White point

A white point (often referred to as reference white or target white in technical documents) is a set of tristimulus values or chromaticity coordinates that serve to define the color 'white' in image capture, encoding, or reproduction. Depending on the application, different definitions of white are needed to give acceptable results. For example, photographs taken indoors may be lit by incandescent lights, which are relatively orange compared to daylight. Defining 'white' as daylight will give unacceptable results when attempting to color-correct a photograph taken with incandescent lighting.An illuminant is characterized by its relative spectral power distribution (SPD). The white point of an illuminant is the chromaticity of a white object under the illuminant, and can be specified by chromaticity coordinates, such as the x, y coordinates on the CIE 1931 chromaticity diagram (hence the use of the relative SPD and not the absolute SPD, because the white point is only related to color and unaffected by intensity).If the color of an object is recorded under one illuminant, then it is possible to estimate the color of that object under another illuminant, given only the white points of the two illuminants. If the image is 'uncalibrated' (the illuminant's white point unknown), the white point has to be estimated. However, if one merely wants to white balance (make neutral objects appear neutral in the recording), this may not be necessary.