Spatial resolution of thermographic cameras.

To obtain an impression of the spatial resolution of a number of thermographic cameras, a method similar to that of DODD et al. [1969] was followed: a test pattern with a series of line groups was scanned and the relative amplitude of the videosignal was plotted as a function of the spatial frequency (MTF). To make it possible to compare the various cameras directly, the spatial frequency was expressed in radians-1 and not in lines per millimeter, which makes the result less dependent on the distance. For one of the cameras, the MTF was also measured densitometrically, which yielded a lower value of spatial resolution. In practice this is of minor importance, because temperature differences are measured directly with the videosignal (isotherm mode) and not by density measurements. In contrast to Macey and Oliver [1972] who assumed that the resolution is determined by the time constant of the detector only, the resolution of the cameras investigated in our laboratory depended only on the dimensions of the detector.