An optical detector uses an auxiliary integrating capacitor is used to determine the optical flux rate when the charge on the well capacitor approaches or reaches linear saturation. The voltage across the primary linearly operating integrating well capacitor is monitored to detect its linear saturation. The auxiliary capacitor charges at a predetermined rate from the start of the integration interval, and the voltage across it is sampled when the monitored voltage across the primary linear integrating well capacitor reaches linear saturation. The voltage across the auxiliary capacitor at this point is function of the rate at which the primary well capacitor was filling, and this voltage is processed as the illumination level for the integration period.