The tapetum lucidum is located in the eye behind the retina. It is a reflective layer of tissue that reflects the incident light. This effect can be observed when the headlights of a car shine into the pupils of a cat: The eyes then light up like small lamps. In the animal itself, the reflection causes the light to pass through the retina twice and to be perceived more intensely.
Electron bombardment or thermal image
For humans to see well in the dark, they need technical assistance. This can be achieved, for example, through a technical residual light amplifier. This collects residual light, directs it into an electron tube and produces an increased light density on a fluorescent screen through accelerated electrons. Produced as a result are the characteristic green pictures in nocturnal images familiar from night-time scenes in action and documentary films.
As its name indicates, the residual light amplifier requires a minimum level of incident light. The flickering of the stars, for example, can suffice here. In a pitch-black night under cloudy skies or during fire fighting operations in an unlit, closed room, this technology is of no use, however. In such situations, heat-dependent infrared radiation comes into play. Thermal imaging cameras typically use medium- and long-wave infrared light for the imaging of objects. These are, in principle, constructed in the same way as a typical digital camera for visible light, only that their light-sensitive sensors are designed for the infrared part of the spectrum. In their images, the "light intensity" corresponds to the temperature profile: The warmer the object, the more strongly and more clearly it can be seen. Such images can, therefore, also be used to identify the sources of heat loss in buildings.
Various technologies for infrared imaging
In addition to the process that is similar to typical photography, there are other physical methods for evaluating infrared radiation. "Thermal" or IR imaging is used to identify the temperature difference between the background and foreground of an object and, of course, between areas with different temperatures. The microbolometer, for example, is a thermal sensor that can detect a very broad spectrum – from millimeter waves to UV and infrared to X-rays. In thermal imaging, it is used mainly for the detection of medium- and long-wave infrared radiation with wavelengths greater than three micrometers.
The quantum-well infrared photodetector, also known as QWIP, consists of thin layers of semiconductors that are arranged alternately. These layers limit the quantum mechanical states that a particle can take on. It responds if infrared waves are incident on the detector, thereby allowing meaningful images to be generated. These images are especially detailed and offer a high resolution that is comparable to colors. The technology is used in various areas, such as in atmospheric and space research.
Another application is active illumination, in which a thermal imaging camera is combined with an infrared light source. Similar to a conventional headlight, this serves as a light source to illuminate the scene, which can then be viewed with a corresponding night-vision device. This process is suitable, e.g., for observing dark rooms.
Image optimization by combining technologies
Different technical approaches are often combined with one another to achieve optimum results. By combining residual light amplification, thermal imaging technology and active illumination, more image information is generated, the resolution increased and the depth of focus of the images improved. Possible sources of interference that could negatively affect one of the methods are compensated for by other methods. Nevertheless, it is necessary in any case to capture, bundle and direct light waves in order to create the images. This process is fundamentally similar to conventional photography in the range of light that is visible to the human eye. For this reason, familiar optical elements such as lenses for focusing and zooming, apertures for adjusting the amount of light, filters for making adjustments and shutters for controlling the exposure are used here as well.
For focus and zoom in conventional as well as in night-vision devices, DC-micromotors of the 1516...SR and 1524...SR series with precious metal commutation are often used. The DC-micromotors of the SR series with diameters of 10 and 13 mm are also very popular. They fit into small lenses thanks to their minimal volume – without loss and with high performance values. The stepper motors of type AM1020 in combination with a lead screw are especially well suited for the positioning of filters and apertures.
Thanks to the wide variety and numerous combination possibilities of the FAULHABER drive components, the optimum solution can be found for nearly every optical application.
