How Laser Printers Use Rotating Mirrors to Write Information

A 1972 Xerox patent describing how to use a spinning mirror to scan a laser beam across a page, adjusting the speed of the data to keep the image sharp.

What it covers

This system uses a laser to project a tiny, high-intensity spot of light onto a light-sensitive surface, like a drum in a printer. A multifaceted rotating polygon mirror reflects this beam, sweeping it across the surface to create a line of information. Because the beam moves faster in the middle of a scan than at the edges, the system uses a function generator to speed up or slow down the data transmission rate (the bit rate) to match the spot's velocity. This ensures the printed image remains uniform and does not look stretched or distorted.

What it doesn't cover

• —Does not cover non-laser light sources that lack collimated, uniform intensity.
• —Does not cover scanning systems that use a fixed data rate regardless of the spot's velocity.
• —Does not cover systems that lack a multifaceted rotating polygon for beam deflection.
• —Does not cover digital image processing or software-based image correction.

The clever bit

The system recognizes that a rotating mirror causes the light spot to speed up as it moves away from the center of the page. By mathematically adjusting the data bit rate using a 1/secant-squared function, it compensates for this mechanical speed change to keep the printed pixels perfectly spaced.

Why it matters

This technology is the fundamental engine behind the laser printer. Before this, printers were mostly impact-based (like typewriters). This invention allowed for high-speed, high-quality document reproduction by synchronizing electronic data with precise mechanical movement, enabling the desktop publishing revolution of the 1980s.

Real-world examples

• 1.Early Xerox laser printers
• 2.Standard office laser printers
• 3.Laser-based barcode scanners
• 4.Laser-based phototypesetting equipment