DEFINING COLOUR BY MEASUREMENTIn Ch

DEFINING COLOUR BY MEASUREMENT

In Chapter 1 it was shown that colour vision is quite a complex process, involving physical, physiological and psychological influences. A system for colour measurement should ideally consider all these influences if it is to correlate with our perception of the colour. In practice, however, it is the three basic factors that are vital to the appearance of colour itself, and which form the basis for systems of colour measurement:
• a source of light,
•an object that will absorb some wavelengths of light and reflect others,
•a visual system that can provide the required sensation in the brain when appropriate wavelengths of light are received.

A colour measurement system must, therefore, consider the:

•definition of the spectral output of the illuminates,
•measurement of the spectral reflectance of the sample,
•definition of normal colour vision (standard observer).

Today`s methods for colour measurement are largely based on systems and standards that have been developed through the Commission Internationale de l’Eclarage (CIE), which is the international body responsible for recommendations for photometry and colorimetry. Within this body, standards have been defined for a range of illuminates and the observer. These standards provide the means for converting a spectral reflectance measurement from a sample, to a meaningful colour description that can be represented in some form of colour space model.

Spectral reflectance

The colour of an object or the print on a sheet of a paper is dependent on the absorption of certain wavelengths of light and the reflection or transmission of others. Therefore measurement of the amount of light reflected by the simple at different wavelengths, provides an indication of its colour. This basic measurement may be used to provide a graphical representation of the colour in the form of a curve. Example curves for the process colours and their overprint are shown in Figure 2.2. These curve are referred to as spectral reflectance curves and they are the fundamental physical measurement of any colour.




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The curves are largely self-explanatory. but for those familiar with viewing these types of curves, some explanation is worthwhile since similar curves are used to indicare the colour of light (spectral power distribution curves) and colour sensitivity (spectral sensitivity curves). The horizontal axis is the wavelengths of light covering the visible part of the spectrum, measurements are taken, normally at 5-20 nanometre intervals, depending on the instrument. The vertical axis is the percentage of light reflected from the sample at each of the measured wavelengths.
From these examples, it can be seen that with some experience, it is possible to view such a curve and obtain a good indication of the colour, without seeing a printed sample. However, while it is possible to state that two printed samples having identical spectral reflectance curves will be perceived to be the same colour when viewed, it is also possible to have two samples that are perceived to be a colour match but have different spectral curves. Such a colour match has been referred to (Chapter 1), as a metameric match.
Although the measurement of spectral reflectance provide the basis for colour measurement, it needs to be related to the colour of the light source and the colour sensitivity of the human visual system, if it is to provide a meaningful method of measurement and definition of colour.


CIE COLOUR STANDARDS

In 1931, the CIE made an important step in standardizing systems for colour measurement and order by specifying spectral characteristics of standard illuminants, and dara relating to the standard observer, along with methods for describing colour.

Standard illuminants

A number of standard illuminants have now been defined but initially the standard illuminants were A, B and C (illuminants B is now obsolete). In addition, there are now a range of standard D illuminants. The most important which are relevant to printers are shown Table 2.1. The spectra power distribution curves for two of the most important standard light illuminants are shown in Figure 2.3.



Tab2.1
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It should be noted that while these standards exit in terms of a specification of their spectral power distribution. and can be used in colorimetry calculations it is not possible to obtain actual light sources that have an identical spectral power distribution. The ‘daylight’ tubes that are use to represent D65 in colour viewing, for example, are the nearest that the lamp manufacturers are able to achieve. A comparison between the D65 standard and a typical fluorescent light source is shown in Figure 2.4.






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The CIE standard observer

So far, we have seen that we can quantify the spectral output of the illuminant and measure the spectral reflectance of the sample. Quantifying the colour sensitivity and perception of the human visual system is more involved. The basis for our current understanding of human colour vision is experimental work carried out in the UK in the mid-1920s,


During the course of this work, a number of observers, with normal colour vision, were required to match monochromatic light of individual wavelengths, by additively mixing portions of red green and blue light. A diagrammaric representation of the type of apparatus used for this is shown in Figure 2.5. From these experiments it was possible to defind, for all colours of the spectrum, the amount of red green and blue light required to be received by the eye in order to match that colour.



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The CIE used the results of this Work in providing a definition of the ‘standard observer’. More specifically, the data that defines this is referred to as the CIE colour matchlng functions. It is provided as a data table, giving values for x (red), y (green) and z (blue), at wavelength increments of 5 nm throughout the visible spectrum.
The original 1920s experimental work used apparatus that provided a viewing field with a cone angle of 2 (CIE 2 ํ Standard Observer). Later experiments used a 10 ํ angular field, and these results are used in the CIE 1964 10 ํ Standard Observer. The 10 ํ standard is more appropriate where the samples being measured are larger areas of colour. The data for the colour matching functions for the two standard observers, are shown in Figure 2.6.




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Tristimulus values

The colour matching functions provide the means for converting any spectral curves into three numbers, known as the X, Y and Z tristimulus values (sometimes referred to as ‘big X Y Z ’), which provide a unique definition of that colour. The tristimulus values represent the amount of red, green and blue cone response required by the ‘standard observer’ to match the particular colour, when viewed under a particular light source. The mathematics used in determining the colour matching functions and calculating tristimulus values are quite involved, and do not concern us in day-to-day colour measurement. However, for those wishing to deal with this and the subject of colour measurement in more detail, see Hunt1. A simple appreciation of how the tristimulus values are calculated can be obtained by considerin Figure 2.7. The diagram shows how the spectral reflectance is modified by the spectral power data of the illuminate and then weighted by the colour matchmg functions, to provide three values for each wavelength increment. This is carried out throughout the spectrum,