Colour Measurement Devices
There are two main types of instruments that are used for measuring the colour of opaque surfaces: reflectance spectrophotometers and colourimeters.
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 16 wavelength intervals centred at 400nm, 420nm, 440nm, ..., 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.
The optical geometry of the instrument is important. In some instruments an integrating sphere is used that enables the sample to be illuminated diffusely (from all angles equally) and the reflected light to be collected at at angle roughly perpendicular to the surface of the sample. Alternatively, other instruments illuminate the sample at a certain angle and collect light at another angle. For example, typically the sample may be illuminated at 45 degrees to the surface and light reflected measured at 0 degrees - this is known as 45/0 geometry. The converse to this is 0/45. The sphere-based geometries are known as D/0 and 0/D. It is extremely difficult to correlate measurements taken between instruments if the optical geometry is not identical.
The four CIE standard geometries are:
- diffuse illumination and light collection at the normal, D/0;
- normal illumination and diffuse light collection, 0/D;
- illumination at 45 degrees and light collection at the normal, 45/0;
- normal illumination and light collection at 45 degrees, 0/45.
Colourimeters measure tristimulus values more directly and operate using three broad-band filters. Consequently, colourimeters cannot provide spectral reflectance data but sometimes they are preferred to spectrophotometers because of their low cost of manufacture and portability.