WHITE PIXEL HDR
Normally exposed RGB
Over exposed W
The dynamic range of RGBW sensors can be extended by over-exposing their W pixels. When the image sensor is exposed for RGB, and the integration time for all pixels is equal, W is automatically over-exposed by a factor of 2. Moreover, W can be further over-exposed by setting the integration time of W pixels greater than that of RGB pixels.
W pixels serve the following functions:
- They aid in reconstructing high frequencies of all demosaicked color planes.
- They help in reducing noise from the reconstructed RGB image.
Loss of W pixels due to overexposure does not affect the color of the highlights captured by RGBW sensors because the demosaicker can ignore W pixels and demosaic only RGB. Additionally, the absence of W-pixel-assisted noise removal is not a problem for highlights because they are already relatively clean.
However, the loss of W pixels due to saturation can impact the reconstruction of high frequencies. It’s worth noting that high contrast edges, where W pixels clip on the bright side but not on the dark side, can still be reconstructed without loss of resolution since the edge is visible in the W color plane.
Bright image features where W is clipped, but R, G, and B are not, including edges where W is clipped even on the dark side, may suffer from a loss of W information. The contrast of these image features is limited by the degree of overexposure of W. As long as the contrast is low, loss of resolution and aliasing resulting from W saturation may not be noticeable.
The overexposure of W pixels that occurs in sensors with equal integration times for all pixels, at a 2:1 ratio, is very difficult to perceive. In our subjective opinion, up to a 16:1 overexposure of W yields good quality images. When combined with split pixel or staggered HDR for automotive applications, important features such as traffic lights and signage can be captured without any loss of quality due to their high contrast.