Leica thermal cameras.

Every body permanently emits electromagnetic radiation: reflected radiation from light sources as well as thermal radiation. However, our eye can only process the reflected radiation, and only in the range of visible light. A large part of the reflected radiation in the ultraviolet and infrared range cannot be perceived by the human eye. Classic hunting optics such as binoculars and riflescopes are optimized to transmit visible light with as little loss as possible. The predestined lens material for this is glass, since it transmits well over 90 % of the visible spectrum.

At night, only very weak light sources are available, so hardly any light is reflected by the environment. The eye reaches its limit, due to the spectrum of visible light, and detailed vision is no longer possible.

This is where thermal imaging technology comes in. Thermal imagers do not use reflected electromagnetic radiation, but with the heat radiating from any body. Even ice radiates heat, as does any object with a temperature higher than -273 °C. This makes thermal imaging completely independent of daylight or artificial radiation sources. In fact, the thermal imager perceives very fine temperature differences, sometimes as small as 0.01 °C, which are converted into a very high-contrast image in the visible spectrum. The higher the temperature of a body, the more energy is absorbed by each pixel of the thermal imager. As a result of the thermal radiation of all individual pixels, a black and white image with many gray levels is produced. Each gray level represents a temperature difference of a few hundredths of a degree Celsius. Digital image processing in the device then converts this black-and-white image into different color scales, e.g. into rainbow colors, White Hot, Black Hot, or Red Hot.

Like a video camera, a thermal imaging device also has a lens that bundles the incident rays (in this case heat) and focuses them on the sensor. However, the lenses here are not made of glass, but of germanium, zinc sulfide, zinc selenide, or silicon materials. These materials transmit the thermal radiation optimally, while glass only transmits a fraction of this spectrum. However, producing the germanium lenses is very complex and expensive, and this is reflected in the price of thermal imaging devices.