Tag Archives: wavelength

why are leaves green?

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Why are leaves green? The most obvious answer is that they contain green pigments, the most abundant being chlorophyll and that chlorophyll absorbs the short and long wavelengths in the visible spectrum leaving the middle wavelengths to be reflected and scattered. However, the deeper question is why should chlorophyll absorb in the short and long wavelengths of the visible spectrum when there is more light available in the middle of the spectrum?

The spectral irradiance of sunlight varies with the time of day, the weather conditions, the time of year, and the latitude/longitude. However, I think it would be reasonable to say that by and large, in most situations, the peak irradiance is in the middle of the spectrum (that which we would normally associate with being green and yellow).

So if one assumes that evolution has produced a perfect engineering solution to this aspect of nature in particular then I think one may expect plants to absorb mainly in the middle part of the spectrum (and this would result in the bluish and reddish wavelengths being reflected and a purplish colour).

So why don’t we have a chlorophyll equivalent that is purple? I have come across a number of arguments.

1. One could go further and say that if a plant wanted to be really efficient it would absorb all wavelengths of the visible spectrum and would therefore appear black. So black, rather than purple, would be the perfect engineering solution. Given that most plants are neither black nor purple then I think we can assume that evolution did not find the perfect engineering problem or that the problem is more complex than we think. For example, it could be that a plant that is black would absorb too much light and overheat. Or it could be that chlorophyll evolved from some earlier light-sensitive chemical and that genetic mutations could lead more easily to chlorophyll than to purple or black pigments.

2. Taking this point further, I have heard it suggested that most plants evolved from earlier plants that lived under water and that absorbed mainly short wavelengths of light (long wavelengths – red – cannot penetrate much more than 1 m of water). These earlier cousins of the modern plant would most likely have been brownish. Indeed, if one looks today ay plants in seawater, green plants are only seen on the surface or at very low depths. So the ancestor of chlorophyll could have been a brown pigment which mutated into green chlorophyll more easily than it could have mutated into a purple pigment.

3. I have also come across the ‘early purple earth’ hypothesis. This suggests that originally most plant life on land was indeed optimally purple and that chlorophyll absorbed to take advantage of those wavelengths that were not already being gobbled up by the dominant species. Subsequently, chlorophyll proved more successful than its purple companion.

4. It could be argued that optimally absorbing light (and being purple) is not the most important thing and that there are other aspects of the problem that are more important. Green chlorophyll could be the optimal solution to this more complex problem.

In short, the real answer is … I don’t know. I am not overwhelmingly convinced by any of the above arguments.

If you enjoyed this post you may like to look at my special christmas post on carrots and why they are orange.

why is hue circular?

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Everyone is familiar with the colour spectrum. If you pass white sunlight through a prism then it splits into the component wavelengths. The shorter wavelengths appear blue, the longer wavelengths appear red, and in between we have the familiar colours that I learned as school as Richard Of York Gave Battle In Vain, for the sequence red, orange, yellow, green, blue, indigo and violet, and that I have since understood is taught in the US as a person: Roy G Biv. I wonder if there are any other mnemonics that people know of? Of course, many people believe that Newton was in error when he identified 7 colours in the spectrum – he was probably influenced by Aristotle who wrote about there being 7 fundamental colours as there are 7 tones in the musical octave. I’ve posted about the indigo issue before – http://colourware.wordpress.com/2009/07/20/indigo-a-colour-of-the-rainbow/ – so won’t repeat that here.

Newton was probably the first person to create a hue circle (others, such as Forsius, created colour cicles but often included white and black in the circles). Newton created a true hue cirlce where he took the colour spectrum and wrapped it around, noticing that the two ends of the spectrum (where the reds become bluish and the blues become reddish) look rather similar.

Of course, there was a gap because the two ends of the spectrum did not quite match and thus Newton had to add in some purplish colours – these are hues that are never seen in the spectrum (and are sometimes called extra-spectral hues or non-spectral hues). The hues in the spectrum can be created by a single wavelength; however, the extra-spectral hues only occur when we see several wavelengths at the same time. For example, when we see short and long wavelengths together we can see purple.

