HDR Pixel Thesis

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Glasgow University, UK, publishes a PhD thesis "High dynamic range image sensor using tone mapping operation" by Waqas Mughal. Here is the HDR pixel principle:

"WDR capture can be performed by introducing a monotonically increasing reference signal Vref. It is possible to capture high intensity information by comparing the integrated voltage at node N to a reference voltage Vref.

The pixel output follows a known reference signal, which is sampled and held at a value when the photo-generated signal on the diode becomes lower than the reference voltage. The potential at which these two signals are equal is recorded and is used as the pixel’s response. In the pixel, M1, M2, M3 and M4 are reset device, reference voltage switch, source follower and row select switch.
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The Vref sweep function can be used for the tone mapping, once the pixel FPN issues are solved:

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