Colorimetry, color science, photography

Red is any of a number of similar colors evoked by light consisting predominantly of the longest wavelengths of light discernible by the human eye, in the wavelength range of approximately 630–700 nm.^[2] Longer wavelengths than this are called infrared, or below red and cannot be seen by human eyes.^[6] Red's wavelength has been an important factor in laser technologies as red lasers, used in early compact disc technologies, are being replaced by blue lasers, as red's longer wavelength causes the laser's recordings to take up more space on the disc than blue lasers.^[7] A main theory for why primates developed sensitivity to red, is that it allowed ripe fruit to be distinguished from unripe fruit and inedible vegetation.^[8] This further drove other adaptations to take advantage of this new ability, such as red faces.^[9] Red light is also used to preserve night vision in low-light or night-time situations, as the rod cells in the human eye are not sensitive to red.^[10][11] Red is one of the three additive primary colors of light, complementary to cyan, in RGB color systems. Red is also one of the three subtractive primary colors of RYB color space but not CMYK color space.^[12]
One common use of red as an additive primary color is in the RGB color model. Because red is not by itself standardized, color mixtures based on red are not exact specifications of color either. The United States government sets certain specifications for what paints to use when red is stated in a design.^[13] In order for computers to produce exact colors, the color red needs to be defined in terms of an absolute color space, such as sRGB.^[14] color correction (so that a standardized red is produced that is not in fact full intensity of only the red colorant).
Red illumination was (and sometimes still is) used as a safelight while working in a darkroom, as it does not expose most photographic paper and some films.^[15] Though many more modern darkrooms use an amber safelight, red illumination is closely associated with the darkroom in the public mind.