What Color Is An Orange ?
    By Patrick Lockerby | August 13th 2011 01:20 PM | 12 comments | Print | E-mail | Track Comments
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    Retired engineer, 60+ years young. Computer builder and programmer. Linguist specialising in language acquisition and computational linguistics....

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    What Color Is An Orange ?

    A question like that, here at, just has to be a trick question.

    It is possible, by the application of common sense arguments, to prove to a scientific level of certainty that an orange is absolutely not orange.

    How don't we see ?

    Between 41% and 67% of participants, depending on the exact way the question was asked, thought that the eye sent out some kind of ray or beam in order for us to see.

    That's not the startling bit.

    The startling bit is that the participants in the experiment were college students who, just a few weeks before, had taken an introductory psychology class on perception. As part of this class students had been explicitly told on a number of occasions how people see.

    It seems that once an idea becomes firmly entrenched in our minds, it becomes very difficult to dislodge.  It doesn't matter how often we are shown the facts if we can't see them.

    How we do see

    Way back in Ancient Greece, the way we see things was widely discussed by philosophers.  From around 500 BCE there were two broad schools of thought: extramission theory supposes that vision is accomplished by something coming from the eyes; intromission theory supposes that vision is accomplished by something entering the eyes.  It seems that the people surveyed, above, held to the extramission theory.  That is very strange, because intromission theory has been held to be correct by more and more philosophers since Lucretius wrote about it in 50 BCE.  Today, the idea that we see by means of light coming into our eyes from the thing seen is not so much a scientific theory as an easily proven fact.

    And thus I say that effigies of things,
    And tenuous shapes from off the things are sent,
    From off the utmost outside of the things,
    Which are like films or may be named a rind,
    Because the image bears like look and form
    With whatso body has shed it fluttering forth ...

    On the Nature of Things, Book IV
    Lucretius. 50 BCE

    What we see

    An object illuminated by white light reflects and scatters some of that light.  In the case of an orange, the light which reaches the eye of any observer is orange.  The light came from whatever source, commonly the sun, and the components of the source light which we call 'orange' were bounced off the fruit.  From whatever angle you view the fruit you intercept orange light.  This is true for all observers.  It follows that the orange light is not a part of the 'essence', the 'is' of an orange.  The property 'orange' is not a property of the fruit but a property of the source of illumination.  Whatever an orange 'is', it is not orange.

    The RGB model

    Human color vision can be readily understood in terms of a very simple model.  If you use a computer to control the three colors created on a phosphor screen by the three electron guns of a cathode ray tube, you have the essence of a computer controlled color graphics display.  The actual type of computer display is of no importance: the logic behind the display of a color image is the same for all display types.

    The human eye, optic nerve and brain all work together as an image processor.  The first stage of that image processing is done by the retina.  In the retina are three types of color receptor.  Roughly speaking these receptors respond to red, green and blue light and are the inputs to our color sensing system.  Television screens and computer monitors  produce three colors and mix them in a way that matches fairly closely the way that we see colors.

    When a computer controls the colors on a screen it does so by converting binary numbers to brightness levels for each of the red, green and blue components or channels of the imaging system.  By convention, computers handle binary numbers as words of 8 bits.  A word of 8 bits can encode any number from 0 to 255 decimal.  By another convention we include zero as a light level, making 256 levels per channel.  Thus the number of colors we can generate by using 8 bit numbers to control the mix of red, green and blue - the RGB channels - is 2563 = 16,777,216 colors.  That is enough colors for most non-professional uses and is the basis of the colors displayed in most current graphic images in html web pages.

    Inverse video and the 'not' colors

    For any set of three RGB values which display a color there is a set of complementary numbers which display the inverse color.  The inverse of any color in this scheme is calculated by subtracting each channel value in turn from 255.  The image below shows the numbers for just one of the many shades of orange in the image, and its blue inverse.

    In the same way that white is not black, and black is not white, we can use the concept 'not' for any color and its inverse.  The exclamation mark makes a handy abbreviation for 'not', so we can write -

    black = !white
    !black = white

    and since the order makes no difference to the logic we can write this four different ways.

