Principle of operation of thermal imaging devices

Principle of operation of thermal imaging devices is based on transformation of thermal emission of the observed object to what the human eye can see. With the help of these devices objects can be detected and recognized in fog or haze, in difficult background, and even through relatively nontransparent barriers, for example leaves.
Can work twenty-four hours, without taking in consideration the illumination level.
But thermal imaging devices don’t allow to detect small details of the object because the thermal image changes when the temperature of the object changes (for example, the human face). It is different from the image you see in the thermal imaging device.

Main characteristics of thermal imaging matrix

Resolution (number of pixels of the receiver): is the number of sensitive elements of the thermal matrix. The number of sensitive elements horizontally and vertically determines the final image sharpness.

Sensitivity: is the smallest temperature difference perceived by two adjacent pixels (expressed in degrees Celsius or Kelvin).

Threshold of temperature sensitivity: characterizes the intrinsic noise of matrix and defines the minimum discernible difference in temperature of the object and its background. Should be better than 0.1 degrees. At a temperature of 30 degrees.

Frame rate: is a value that characterizes the speed of image changing on the screen of thermal imager. Is expressed in Herz. The larger the value, the lower is the delay in the arrival of the image to the screen of thermal imager.

Main parameters of lenses used in thermal imaging devices

Aperture: lens aperture is determined by fraction, for example, 1:1,5, where 1,5 – is a ratio of the focal length of the lens to the diameter of the entrance pupil. It means that increasing the focal length at one and the same diameter of the front lens the aperture ratio will decrease. The lens will let through less light that means that the characteristics of the device will be lower. For good quality thermal imaging lenses this parameter should be from 1:0,7 to 1:1,2.

Focal lengh: characterizes the magnification of the device. The larger the focal length is the larger is the magnification, and as a result the larger is the range of detection. The larger the focal length of the lens is the less is the instantaneous field of view which forms a larger number of pixels covering the target at a fixed range.

Range of vision: this function is a set of variables. Thus, there is no single answer to the question “how far can I see with a thermal imager?” This depends on a large number of parameters of the environment and the system, including the nature of the target (for example, parked and moving car), background characteristics (hot desert or cold snow) and weather conditions (clear sky or haze). Rain and fog can seriously limit the range of thermal imaging systems due to scattering of heat radiation by water drops. The same happens when heavy rain and snowfall. It also depends on the choice of the camera and its lens.