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.
Original patent title: “Flying spot scanner”
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. Granted to Xerox Corp in 1975 with 17 claims and 22 forward citations, and it is now in the public domain.
Key facts
Coverage
What does this patent actually cover?
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.
The gap
What does this patent 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.
These exclusions are unique to PatentBrief — derived from the actual claim language, not patent-office boilerplate.
What made this novel
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.
The Patent Drawing
Schematic visualization of the patent's claim structure. Hand-drawn diagrams in progress for each landmark patent.
Where you've seen this
Real-world examples
Early Xerox laser printers
Standard office laser printers
Laser-based barcode scanners
Laser-based phototypesetting equipment
Why it matters
The bigger picture
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.
Filed
November 27, 1972
Granted
February 18, 1975
Market context
Who's building on this
Companies in this space
Xerox pioneered this, but the technology was quickly adopted and refined by companies like HP, Canon, and Brother, who dominated the office printer market for decades. Today, the core principles remain in use for high-end industrial laser marking and specialized printing hardware.
Market impact
This patent helped move printing from mechanical impact systems to digital laser systems, creating the multi-billion dollar laser printer industry. It enabled the transition from analog document handling to the digital office, making it possible to print high-resolution text and graphics at high speeds.
Claim 1 — Plain English
What this patent 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.
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.
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.
Patent Journey
From filing to expiry
PatentBrief Score
Impact Score
Moderate
Citation count
27/40
Moderately cited
Claim breadth
11/20
Broad claimsclaimsThe numbered statements at the end of a patent that legally define what the inventor owns.Read more →
Recency
0/20
Older than 20 years
Assignee scale
20/20
Major company or institution
PatentBrief Impact Score — based on citation count, claim breadth, recency, and assignee scale. Not a legal assessment.
Heuristic Value Estimate
What this patent might be worth
$20K – $66K
Midpoint $41K · expired or expiring · industry ×1.4
Heuristic only — blends forward/backward citation counts, claim scope, time remaining, litigation history, and CPC-derived industry baseline. Real valuations need a professional appraisal.
The original legal language
Original claims
17 claims as filed with the patent office.
Concepts involved
Citations
Patent lineage
Cite this patent
Damouth, D. E., & Starkweather, G. K. (1975). How Laser Printers Use Rotating Mirrors to Write Information (U.S. Patent No. 3,867,571). U.S. Patent and Trademark Office. https://patentbrief.org/patent/us/3867571/laser-printer-starkweather
Auto-generated from the patent record. Double-check author order and the issue date against the official USPTO document before submitting.
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Common Questions
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.
Same assignee
More from Xerox Corp
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