A series of time delay integration TDI stages each integrate a photocurrent from a separate detector such as detectors in an array. In a first stage 20, a first integrator is initialized with a fixed bias 30, and integrates a signal from a first detector 22 during a first time interval. Next, a reset switch 26n causes that integrated first detector signal to bias a second integrator 24n. During a second integration interval, the second integrator integrates a signal from a second detector 22n. Multiple stages may be arranged in series so that an integrated signal from a previous stage biases an integrator in the current stage. At a final stage, an N.sup.th integrator outputs the resulting signal V.sub.final. Any bias used to initialize the first integrator is removed from V.sub.final to achieve a total integrated signal from the detectors. A bi-directional switch 38 at each stage enables a forward or backward scan of the detectors.
