How Early Television Systems Isolated Specific Colors for Special Effects
A 1971 circuit design that allowed television equipment to detect a specific color in a video signal, enabling the green-screen effects we see in modern weather forecasts and movies.
Original patent title: “Chroma key circuit”
A 1971 circuit design that allowed television equipment to detect a specific color in a video signal, enabling the green-screen effects we see in modern weather forecasts and movies. Granted to Philips Broadcast Equipment Corp in 1972 with 12 claims and 17 forward citations, and it is now in the public domain.
Key facts
Coverage
What does this patent actually cover?
This patent describes an analog circuit designed to identify a precise color within a video feed. It works by taking color difference signals and multiplying them by sine and cosine values of a control voltage, which effectively maps the color space to a coordinate system. By using a series of amplifiers, summers, and AND gates, the circuit creates an output pulse only when the incoming video signal matches the specific color selected by the operator. This pulse acts as a switch, telling the broadcast equipment to replace that specific color with another image, which is the fundamental mechanism behind chroma keying.
The gap
What does this patent NOT cover?
- Does not cover digital image processing or software-based color keying algorithms.
- Does not cover the actual display or projection of the background image.
- Does not cover methods for generating the original red, green, and blue color signals.
- Does not cover non-analog signal processing techniques.
These exclusions are unique to PatentBrief — derived from the actual claim language, not patent-office boilerplate.
What made this novel
The inventors used trigonometric identities (sine and cosine) to rotate and isolate color vectors in an analog circuit, allowing for a much more precise selection of a target color than simple thresholding.
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
Television weather forecast backgrounds
News broadcast studio green screens
Early 1970s film compositing equipment
Why it matters
The bigger picture
This technology was essential for the evolution of broadcast television in the 1970s. It allowed news stations and film studios to move beyond physical sets and use compositing to place presenters in front of weather maps or fictional environments. It represents a critical bridge between early mechanical television and the sophisticated digital visual effects used today.
Filed
March 12, 1971
Granted
July 18, 1972
Market context
Who's building on this
Companies in this space
While the specific analog circuitry is largely obsolete, the underlying logic of color space manipulation is the foundation for all modern video editing software. Companies like Adobe, Blackmagic Design, and Avid build upon these principles, though they now implement them through digital signal processing rather than discrete hardware components.
Market impact
This patent helped standardize the chroma key process for the broadcast industry, enabling the rapid adoption of virtual sets in the 1970s and 80s. It shifted the industry toward more flexible, cost-effective production methods that remain standard in television broadcasting today.
Claim 1 — Plain English
What this patent covers
This patent describes an analog circuit designed to identify a precise color within a video feed. It works by taking color difference signals and multiplying them by sine and cosine values of a control voltage, which effectively maps the color space to a coordinate system. By using a series of amplifiers, summers, and AND gates, the circuit creates an output pulse only when the incoming video signal matches the specific color selected by the operator. This pulse acts as a switch, telling the broadcast equipment to replace that specific color with another image, which is the fundamental mechanism behind chroma keying.
The clever bit
The inventors used trigonometric identities (sine and cosine) to rotate and isolate color vectors in an analog circuit, allowing for a much more precise selection of a target color than simple thresholding.
What it does not cover
- Does not cover digital image processing or software-based color keying algorithms.
- Does not cover the actual display or projection of the background image.
- Does not cover methods for generating the original red, green, and blue color signals.
- Does not cover non-analog signal processing techniques.
Patent Journey
From filing to expiry
PatentBrief Score
Impact Score
Early stage
Citation count
25/40
Moderately cited
Claim breadth
8/20
Moderate scope
Recency
0/20
Older than 20 years
Assignee scale
0/20
Independent or smaller assigneeassigneeThe entity that owns the patent — usually the inventor's employer or a company.Read more →
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
$16K – $50K
Midpoint $32K · 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
12 claims as filed with the patent office.
Concepts involved
Citations
Patent lineage
Cite this patent
Boxman, P., & Roessel, F. J. V. (1972). How Early Television Systems Isolated Specific Colors for Special Effects (U.S. Patent No. 3,678,182). U.S. Patent and Trademark Office. https://patentbrief.org/patent/us/3678182/chroma-key-green-screen
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 Early Television Systems Isolated Specific Colors for Special Effects cover?
A 1971 circuit design that allowed television equipment to detect a specific color in a video signal, enabling the green-screen effects we see in modern weather forecasts and movies.
Who owns patent US 3678182?
Philips Broadcast Equipment Corp owns this patent, granted in 1972.
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 3678182 cited by?
This patent has been cited by 17 later patents that build on its ideas.
What problem does this patent solve?
This technology was essential for the evolution of broadcast television in the 1970s. It allowed news stations and film studios to move beyond physical sets and use compositing to place presenters in front of weather maps or fictional environments. It represents a critical bridge between early mechanical television and the sophisticated digital visual effects used today.
What does this patent NOT cover?
Does not cover digital image processing or software-based color keying algorithms.
Patent monitoring