An Approach to Improve Cell-Phone Cameras' Dynamic Range Using A Non-linear Lens Correction

摘要

Most cell-phone cameras today use CMOS sensors with higher and higher pixel counts, which in turn, results in smaller pixel sizes. To achieve good performance in current technologies, pixel structures are fairy complicated. Increasing complexity in pixel structure, coupled with optical constraints specific to cell-phone cameras, results in non-uniform light response over the pixel array. A cell-phone camera sensor module typically has a light-falloff of -40% from center relative to an edge. This high fall-off usually has non-radial spatial distribution making lens fall-off corrections complicated. The standard method of reducing light fall-off is linear (i.e. multiplicative gain), resulting in close to a ～2x peripheral gain and a corrected image with lower dynamic range. To address this issue, a novel idea is explored where the fall-off is used to increase the dynamic range of the captured image. As a typical lens fall-off needs a gain of up to 2x centre vs edge, the fall-off can be thought of as a 2D neutral density filter which allows up to 2x more light to be sensed towards the periphery of the sensor. The proposed solution uses a 2D scaled down gain map to correct the fall-off. For each pixel, using the gain map, an inflection point is calculated which is used to estimate the associated pixel transfer characteristic which is linear up to the inflection point and then becomes logarithmic.