The current standard to characterize Electro-Optical system performance is the MRTD (Minimum Resolvable Temperature Difference) for thermal imagers and the MRC (Minimum Resolvable Contrast) for visual devices. This standard has at least three serious disadvantages: (1) the standard 4-bar test pattern is theoretically and practically unsuitable for 1D or 2D spatially sampled systems such as pixel-array camera's, (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: TOD (Triangle Orientation Discrimination threshold). The TOD is based on an improved test pattern, a better defined observer task, and a solid psychophysical measurement procedure. The new method has theoretical and practical advantages: it is suitable for pixel-array camera's, scanning systems and other (Electro-) Optical imaging systems in both the thermal and vision domain, 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 for automatic measurement, and can be extended for future sensor systems that include advanced image processing. The TOD curve can be implemented easily in a TA model such as ACQUIRE. An observer performance study with real targets shows that the TOD curve predicts TA performance better than the MRC does. The method has been implemented successfully in a thermal imager field test apparatus called TIPI, and may be implemented in current MRTD and MRC test equipment with little effort.