T-CAT, a new Thermal camera aquity tester

T-CAT (Thermal Camera Acuity Tester) is a thermal 'eye chart' for measuring the spatial resolution ('sensor acuity') of thermal imaging systems. It is a small, portable system, that is used in a similar way as the optometrists' visual acuity charts. The design is an implementation of the TOD (Triangle Orientation Discrimination) method for Electro-Optical sensor characterization that bas recently been introduced. The purpose of the T-CAT is do quick and easy assessments of thermal imager performance, e.g. for routine testing such as go/nogo decisions. T-CAT measures system performance including the human observer, which is still the best way to assess actual field performance of an imaging device.
The 'test chart' consists of a 20´28 cm blackened aluminum plate with 5 rows of 4 equilateral triangle shaped cutouts. A black body plate is placed 3 cm behind the test plate. The temperature difference can be set to ±2K, ±5K, ±l0K or ±20K. The triangle size is different for each row, and the orientation (up, down, left or right) is randomized per row. A choice of test plates with different triangle size ranges is available, and each test plate can be placed in T-CAT in 4 different orientations (to minimize the effects of observers memorizing the triangle orientations).
The camera to be tested is aimed at the test plate, and the distance is chosen such that the largest triangles are just discernable on the image. An observer marks the orientation of each triangle as he/she perceives it. If the orientation cannot be discerned, the observer bas to guess. A triangle angular size corresponding to 75% correct responses (S75) is calculated from the observer's score. Sensor acuity is defined as I/S75, and is measured in mrad-1, i.e. the same unit as spatial frequency. Sensor acuity is in fact an improved way to define the 'MRTD' cut-off frequency.