A friend wanted to measure the linearity of his CCD camera, and asked for my help. He needed to measure linearity to 0.1% and wanted to do so by exposing the CCD sensor to a constant light source for varying time intervals. To decrease the effect of dark current, the smallest time interval needed to be extremely short, shorter than the shutter of the camera could achieve. Instead he asked if I could design a circuit to accurately turn on and off an LED, allowing the CCD sensor to be exposed for a much shorter interval than the shutter is capable of.
To measure the camera response to 0.1%, a range of 1000:1 is needed. This requires at least a 10 bit counter (2^10 = 1024). A constant current source is required to drive the LED so that its luminosity is constant. With those specifications in mind, I came up with a design.