|MadSci Network: Chemistry|
Brian - an interesting observation you have made. However, it is a little dangerous to generalise and assume that it is always the case that the red colour fades fastest – because it will depend on what method of colour printing is used. This makes your question a little complicated to answer, but let’s see if I can help you understand what sort of processes are occurring, and why colour changes happen.
First it’s good to understand that white light is made up of three primary colours – red, green and blue which correspond to the longest third, the middle third and the shortest wavelength third of the visible range of radiation that our eyes detect. It is this way because our eyes have three colour sensors, which separate these parts of the visible spectrum, and we get all of our colour sensations by various mixtures of sensing in these three areas. One of the most surprising results is that we see yellow when only our green and red sensors are stimulated. This means that yellow is the sensation we see when blue is absent from what would otherwise be white light. Similarly we sense cyan (a turquoise colour) when red is absent – i.e. only our green and blue sensors are stimulated, and magenta (a purplish hue) when green is absent – i.e. only our red and blue sensors are stimulated. I need to tell you this to explain how a lot of colours are formed in printing and in photography. Red, blue and green are called the “additive” primaries – simply because they add up to white light. Cyan, magenta and yellow (the “opposites” of red, green and blue) are called the subtractive primaries because each subtracts one of the additive primaries from white light.
In printing and in photography cyan, magenta and yellow dyes are most commonly used to form the image colours. This is better than using red, green and blue dyes because C, M & Y only absorb one third of white light, whereas R,G & B dyes each absorb two thirds. This makes twice as much use of light when viewing images.
Now, if you imagine a red image in a sign, which is made using cyan, magenta and yellow dyes, the red colour is formed by a mixture of yellow and magenta dyes. The white light falls onto the mixture and the yellow dye absorbs all the blue light. Similarly the magenta absorbs all the green light, and so the remaining light – red is the only light reflected back to your eye. If you see the red fading – i.e. your eye is seeing other colours coming back reflected from the image area that is supposed to be red this is because the yellow or the magenta or both are themselves fading and failing to fully absorb the blue and green light. Now it is usually the case in photography that the magenta and yellow dyes in colour prints fade faster than the cyan. You may have noticed that photographs faded in shop windows go cyan in colour – this is why. Remember cyan is the opposite of red – so the red is fading fastest, just as your observation stated. This is because the yellow and magenta dyes are more susceptible to oxidative fade induced when light reacts within the dye layer to produce oxygen radicals which tend to react with the dye and bleach it – destroy the part of the molecule which absorbs the light to give the dye its colour. Cyan dyes tend to bleach by a reductive process and are actually less stable in dark, hot conditions than are yellow and magenta.
BUT – this all depends on the particular dyes used, and also on how they are contained in the substrate, and on what protection from UV light may be coated on top. Another factor is the incorporation of “stabilisers” – things that preferentially react with oxygen radicals to prevent dye bleaching. Now some printing systems don’t use yellow, magenta and cyan – but use red, green and blue pigments. My guess is your observations relate to the YMC systems – they are most common – photography, inkjet and most colour printing use them (the latter two also use black) In the business they are called CMYK systems as opposed to RGB – look closely at your TV screen and look closely at an inkjet image or colour magazine with a magnifying glass to see the dots that make up the image and you will see the two systems. TV uses RGB because it give out light – it isn’t based on absorbing or subtraction from white light – it is a true additive system. That’s it – did you get this far?
I hope this has been helpful. Get back to me if you have questions - email@example.com
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