3.2 basic principles

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3.2 basic principles

  1. 1. BASIC PRINCIPLESColorimetry and the CIE system 1
  2. 2. BASIC PRINCIPLESIt would obviously be difficult to design a system of colour measurement that attempted to describe • the colours that we see.We simply have to think how we use words to describe colours. Colour names such asred, yellow, green and blue are reasonably well understood,but names such as rose, salmon and cerise are • less well standardised. • What might be called rose by one person could be called pink by another. 2
  3. 3. BASIC PRICIPLESEven for red there is a considerable range of colourswhich any one person might accept as red.This is not necessarily the same range as for a second person.Experiments with spectrum colours have shown that if one person chooses thewavelength considered closest to a true yellow,a second person might well not agree and claim that the chosencolour is too green or too orange. 3
  4. 4. The aim of the CIE systemThe aim of the CIE system is to tell ushow a colour might be reproduced(by a mixture of three primary light sources)rather than described.The amounts of the three primaries required to match a particular colourprovidea numerical specification of that colour.A different colour would require different amounts of the primaries and hence the specification would be different.It turns out that in many applications this is all that is required. 4
  5. 5. Colour is thr ee-dimensional,Colour is three-dimensional, a property that is apparent in various ways.Colour atlases arrange colours using three scales(hue, value and chroma in the case of the Munsell system, for example).If we apply a range of concentrations of, say, a yellow dye we can produce a range of yellow samples, but there will generally be many different yellowsthat cannot be matched –for example, those that are slightly redder thanour yellows. With mixtures of our yellow with a red dye we will be able to produce arange of reds, oranges and yellows, but there will be many oranges that we cannotmatch, including colours that are browner or greyer or less saturated than thoseactually produced. 5
  6. 6. M IXTURES OF 3 DYESIf we use mixtures of three dyes, however, such as our yellow and reddyes together with a blue dye, we can match a wide range of colours:yellows, oranges, reds and blues, and also browns, grays and so forth. In fact, the only colours that we will not be able to match will be verysaturated or very pure colours, such as a purer yellowthan that produced by our yellow dye.By choosing particularly bright or pure dyes wewill reduce to a minimum the range of colours that cannot be matched.(The widest range of colours can be matched if magenta, yellow and cyan areused as the primaries.) 6
  7. 7. System of Color SpecificationsIgnoring the few colours that cannot be produced in this way, we couldimagine a system of colour specificationin which the concentrations of three specific dyesRequired on a particular substrateto match a colour could be used to specify that colour.Of course we would need to specify the light source under which the colourwas seen, but the three concentrations wouldgive a numerical specification of the colour;the specification would be different for different colours and it would bepossible for someone else to reproduce the colour.Unfortunately there would be serious disadvantages to such a system.The relationship between dye concentrations and any measurableproperties of a colour are complex, although definable with modern computers 7
  8. 8. Moreover, dyes or pigments arerarely pure,and their precise colour depends onthe method of manufacture.Even repeat batches from the sameplant will not match exactly,and again properties of mixtures arenot completely predictable. 8
  9. 9. Color ed LightsIn contrast, coloured lights are much easier to define and reproduce.Imagine a red light obtained byisolating the wavelength 700 nm from the spectrum.Any laboratoryin the world capable of measuring wavelength accurately (an objective physical measurement)could produce the same red colour. 9
  10. 10. Color ed light gr eenA green colour corresponding to 546.1 nmcould be produced even more readily,since a mercury lamp emits light atonly four wavelengths in the visible region(404.7, 435.8, 546.1 and 577.8 nm).By filtering out the other three, the required green wavelength could be obtained.The wavelengths 404.7 and 435.8 nm could be obtained in a similar manner.Small variations in the operating conditions have no significant effect on thewavelengths emitted by a mercury lamp.Hence three primary light sources could be defined simply as appropriatewavelengths and easily reproduced. 10
  11. 11. M ixtur es of thr ee colour ed lightsMixtures of three coloured lights can be produced in various ways,but the simplest is to imagine three spotlightsshining on the same area on a white screen (Figure 3.1).The colour produced would be a mixture of the three colours, and a widerange of colours could be produced by varying the amounts of the three primaries.The colours to be matched could includesurface colours illuminated by a particular light source. 11
  12. 12. Elementswww.animationfactory.com 12

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