Electrons and Holes blog referred to an interesting paper from the open-access Sensors magazine discussing the issue of non-linearity in dual-integration HDR mode. This non-linearity is often omitted from the HDR principle explanation, although it can greatly affect the final image quality.
Non-Linearity in Wide Dynamic Range CMOS Image Sensors Utilizing a Partial Charge Transfer Technique
Suhaidi Shafie (1), Shoji Kawahito (2), Izhal Abdul Halin (1) and Wan Zuha Wan Hasan (2)
(1) Department of Electrical and Electronic Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia
(2) Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Nakaku, Hamamatsu 432-8011, Japan
The non-linearity is caused by two factors, namely the current diffusion, which has an exponential relation with the potential barrier, and the initial condition of photodiodes in which it shows that the error in the high illumination region increases as the ratio of the long to the short accumulation time raises. Moreover, the increment of the saturation level of photodiodes also increases the error in the high illumination region.
To make the analysis more complete, I'd like to see the influence of pixel to pixel mismatches on this non-linearity and see whether these mismatches make the non-linearity less visible in the final picture. The paper relies on Link Research' SPECTRA simulator - a good demo of the simulator capabilities.