Leica thermal cameras.
How the image is created at night.
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.
Technical aids such as residual light amplifiers are able to detect the “near infrared spectrum” that is invisible to the naked eye, and convert it into visible light on a screen via electronics. In complete darkness, artificial reflection can be produced by additional infrared emitters. This technique has three disadvantages, namely the limited range, the dependence on infrared light sources, and the lack of contrast.
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.
The focused heat rays now hit a special sensor, also called a microbolometer. This sensor detects infrared radiation of the medium and long-wave range and serves as an image sensor in a two-dimensional array. The sensor sends an electronic signal to a processor, which evaluates the information from the individual pixels and converts them into an image on a screen, recognizable by the eye. Thermal imaging devices don’t have traditional eyepieces; magnification is usually done via digital zoom.
Resolution is the number of pixels (detector cells) in horizontal and vertical arrangement (e.g. 640 x 480). As a rule, the higher the number of pixels, the higher the quality of the image obtained. Higher resolution usually means a higher purchase price.
Pitch is the size of an individual detector cell (a pixel) in micrometers (e.g. 17 μm or 12 μm). The current state of the art is 12 μ m. A smaller pixel pitch does not necessarily mean a better image. Often, for a similarly good image at 12 μm compared to 17 μm, more elaborate optics are required.
NETD (Noise Equivalent Temperature Difference)
NETD (Noise Equivalent Temperature Difference) is a measure of detector sensitivity. NETD indicates the smallest temperature difference perceived by the sensor and is given in millikelvin (mK). In principle, a lower value is better, but NETD measurements are not standardized. Device performance can only be defined to a limited extent via NETD; the optics concept and the image processing are decisive.
Frame rate in hertz (Hz), or sometimes fps (frames per second), is a criterion for smooth image display during moving observation. Here, higher is better. The current state of the art is 50 Hz.
f-number (or f-stop) is the ratio of focal length to the effective lens diameter. An f-number around 1.0 is ideal.
Various aspects are decisive when choosing a material for the housing. Cost-effectiveness, manufacturability and – increasingly important at present – material availability. The properties of modern plastics are no longer inferior to those of traditional materials such as aluminum or magnesium alloys.
a-Si and VOx
a-Si and VOx describe the carrier material of the detector. VOx stands for vanadium oxide, a-Si for amorphous silicon. While a-Si was the preferred material years ago, VOx is now in the lead.
Today’s thermal cameras are equipped with organic LED or AMOLED displays, or LCD or LCoS displays with high resolution. However, display size should always be considered in relation to the sensor’s resolution. Resolution, in conjunction with the optics, determines the performance of the device.
NUC (Non Uniformity Correction) is the internal calibration of the sensor. It compensates for adverse influences such as the temperature increase of the device/ optics during operation, which can have a negative effect on image quality. This is done either by a mechanical shutter, by image processing algorithms, or simply by covering the lens with a protective cap or by hand.