Laser scanner power compensation circuit

Claim 1

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

Claim 6

A system for regulating the intensity of a laser beam to a constant value as said beam 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 said modulator and said drum for continuously reflecting a portion of said beam, a photomultiplier tube for converting said reflected beam into an analog voltage, an analog to digital converter for converting said analog signal into a digital signal, a reference generator for generating a digital reference, a comparator for comparing said reference to said digital signal, a start of scan detector for generating a signal at the start of every scan, an end of scan detector for generating a signal at the end of every scan, a clock generator for generating clock pulses, an address generator which counts said clock pulses, is started by said start of scan signal and is stopped by said end of scan signal for generating an address, a memory responsive to said address for loading the output of said comparator into a memory location addressed by said address generator, and for outputting the contents of said memory addressed by said address generator, a digital to analog converter for converting the output of said memory into an analog signal, a driver responsive to said digital to analog output for supplying to said acousto-optic modulator a signal for varying the amount of the laser beam which is transmitted through said acousto-optic modulator, and means for disabling or enabling the writing of the output of said comparator into said memory so that the output of said memory will be either the output of said comparator for a previous scan or for the current scan.

Claim 7

A system for regulating the intensity of a laser beam to a constant value as said beam 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 said modulator and said drum for continuously reflecting a portion of said beam, a photomultiplier tube for converting said reflected beam into an analog voltage, an analog to digital converter for converting said analog signal into a digital signal, a reference generator for generating a digital reference, a comparator for comparing said reference to said digital signal, a start of scan detector for generating a signal at the start of every scan, an end of scan detector for generating a signal at the end of every scan, a clock generator for generating clock pulses, an address generator which counts said clock pulses, is started by said start of scan signal and is stopped by said end of scan signal for generating an address, a memory responsive to said address for loading the output of said comparator into a memory location addressed by said address generator, and for outputting the contents of said memory addressed by said address generator, a digital to analog converter for converting the output of said memory into an analog signal, a driver responsive to said digital to analog output for supplying to said acousto-optic modulator a signal for varying the amount of the laser beam which is transmitted through said acousto-optic modulator, and means for disabling or enabling the writing of the output of said comparator into said memory so that the output of said memory will be either the output of said comparator for a previous scan or for the current scan.