Image Quality in Thermal Imaging Systems: SENSITIVITY

How can you achieve a superb quality of thermal video image? Only by having a high-resolution thermal detector? There is another parameter that plays an important role in providing crisp image:thermal sensitivity. Sensitivity of a detector (FPA) determines the minimum temperature difference that a thermal detector can discern. It is a temperature value and usually expressed in milliKelvin (one-thousandth of a Kelvin). The lower the value, the higher sensitivity and, hence, the more detailed images a thermal camera can produce.

However, there is a direct correlation between detector’s thermal sensitivity and objective lens f-number. F-number is responsible for sensitivity and operating distance of an electro-optical device. Therefore detector’s sensitivity in conjunction with lens f-number define overall sensitivity of a thermal imaging system:

Overall Sensitivity = Detector Sensitivity x Lens F-Number

For example, you have a thermal device with detector sensitivity of 50mK and front lens f-number f/1.4 then the overall device’s sensitivity is 50x1.4=70mK. The result implies that thermal image will be 40% less clear and accurate than that with f/1.0 front lens. So even a device with a very sensitive thermal detector would deliver poor performance if accompanied by low-quality optics.

Let’s not forget that overall sensitivity largely affects DRI (Detection/Recognition/Identification) ranges especially in complex weather conditions: heat, extreme cold, rain, snow, hail, high humidity, and so on. These environmental factors may substantially impair operating distance and image clarity. 

Some manufacturers of thermal imaging devices may hide overall sensitivity of their systems claiming only high sensitivity of their detectors  while conveniently omitting f-number of the front lenses the devices are equipped with. Detectors can be extremely sensitive, however in the real world conditions it means nothing if the device is paired to a poor quality lens system.

Sensitivity of 50mK means that the detector is able to distinguish two objects with temperatures of 11.25 and 11.75 degrees respectively.

At high ambient temperatures (on a hot day or in a desert) it is much harder for a thermal imager to differentiate objects. Likewise, in extremely cold environment objects’ temperatures tend to level out making it challenging for a thermal imager to distinguish between them. These are just a couple of examples when overall thermal sensitivity becomes a key performance factor.
 
Provided by GSCI