Ccd Day Night
The Three Primary Technologies for Night-time Video Surveillance
Introduction
There are lots of challenges in performing video surveillance at night. Selecting the best video cameras for a particular application will depend on the specific requirements that are deemed necessary. For example,
- Are both day and night video operation desired?
- Does the surveillance system need to be covert (so that no visible illumination is used at night)?
- Does the surveillance system need to be undetectable and completely "passive"?
- What is the size and shape of the area to be monitored? How far are objects to be observed?
- Is the goal of the surveillance to detect, recognize or identify objects in the field of view?
The "best" system design for a specific night-time video surveillance system will use one of the three primary night vision technologies: low-light imaging, thermal imaging and near-infrared illumination. In this article, these three technologies are explained and the advantages of each reviewed.
As shown in the figure that can be referenced at link below, each night vision technology exploits a different range of the electromagnetic spectrum. Low-light imaging devices basically work in the visible light range, normally 400-750 nm. Near-infrared illumination operates in the non-visible near-infrared spectrum, wavelengths slightly longer than visible light. Thermal imaging operates in the infrared range of wavelengths, normally 3-5 microns or 8-12 microns.
Electromagnetic Spectrum
The table that can be referenced at the link below summarizes some of the differences in the technologies.
Low-light Imaging Although image intensifiers are the most common night vision technology, they are predominant in portable applications such as night vision photography and night-time electronic news gathering. For video monitoring applications, other technologies exist that produce results that are in many ways superior. Specifically, electronic multiplication CCD detectors, or EMCCD cameras (described below), have become popularized for performing low-light security and surveillance monitoring. Other enhanced CCD cameras including integrating CCD cameras are also available that perform image processing tricks to enhance certain aspects of camera performance at the expense of other performance specifications.
EMCCD Cameras Electron multiplication CCD or EMCCD cameras incorporate a significant advancement to CCD detector technology that results in performance better than intensified CCDs. At the heart of the EMCCD is a highly sensitive CCD detector that has its image signal amplified on the chip while it is still in the charge domain. The multiplication gain (which takes place after photons have been detected in the device's active area but before one of the detector's primary noise sources) amplifies signal electrons up to 1000 times so that they may be detected above the CCD output and camera electronics noise.This allows the sensor to operate in real time with sub-electron equivalent readout noise, enabling very dim sources and very dark scenes to be imaged. These sensors maintain full well capacity and high quantum efficiency while minimizing blooming from strong light sources in the object being imaged. As a result, EMCCDs are now preferred in many low-light applications such as night-time surveillance and astronomy. In addition, EMCCDs have the capability to operate across the full range of light levels which results in it's usefulness for 24-hour surveillance.
Advantages of EMCCD Cameras:
- High sensitivity to low-light
- High spatial resolution & contrast
- No halo from bright sources allowing visible detail in adjacent pixels
- Not susceptible to damage from bright lights
- No scintillation giving improved image quality
- Solid state giving lower lifecycle cost
- Excellent for day and night operation
Integrating CCD Cameras Another method for improving a CCD camera's sensitivity is to perform averaging to reduce noise either temporally (where sequential video frames are averaged) or spatially (where neighboring pixels are "binned" or added together). By performing special processing of the signal inside the CCD, superior performance over standard CCD cameras can be achieved. Certain CCD cameras have the capability to automatically perform binning (where the signal from adjacent pixels are averaged) and frame averaging (where the pixel signal from sequential video images are averaged) when light levels fall below acceptable values. These image processing techniques decrease noise levels and result in superior low-light performance up to 100 times lower than standard CCDs. However, this technique does have compromised performance. Binning will result in a decrease in resolution. Normally, binning is performed in the vertical dimension first, retaining the level of horizontal resolution normally important when imaging objects at the horizon. Temporal integration causes the apparent sensitivity to still objects increases while moving objects may become blurry.
Thermal Imaging Different from low-light imaging methods of night vision (which require some ambient light in order to produce an image), thermal imaging does not require any ambient light at all. Thermal infrared cameras operate on the principal that all objects emit infrared energy as a function of their temperature. In general, the hotter an object is, the more radiation it emits. An infrared camera is a product that collects the infrared radiation from objects in the field-of-view and creates an electronic image based on the level of radiation detected. Since they do not rely on reflected ambient light, infrared cameras are entirely ambient light-level independent. In addition, they also are able to penetrate obscurants such as smoke, fog and haze. There are two types of thermal imaging detectors: cooled and uncooled. Cooled detector infrared cameras require cryogenic cooling to very cold temperatures (below 200K). Uncooled detector infrared cameras are normally either temperature stabilized (at room temperatures) or entirely unstabilized. Thermal images are normally black and white in nature, where black objects represent cold objects and white objects represent hot objects. Some thermal cameras show images in color. This false color is an excellent way of better distinguishing between objects at different temperatures.
Thermal infrared cameras operate on the principal that all objects emit infrared energy as a function of their temperature.