    In the image above we can see that a specific set of RGB values makes a specific shade of orange, and its inverse makes a specific shade of blue.  For the specific shades shown, we can say that blue = !orange, and per contra.  We can also say that for any color C and its inverse !C  C + !C = white:  the two colors added together across the three channels produce  255,255,255 - or white light.

    When white light shines on an object, the colors seen by the eye are the colors scattered by the object.  If the object is all of one color - let us call it skypink - then we can say with some certainty that the object receives white light and scatters skypink.  We have seen that skypink + !skypink = white, so a very obvious question to ask now is: what happened to all that !skypink ?

    Light absorption

    If an object absorbs all the light which it doesn't scatter, what happens to that light?  Does the absorption affect the object in some fashion?  It is now accepted that the light absorbed by a body is converted into heat.  Today we have cameras which can photograph this heat.

    Objects scatter some of the light that falls on them and that is how we see them.  The light which they do not scatter, they convert into heat.  The beginnings of the discovery of that particular piece of science can be traced back to Emile du Chatelet, whose paper Dissertation sur la nature et la propagation du feu was published in 1744.  In that paper, she showed that heat and light are two effects of a common cause.  She thus foreshadowed the discovery of infra red radiation.

    Gabrielle Émilie Le Tonnelier de Breteuil, marquise du Châtelet,
    more commonly known as Émilie du Châtelet.


    Every object absorbs some of the light that falls on it and converts it to heat.  Every illuminated object thus contains within itself, and emits, thermal energy.  We are surrounded by that thermal energy, or infrared radiation, but it is invisible to us.  Unless heat radiation is somewhat concentrated we remain entirely unaware of it.  But it is around us everywhere.

    So, what color is an orange ?


    I have actually thought about this a lot in the past, so I am glad to see you write something on it here. A sort of validation I guess. I thought of it in terms of 'true color', but I am unsure of how appropriate that terminology would be. When you think about it in terms of skin color it becomes even more... interesting. Thank you for the article!

    had taken an introductory psychology class on perception.
    There is something deeply wrong with a large fraction of people who take psychology classes; that should be one part of the explanation. A large portion goes there to cope with their own problems and they learn surprisingly little about anything. It is also deeply troubling how many people run around with psychology degrees having no idea about science whatsoever. Scary really.
    The property 'orange' is not a property of the fruit but a property of the source of illumination.

    I want to disagree with this, it is a property of an orange to reflect "orange" wave lengths of light(and absorb all the other visible wave lengths). The same as how a Red Delicious apple reflects red wave lengths of light.

    It is however true that to be reflected, the wave length has to be present in the illuminating light.
    I had a Red car that turned gray in mercury vapor light, which was a really cool effect btw.
    Never is a long time.
    This is a bit of a snafu. If someone asks about the color of something, without further clarification, it is assumed to be the color perceived in direct sunlight by a person of normal vision. We have to make assumptions in conversations as to do otherwise would require an endless descriptive list. In fact we couldn't have a conversations about anything without these assumptions. Of course, the orange reflects a spectrum of colors. Something can be orange because it reflects reds and yellows and doesn't have to have any orange spectra at all. I thought that you would have shown us the spectrum that belongs to the orange. I will have to deem this article a dismal failure.

    Arrogantly Yours
    Sodium Crapolate

    Considering the assumptions you mention, how would one know this:

    Something can be orange because it reflects reds and yellows and doesn't have to have any orange spectra at all.

    Without a spectrometer?

    And in fact if you're seeing an Orange on your normal TV(or computer monitor), you're seeing Red and Green :)
    Never is a long time.
    Yes Mi Cro, you can tell if something is orange without a spectrometer (clue: just look at it). If you want to know why it's orange, then a spectrometer is useful.
    As I mentioned, the natural color of something is determined in direct sunlight, not by a TV or computer monitor. I think that the RGB phosphor color space is different than that of sunlight.

    Yes the colour space is very different. With sunlight you have the option of picking any spectrum you please - extreme ones may be a bit dark but the colour will be whatever you want it to be - or to put it another way, there's plenty of sunlight to use.

