How does a reflectance spectrophotometer work?

Reflectance spectrophotometers measure the amount of light reflected by a surface as a function of wavelength to produce a reflectance spectrum. The reflectance spectrum of a sample can be used, in conjunction with the CIE standard observer function and the relative spectral energy distribution of an illuminant, to calculate the CIE XYZ tristimulus values for that sample under that illuminant.

The operation of a spectrophotometer is basically to illuminate the sample with white light and to calculate the amount of light that is reflected by the sample at each wavelength interval. Typically data are measured for 31 wavelength intervals centred at 400nm, 410nm, 420nm, ..., 700nm. This is done by passing the reflected light though a monochromating device that splits the light up into separate wavelength intervals. The instrument is calibrated using a white tile whose reflectance at each wavelength is known compared to a perfect diffuse reflecting surface. The reflectance of a sample is expressed between 0 and 1 (as a fraction) or between 0 and 100 (as a percentage). It is important to realize that the reflectance values obtained are relative values and, for non-fluorescent samples, are independent of the quality and quantity of the light used to illuminate the sample.