SWIR imagers (short wavelength infrared) are an important group of surveillance electro-optical imagers due several reasons. First, advances in InGaAs technology enabled design of relatively low cost SWIR imagers. Second, InGaAs imagers are very sensitive and can generate images of observed scenery even at dark nights. Third, SWIR imagers are less vulnerable to bad atmospheric conditions than TV/LLLTV cameras working in visible/near infrared band. Fourth, SWIR imagers can generate hi-res images even if they are built using much smaller optics than used for design of thermal imagers.
SWIR imagers can use radiation reflected by observed targets to create images of these targets similarly to VIS-NIR cameras. SWIR imagers can also use thermal radiation emitted by observed targets to create images of these targets like thermal imagers. Due these features SWIR can be tested using both methodology for testing VIS-NIR cameras or methodology for testing thermal imagers. Parameters of InGaAs FPAs can be also used to characterize SWIR imagers.
Inframet proposes to characterize SWIR imagers by three ways: A)measurement of parameters typical for VIS-NIR cameras (Resolution, Minimal Resolvable Contrast, MTF, Distortion, FOV, Sensitivity, SNR, Noise Equivalent Input, Fixed Pattern Noise, Non Uniformity), B) measurement of parameters typical for thermal imagers (MRT, MDT, MTF, NETD, FPN, non uniformity, distortion, FOV), C) measurement of parameters of InGaAs FPA module at 1550 nm (Mean Detectivity, Noise Equivalent Irradiance, Noise, Dynamic range). Limiting tests to measurement of only type A parameters is acceptable in typical case of non-cooled SWIR cameras sensitive up to about 1700nm. Expanded tests including measurement of type B parameters are recommended in case of cooled SWIR cameras sensitive up to 2200nm.
Fig. 1. ST200 test system for testing long range SWIR cameras
Fig. 2. SINIS60 test system for testing short range SWIR cameras
Inframet offers two types of test sytems for testing SWIR cameras:
- ST test systems,
- SINIS test systems.
ST test system is in general a variable target measuring system that uses a series of different targets to project their images into direction of a tested SWIR imager. The imager generates distorted copies of the projected images. Quality of the images generated by the imager is evaluated by human observers or by software and its important characteristics of SWIR imagers are measured. The ST test system consists of a reflective off axis collimator, broadband light source, medium temperature blackbody, motorized rotary wheel, a set of targets, a set of filters, PC, frame grabber, and test software.