In my lecture at the University of Leeds (www.leeds.ac.uk) this week someone asked “Why do the two ends of the spectrum look similar at all when the light is so different physically (at one end the waves are short and high energy and at the other they are long and low energy)?” Very very good question – if changes in wavelengths change the hue why should wavelengths that are so different look so similar?

So, why is hue circular? The answer is that it has very little to do with wavelengths and physics and more to do with human physiology. The human visual system captures light with three classes of cell (called cones) in the retinae of the eye. The signals from these cones are processed by the human visual system to create opponent signals (red-green and yellow-blue). This puts red and green opposite each other and yellow blue opposite each other and results in the perception of hue being circular. It also explains why some hues particularly contrast – sometimes called complementary colour harmony.

What is a colour space?

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In my job I probably use the phrase “colour space” every day and have done for the last 20 years. So imagine my surprise when I was talking with a colleague recently and after a few minutes he said “Can I stop you for a second there Steve – when you say colour space, what exactly do you mean?”.

A colour space is like a map. A map of New York, for example, shows the location of various landmarks with reference to the xy coordinates (the position in horizontal x and vertical y units on the map). A colour space or colour map does the same thing with colours. Perhaps the simplest colour space is the spectrum, see below:
 myspectrum

As we look from right to left on the spectrum the wavelengths changes from around 700nm on the far left to about 400nm on the far right. So this map shows colour with reference to wavelength. Although it is a commonly used colour space it is limited because it only really describes how hue changes with wavelength. Hue is only one of three ways in which colour can change or vary.

The most well-known really useful colour space then is the CIE chromaticity diagram – see below.

chromdiagram

The CIE chromaticity diagram shows colours arranged on a 2-D plane. We can easily refer to any colour by how far from the left it is (the x coordinate) and how far from the bottom it is (the y coordinate). This space only shows two of the dimensions of colour; the hues are arranged in a somewhat circular way and the colourfulness increases as we move outwards from the white point (a position near to the centre of the diagram). However, we can also consider the third component of colour (brightness) if we imagine a dimension coming out of the page towards you (http://colourware.wordpress.com/2009/07/18/cie-system-of-colorimetry/). The CIE defines several different colour spaces; the CIELAB colour space, for example, is another 3-D space that defines a colour by its L*, a* and b* values.

It is useful to think of an image-display device as also having a colour space. Consider the display on which you are probably reading this blog. The display shows colour by changing the amount of the red, green and blue light emitted at each point on the screen. The diagram below is a representation of what the RGB colour space of your display device may look like.

 

rgb

In the RGB cube, black is in the bottom left. As the RGB values increase colours are created and white results from each of the RGB primaries at full strength. So the RGB colour space defines the relationship between RGB values and colour. However, here’s the really interesting thing: The colour space for different display devices is very different. Even if we take a single device – such as the one that you are reading this blog on – then as we change settings (the brightness, the contrast, the gamma, the colour temperature, etc.) then the colour space changes. That is, the relationship between RGB and colour changes as you change those settings. This is a huge problem. Imagine if there were many maps of New York and each showed the position of, say, the Empire State Building to be in a different position. How confusing would that be? Well, that’s the problem with colour-display technology. If we didn’t do anything about this problem then every time we looked at a colour image on a different display device the colours could change markedly. This is why we need colour management. Colour management can make compensations to the RGB values that are sent to each display device so that the colours always appear the same (well, nearly the same). To make this compensation the colour management software (which is embedded in your Windows or Apple operating system) needs to know about the colour space of each device connected to the computer. Each device needs to have a profile that describes the relationship of its own colour space with respect to some standard colour space. 

