# 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. - **Patent:** US 3678182 - **Original title:** Chroma key circuit - **Owner:** Philips Broadcast Equipment Corp - **Granted:** 1972 - **Status:** Public domain (expired) - **Times cited:** 17 - **Field:** consumer_electronics, mechanical, telecommunications ## What it does 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. ## 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. ## 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. ## Real-world examples 1. Television weather forecast backgrounds 2. News broadcast studio green screens 3. Early 1970s film compositing equipment ## Why it matters 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. ## 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. **Full plain-English explainer:** https://patentbrief.org/patent/us/3678182/chroma-key-green-screen **Original patent:** https://patents.google.com/patent/US3678182 --- _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: - [How Wozniak Made the Apple II Display Color Graphics](https://patentbrief.org/patent/us/4136359/apple-ii-personal-computer) — Steve Wozniak's 1977 patent for a circuit that allowed a home computer to display stable, sharp color graphics on a standard television screen. - [How the First Home Video Game Console Worked](https://patentbrief.org/patent/us/3659285/video-game-console-magnavox) — Ralph Baer's 1969 patent for the Magnavox Odyssey, the first home video game system, which generated controllable dots on a standard television screen using analog circuitry. - [The First Digital Camera's Core Technology](https://patentbrief.org/patent/us/4131919/digital-camera-electronic-still) — Kodak's 1978 patent on the fundamental technology for capturing, processing, and storing digital images using a CCD sensor and magnetic tape. - [How Liquid Crystal Displays (LCDs) Were Invented](https://patentbrief.org/patent/us/3322485/lcd-liquid-crystal-display) — This 1962 patent describes the first practical way to use organic liquid crystals to create a display that scatters light when an electric current is applied. - [How a Modern Camera Sensor Captures Light and Converts It to Data](https://patentbrief.org/patent/us/5471515/cmos-active-pixel-image-sensor) — This patent describes a camera sensor technology that combines light-capturing elements with a special circuit to read out the image data quickly and efficiently, all on a single chip.