be matched by an analog CCTV camera. The network camera, the video encoder and the video management software are considered the cornerstones of an IP-Surveillance solution.
The network, the server and storage components are all standard IT equipment. The ability to use common off-the-shelf equipment is one of the main benefits of network video. Other components of a network video system include accessories, such as camera housings and PoE mid spans and active splitters.
Benefits of network video surveillance
The digital, network video surveillance system provides a host of benefits and advanced functionalists that cannot be provided by an analog video surveillance system. The advantages include remote accessibility, high image quality, event management and intelligent video capabilities, easy integration possibilities and better scalability, flexibility and cost-effectiveness.
Remote accessibility
Network cameras and video encoders can be configured and accessed remotely, enabling multiple, authorized users to view live and recorded video at any time and from virtually any networked location in the world. This is advantageous if users would like a third-party company, such as a security firm, to also gain access to the video. In a traditional analog CCTV system, users would need to be at a specific, on-site monitoring location to view and manage video, and off-site video access would not be possible without such equipment as a video encoder or a network digital video recorder (DVR). A DVR is the digital replacement for the video cassette recorder.
High image quality
In a video surveillance application, high image quality is essential to be able to clearly capture an incident in progress and identify persons or objects involved. With progressive scan and megapixel technologies, a network camera can deliver better image quality and higher resolution than an analog CCTV camera.
Image quality can also be more easily retained in a network video system than in an analog surveillance system. With analog systems today that use a DVR as the recording medium, many analog-to-digital conversions take place: first, analog signals are converted in the camera to digital and then back to analog for transportation; then the analog signals are digitized for recording. Captured images are degraded with every conversion between analog and digital formats and with the cabling distance. The further the analog video signals have to travel, the weaker they become.
In a fully digital IP- Surveillance system, images from a network camera are digitized once and they stay digital with no unnecessary conversions and no image degradation due to distance traveled over a network. In addition, digital images can be more easily stored and retrieved than in cases where analog video tapes are used.
Event management and intelligent video
There is often too much video recorded and lack of time to properly analyze them. Advanced network cameras and video encoders with built-intelligence or analytics take care of this problem by reducing the amount of uninteresting recordings and enabling programmed responses. Such functionalists are not available in an analog system.
Techno Crate network cameras and video encoders have built-in features such as video motion detection, audio detection alarm, active tampering alarm, I/O (input/output) connections, and alarm and event management functionalists. These features enable the network cameras and video encoders to constantly analyze inputs to detect an event and to automatically respond to an event with actions such as video recording and sending alarm notifications.
Event management functionalists can be configured using the network video product’s user interface or a video management software program. Users can define the alarms or events by setting the type of triggers to be used and when. Responses can also be configured (e.g., recording to one or multiple sites, whether local and/or off-site for security purposes; activation of external devices such as alarms, lights and doors; and sending notification messages to users).
Easy, future-proof integration
Network video products based on open standards can be easily integrated with computer and Ethernet-based information systems, audio or security systems and other digital devices, in addition to video management and application software. For instance, video from a network camera can be integrated into a Point of Sales system or a building management system.
Scalability and flexibility
A network video system can grow with a user’s needs. IP-based systems provide a means for many network cameras and video encoders, as well as other types of applications, to share the same wired or wireless network for communicating data; so any number of network video products can be added to the system without significant or costly changes to the network infrastructure. This is not the case with an analog system. In an analog video system, a dedicated coaxial cable must run directly from each camera to a viewing/recording station. Separate audio cables must also be used if audio is required. Network video products can also be placed and networked from virtually any location, and the system can be as open or as closed as desired.
Cost-effectiveness
An IP- Surveillance system typically has a lower total cost of ownership than a traditional analog CCTV system. An IP network infrastructure is often already in place and used for other applications within an organization, so a network video application can piggyback off the existing infrastructure. IP-based networks and wireless options are also much less expensive alternatives than traditional coaxial and fiber cabling for an analog CCTV system. In addition, digital video streams can be routed around the world using a variety of interoperable infrastructure. Management and equipment costs are also lower since back-end applications and storage run on industry standard, open systems-based servers, not on proprietary hardware such as a DVR in the case of an analog CCTV system.
Furthermore, Power over Ethernet technology, which cannot be applied in an analog video system, can be used in a network video system. PoE enables networked devices to receive power from a PoE-enabled switch or midspan through the same Ethernet cable that transports data (video). PoE provides substantial savings in installation costs and can increase the reliability of the system.
What is a network camera?
A network camera, often also called an IP camera, can be described as a camera and computer combined in one unit. The main components of a network camera include a lens, an image sensor, one or several processors, and memory. The processors are used for image processing, compression, video analysis and networking functionalist. The memory is used for storing the network camera’s firmware (computer program) and for local recording of video sequences.
Like a computer, the network camera has its own IP address, is connected directly to a network and can be placed wherever there is a network connection. This differs from a web camera, which can only operate when it is connected to a personal computer (PC) via the USB or IEEE 1394 port, and to use it, software must be installed on the PC. A network camera provides web server, FTP (File Transfer Protocol), and e-mail functionalist, and includes many other IP network and security protocols.
A network camera can be configured to send video over an IP network for live viewing and/or recording either continuously, at scheduled times, on an event or on request from authorized users. Captured images can be streamed as Motion JPEG, MPEG-4 or H.264 video using various networking protocols, or uploaded as individual JPEG images using FTP, e-mail or HTTP (Hypertext Transfer Protocol).
In addition to capturing video, Techno Crate network cameras provide event management and intelligent video functionalists such as video motion detection, audio detection, active tampering alarm and auto-tracking. Most network cameras also offer input/output (I/O) ports that enable connections to external devices such as sensors and relays. Other features may include audio capabilities and built-in support for Power over Ethernet (PoE). Techno Crate network cameras also support advanced security and network management features.

  To be Continue.