The minimum resolvable temperature difference (MRTD), the minimum resolvable contrast (MRC), and the triangle orientation discrimination (TOD) are end-to-end EO system performance measures; that is, laboratory measures that characterize EO system performance with a human observer in the loop. Such system performance measures are used: 1) to verify if a system is working properly; 2) to compare competing sensor systems; or 3) to predict field performance. The MRTD or MRT is defined in NATO STANAG 4349 (NATO Standard Agreement[1]). The primary measure of performance for IR/EO imaging systems is the MRT/MRC. It is a visual acuity test that combines sensitivity and resolution in a manner that reflects human performance through a sensor. Basically, a standard periodic four-bar pattern on a uniform background (Fig. 1) is presented to an observer using the camera under test. The observer adjusts the lowest contrast at which the four bars can be resolved. The procedure is repeated at several spatial frequencies and yields a relation or threshold curve between spatial frequency and minimum resolvable temperature difference. Theory, application, and measurement method are described elsewhere in this encyclopedia.[2] The TOD is described in detail by Bijl and Valeton.[3,4] A short description is given in this entry.