The Effects of Gain, Integration, Frame Rate and Optics on NETD in Microbolometer Infrared Camera Systems
To better understand NETD and its relationship to infrared imaging systems we must first take a look at the variables, and their effects, as they apply to any infrared camera system and how this might also affect a particular application.
For purposes of this discussion we will use the (640 X 480 Vanadium Oxide) microbolometer. Its output will be sampled by a 14 bit ADC (Analog to Digital Converter) to give an Intensity Count (the perceived amount of radiation) as an integer between 0 and 16,384. This Intensity Count is relative to an initial sampling of temperature, via a Black Body source, which is set as the center of the Temperature Range, i.e. 8,192. We refer to this as a “Cold Calibration” which simply serves as the starting point for our Radiometric Calibration. Variables, such as Gain, Integration Time, Frame Rate and Optics all play a role and directly affect both the NETD (Noise-Equivalent Temperature Difference) and the Temperature Range that a radiometric calibration will be able to perceive. NETD is a measurement of the Standard Deviation of the full array divided by the perceived Intensity Counts per degree of Celsius
The internal gain setting of the (640 X 480 Vanadium Oxide) microbolometer has 8 settings including: 1.0, 1.4, 1.8, 2.5, 3.5, 4.2, 5.6 and 8.0. Simply put, the higher the gain setting, the lower the NETD and Range. Giving all other things equal, lowering the Gain will increase the temperature range you are able to perceive. But, this will also increase the NETD, decreasing sensitivity and causing the image to become grainy.
The internal integration settings of the (640 X 480 Vanadium Oxide) microbolometer has 8 settings from 0 to 7. The higher the Integration Time the lower the NETD and Range. It is our experience that lowering the integration setting will increase the range without affecting the NETD as much as simply decreasing the Gain.
Our ICI 7640-P which uses the (640 X 480 Vanadium Oxide) microbolometer, has 10 different settings which allow us the ability to set the frame rate from 5 Hz to 22 Hz. As we slow the frame rate, the amount of time each pixel is allowed to perceive the amount of radiation that is focused on it increases. We refer to this as Dwell Time and it can have a large impact on NETD and Range. For instance, in a calibration where the frame rate is 22 Hz we can achieve a 42mk NETD with a range of 104C. By simply lowering the frame rate to 6 Hz we also lower our NETD to 30mk and our Range to 50C.
The optic of an infrared camera system has a very important role in NETD and Range. An F/1.0 optic, for instance, is much more efficient at focusing radiation than an F/1.4 optic. Other factors such as optic coating, filters and athermalization are key factors but are outside the scope of this discussion.
In applications for Medical use, where NETD, accuracy and image quality are essential, it is ICI’s goal to build a camera with the lowest NETD possible. This is achieved by using very high grade f/1.2 or less optics, a slower frame rate camera, and the highest gain setting with the lowest effective temperature range which is centered at it’s would be target…the Human Body.
Knowing this, the ICI 7640-P and 7320-P use the following calibration parameters. Cold Calibration is performed at 35C and Gain and Integration are set to limit the camera’s range from 10C to 60C. This increases accuracy as the 16,384 Intensity Counts are divided into this small range of 50C. This yields us approximately 300 Intensity Counts per degree of Celsius which allows for better accuracy and better NETD.
Cameras that have been calibrated to operate within higher ranges (i.e. -20C to 200C) must divide these Intensity Counts over a broader range thus increasing the NETD.
All infrared camera manufacturers give the NETD values of their cameras. It is my belief that these values are sampled during the optimum setting as discussed above. This is not the true NETD of the camera as it is calibrated at a higher range for most end user applications but used more to show the efficiency that microbolometer is capable of achieving.