How good is colour management? Well, that depends upon many factors. Most printers, cameras, scanners, and screens (LCD, CRT, etc.) come with a driver that includes a crude colour profile. This ensures that there is a basic level of colour management and for a great majority of users this is more than adequate. However, if you want better performance then you need to think about making some measurements that will allow a more accurate colour profile to be built. In a recent blog I described a new device that you can buy to enable you to do this – http://colourware.wordpress.com/2009/07/29/colormunki-colour-management/. There are many such devices on the market. I highly recommend Andrew Rodney’s book Color Management for Photographers which is both clear and accurate (though the edition I have works on Adobe’s CS2 package whereas the latest package is CS4).

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However, no matter how hard you try, colour management is never likely to be perfect. This is because different devices have different colour gamuts; a printer is likely to be able to display some colours that your display physically cannot and vice versa.

cure for colour blindness

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One of the reasons I enjoy travelling by train is that it gives me an opportunity to read a newspaper from front to back (something I very much enjoy but rarely have time to do). Yesterday I was travelling to Bristol where I was delivering a lecture at the IMPACT6 Printmaking conference on colour management and took the train from Leeds to Bristol during which I was able to read The Times. I couldn’t fail to notice the story about a potential cure for colour blindness – http://www.timesonline.co.uk/tol/news/science/medicine/article6837392.ece

Congenital red-green colour blindness occurs when either the L- or M-cone class is either missing (making the sufferer a dicromat) or shifted in terms of peak wavelength of sensitivity (resulting in anomalous trichromacy) – see http://colourware.wordpress.com/2009/07/04/colour-blindness-news/.

Scientists working at the universities of Seattle and Florida have restored normal colour vision to two colour-blind monkeys by injecting a virus with a modified gene (called L opsin) that is known to be responsible for red-green colour blindness. The success of this work is remarkable in that it suggests that the brain is able to rewire itself to take advantage of the new receptors. 24 weeks after the injection the monkeys were able to correctly distinguish patterns of grey, green and red dots that they had previously been unable to distinguish.

colour_385x185_615124a

Jay Neitz, professor ophthalmology at the University of Washington, is now looking to start work that could lead to a similar treatment for humans.

The work has just been published in Nature – http://www.nature.com/news/2009/090916/full/news.2009.921.html 

The rays are not coloured

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So when Newton wrote that the rays are not coloured, what exactly did he mean?

Well, he meant that even though we may say loosely that light at 400nm is blue and light at 700nm is red this implies that the blueness and the redness are properties of light.  Although there are philosophical arguments that would support colour as a property of light (and we’ll get on to those arguments in a later post) for now I would like to put forward my view (which is, I believe, consistent with Newton’s) that colour is not the property of light.

The evidence that supports my view is that light at 700nm may look red to most people most of the time, it doesn’t look red to all of the people most of the time or even to most of the people all of the time. For a very striking example please consider the image below:

illusion

In this example, you will see some blue spirals and some green spirals. But physically the blue and green are the same. In terms of wavelengths, exactly the same wavelengths (in exactly the same proportions) are being reflected from the areas that you perceive as being green and the those you perceive as being blue. If you think in terms of digital (RGB) terms, the RGB values of the green areas and the blue areas are the same – both are about R = 9, G = 20, B = 160. We know now that the colour that you perceive for a wavelength of light or a group of wavelengths depends upon the colours that are close by. This is often expressed as contrast or assimilation. When contrast occurs colours become less like the colours that they are next to an image; when assimilation occurs colours become more like the colours that they are next to. Contrast and assimilation effects result in you seeing two colours, a blue and a green, when physically only one colour exists.

Straight away some of you can see that I am falling into loose language straight away because I am using colour in two different ways. On the one hand I am saying the two colours are physically the same and on the other hand I am saying that the two colours are perceptually different (blue and green). Which is it? It all depends upon how you define colour. My stance is that I define colour as a perceptual phenomonon – it’s something we see and experience. Others may argue that the two colours are really the same and that it is a mere illusion that they look different – I, on the other hand, would argue that the two colours are different. It’s not an illusion – you see a blue and a green, don’t you?

This is what Newton was referring to when he said that “to speak properly, the rays are not coloured” – I believe that Newton was aware of this problem with language – that colour can be used to represent several things. But when we speak properly we realise that the rays are not coloured.