# The Naked Scientists Forum

### Author Topic: colour...how do we see it  (Read 5558 times)

#### mdigweed

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##### colour...how do we see it
« on: 14/02/2006 15:33:48 »
do we see colour consistently? in other words is the blue i see the same as the blue you see?

Doh!

#### tony6789

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##### Re: colour...how do we see it
« Reply #1 on: 14/02/2006 16:07:47 »
yes unless you are blind

- Big T

#### harryneild

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##### Re: colour...how do we see it
« Reply #2 on: 14/02/2006 17:20:27 »
hmm i suspect that determining whether we interpret colours differently would be difficult, if not impossible. How can you describe blue?

"Knowledge has to be improved, challenged, and increased constantly, or it vanishes." Peter F. Drucker

#### another_someone

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##### Re: colour...how do we see it
« Reply #3 on: 14/02/2006 17:23:12 »
quote:
Originally posted by mdigweed

do we see colour consistently? in other words is the blue i see the same as the blue you see?

Doh!

Not sure that this is the right question to be asking.

When you look at something (e.g. the sky) and see something, and I look at the sky, and see something, we both see the same thing, and we both agree to call it blue.

If I then look at a bluebell, and you look at a bluebell, we will also see the same thing, and we can agree to call it blue.

The problem arises when one asks whether the colour we see in a bluebell is the same as the colour we see in the sky, and this is a point upon which we may disagree.

When someone who is red/green colour blind looks at a red traffic light, he and I can agree that the traffic light is red.  The problem is that when we look at a green traffic light, we can both agree that it is a green traffic light, but he cannot see that it is any different in colour from the red traffic light he saw earlier.  The difference is a relative difference, in that I can see two different colours, whereas he can only see one colour.  There is no absolute measure of colour that could possibly say whether when he saw as red or green was actually red or actually green, all we can measure is that he cannot tell them apart.

George.

#### neilep

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##### Re: colour...how do we see it
« Reply #4 on: 14/02/2006 21:48:19 »
quote:
Originally posted by mdigweed

do we see colour consistently? in other words is the blue i see the same as the blue you see?

Doh!

I asked a similar question once before..here it is with all the cooments that ensued http://www.thenakedscientists.com/forum/topic.asp?TOPIC_ID=689

Men are the same as women.... just inside out !!

#### Dr B

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##### Re: colour...how do we see it
« Reply #5 on: 18/02/2006 18:01:53 »
How do we see black?

Dr B
Istanbul

#### another_someone

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##### Re: colour...how do we see it
« Reply #6 on: 18/02/2006 21:46:53 »
quote:
Originally posted by Dr B

How do we see black?

Do we see black, or merely imagine black in the space where we see nothing else?

George

#### tonycsm

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##### Re: colour...how do we see it
« Reply #7 on: 19/02/2006 00:49:17 »
If I remember correctly from my early studies, when a light source in our visible spectrum such as daylight strikes an object, the result is that energy is released from the object's atomic structure in the form of photons. These released photons usually have a specific wavelength, depending upon the release energy, which our brain then interprets as a specific colour, therefore I would presume, if we all have similar abilities to detect and interpret these wavelengths correctly ( excluding those with defects such as colour blindness etc) then we should all see the same colour.
However, what we actually call these colours we see is abitrary!
Objects which are black, have the ability to absorb the light source, hence very low reflectance.
Objects left in light sources such as strong sunlight will eventually suffer some atomic degeneration such as we see in colour fading!

#### another_someone

• Guest
##### Re: colour...how do we see it
« Reply #8 on: 19/02/2006 05:58:21 »
quote:
Originally posted by tonycsm

If I remember correctly from my early studies, when a light source in our visible spectrum such as daylight strikes an object, the result is that energy is released from the object's atomic structure in the form of photons. These released photons usually have a specific wavelength, depending upon the release energy, which our brain then interprets as a specific colour, therefore I would presume, if we all have similar abilities to detect and interpret these wavelengths correctly ( excluding those with defects such as colour blindness etc) then we should all see the same colour.
However, what we actually call these colours we see is abitrary!
Objects which are black, have the ability to absorb the light source, hence very low reflectance.
Objects left in light sources such as strong sunlight will eventually suffer some atomic degeneration such as we see in colour fading!

