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Thermal imaging cameras in video surveillance systems
The roots of thermography date from 1800, when the German astronomer Sir William Herschel performed experiments with sunlight and noticed "dark heat" beyond red light. In 1840, Sir William Herschel’s son, Sir John Herschel, produced the first infrared image, using a device called an evaporograph, based on different evaporation of a thin film of oil, caused by diverse intensity of the infrared radiation falling on the film.
Practical applications of this phenomenon took place only at the end of the First World War and during the Second World War. The first active and passive systems were able to detect people, ships and aircraft. The military potential of such systems was the reason to keep them secret - first civil applications saw the light of day in the late 50s of the twentieth century. From that moment on, one can see systematic development and popularization of that infrared technology. Achievements in the construction of thermal imaging cameras (also called thermographic cameras, thermal imagers, or simply thermal cameras) enable the reduction of their weight, size, and offer the possibility of commercial use. A recent breakthrough in the construction of the cameras has been possible by the use of much cheaper bolometric sensors based on the change of electrical resistance caused by the incident radiation. Since then, the cameras have become available not only for the army and scientists. Currently, the cameras have similar sizes and weights to "normal" optical devices and are affordable for a wide range of users.
The use of thermal imaging cameras is no longer limited to buildings or installations critical to the defense of a country, or scientific applications. Cameras of this kind are employed to monitor unlit areas with dense vegetation, various processes in factories, power plants, oil and gas installations, at airports, etc. The best results can be achieved by creating systems consisting of optical and thermal cameras. This is due to different principles of operation and diverse forms of the generated images. An important part of such systems is image analysis. Thermal imaging cameras used for monitoring purposes usually have essential tools for image analysis, such as motion detection and detection of temperature changes beyond a preset range.
Principle of operation
Comparison of "normal" CCTV cameras and a thermal imaging camera (in the bottom): the "normal" cameras capture visible or infrared light reflected by objects illuminated by daylight or artificial sources of visible or near-infrared light; the thermal imaging camera captures mid-infrared radiation generated by objects, so that it does not need any source of visible or IR light
"Normal" (optical) analog or IP cameras used in video monitoring systems capture visible and (usually in night mode) near-infrared radiation (in total: 300-950 nm) reflected from objects in the field of view. The sources of the radiation can be the daylight or artificial lighting systems, including IR illuminators. Thermal imaging cameras work properly without any additional source of lighting. Each body with a temperature above absolute zero emits infrared radiation depending on the temperature and physical properties of its surface. Infrared cameras designed for security systems typically detect wavelengths between several to 20 micrometers (mid-infrared range).
Images from a CCTV camera, on the left - in low-light conditions, on the right - in IR mode (with the built-in IR illuminator switched-on)
An image from a thermal camera in the dark, without any additional source of illumination
Thanks to the relationship between temperature and the intensity and spectral characteristics of the mid-infrared radiation, taking into consideration the features of the emitting surfaces, the image from a thermal camera can show the distribution of temperature on the surfaces of the objects in the field of view, so it can be graduated in temperature scale divisions. This enables the user to read the temperatures of the interesting points on the surface of various objects. Due to no lighting required for proper operation of the camera, the device remains invisible in the night (an intruder can be unaware of its presence).
A thermal camera can detect and clearly present temperature differences in the scene. Therefore, objects invisible for an ordinary camera (e.g. people or animals hidden in bushes) are visible in the image from the thermal camera.
Due to no lighting required for proper operation of thermal cameras, the devices are invisible in the night (an intruder can be unaware of its presence). They are ideal for monitoring large areas with no or scarce artificial lighting (forests, ports, unlit parking lots, etc).
Of course, thermal imaging cameras have some limitations. Adverse weather conditions such as fog, snow and rain suppress infrared radiation, which reduces the range of operation and temperature resolution (they "flatten" the temperature variation of the observed scene). However, the cameras are still far more effective in such difficult conditions than "normal" CCTV cameras.
An image from the K1606 thermographic camera with a man in the field of view
The use of thermal imaging cameras for temperature measurements
Thermal imaging cameras are often used for remote temperature measurements of various objects, with notifications about exceeding arbitrary thresholds. The problem of calibration of such temperature measurements is not straightforward and depends on many factors. However, when the measurements do not have to be very accurate, the calibration issue can be reduced to two presets of the camera, the emissivity of the surface of the examined body and the temperature of the environment.
The emissivity is a feature of the surface of an object. It is the ratio of the thermal radiation from the surface to the radiation from an ideal black surface at the same temperature. For example, the emissivity of polished aluminum is ca. 0.05 and of paper is ca. 0.9.
The ambient temperature setting is required to compensate for the radiation reflected from surface of the examined object. If the emissivity of the object is low, the setting of the ambient temperature is crucial.
When a thermal imaging camera is used only to diagnose temperature differences, for example to detect the hottest places or record temperature changes, the emissivity and ambient temperature settings are usually not required.
A temperature image of the inside of an electric circuit panel (in false colors)
Optical systems and resolution
Relatively high prices of thermal cameras are dictated by the necessity to use special thermal detectors and dedicated lenses. The lenses must be made of a material that transmits far-infrared radiation (ordinary glass strongly absorbs it), and have to be equipped with special filters minimizing the noise (i.e. the unwanted wavelengths).
The focal length of the lens should be adjusted to the required width of the screen and its resolution. Just as in ordinary cameras, there are rules governing the ability of detection and recognition of various objects (the important parameter is the suitable number of pixels in width or height of the image necessary for the required purpose).
The principle of operation of thermal imaging cameras
Sunell thermal imaging cameras
For years now, thermal Sunell cameras have been among the world's best thermal imaging cameras for applications in video surveillance systems. Some of the cameras are equipped with network interface and are compliant with Onvif 2.4, so they can be easily integrated in IP CCTV systems with other IP devices. Thanks to using high-end infrared detectors, proprietary image processing algorithms, and new-generation lens with high transmission factor in the infrared range, the cameras work perfectly in any weather conditions (independently from fog, rain, etc). In addition to typical features and functions of IP cameras (e.g. motion detection), the cameras have additional menu associated with thermal imaging. The indicator in the main field of view of such camera shows the point with the highest temperature that is displayed in digital form. With a mouse, the user can place the pointer anywhere in the field of view and read the local temperature. The user of the cameras can set up to 5 different detection areas with different types of temperature alarms. Aside from motion detection, the cameras support two types of warnings/alarms with independent thresholds for warnings and alarms:
  • Threshold Alarm - the camera triggers warning/alarm after defined temperature thresholds have been exceeded
  • Temperature Difference Alarm - the camera triggers warning/alarm after a defined temperature difference has been exceeded between the coldest and hottest point in the detection area
The SN-TPC4200KT/F K1606 camera is an IP device for fixed applications. The camera can be connected directly to an NVR or be operated via IP network. By integrating in a single system "normal" IP CCTV cameras and thermal imaging cameras, the effectiveness of the video surveillance system can be greatly increased. Thanks to IP technology, the users can have direct access to live images from the optical and thermal cameras, as well to the recordings from the both types of the cameras via local network and also via the Internet (if so configured).