The Analysis of Selecting Infrared Bands
Infrared imaging technology with wavelengths ranging from 3 to 5 μm and 8 to 12 μm has always been the focus of the development of infrared imaging technology. Which of
these two bands is superior in long-range detection? The conclusions are
different because of different assumptions, application occasions and selected
data. This issue has been under discussion since the 1970s, showing its
importance. The sensitive detector of the infrared imaging system responds to
the radiant power of the target and the background. Therefore, specifically,
infrared imaging is realized by using the difference of the radiant exitance
between the target and the background. This difference in radiance is called Radiant
Contrast, which is not equivalent to the difference in radiation temperature.
For the same temperature difference, the radiant contrast varies from band to
band. Therefore, the maximum target background radiant contrast can be realized
by selecting an appropriate spectral passband.
Practical comparison of two infrared bands
Relatively high temperature targets have strong radiation in the 3-5 μm band. For side or tail chase detection, the 3-5μm band of the tail flame radiation from the nozzle accounts for more than 60% of the total energy. At this time, the skin radiation is relatively small, which is suitable for detecting air targets. In hot and humid areas or areas with high atmospheric moisture, the 3-5 μm band is better than the 8-12 μm band. The 8-12 μm band is suitable for working in low temperature and dry climate conditions. Considering the characteristics of the target detected, it is better to use the 3-5μm band to detect supersonic missiles; it is better to use the 8-12 μm band to detect subsonic missiles.
For infrared systems, under the condition of optical diffraction limitation and the same resolution angle, the optical aperture in the 3-5 μm band is half smaller than that in the 8-12 μm band. The more portable optics makes a great difference in reducing the size and weight of the infrared system. Moreover, after comparing the cost and price of detectors and optical systems of the same scale in the two bands, it can be found that the price of the 3-5 μm band is lower than the price of the 8-12 μm band.
Correct selection of the spectral passband is essential to fully utilize the performance of the infrared imaging system and improve the sensitivity. Of course, when we need to selection the band, we should also fully consider factors such as the atmospheric transmission window, the detection object, and the working environment. These factors are the main guidelines for infrared system design. It is remarkable that the 3-5 μm band and the 8-12 μm band have their own advantages and disadvantages. Therefore, in the long run, it is a development trend to use dual-band detection and even short-wave infrared detection.
For targets around 300K (such as helicopters, aircraft and head-on missiles), the skin radiation is mainly concentrated in the 8-12μm band. Because the peak wavelength of radiation at 300K is 10μm, and the 8-12μm band is wide enough. This is one of the main reasons that a certain type of infrared system selects the 8-12 μm band when a single band is used.