The creation of colour is easy, but we do not actually see real colour, we only see certain points in the colour spectrum, and try and guess at the real colour.

We have three points in the colour spectrum which we actually detect (hence why all our colour monitors only bother to transmit 3 colours), although there is actually an infinite range of colour.

http://www.cs.bath.ac.uk/%7Epjw/NOTES/75-ACG/ch7-colour.pdf
quote:

Cones come in three kinds, with peak sensitivities in different parts of the spectrum. These are conveniently called the red, green and blue cones. However, the responses are quite broad, with considerable overlap. Green cones have the strongest response, red is close but slightly weaker, while blue is 20 times lower, i.e., the eye’s response to blue is much lower than red and green. The blue peaks at about 440nm, green about 545nm, and red about 580nm. Interestingly, the red and green peaks are within the yellow part of the spectrum. Furthermore, red cones have a secondary response in the blue part of the spectrum. The eye’s limits are about 400nm for violet and 700nm for red.
This use of three receptors to explain low-level colour is called the trichromatic theory or the tri-stimulus model. Basically this states that all perceivable colours can be represented as RGB stimuli to the eye. (This is almost true but not quite, as we will see shortly.) Thus, when we see, say, yellow light, what is really happening is that the red and green cones are being excited to certain relative levels. If we were to take red light and green light of the right intensities and mix them, the relative excitement of the cones would be the same, and we would perceive yellow. This, of course, is the principle of TVs. (The technical term for two colours which are visually the same but generated by different mixtures of wavelengths is ‘metamers’).
Overall, the eye’s response to light of constant luminance has a peak at about 550nm, roughly the colour of sunlight.
Even though we have a completely non-linear response in the eye, we will start with a simple
tri-stimulus system. This means we shall represent a colour by a mixture of standard red, green
and blue sources, mixed in some proportion. These are our primary colours.
We get secondary colours by mixing the primary ones in equal measure:
yellow = red + green;

cyan = blue + green;

magenta = red + blue.

We get white by adding all three primaries in equal proportion:
white = red + green + blue.

It is important to understand that the basis of the tri-stimulus model is the human eye, not physics. The spectrum is a continous, linear concept. If you were to invent something to approximate it, you might choose one light at each end of the visible part - blue and red say - and hope you could linearly interpolate the colours in-between. The reason this does not work fully is exactly because of the need to stimulate eye’s cones. Equally, the existence of metamers is a consequence of the overlap of the cones’ responses: there is no unique physical stimulus for any given perceived colour. One immediate consequence is that our computer screens depend on the viewing eyes having three types of cone: an alien - or even a bee - will not see what we
see.
As an interesting digression, you might like to know that a bee has three receptors too, but one is in the ultraviolet and bees are less sensitive to red than we are. Many birds can also see in the ultraviolet but in their case they have four receptors and so have a wider viewable range than we do. So do some fish and turtles. Sometimes male-female pairs of birds look similar to us but are very different in the UV, so they can find the opposite sex more easily: the blue in a blue tit’s head is different in the UV, according to the sex, but is very similar to us. Some white feathers reflect strongly in the UV, others do not, so they will look different to other birds. Blue tits, zebra finches and even starlings use UV in mate selection. Maybe that UV
“black light”, once popular at discos, had its uses after all.

The point is that one can have a colour at 570nm that is indistinguishable from a mix of colours at 545nm and 580nm, or a different proportion of mix of colours at 550nm, 560nm, and 580nm.  All our eyes can see is an equivalence between those colours, even though they actually represent three very different combination in the light spectrum, and for other animals might even be seen as very different colours to the way humans would see those colours.  The difference is even more significant if the illuminating light has gaps in its spectrum – for instance, if the illuminating light has a gap at 560nm, then it is clear that a body that reflects light at 560nm would look black, but a body that reflects light at 550nm and 570nm would have an apparent colour as if it were reflecting light at 560nm (which ofcourse, in that environment, would be an impossibility, because there is no light in that environment to reflect at 560nm).

