At some time later depending on the desired exposure time, the second curtain is released. The width of the slit determines the total time any part of the film is exposed. In a similar way, the pre-charge voltage is applied simultaneously a number of lines of the array. When the pre-charge voltage is removed from a line, exposure begins. At some point in the array scan period, the line is read out thus ending the exposure period.
Exposure is varied by holding a variable number of lines in a state of pre-charge.
Removing pre-charge from a line is analogous to releasing the first curtain in a focal plane shutter and readout is analogous to the second curtain in the shutter mechanism. Now that you have a handle on the exposure mechanism, here's an overview of automatic exposure and gain circuits. It's a fairly simple process and easy to understand. First, we start with a preset minimum gain setting and about midway through the range of exposure or integration period. At the end of a frame scan, we average the luminance signal for the entire frame.