Laser-ScanLaser-Scan was a British technology company based in Cambridge, England, which produced world-leading graphics hardware and then advanced software for computer graphics, digital mapping, charting, and geographic information handling.
Read the LSL History for a summary of company history, then the LSL Hardware page for a summary of hardware products, and then see below for more detail on the company's principal graphic display and plotting devices.
The main mirrors have a second measuring laser bounced off the back of them, measuring their position to a microradian of angle. We know at all times where they should be pointing (that's where we told them to go), and the difference between the ideal and actual positions is negated, turned into a voltage and used to swing the minimirrors to compensate. The result of the writing laser beam passing through all four mirrors is that it can be positioned accurately and quickly.
The HRD-1 is not called high resolution for nothing - it has an addressability of 140,000 by 100,000 addressible points, and a line resolution in excess of 7000 x 5000.
The writing laser in an HRD-1 was a blue Argon ion laser, consuming 30A on three-phase in order to produce a bit less than one watt of turquoise light focussed into a spot about 10 microns across. The measuring laser was a red Helium-Neon laser of much lower power.
More informnation is in the following brochures:
See LASERPLOT - Very High Speed LaserPlotter for a description of the LASERPLOT A3 film plotter.