3.3 additive and subtractive mixing

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3.3 additive and subtractive mixing

  1. 1. ADDITIVE AND SUBTRACTIVE MIXINGMost of us are familiar with the colours produced bymixing paints or coloured solutions.Many will be equally familiar withthe colours produced on fibres by mixtures of dyes. – Obviously the results depend on the exact colours mixed, – but roughly: 1
  2. 2. Additive and subtractive• red + blue gives purple• red + yellow gives orange• yellow + blue gives green• while red + yellow + blue in the correct proportions gives grey or black 2
  3. 3. Mixing lights• To people accustomed to mixing dyes or pigments, the colours produced by mixing lights may sometimes be surprising. For example,• a blue light mixed with a yellow light might well give white,• while red and green lights could be mixed to give a yellow.• (Yellow and blue dyes would be expected to give green,• while red and green dyes would probably give a dirty brown colour.)• Quite obviously, mixing dyes and pigments is fundamentally• different from mixing coloured lights.• Since the CIE system is based on mixtures of lights, these must be considered further. 3
  4. 4. Additive mixing• Additive mixing occurs – when two or more coloured lights – are shone at the same time – so that we see the two lights together.• Consider red and green lights shining on to a white screen using an arrangement similar to that shown in Figure 3.1.• The screen will reflect almost all the light incident upon it,• and the mixture of red plus green in the same appropriate proportions• will reach the observer’s eye.• If the colour seen (yellow)• is surprising this is simply because we are not used to mixing colours in this way.• The two colours do not interact with each other at all. If the red and green are single wavelengths,• both wavelengths reach our eye and do not interfere with each other in any way. We see the red wavelength plus the green wavelength: hence we call the mixture an additive mixture. 4
  5. 5. ADDITIVE MIXING (Maxwell disc) • Another simple method of demonstrating additive colour mixing is • the Maxwell disc. • This is a disc made of sectors of various colours, • which is spun at increasing speed. • Above a certain speed we see – the colours blending together – in an additive manner. • The colours produced can be varied by – altering the relative areas – of the differently coloured sectors. 5
  6. 6. Subtractive mixingSubtractive mixing is much more common but usually involves more complex processes. 6
  7. 7. SUBTRACTIVE MIXING• In the simplest case,• we shine light through two coloured glass filters (Figure 3.2).• The light passes through the two filters in succession: – only the light transmitted by the first filter (F1) – reaches the second filter (F2). – Each filter ‘subtracts’ light – and the only light seen is that which has succeeded in passing through both.• This is completely different from additive mixing, where all the light from the mixture reaches the eye. 7
  8. 8. SUBTRACTIVE COLOR MIXING• Suppose that F1 is made of red glass• and only transmits light of wavelengths longer than 650 nm. – The light reaching F2 will therefore consist only of wavelengths longer than 650 nm and will appear red.• Suppose that F2 is green and transmits only wavelengths between 500 and 550 nm. If white light were to be shone on to this filter the• wavelengths between 500 and 550 nm would be transmitted and would look green. 8
  9. 9. SUBTRACTIVE COLOR• In our example the only light reaching F2 is the red light of wavelengths greater than 650 nm,• and therefore no light is transmitted through F2. – Hence no light reaches the eye – and the two filters together produce black.• In this case F1 has subtracted all the wavelengths except those of 650 nm or longerand F2 has subtracted the rest.It is easy to see that the mixture should be considered to be a subtractive mixture.The order in which the filters are placed does not affect the final colour as long as the light seen has passed through both filters. 9
  10. 10. SUBTRACTIVE COLOR MIXING• Not all red glass transmits long wavelengths only.• It is possible that some red glass (F3) may transmit light of all wavelengths. – As long as there is a preponderance of red wavelengths, – the transmitted light will look red.• If F1 is replaced by F3 – some light of all wavelengths will reach F2, – and the wavelengths between 500 and 550 nm will reach the eye.• In this case a subtractive mixture of red plus green will give green.• This will be a very dark green because F3 will only – transmit a small proportion of each of the green wavelengths. 10
  11. 11. SUBTRACTIVE MIXING• If we use a new green filter (F4) which transmits some light at all wavelengths, – the result is more difficult to predict. – If the fractions of light transmitted by F3 and F4 – at a particular wavelength are f3 and f4 respectively, – the fraction transmitted by the subtractive mixture is f3 ´ f4 – (ignoring the small amount of light reflected from the surfaces of the filters).• For example if, at a particular wavelength, – F3 transmits half the light – (f3 = 0.5) and F4 transmits 10% (f4 = 0.1), – then the two filters together transmit only 0.5 ´ 0.1 = 0.05, – i.e. one-twentieth of the light.• Although we can carry out this Calculation at all wavelengths throughout the visible region it is not always easy to deduce the colour of the mixture from the result:• even in simple cases like this, the effects of subtractive mixing are often difficult to predict. 11
  12. 12. Subtractive mixing• With coloured solutions, possible chemical reactions make the situation – even more complicated.• For example, – the addition of (colourless) phenolphthalein to – (colourless) sodium hydroxide solution – gives a bright pink/magenta colour.• Other indicators give equally unpredictable results.• Obviously these are extreme examples, but with certain• dyes interactions do occur, making the results of mixing not completely predictable. 12
  13. 13. Subtractive Mixing• The most important examples of subtractive mixing are the mixing of paints, and dyeing with a mixture of dyes. The results are often predictable on the basis of everyday experience, but the details of the process are much more complicated than those discussed so far. 13
  14. 14. Subtractive Mixing• Consider a red dye and a green dye, – applied first separately – and then in mixture, – to samples of a white substrate. – (In this and in all similar examples, – if the light source is not specified – it should be considered to be – a white light – consisting of approximately equal amounts of all the wavelengths in the visible region; – daylight is one such source.)• The red dye on its own would produce a sample which reflected some light at all wavelengths,• but less light at certain wavelengths• (particularly around 500 nm) than others. 14
  15. 15. SUBTRACTIVE MIXING• Consider a red dye and a green dye,• Our eyes would see a mixture (additive!) of all the wavelengths,• but because most of the reflected light would be red, the colour seen would be red.• Similarly the substrate sample dyed with green dye alone would absorb some of the light at all wavelengths,• But reflect more light of the green wavelengths than the others, giving a green colour.• For the mixture of the two dyes we need to consider each wavelength in turn/sequential/following.• For any one wavelength both dyes will• absorb some of the light,• but the amounts absorbed will be different at different wavelengths. 15
  16. 16. SUBTRACTIVE MIXING• Qualitatively we would expect that both dyes will – subtract from the incident light, – particularly at the wavelengths – where they absorb strongly – (around 500 nm for the red dye – and around 400 and 600 nm for the green dye).• Hence the mixture is called a – subtractive mixture – and we see the light that has not been absorbed by either dye; – In this case we would see a dirty brown colour.• The fact that the results of subtractive mixtures are normally predictable simply comes from our experience; accurate calculations of the results are difficult (even ignoring interactions)• and the effects of interactions• are not predictable from the colours of the dyes being mixed. 16

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