    The traditional colour wheel is just a qualitative picture. A colour triangle is better and reflects the fact that for many purposes you can get away with 3 primary colours. To quantify colour, the tiangle is distorted into the chromaticity diagram on the right. Here, pure monochromatic colours are drawn in an arc. The shape is chosen so that the result of mixing colours can be obtained geometrically: the eye is sufficiently linear for this to work. Three monchromatic lasers can cover most of the colours -as the black inscribed triangle shows. Phosphors are not so good and can be represented by the grey triangle. Pigments or ordinary (stage) filters are more like the white triangle. In each case colours outside the triangle simply cannot be made.

    I once had a set of colour separation filters for photographic use. They were all very narrow-band and consequently quite dark. However the green and the blue both leaked a little red so you could put the green over the blue expecting a dark green-blue to result, but instead you got a very deep red. 

    Oddly enough, it is possible to have colours outside the chromaticity diagram too. There is considerable overlap in the response of the three colour receptors in the eye, so it is possible (don't ask!) to stimulate one on its own in a way that light cannot.

    And, of course, you can imagine impossible colours. I, at any rate, have no difficulty with "reddish green" but I know I will never see it.

    The pleasantest lighting is generally that from a white-hot object, be it sunlight or a filament lamp. Florescent lighting and LED lighting uses whatever wavelengths make up a reasonable white, but as the diagram shows, this could be a mix of blue-green and yellow with no red at all. This is one reason such lighting is so unpleasant - people look like corpses.

    Which brings us back to oranges. People say an orange "looks pale" under sodium (yellow) street lighting. What then is its true colour? Clearly, common usage refers to its innate colour not whatever colour sensation it's giving under a particular light source. While this may be ascertained by looking at it :) it can also be quantified so that even under an intense blue light - where everything looks blue - it still "is" orange. The answer, of course, is to use the reflectivity spectrum as this does not change at normal levels of illumination. 

    Orange, by definition. According to Wikipedia, the color orange was named after the fruit, not the other way around. Before the introduction of the orange, the color was known simply as red-yellow.

    Thank you all for your very intelligent and critical comments.

    I wrote this piece for a few reasons.  Firstly, I wanted to present color vision as applied to ordinary objects in a very simple way, so as to explain light scattering and light absorption.  Secondly, my intention was to show that the idea of vision as 'rays coming from the eyes' was false.  Lastly, I wanted to briefly introduce the idea that absorbed light energy gets converted into low grade heat.

    I didn't go into the idea of light as a mix of spectra because that is covered just as well by the three color model - as far as human vision is concerned.  Also, one can say either that an orange scatters orange light, or that it absorbs blue.  In either case we are looking at a special property of electrons, and one mode of operation implies the other, so there is no need for us to multiply causes.

    Our sensory system seems to be very capable of taking a physical reality and making us completely misunderstand it.  The number of sensory illusions is uncountable.  Just as our sense of color tells us that we see an object, rather than the light scattered from it, so our sense of touch tells us that a number of objects at a single temperature are of various temperatures.  We sense the heat conductivity of objects, but we interpret that conductivity as temperature.  But that topic is for another occasion.

    Two thoughts.  One is that it is our friend Ibn Al-Haytham, who is understood to have been the first, roughly about 1000 AD, to have demonstrated that light travels from the object to the eye, not the other way round.

    The other: did you see the BBC Horizon programme Do you see the same colours as me?  It included some time with the Himba people of Namibia, who tend to differentiate colour by saturation and value rather than hue.  The point was made that our colour perception is very much language-driven.

    Robert H. Olley / Quondam Physics Department / University of Reading / England
    What color is an orange?
    technically, colors don't exist, the way temperature doesn't exist, the way all senses felt by living things are just ways for them to adapt to the universe. or words our mind made up.
    the more human we get, the further we stray away from finding out what is the true concept of the universe and what is reality. everything we know about reality now is something our minds created to cope with the vast incomprehensible universe. its possible that a human's purpose in life is to become one with this true reality and throw away the illusion of the current reality.
    or we could give in to what is in front of us and become less civilized like "animals." or "nature." even though we are products of nature.

    like my name this is just random blabbering but perhaps it could provide some new topics
    like the purpose of life
    yea im a troll...

    ...everything we know about reality now is something our minds created to cope with the vast incomprehensible universe.


    yea im a troll...

    I never saw 'philosopher' spelled that way before. ;-)