Intensity controller for an aperture card plotter

Abstract

A system for controlling the intensity of a scanning laser beam used to plot a computer-output microfilm image on an aperture card. The intensity of the beam is controlled by an acousto-optic modulator, which varies the light contained in various components of the modulated output. The first order beam is used for scanning the microfilm because it can be varied from its maximum intensity down to essentially cut-off without requiring large amounts of modulator drive. The intensity control is accomplished in two stages. First, before each scan across the microfilm, a known fraction of the first order beam is split off and sensed for intensity level. This level is directly proportional to the scan beam intensity. The sensed level is then compared to a preset voltage representing the proper exposure operating point for that manufacturer's microfilm characteristics. An error signal is produced by the comparison and is fed into a hold circuit. During this non-microfilm scanning portion of the cycle, the error signal updates the modulator drive level to continuously vary the modulated beam intensity in a way to minimize the error signal, thus, in closed-loop fashion the scan beam intensity is controlled to the proper exposure operating point. Second, as each scan line of the microfilm begins, the hold circuit is switched to 'hold' mode so the intensity level sensed before the scan is the level that is held during the subsequent scan. During the scan, pixel data is modulated to the 'held' intensity level in order to control the modulation of the scan beam intensity to the operating level. By modulating the beam in a purely binary fashion according to pixel data as it scans across the microfilm, a microfilm image is reproduced.