Raster output scanner

Claim 1

A raster output scanner comprising means for generating a beam of coherent radiation; a write drum an optical system for scanning the drum with the beam, and a beam output power regulation circuit comprising, a detector for generating a start-of-scan signal, a detector for generating an end-of-scan signal, a generator of clock pulses, a counter for counting the clock pulses, a counter for counting the clock pules, the counter using the start-of-scan signal to start counting, and the end-of-scan signal to stop counting, to produce a set of counts which correspond to scan positions across the drum, means located between the optical system and the drum for reflecting a fraction of the beam that would otherwise fall on the drum, means for receiving the reflected radiation and converting it into an electrical signal, means for generating a reference voltage, means for comparing the electrical signal with the reference voltage to produce a difference signal, memory addressed by the set of counts for storing the difference signal for each scan position, means for enabling or disabling the loading of the memory with the difference signal so that the output of the memory is either the difference signal of the scan position for the current scan or for a previous scan, and means responsive to the output of the memory for controlling the amount of light allowed to pass from the beam generator to the reflecting means.

Claim 6

A system for regulating the intensity of a beam of coherent radiation to a constant value as it scans a write drum from one side to the other, comprising: an acousto-optic modulator positioned in the beam path, a partial mirror positioned between the modulator and the drum for reflecting a portion of the beam, a photomultiplier tube for converting the reflected beam into an analog signal, an analog-to-digital converter for converting the analog signal into a digital signal, a generator for a digital reference signal, a comparator for comparing the reference with the digital signal, a detector for generating a signal at the start of every scan, a detector for generating a signal at the end of every scan, a generator of clock pulses, an address generator which counts the clock pulses, is started by the start-of-scan signal and is stopped by the end-of-scan signal, for generating an address, a memory responsive to the address for loading the output of the comparator into a memory location addressed by the address generator, and for outputting the contents of the memory addressed by the address generator, a digital-to-analog converter for converting the output of the memory into an analog signal, a driver responsive to the digital to analog output for supplying to the acousto-optic modulator a signal for varying the amount of the beam which is transmitted through the modulator, and means for disabling or enabling the writing of the output of the comparator into the memory, so that the output of the memory will be either the output of the comparator for a previous scan, or for the current scan.

Claim 7

A system for regulating the intensity of a beam of coherent radiation to a constant value as it scans a write drum from one side to the other, comprising: an acousto-optic modulator positioned in the beam path, a partial mirror positioned between the modulator and the drum for reflecting a portion of the beam, a photomultiplier tube for converting the reflected beam into an analog signal, an analog-to-digital converter for converting the analog signal into a digital signal, a generator for a digital reference signal, a comparator for comparing the reference with the digital signal, a detector for generating a signal at the start of every scan, a detector for generating a signal at the end of every scan, a generator of clock pulses, an address generator which counts the clock pulses, is started by the start-of-scan signal and is stopped by the end-of-scan signal, for generating an address, a memory responsive to the address for loading the output of the comparator into a memory location addressed by the address generator, and for outputting the contents of the memory addressed by the address generator, a digital-to-analog converter for converting the output of the memory into an analog signal, a driver responsive to the digital to analog output for supplying to the acousto-optic modulator a signal for varying the amount of the beam which is transmitted through the modulator, and means for disabling or enabling the writing of the output of the comparator into the memory, so that the output of the memory will be either the output of the comparator for a previous scan, or for the current scan.