Measurement of the optical transfer function using a white-dot pattern presented on a liquid-crystal display

Abstract

The optical transfer function (OTF) and its modulus, the modulation transfer function (MTF), are widely accepted measurements of the quality of optical systems. There are different ways of estimating both OTF and MTF. Random-dot-pattern methods have some advantages when computing MTFs, especially those which present the pattern on a liquid crystal-display (LCD) screen because no additional light source is needed. Nevertheless spatial information is not usually available in the image plane because MTFs are computed for the whole image in a finite number of directions only. We propose a way of providing spatial information by measuring a number of point-spread functions (PSFs). Created by a white-dot pattern on a LCD screen, white pixels operate as point sources and PSFs are calculated to eventually result in the OTF of the system. MTFs in the main directions are computed to compare with reference values obtained by the random-dot method. Sensor and LCD resolutions define the achievable MTF range. Our proposed method is used to characterize a liquid-crystal tunable filter (LCTF) attached to a monochrome camera at different wavelengths. This method, which is both easy to install and to use, achieves results with errors of less than 3%, and has advantages over classical OTF estimation methods: spatial information provided in the image plane, all frequencies and directions covered in a single capture, no additional light source needed and derivative-dependent noise avoided.

Keywords

• optical transfer function
• modulation transfer function
• spatial information
• liquid-crystal display point-spread function