A spectral color is a color that is evoked by monochromatic light, i.e. either a single wavelength of light in the visible spectrum, or a relatively narrow band of wavelengths (e.g. lasers). Every wavelength of visible light is perceived as a spectral color; when viewed as a continuous spectrum, these colors are seen as the familiar rainbow. All colors that do not qualify as a spectral color are called non-spectral colors or extra-spectral colors. Extra-spectral colors cannot be evoked with a single wavelength of light, but rather by a combination of wavelengths. Likewise, light comprising several wavelengths cannot evoke a spectral color. In color spaces In color spaces which include all, or most spectral colors, they form a part of boundary of the set of all real colors. When considering a three-dimensional color space (which including luminance), the spectral colors form a surface. When excluding luminance and considering a two-dimensional color space (chromaticity diagram), the spectral colors form a curve known as the spectral locus. For example, the spectral locus of the CIEXYZ chromaticity diagram contains all the spectral colors (to the eye of the standard observer). A trichromatic color space is defined by three primary colors, which can theoretically be spectral colors. In this case, all other colors are inherently non-spectral. In reality, the spectral bandwidth of most primaries means that most color spaces are entirely non-spectral. Due to different chromaticity properties of different spectral segments, and also due to practical limitations of light sources, the actual distance between RGB pure color wheel colors and spectral colors shows a complicated dependence on the hue. Due to the location of R and G primaries near the ‘almost flat’ spectral segment, RGB color space is reasonably good with approximating spectral orange, yellow, and bright (yellowish) green, but is especially poor in reproducing the visual appearance of spectral colors in the vicinity of central green, and between green and blue, as well as extreme spectral colors approaching IR or UV. Note that spectral color is universally included in scientific color models such as CIE 1931, but industrial and consumer color spaces such as sRGB, CMYK, and Pantone, do not typically include any spectral colors. Exceptions include Rec. 2020, which uses three spectral colors as primaries (and therefore only includes precisely those three spectral colors), and color spaces such as the ProPhoto RGB color space which use imaginary colors as primaries. In color