Method and apparatus for providing offset for light detector

A light imaging system comprising:

• a plurality of row electrodes and column electrodes;
• an array of rows and columns of photo detector elements, each said element including a photo diode having an anode and a cathode, and a transistor having first and second power electrodes and a control electrode for controlling current flow between said first and second power electrodes, said control electrode being connected to one of said row electrodes, said first power electrode being connected to one of said column electrodes, said second power electrode being connected to said cathode of said photo diode, and said anode of said photo diode being connected to a negative voltage -V.sub.b ;
• means for charging said photo diodes by connecting said column electrodes to a voltage V.sub.c and connecting said row electrodes to a voltage V.sub.on which is positive relative to V.sub.c ;
• means for exposing said array to photons after said charging means charges said photo diodes;
• means for impeding said photo diodes from being recharged, while said exposing means exposes said array to photons and after said exposing means exposes said array to photons, said impeding means including said transistors and including means for connecting said row electrodes, prior to said array being exposed to photons but after said photo diodes are charged, to a voltage which is negative relative to V.sub.c and negative relative to -V.sub.b ;
• means for sequentially connecting the rows to the voltage V.sub.on, for a predetermined period of time, after said exposing means exposes said array to photons and after said impeding means impedes said photo diodes from being recharged;
• means for respectively measuring, at the column electrode for each photo diode in the row connected to V.sub.on by said connecting means, the charge required to restore the photo diode to its voltage before it was exposed to photons and after it was charged by said charging means, for a period of time longer than the predetermined period of time so as to provide a settling period;
• means for resetting the measuring means during a reset period after the settling period and before the next row is connected to V.sub.on ;
• and
• means for providing, on a row other than the row being measured by said measuring means, a negative voltage transition, starting when or before the row being measured is connected to V.sub.on and ending during the reset period, so as to provide an offset against charge retention by the transistors in the row being measured.