Triangle orientation discrimination: the alternative to minimum resolvable temperature difference and minimum resolvable contrast

The characterization of electro-optical system performance by means of the minimum resolvable temperature difference (MRTD) or the minimum resolvable contrast (MRC) has at least three serious disadvantages: (1) the bar pattern stimulus is theoretically and practically unsuitable for 1-D or 2-D spatially sampled systems such as pixel-array cameras, (2) spatial phase is not taken into account, and (3) the results depend on the observer's subjective decision criterion. We propose an adequate and easily applicable alternative: the triangle orientation discrimination (TOD) threshold. The TOD is based on an improved test pattern, a better defined observer task, and a solid psychophysical measurement procedure. The method has a large number of theoretical and practical advantages: it is suitable for pixel-array cameras, scanning systems and other electro-optical and optical imaging systems in both the thermal and visual domains, it has a close relationship to real target acquisition, and the observer task is easy. The results are free from observer bias and allow statistical significance tests. The method lends itself very well to automatic measurement, and can be extended for future sensor systems that include advanced image processing. The TOD curve can be implemented easily in a target acquisition (TA) model such as ACQUIRE. An observer performance study with real targets shows that the TOD curve better predicts TA performance than the MRC does. The method has been implemented succesfully in a thermal imager field test apparatus called the thermal imager performance indicator (TIPI) and may be implemented in current MRTD test equipment with little effort. (C) 1998 Society of Photo-Optical instrumentation Engineers.