Colour measurement principles and methods

The scientific basis for the measurement of colour is the existence of three different types of response signals in the human eye. Though four different types of receptor - p, 7 and / cones, and rods - have been identified, the messages from these are encoded (in a way still not fully understood) to give three types of signal. 

In a tristimulus colorimeter, three or four filters duplicate the response of the standard observer. The filters, which correspond to the three primary colours in the spectrum (red, green and blue), can be combined to match most colours. The more sophisticated instruments carry a fourth filter to simulate the blue part of the CIE [[X bar]] function shown in Fig. 5.1. The other essential parts of a tristimulus colorimeter are a white light source, an array of photometers and, nowadays, a computer or an interface to one, as shown in Fig. 5.2a. The computer can collect responses as well as carry out data transformations between CIE and other colour scale systems or between different standard white light sources or white diffusers. Hunter and Harold (1987) give a good summary of the transformation formulae employed. Data from a colorimeter is given as a three-point output, commonly CIELAB, HunterLab or 7, x, y. 

Unlike the colorimeter s filter system, a spectrophotometer works by measuring the whole spectrum of visible light reflected from a sample, i.e., between 380nm and 700nm. Commonly, measurements are taken, via an integrating sphere and then a diffraction grating (see Fig. 5.2b) with the results being expressed as the 

The two commonly used instruments discussed allow both laboratory and nearline colour measurements to be made. With the advent of almost fully automated lines, true on-line colour measurement would be of great value in many factories. McFarlane (1988) discusses the economic benefits of on-line and closed-loop systems. His paper highlights closer adherence to specifications, minimisation of waste during start-up and product change, as well as an increase in process efficiency and plant throughput as key issues. 

As mentioned previously, the now routine integration of a pc means that the data collected from either a colorimeter or spectrophotometer can be transformed to XYZ, CIELAB, HunterLab or other colour system as specified by the user and provided by the software. The initial step from instmment response to XYZ requires the definition of standard observer functions. 

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