Green Screen & Blue Screen Basics

Professional TV equipment by Gerriets

At this point we would like to explain some basic technical terms on the subject Green Screen / Blue Screen:

The term "chroma keying" is often used as a general term for this technique. The use of this technique is well known, for example, in weather forecasts on TV, showing the frequent and rapid change of different weather maps. The first cinema movie to use a blue screen and the associated technology was already in 1940 in "The Thief of Bagdad".

The Green Screen or Blue Screen technique is a colour-based technique for keying out a person or an object in order to merge it into another, virtual background. Basically, chroma keying works with every possible colour of the visible colour spectrum in the wavelengths of approx. 380 – 780 nm. However, certain shades of green or blue rarely occur in nature and therefore meet the demands for using this technique.
There are different opinions about which of the two colours (green or blue) is better suited for this purpose. Green is less common in clothing than blue. In addition, blue eyes can lead to irritations, and almost no pigments of green are found in skin colour.
Another argument in favour of Green Screen is that each pixel field in modern cameras has a blue and a red, but two green pixels. In this way, the green colour range is recorded and displayed in a much different way.
Furthermore, any other shade of colour rarely occurring in nature, such as mint or pink, can be used. A subject can also be isolated by using different levels of brightness. If a uniform background is evenly lit but the person standing in front of it is not, this person can also be chroma keyed in this way.

The process of editing out a person or an object and merging it onto a virtual background is called “Keying“.

Here are some important explanations of the Green Screen and Blue Screen techniques:

  • Illumination
    The adjustment of the people and the background are most important for an optimal use of chroma keying. Although modern keyers can cope with certain differences in levels of brightness, the grade of reflection of the material is a crucial criteria. If possible, the background and the person (or object) in front of it should be lit/illuminated separately, in order to avoid shadows to fall on the background.

  • Lighting system / spotlights
    The lighting should be as soft as possible to avoid strong contrasts and prominent shadows. The set and background may require separate lamp types (e.g. asymmetrical spotlights).

  • Colour point
    In additive colour mixing, the colour point describes the respective proportion of the colour values red, green and blue (RGB) of a colour in values from 0 to 255. For Green Screen, the colour coordinates of 0/177/64 (RGB) are recommended as standard. For Blue Screen: 8/39/245 (RGB). Experienced keyers are quite tolerant, so they can certainly handle deviations from this. Pink or mint may also be usable as mentioned above. To clarify: white has the values (255/255/255) in the colour point in additive colour mixing. When backlighting studio sets using a projection screen for example with our TRANSMISSION screen with LEDs in the background, the colour point values can be set accordingly on the lighting console.

  • Cove
    A cove describes the concave rounding of an edge. In the Green Screen or Blue Screen studio, this transition between the floor, for example made of VARIO CLASSIC, and the back wall, for example made of CHROMAKEY SUPRA, serves to illuminate the background softly and without any edges. A cove can be made of different building materials, often bent plywood boards, which are filled and painted or covered with a textile. For temporary solutions, the textile on the back (for example TELEVISION CS) can also be used as flooring.

  • Even Luminance
    Wrinkles or strongly differing materials on the wall and floor can be a problem. Here, for example, the TELESTRETCH offers the possibility of producing really perfectly smooth surfaces, fixed at the top and sides to a wooden construction and at the bottom with the Gerriets' Mounting Flange bracket. Alternatively, clamping to a TRUSS FRAME is also an option, in which case the chroma key fabric can be viewed on three sides or all around. Shadows that are too strong and sharp also cause difficulties. The more even the luminance of the set, the better the keying behaviour.

  • Spill
    Spill is a reflection or blur that disturbs the image. If a strong light in the green background causes reflections on a person's white clothing, clean image editing will be difficult in the post-production process. To prevent this effect to a large extent, the areas in the green screen that are not needed can be covered with a black cloth before shooting. Spill also often appears in the area around the head where the hair already has a certain transparency to the background.

  • Room size / relations
    Too small a distance between the people and the background is not effective. Sometimes, however, the production space does not allow other options. Shadows can be cast and spill effects may occur on the people or objects. In small rooms or rooms which are intended for different multiple uses, a track system such as the TRUMPF 95 from Gerriets will provide more flexibility.

  • Reflective behaviour
    Matte and low-reflective surfaces are to be preferred. They prevent too much light from being reflected onto the set. However, special attention must be paid to illumination. DUVETYNE R 55, DUVETYNE CS or CHROMAKEY SUPRA are suitable for many applications, but often require extra illumination additional to the set. TELEVISION CS is easy to handle, can be washed without losing its flame-retardant feature, but can cause reflection due to improper lighting conditions.

"Many factors are crucial for the successful presentation in proper chroma key sets. Room size, illumination and the motif itself have a major influence on the result. Herein, the choice of keyable material is essential. Today‘s keyers do have a certain tolerance. But also they cannot save poorly set up Green Screen/Blue Screen settings."
Birger Bustorff (1st lighting cameraman at SWR from 1997–2019)

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