Modulation Transfer Function (MTF)
The Modulation Transfer Function (MTF) describes how well a camera can represent the detail contrast of an object in the image.
The MTF is measured by, for example, recording a line grid with the camera. The brightness distribution of such a grid corresponds to a sinusoidal curve. The width of the line pattern (line pairs per millimeter – Lp/mm) describes the so-called spatial frequency. The MTF is a function dependent on the spatial frequency, which is defined as the ratio between the amplitude of the sinusoidal curve on the image, and the original sinusoidal curve. The value is always between 1 (optimal transmission) and 0. In contrast to an optical camera, infrared cameras measure thermal contrast. This is called SRF (Slit Response Function).
Within the Measurement Field of View (MFOV) of 3 x 3 pixels, the energy transfer must be greater than 90%.
Since the SRF also depends on where the measurement is taken within the image section, in the Final Inspection Report for its Precision Line infrared cameras Optris states the values for the center of the image as well as for all four corners. The spatial frequency is 9.8 Lp/mm.