Cooled-detector Infrared Cameras
Cooled infrared detectors are typically housed in a vacuum-sealed case and cryogenically cooled. The detector designs are similar to other more common imaging detectors and use semiconductor materials. However, it is the effect of absorbed infrared energy that causes changes to detector carrier concentrations which in turn affect the detector's electrical properties. Cooling the detectors (typically to temperatures below 110 K, a value much lower than the temperature of objects being detected) greatly increases their sensitivity. Without cooling, the detectors would be flooded by their own self-radiation. Materials used for infrared detection include a wide range of narrow gap semiconductor devices, where mercury cadmium telluride (HgCdTe) and indium antimonide (InSb) are the most common. Cameras based on cooled HgCdTe and InSb detectors have similar performance, however HgCdTe detectors are often preferred because they exhibit higher quantum efficiency and wavelength tunability (including performance in shortwave, midwave and longwave bands as well as dual-band).
Advantages of Cooled-detector Infrared Cameras:
- The highest possible thermal sensitivity
- Able to detect people and vehicles at great distances
- Not affected by bright light sources
- Able to perform high speed infrared imaging
- Able to perform multi-spectral infrared imaging
Uncooled-detector Infrared Cameras Unlike the cooled detector cameras described above, uncooled infrared detectors operate at or near room temperature rather than being cooled to extremely low temperatures by bulky and expensive cryogenic coolers. When infrared radiation from night-time scenes are focused onto uncooled detectors, the heat absorbed causes changes to the resistance properties of the detector material. These changes are then compared to baseline values and a thermal image is created. Despite lower sensitivity than realized in cameras using cooled detectors, uncooled detector infrared cameras are smaller, much less costly and have higher operating reliability over time. As a result, these new technology infrared cameras open many viable commercial applications.
Advantages of Uncooled Infrared Cameras:
- Relatively inexpensive compared to other thermal imaging technologies
- High contrast in most night-time scenarios
- Easily detects people and vehicles
- Not affected by bright light sources
- Higher reliability than cooled detector thermal imagers
Near Infrared Illumination A popular and sometimes inexpensive method for performing night vision is by near infrared illumination. In this method, a camera that uses a detector that is sensitive to invisible near infrared radiation is used in conjunction with an infrared illuminator. The Sony Night Shot camcorder popularized this method. Because of the IR sensitivity of the camcorder's CCD detector and since Sony installed an infrared light source in the camcorder, infrared illumination was available to augment otherwise low-light video scenes and produce reasonable image quality in low-light situations without the use of visible light sources.
The method of near-infrared illumination has been used in a variety of night vision applications including perimeter protection where, by integrating with video motion detection and intelligent scene analysis devices, a reliable low-light video security system can be developed.
Several different near infrared illumination devices are available today, including:
- Filtered incandescent lamps: A standard high power lamp covered by a filter designed to pass the lamp's near infrared radiation and block the visible.
- LED type illuminators: These illuminators utilize an array of standard infrared emitting LEDs.
- Laser type: The most efficient infrared illuminator, these devices are based on an infrared laser diode that emits near infrared energy.
Near infrared illuminators are typically available in a range of wavelengths (e.g. 730nm, 830nm, 920nm). Providing supplemental infrared illumination of an appropriate wavelength not only eliminates the variability of available ambient light, but also allows the observer to illuminate only specific areas of interest while eliminating shadows and enhancing image contrast. The supplemental near infrared lighting improves the quality of solid state cameras that have the ability to convert near infrared images to visible.
Advantages:
- Lowest cost compared to other night vision technologies.
- Eliminate shadows and reveal identifying lettering, numbers and objects. Can also be used to perform facial identification.
- Able to perform high-speed video capture (such as reading license plates of moving vehicles).
- IR illuminators can see through night-time fog, mist, rain and snowfall as well as windows
- Eliminates the variability of ambient light.
Summary Clearly, the optimal solution for a particular night-time video surveillance application will depend on the requirements for that application. The "best" design will utilize one of the three primary night vision technologies: low-light imaging, thermal imaging and near-infrared illumination.
The table that can be referenced at the link below summarizes some of the differences in the technologies.
The following refer to each capability in the table:
24-hour surveillance - All three technologies permit (to some capacity) 24-hour surveillance.
Detection - Thermal imaging is considered the superior technology when detection is required. Most objects under surveillance, whether they are people or vehicles in motion, have thermal signatures that are distinct from the background. As a result, these objects normally show high contrast with respect to the background and are easy to spot.
Recognition/identification - For recognition/identification applications, thermal imaging is not the preferred technology since thermal signatures are in many cases difficult to identify and distinguish. Low-light imaging is best since it generates an amplified version of the visible light signature. Near-infrared illumination is also useful, but it is important to realize that reflectivity in this spectral range may differ from the reflectivity in the visible range. As a result, identifying marks on vehicles may be difficult to read, hair color and complexion will also differ when imaging in these wavelengths.
Passive operation - Low-light and thermal imaging are both passive technologies since neither surveillance method is easily detected. Near-infrared illumination, on the other hand, is considered active since it emits a beam of near infrared light that can be detected with other near-infrared sensitive devices.
Ambient light level independent - Thermal imaging and near infrared illumination are somewhat light level independent. The presence of sunlight will cause solar heating that will impact thermal images, creating shadows and hot spots that are different than night-time. Near-infrared systems are often considered independent of ambient light since they emit their own illumination, but quite often these systems take advantage of the sun's near-infrared radiation and may disable the separate illuminators. Image quality for low-light imagers will depend greatly on the ambient conditions.
For an online version of this article with all referenced illustrations, please click here.
About the Author
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Electrophysics - Night Vision for Digital Photo/Video
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