# 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. - **Patent:** US 3867571 - **Original title:** Flying spot scanner - **Owner:** Xerox Corp - **Granted:** 1975 - **Status:** Public domain (expired) - **Times cited:** 22 - **Field:** consumer_electronics, mechanical, semiconductors ## What it does 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 does NOT 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. ## Real-world examples 1. Early Xerox laser printers 2. Standard office laser printers 3. Laser-based barcode scanners 4. Laser-based phototypesetting equipment ## 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. ## Frequently asked questions ### What does How Laser Printers Use Rotating Mirrors to Write Information cover? 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. ### Who owns patent US 3867571? Xerox Corp owns this patent, granted in 1975. ### When does this patent expire? This patent has expired and is now in the public domain — anyone can use the invention freely. ### What is patent US 3867571 cited by? This patent has been cited by 22 later patents that build on its ideas. ### What problem does this patent solve? 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. ### What does this patent NOT cover? Does not cover non-laser light sources that lack collimated, uniform intensity. **Full plain-English explainer:** https://patentbrief.org/patent/us/3867571/laser-printer-starkweather **Original patent:** https://patents.google.com/patent/US3867571 --- _Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._ ## Related patents Semantically similar inventions in the PatentBrief corpus: - [Chester Carlson's Original Xerography Patent](https://patentbrief.org/patent/us/2297691/xerography-electrophotography-photocopier) — Chester Carlson's 1942 patent for xerography, the dry copying process that became the foundation for Xerox machines. - [How Printers Save Toner by Dropping Light-Colored Pixels](https://patentbrief.org/patent/us/6476836/image-forming-apparatus-having-a-function-of-saving-developing-agent) — A Ricoh patent for printers that save toner by completely ignoring very light pixels and using a special dot pattern for darker ones. - [How Piezoelectric Inkjet Printing Works](https://patentbrief.org/patent/us/3946398/drop-on-demand-inkjet) — A 1970 patent describing how to print images by using electrical pulses to bend a tiny crystal plate, squeezing individual ink drops out of a nozzle on demand. - [How 3D Printers Build Objects Layer by Layer from Liquid](https://patentbrief.org/patent/us/4575330/stereolithography-3d-printing) — This patent describes the foundational method for 3D printing, where a machine builds a three-dimensional object layer by layer by hardening a liquid material with light or other energy. - [How James Russell Invented the Digital Optical Disc](https://patentbrief.org/patent/us/3501586/optical-digital-recording-russell) — A 1966 invention that replaced physical needles on vinyl records with a laser beam reading digital data from a spinning disc.