Another factor that would seem obvious to me is that although we all (with the exception of some people with true colour blindness) have 3 colour receptors, it does not follow that we are all equally efficient at seeing all three colours – we may have different numbers of the different receptors, or some may have some slight mutation that effects the efficiency of some of the receptors,  these mutations may even alter the core frequency at which the receptor absorbs light – after all, if different animals have different frequencies at which the absorb light, then could not natural mutations in humans also shift these core frequencies.  Any of these variations could alter the way we perceive, and more importantly, the way we discriminate, between colours.

George
« Last Edit: 19/02/2006 07:06:29 by another_someone »

#### DoctorBeaver

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##### Re: colour...how do we see it
« Reply #9 on: 19/02/2006 07:41:00 »
quote:
Originally posted by Dr B

How do we see black?

Dr B
Istanbul

I asked that very question on the Naked Scientists radio show a while back. The explanation given was that light doesn't activate the cones & rods in our eyes, it actually switches them off. So, when we percieve something black, all the rods & cones in our eyes are switched on. What wasn't explained, though, is how we see white. [xx(]

#### Atomic-S

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##### Re: colour...how do we see it
« Reply #10 on: 20/02/2006 03:16:41 »
I heard someplace that the eye contains not 3 types of cones like a color TV camera, but 2 types, and one type senses the relative proportions of red and green, the other the relative proportion of blue to the sum of the former. Does anyone know the facts about that, and have a reference that would definitively answer the question?

Secondly: Vision involves not only the reception of the image, but also the processing of it. The image is relayed to the brain via the optic nerve. In what format is it relayed? 3 separate channels (red, green, blue), or is it turned into something else first (as in standard broadcast television, where the color values are translated into a subcarrier, the phase of which determines the hue) and relayed that way?  Then, how does the brain process and store color? It is evident that if all of these processes proceed according to identical mechanism in different individuals, then we can assert with some confidence that they all see any one color alike. But a problem or non-similarity in any of these areas could indicate that the individuals do not in fact percieve the same sensation for a given light source.

Also, I read someplace that experiments have shown that when displayed a light having a mix of significantly dissimilar wavelengths, individuals differ as to which monochromatic light source it most closely matches -- indicating that they are not seeing it the same way.

#### Atomic-S

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##### Re: colour...how do we see it
« Reply #11 on: 06/03/2006 05:53:05 »
In view of the appearance of the sun at certain times and weather conditions, and the appearances of certain other strongly colored objects, it may be worthwhile to place the trichromatic theory of color to a tough test.

I propose that two grays be created: One by combining a narrow waveband strongly red light with a narrow waveband strongly cyan light, such that they are visually equal and balance to create gray. That will be projected on one screen. Next to it on another screen will be projected another gray, created by combining a strongly yellow, narrow spectrum light and a strongly indigo narrow spectrum light, again in proportions to chromatically balance and create gray.

Actually, we must fit the apparatus with the following controls: On the red-cyan combination we will have a control allowing the viewer to adjust the relative porportions of red and cyan so that the result is as neutral as possible. This is necessary to allow for the possibility that not all eyes are identically responsive to these two lights, and that what may look neutral gray to one observer may be tilted toward the red or cyan to another. So this allows the observer to adjust it to his individual eyes. For the other gray we need 2 controls: One to do a balance adjustment such as just described, and the other to allow the wavelength of the yellow (but not the indigo) to be tweaked either torward the red or toward the green. This is to guarantee that all shades of gray that can be created by substantially these wavelengths, will be reachable.

The experiment is conducted as follows: The observer tweaks the red-cyan combination till it is as neutral as possible to his eyes. Then he adjusts the yellow-indigo combination so as to match, as closely as possible, the gray produced by the red-cyan combination.

If the trichromatic theory of color is correct, it will be possible for every observer with reasonably normal eyes to achieve a perfect match.

If, however, the observer cannot achieve a perfect match, no matter what combination of adjustments is used, then the eye's response to color is actually more complex than the trichromatic model.

Essential to the experiment is that the lights involved occupy narrow spectral bands.

I wonder if this has ever been done, and if so, whether there is a reference.

#### The Naked Scientists Forum

##### Re: colour...how do we see it
« Reply #11 on: 06/03/2006 05:53:05 »