Using luminance key, all content of an image that has a brightness level over (or under) a certain level is replaced by another image or color. This can be used for instance for a black or white title cards in front of a camera or any other title generation. It will not work well with full images with a large color and brightness range.
The chroma key technology is also called "color separation overlay" CSO. With a chroma key just one single color will be replaced by another color or by another complete image. The color can be fine-tuned by color tint, chroma level and luminance level. The problem areas are always the edges between the keyed area and the remaining image area. But todays key technology creates images there the key area is nearly undetectable.
For many years the color blue was favored for the chroma key. That was mainly because the film material and cameras were most sensitive to blue light. The cloths and accessories of a person sitting in front of the blue screen would be chosen without any blue content. As soon as the blue color was keyed out, anything else was remaining in the image.
Actually the green color is better fitting to the chroma key technology because green content can be more easily left out of all cloths and other things visible in the final image. Because todays video camera are more sensitive to the green color and have the least noise with the green color and green paint has greater reflectance than blue paint, green is the color of choice for todays chroma key technology. Today green screens are used also for outside filming with often large green screens spanning through the entire set.
|blue screen and green screen
A new development is the retro-reflective screen. This screen is color neutral and highly reflective because of embedded tiny glass beads. The camera is equipped with a LED ring that emits a highly directional blue or green light (can be chosen). This projected light creates a perfect blue or green background for keying.