Noise evaluation standard of image sensor using visual spatio-temporal frequency characteristics

Abstract

Regarding the noise evaluation of image sensor, it is important to establish the objective evaluation method which has high correlation with appearance. It is well known that visual noise standard is a noise evaluation metric using human visual characteristics. The visual noise level can vary depending on the viewing distance, spatial frequency, color and viewing conditions. A method of measuring the visual noise level is provided in ISO 15739.^[1][2] Furthermore it was discovered that visual characteristics depend on contrast and frame rate; however, the ISO method doesn't consider that. For example, since ISO15739 focus the absolute threshold of human visual system for still image, in some case, the correlation between subjective evaluation and objective evaluation was not so high. And in moving image sequences case, the faster frame rate becomes, the lower perception of noise becomes. We propose solutions to solve those problems using visual spatio-temporal frequency characteristics. Firstly, we investigated visual spatial frequency characteristics that depend on contrast and propose a new evaluation method. It shows that the image sensor with large pixel count is effective in noise reduction. Secondly, we investigated visual temporal frequency characteristics and propose a new evaluation method for the moving image sequences. It shows that the image sensor with high frame rate is effective in noise reduction. Finally, by combining two proposed methods, we show the method in which a noise evaluation is possible in both a still image and in moving image sequences. We applied the proposal method to moving image sequences acquired by the image sensor and investigated the validity of the method.