Signal intensity transfer function determination on thermal systems with stray light or scattering present

Abstract

Accurate Signal Intensity Transfer Functions (SITF) measurements are necessary to determine the calibration factor in the 3D noise calculation of an electro-optical imaging system. The typical means for measuring a sensor's SITF is to place the sensor in a flooded field environment at a distance that is relatively close to the aperture of the emitter. Unfortunately, this arrangement has the potential to allow for additional contributions to the SITF in the form of scattering or stray light if the optics are not designed properly in the system under test. Engineers at the US Army Night Vision and Electronic Sensors Directorate are working to determine a means of evaluating the contribution due to scatting or stray light. DISTRIBUTIONApproved for public release; distribution is unlimited. KeywordsSITF, Signal Intensity Transfer Function, 3D Noise, LWIR, MWIR, Undersampled Imaging Systems BACKGROUNDIn the current procurement environment for programs that require thermal imaging systems, thermal performance can sometimes be minimized in order to maintain a competitive cost. While there are many tradeoffs that could occur in a traditional design for a thermal imaging system, the trait that is most easily sacrificed is performance for size, weight, power, and cost so long as the minimum performance requirements are still met. Because the margin between a passing thermal system and a non-passing one are becoming increasingly small, there is a need for total transparency and repeatability by government testers.The Signal Intensity Transfer Function (SITF) measurement is a critical evaluation of performance on a thermal imaging system under test (SUT) for two reasons: it defines the characteristic equation for a thermal imaging system that relates input to output counts (and is thusly used to scale 3D Noise values). The SITF is used to determine the saturation points and the full dynamic range of that same imaging system. A properly measured SITF will account for any transmission and reflectivity losses between the source emitter and the sensor under test, and the SITF test hardware should be calibrated.In this correspondence, we present our observations on the experimental geometry dependence of the SiTF measurement along with a simple model based on stray light. This work is on-going, and we expect to refine the model as more data becomes available. However, it is important to make these observations available to the test and evaluation community in order to raise awareness and to avoid potential discrepancies between laboratories, we welcome and encourage any dialog on this subject.Recent research has posited that it is the geometry of the SITF measurement can lead to erroneous measurements for SITF and thus NETD. 1 One of the more interesting claims of this work is that the SITF of a thermal imager would change as a function of distance from a blackbody source. It is important to note that the observations that are made in the previous paper are consistent with those that are made in this research (there is a differen...