A Complementary Metal-Oxide-Semiconductor Image Sensor with Analog Gamma Correction Using a Nonlinear Single-Slope Analog-to-Digital Converter

摘要

An image sensor has limited dynamic range while the human eye has a logarithmic response over a wide range of light intensity. Although the sensor gain can be set high to identify details in darker areas on an image, this high gain results in saturation in brighter areas. Therefore, gamma correction is essential to match the human eye response. However, the digital gamma correction degrades image quality, especially for darker areas on the image, due to the limited resolution and dynamic range of the analog-to-digital converter (ADC). In this paper, we propose a complementary metal-oxide-semiconductor (CMOS) image sensor (CIS) with a compact nonlinear ADC which performs analog gamma corrections that use the full dynamic range. A CIS with the proposed nonlinear ADC was fabricated with a 0.35-μm CMOS process. The test results show that the analog gamma correction provides a 2.2 dB peak-signal-to-noise-ratio (PSNR) improved image quality, which is better than conventional digital gamma corrections.