Accurate focusing of knife-edge imaging optical system based on the area under MTF curve

Abstract

The method of knife-edge scanning is always adopted in testing the MTF of infrared focal plane array (IRFPA), and accurate focusing is one of the most important preconditions in the measurement procedure, because the measurement accuracy of MTF is ensured only when IRFPA is placed in the focal plane of the knife-edge imaging optical system. In this paper, a focusing method based on the area value under normalized MTF curve is proposed. Firstly, we analyzed MTF calculation model, which contained derivation of edge spread function (ESF) and then Fourier transformation. In this way, the issue of larger area under normalized MTF curve meant better focusing degree of the measurement system was proved. Next, the quasi-focal plane position of knife-edge optical system was determined according to the output voltage values of pixels of IRFPA. Then a series of MTF curves were measured when IRFPA was placed at different positions located in the optical axis beside the determined quasi-focal plane at an increment of 60 μm. Subsequently, the area under each normalized MTF curve was calculated, and the result showed the area values presented a perfect Lorentzian distribution against the positions of IRFPA. It was concluded the peak position of the fitted Lorentzian curve corresponded to the position of focal plane of knife-edge optical system. With this method, the focal plane position of knife-edge optical system was determined. The method presented here can be used to focus the MTF testing system with a high accuracy, which is good for the following MTF measurement.