Characteristic-analysis of optical low pass filter used in digital camera

Abstract

Optical Low Pass Filter (OLPF) is composed of several birefringent crystals (e.g. three quartz-crystals). A light spot can be separated into several divergent light spots after passing OLPF. This characteristic can be widely used in digital camera and other image-forming system. The imaging beam in practical use is wide spectral and convergent, which causes dispersion and deformation. This paper hereby analyzes and simulates the characteristics of frequency response, dispersion and broad-angle-incident of the OLPF in theory and in experiment. Firstly, several birefringent crystals with proper alignment are introduced into OLPF to get a better frequency response. Compared with the OLPF with one crystal, its side-lobe is suppressed to a great extent without obvious deterioration in the main-lobe. Secondly, although the original dispersion is already acceptable according to optical aberration allowance, the maximum dispersion can be reduced from 8.49% to 5.66% of the pixel-pitch of CCD by changing one of the crystals from positive crystal (quartz) to negative crystal (e.g. calcite) with the same frequency response. Thirdly, broad-angle-incident is analyzed and simulated as a further research. Suppose a conical beam which converging at the CCD passes a birefringent crystal, a circle and ellipse-like faculae instead of two circles will be got. The error is less than 25% of the ideal offset when the incident angle is as large as 14o. The OLPF with two or three crystals can be analyzed in the same way. The characteristics analyzed above are coincident perfectly with simulation in Tracepro and experiment results. Consequently, regular OLPF can meet the requirements of wide spectral range and broad-angle-incident optical system.