the response of an x 1 sensor of the sensor arrangement e g the photodiode response after the light was transmitted through an x 1 filter and another sensing corresponds to the response of the x 2 sensor having an x 2 filter x 1 filters and x 2 filters mimic the response to of a human standard observer 39 s eyes cone cells in different spectral regions one of the primaries for which values are produced by the colorimeter is an achromatic primary the achromatic primary of a human standard observer based color space is the primary that yields rather the intensity of light rather than its color examples for human standard observer based color spaces are the x y z color space where the x y and z responses are the tristimulus values of a human standard observer and y is the achromatic primary defined as luminance the xyy color space with y as the achromatic primary the cielab l a b color space where l is the achromatic primary also referred to as u201c lightness u201d or the cieluv l u v color space having the same achromatic primary l in a first activity m sets of n sensings are generated for m calibration illuminants with known values of the three primaries wherein m is a number greater than n hence n sensings are obtained for each calibration illuminant of altogether m calibration illuminants the term u201c calibration illuminant u201d means for example a real light source used to illuminate a real colorimeter however also a simulated.