The relation between the radius of the lens aberration circle of confusion and the array period of thermal detector matrix is established, which provides the best image quality. Thermal imaging is determined by the spatial resolution and contrast. This fact has allowed us to offer new evaluation criterion for the thermal imager effectiveness - efficient spatial bandwidth, which is determined by the product of the Nyquist frequency and modulation transfer function (MTF) of the thermal imager at this frequency. We consider two criteria to match MTF lens matrix and the radiation detector, which allowed us to estimate the influence of the lens radius of the circle of confusion and the detector matrix period on thermal image quality. An example of application criteria considered for assessing the operation effectiveness of thermal imager with 25 micron pixels microbolometer arrays. It was showed, that the use of a diffraction-limited lens with a relative aperture 1:1 improves the picture quality by 19 % compared with a lens that has the same with detector"s MTF at 0.5. It has been established that use of 17 microns imager matrix can improve image quality by 22 % in comparison with thermal matrix, which has a period of 25 microns.
