Adaptation to the Scene in Color Imaging

In the field of digital imaging, there are pixel topologies whose color detection capability relies on the Silicon absorption properties rather than on the presence of a Color Filter Array (CFA) deposited on top of the pixels. The spectral response of the different color channels is thus not always determined a priori as in the case of a CFA, but it depends on the way photocarriers are collected from the Silicon bulk. In some cases, the spectral responses can be tuned so that the sensor has pixels with an adjustable color space. Color space tunability potentially enables new features to be implemented at pixel level. In this paper, the discussion and the experimental results are focused on the Transverse Field Detector (TFD), a CMOS photosensitive device for imaging applications that performs color detection without color filters. Thanks to the innovative working principle, based on the generation of transverse electric fields in a depleted region of a semiconductor active layer, the color space of a TFD can be real-time adjusted by means of voltage tuning. Three different applications of color space tunability are discussed: the increase/decrease of color saturation, the implementation of white balance by tunable spectral responsivities, and the improvement of interleaved imaging for high dynamic range sensors.