ST patent application US20110221944 proposes linearity improvement on Toshiba double-exposure HDR idea based on 4T pixel with skimming and described in the patent US7586523 and in white paper here. ST says that Toshiba's idea suffers from a significant non-linearity around the long and short exposures connection point:
Toshiba patent calculates the final signal using the equation:
where LS and SS are samplings of the signals LPS and SPS, GA is a coefficient that may be equal to 1, and R=TL/TS is the ratio between the long integration TL and short integration TS times.
ST proposes not to reset floating diffusion node between the long and the short exposures. Together with the new signal calculation equation, this reduces the connection area non-linearity:
The proposed signal calculation equation is:
where A is an optimization coefficient that may be equal to 0. The resulting improvement in linearity is shown on the graph: