What decides the colour of light?

[Rishi Gangadhar]

Consider a beam of light passing through a slab of some refractive index.
We know that the speed and wavelength of the light changes, but its frequency remains the same.
Since the wavelength of the light changes, does its colour change, or does it remain the same as its frequency remains the same.

 

[DrClaude]

Color only has meaning when the light reaches the eye. All that counts is the frequency.

 

[sophiecentaur]

Although people tend to talk in terms of the wavelength of light, that quantity keeps changing as it passes through different media (in particular, it is different whilst it is actually in the eye). Frequency is not changed (as Dr Claude pointed out). But pretty well all the light we see consists of a mixture of frequencies. We seldom come across monochromatic light in nature. Apart from lasers and some electric discharge lamps, the light is far from pure.
It is vital to distinguish between colour and wavelength at all times.

 

[UncertaintyAjay]

Correct me if I'm wrong, but colour is, essentially, the minds way of detecting frequencies. Instead of X Hz or Y Hz, our brain recognizes different frequencies as different colors.

 

[sophiecentaur]

Correct me if I'm wrong, but colour is, essentially, the minds way of detecting frequencies. Instead of X Hz or Y Hz, our brain recognizes different frequencies as different colors.

A partial correction. There is a combination of frequencies involved with most colours.( the ones that are not "spectral" colours). Think in terms of musical chords rather than just tones.

 

[phinds]

...
We know that the speed and wavelength of the light changes, but its frequency remains the same ...

... the wavelength of light, that quantity keeps changing as it passes through different media (in particular, it is different whilst it is actually in the eye). Frequency is not changed ...

HUH? Frequency and wavelength are inverse properties. How can one change and not the other?

 

[phinds]

Correct me if I'm wrong, but colour is, essentially, the minds way of detecting frequencies. Instead of X Hz or Y Hz, our brain recognizes different frequencies as different colors.

I think it's probably more correct to say that color is how the brain categorizes the electrical signal sent to it by the optical nerve rather than that the brain "detects frequencies" directly.

 

[UncertaintyAjay]

Think in terms of musical chords rather than just tones.

Love the analogy . Thanks.

Frequency and wavelength are inverse properties. How can one change and not the other?

Because velocity decreases. Sophiecentaur was referring to light traveling through different media.

 

[nasu]

HUH? Frequency and wavelength are inverse properties. How can one change and not the other?

The proportionality constant (speed of light) changes so that the frequency stays the same.
The wavelength changes due to the changes in speed of light in various media.

 

[phinds]

The proportionality constant (speed of light) changes so that the frequency stays the same.
The wavelength changes due to the changes in speed of light in various media.

Got it. Thanks. I'm slow today. Well, OK, I"m slow every day, but ...

 

[sophiecentaur]

I think it's probably more correct to say that color is how the brain categorizes the electrical signal sent to it by the optical nerve rather than that the brain "detects frequencies" directly.

I always say that human colour perception is a really poor spectrometer. It is sooo easy to fool. And it doesn't matter at all.

 

[phinds]

I always say that human colour perception is a really poor spectrometer. It is sooo easy to fool. And it doesn't matter at all.

I disagree that it doesn't matter. There are situations where subtleties in color do matter.

 

[sophiecentaur]

I disagree that it doesn't matter. There are situations where subtleties in color do matter.

Of course - but the eye is still a lousy spectrometer. That is what doesn't matter because it is never called on to do that job. Colour is not wavelength, is it?
The eye is very good at resolving small differences in perceived colour (when it matters) but that doesn't involve measuring wavelength but combinations of the outputs of just three groups of sensors.

 

[artyb]

Of course - but the eye is still a lousy spectrometer. That is what doesn't matter because it is never called on to do that job. Colour is not wavelength, is it?
The eye is very good at resolving small differences in perceived colour (when it matters) but that doesn't involve measuring wavelength but combinations of the outputs of just three groups of sensors.

This seems similar to Edwin Land's Retinex theory:

"The Retinex Theory of Color Vision

Λ retina-and-cortex system (retinex) may treat a color as a code for a three-part report from the retina, independent of the flux of radiant energy but correlated with the reflectance of objects "

The Retinex Theory of Color Vision SCIENTIFIC - CiteSeer

 

[phinds]

Of course - but the eye is still a lousy spectrometer. That is what doesn't matter because it is never called on to do that job. Colour is not wavelength, is it? The eye is very good at resolving small differences in perceived colour (when it matters) but that doesn't involve measuring wavelength but combinations of the outputs of just three groups of sensors.

Ah. That I agree with. There are people who can reliably discern very subtle differences in color (which is what I mean that mattered) but if they were to look at one of those colors one day and one close to it the next day I doubt they could tell the difference.

 

[sophiecentaur]

This seems similar to Edwin Land's Retinex theory:

"The Retinex Theory of Color Vision

Λ retina-and-cortex system (retinex) may treat a color as a code for a three-part report from the retina, independent of the flux of radiant energy but correlated with the reflectance of objects "

The Retinex Theory of Color Vision SCIENTIFIC - CiteSeer

I remember reading that paper a long time ago when I was involved in colour TV. The well known tristimulus theory of colour vision seems to overlap the Retinex Theory. Reflectance is not the only thing that counts for a lot of our visual input these days (TV displays and projected film) and it is altogether a very complicated business. The eye manages to process out things like the illumination in assessing the colour of an object. That is truly amazing and the description of the process - 'integrating to grey' is a bit of an oversimplification. It's about all that your automatic digital camera colour correction can manage. The eye seems to extract information at a far deeper level, based on context and memory. Brilliant and the Land paper makes a good effort at describing what goes on.

 

[DaveC426913]

... combinations of the outputs of just three groups of sensors.

Or four, in some rare cases...

 

[sophiecentaur]

Or four, in some rare cases...

Tetrachromacy is one extreme of colour vision, I guess. Colour vision is a very personal thing and those tristimulus response curves (look em up folks) are the result of a lot of statistics, conducted on a lot of subjective results with a lot of people (probably a limited racial spread, though), I believe. There is much more spread in the responses between different people than your average conversation acknowledges. The system was aimed mostly at getting a good enough display and printing method to satisfy enough people with the 'accuracy' of copied colours. The proof of the pudding seems to suggest that RGB and CMY depiction of colours is 'near enough'.
One of these days, perhaps, someone will come up with a TV system that uses more than three analyses and more than three basic phosphors. That could be really impressive and we would then start to realize the limitations of what we have at the moment. Colour printing just can't get away with three colours when the colours really count.
There are parallels with Stereoscopic displays, which are impressive but very limited, in fact and multi channel surround sound systems which do better than bog standard stereo sound.
Our brains are definitely on the side of the manufacturers, though. They desperately try to get sense out of these artificially presented sensations, despite the imperfections in the reproduction system.

 

[UncertaintyAjay]

I always say that human colour perception is a really poor spectrometer. It is sooo easy to fool. And it doesn't matter at all.

Its still pretty damn impressive for all that. And as a spectrometer it serves us very well.

 

[sophiecentaur]

Its still pretty damn impressive for all that. And as a spectrometer it serves us very well.

Of course, our colour vision serves us very well - in terms of our survival, and we should not expect anything more than that. (Evolution / Nature never does more than necessary). As a spectrometer, the eye is actually totally inadequate. It cannot even tell the difference between spectral Yellow and a combination of two monochromatic Red and Green lights. If I bought a spectrum analyser that could be fooled as easily as that then I would send it back to the shop.
I don't understand why people get all defensive about their bodily system when someone points out its inadequacies. Our vision is what it is. It has no evolutionary advantage in being a spectrometer - so it never developed to be one.
"It doesn't mean you're a bad person."

 

[wud-wurks]

The cones in the retina of the eye are stimulated this way, 64% react to red light of frequency centred about 650 nanometers about 33 % react to green light centred on 540/550 nanometers and just 2 % react to blue light centred on 450nanometers. SO if you look at a banana for example, all but the frequencies between 570 and 580 are absorbed and the 570/580nanometer light is reflected to your eyes. The cones respond as indicated and the signals in terms of amplitudes from the cones are transmitted down the optic nerve to the brain. The brain interprets these amplitudes/ decodes them if you will, and responds with the result that you are looking at something with a colour we call yellow.

 

[sophiecentaur]

A very nearly monochromatic bannana skin? Hardly likely. It's surface colour, under white illuminant will probably sit around half way between white and spectral yellow on the CIE chart. The pigment will probably be a mix of several natural dyes - at least it could well be. It sure ain't spectral.
If I'm being picky, it's to raise a bit more awareness about the nature of colour.

 

[nasu]

Some people actually study reflection spectra of bananas. :)
http://ucce.ucdavis.edu/files/datastore/234-953.pdf
The graph in figure 3 tells it all.
For a well ripened banana, the reflection coefficient (for visible light) is maximum in the range 550-680 nm. And is at least 20% for the rest of the visible range.
One of the pigments is chlorophyll but obviously, not the only one.

 

[sophiecentaur]

Some people actually study reflection spectra of bananas. :)

And probably the curvature, too. (I just watched the Hugh Fearnley-Whittingstall programme on Cosmetic vegetables; we are so very fussy about curvature in veg)

That spectrograph is interesting as it shows a peak that is 'identifiable' as 'a yellow' (as expected). We would all agree that bannana colour could be described as a bright or strong yellow. However, looking at the total area of the rest of the curve, in the visible range, it is nearly the same as the area of the portion that you could describe as yellows. So, it's fairly desaturated and far from spectral.
I was looking in my massive photo library for a convincing picture of a bannana so that I could look at the RGB components of its yellow colour. I haven't found a picture yet but I may take the trouble to photograph one later today. If it's anything like the other bright coloured objects I have on file, there will be very significant B contributions, along with the G and R (which produce the recognisable yellow). Looking at a brightly clothed audience at an outdoor sport event on a sunny day, it is hard to find objects that are actually 'saturated' colours. Of course, your TV display will never give you spectral colours because they lie outside the gamut, encompassed by the phosphors.

 

[ToddSformo]

Are you saying that it's more proper to note that we are seeing
red at 400–484 THz rather than seeing at 620–750 nm. I know these are equivalent, but in terms of perceived color we are responding to frequency?

 

[phinds]

Are you saying that it's more proper to note that we are seeing
red at 400–484 THz rather than seeing at 620–750 nm. I know these are equivalent, but in terms of perceived color we are responding to frequency?

Your optical nerves respond to how often they are hit by the peak/trough of a wave. Why would they care how far apart the peaks are as they travel towards your eye?

 

[nasu]

As long as is understood that the wavelengths are in air there will be no confusion. The practice of using wavelengths is already well established and the numbers (wavelengths) are easier to remember, I think.

Of course, these wavelengths do not apply to the light actually reaching the retina. It is not in air, I suppose. But it does not matter.

 

[sophiecentaur]

Are you saying that it's more proper to note that we are seeing
red at 400–484 THz rather than seeing at 620–750 nm. I know these are equivalent, but in terms of perceived color we are responding to frequency?

When light was first studied, they couldn't measure the frequency (or even be sure what c was, exactly). However, any Tom, Dick or Isaak could measure the wavelength of the light he was using to a high degree of accuracy, starting from scratch. So wavelength was, and still is, the common currency. But chemicals, sensors and other systems of microscopic charges, work on Energy, which is best quantified in terms of frequency. So both your alternatives are fine. It is the Photon energy that your receptors work on - but you could say that the 'optics' (lens etc) are basically wavelength orientated.

 

[Don Jusko]

Got it. Thanks. I'm slow today. Well, OK, I"m slow every day, but ...

Me too, I've got to get this physics down. This topic was up my ally, it's not a street yet, so this is what I started with.
Science Advisor said a spectrometer could tell the difference between making yellow from two lights, red and green, and the spectrum pigment color.
That being said, I would like to know the difference between the printed yellow photographed and actual pigment yellow, and, the difference between the two yellows examined with a spectrograph. If there is any difference at all I think it is the fault of the electrical receptors in the camera and spectrometer, the cadmium element making spectra yellow can't be wrong but the photo mechanical electrical results can be. I wish I had a spectrometer to compare all three yellow's.
http://www.realcolorwheel.com/final.htm

 

[sophiecentaur]

I can't read your attachments, I'm afraid.
There are a million and one ways of synthesing any particular colour (i.e. producing a match). Your "actual pigment" can be a mixture of natural substances but even a single substance will not reflect only spectral yellow. If it only reflected a very narrow band of colour then it would, unfortunately, look very dark so it can be a very difficult job. This is one reason that they use 'spot colours' in colour printing, because they can end up brighter ( and a better match - say to the coca cola red) than when made with the basic palette of inks)
Cine film was a nightmare to get right and for the reels of different stock to be made near enough so that the audience couldn't spot the reel change. But it was found that people are not as fussy about colourimetry in a dark cinema than they are in their own homes, watching TV, with familiar colours all round them in their living rooms.

 

[Don Jusko]

If it only reflected a very narrow band of colour then it would, unfortunately, look very dark so it can be a very difficult job.

That's not so unfortunate, it would be yellow's dark without the black (that's the way computers work, subtract light to make a darker color).
If you want a lighter color just add it's opposite color, blue light.
I'm a little curious as to just how dark of a yellow pure spectral yellow from a light source would be.

PY100 pigment from 1 light yellow to 9 dark brown.
Red to yellow darken to brown in the Real Color Wheel and element crystals.
Here is a link to show how dark spectral yellow is in pigment, plus the story.
http://www.realcolorwheel.com/colorwheel.htm

 

[DaveC49]

I always say that human colour perception is a really poor spectrometer. It is sooo easy to fool. And it doesn't matter at all.

Hi Sophie I don't think the operation of the human eye can be described as a spectrometer as such ( even a poor one) as the action of a spectrometer is to measure the intensity at a given wavelength whereas the brain ( not the eye) is inferring a color from the differential responses of three sets of chromo-receptors which each respond to different frequency/wavelength ranges which have some degree of overlap in the optical spectrum.

 

[sophiecentaur]

Hi Sophie I don't think the operation of the human eye can be described as a spectrometer as such ( even a poor one) as the action of a spectrometer is to measure the intensity at a given wavelength whereas the brain ( not the eye) is inferring a color from the differential responses of three sets of chromo-receptors which each respond to different frequency/wavelength ranges which have some degree of overlap in the optical spectrum.

As with all things 'evolutionary', the brain expends just enough effort on a problem to get by satisfactorily. A three filter analysis is good enough to distinguish between the spectra we see, reflected from most of the important objects in out lives. Skin can take on may different hues, depending on emotion, health and where we've come from. The tristimulus system does a great job there. Also, the range of colours from greenery / brownery is relevant to us and we do a great job there, too. No animal really cares about wavelength so we can't 'see' it. It was only when they started teaching kids about wavelength that this idea that 'wavelength = colour' and vice versa that any confusion arose.

 

[sophiecentaur]

If you want a lighter color just add it's opposite color, blue light.

You cannot "add blue light" in a subtractive colour mixing process (pigments or filters). If you want a more saturated colour with pigments, all you can do is subtract more light, making the surface darker. RGB colour synthesis is a lot easier to follow because the phosphors are fairly pure and bright. With CMY mixing, you can go more and more saturated but at the expense of brightness. The only way to produce a proper spectral yellow (i.e. a narrow band of wavelengths around the sodium yellow) is with a dielectric (interference) filter. A pigment can't do it - and it's particularly hard with a reflective surface. Insects and birds can have very saturated / bright colkours but not with pigments.
Those pictures of beakers of coloured water do not show spectral colours. How can they?
Printers and artists are pragmatic practitioners. They do the best they can to produce the colours they want. They do not claim to (or want to) produce spectral colours. I think there is an overlap in terminology which suggests some Physics that isn't really there in colour work.That doesn't matter at all - unless you try to equate the two fields of study.
That paper by Land, in an earlier post, is well worth reading in detail as it explains a lot about our perception of the colours of illuminated objects.

 

[Graeme M]

Sophiecentaur I will have to read that paper when I get a moment - I find the whole matter of colour intriguing, in particular that it is an entirely internal property or quality. Something I never realized before and which illustrates how little I actually know about this. So, a question from ignorance.

As I understand what's been said, the colour perceived is generated by the summing of responses from the cone cells which respond to the frequencies of incoming light. So I understand that colour is not a real physical property, rather light has frequency and wavelength and our brains work with frequency to generate a representational quality.

But I'm not clear about the matter of the shades or hues of colours. For example, the mention of say spectral yellow. I assume this term means 'pure' yellow? How have we derived (or more exactly, agreed upon) the values for those spectral colours? I would have thought that the frequencies of light at any particular point on the spectrum don't all necessarily sum exactly the same in all brains, so how do we know which specific frequencies are spectral yellow? Or does that not matter, it's just the statistically averaged perception of that colour?

Or am I just missing the point entirely?

 

[sophiecentaur]

I will have to read that paper when I get a moment

Yes - it is full of good stuff. Perhaps it's starting in on the subject a bit far along the road and you may find it 'challenging'.

. For example, the mention of say spectral yellow. I assume this term means 'pure'

When you meet up with a fresh term like 'spectral colours' in a post, Google can be your friend. I put in that term and the first hit was a wiki article with just what you need. Look at the CIE chromaticity diagram on that link. You can't expect to get all you need to know from conversations on PF (Q and A can be a very inefficient form of learning - when you don't actually have a personal tutor, sitting next to you).

 

[Graeme M]

Well yes, but that doesn't answer my musings. The idea of spectral colours described there is pretty much as I assumed it to mean. That is, it's evoked by a single or narrow set of wavelengths. Though here I am not sure why it talks of wavelength rather than frequency but that just means I know little about em radiation. Regardless I can see how that works.

What I am more getting at is that if we consider yellow which is evoked by way of light stimulating the L and M cones in some proportion, and we have some 6 million cone cells in total, there seems to me to be a fair amount of potential latitude in the exact numbers of cone cells stimulated. That is, how likely is it that for arguments sake exactly 1.8 million Ls and 1.65 million Ms are stimulated in each person's retina.

As colour is not a real thing how is it that we can agree on what spectral yellow looks like? Yes it might be light at 570nm, but does it follow that every human retina responds to that in exactly the same proportion? Or does it not matter in that near enough is good enough? Or do we derive the agreement on what spectral yellow looks like by statistical sampling? That is, if we want to create spectral yellow we can't simply produce something that reflects at 570nm, we have to judge by eye surely?

Yes I realize we can measure the frequency and wavelength via an instrument, but the instrument has no idea what yellow looks like, it can only measure the physical property. We still have to agree on what spectral yellow is before we can assign a wavelength to THAT colour.

Don't we?

 

[sophiecentaur]

Though here I am not sure why it talks of wavelength rather than frequency

I went into that. It's just historical and it is now the convention. There would be no point in changing, at this stage.

there seems to me to be a fair amount of potential latitude in the exact numbers of cone cells stimulated.

Yes, I'm sure there is. Our actual memory for colours is pretty poor (which is why we have to take the curtains into the shop when choosing the wallpaper and one's wife would not allow you to go out and choose a top for her birthday, without taking the skirt with you). But we can distinguish between the 'millions of colours' that your TV monitor can give you, in some really critical material (large areas of nearly the same colour). The 'experiment' you describe would not be a good one because it would be illuminating the whole retina so there would be no reference with which the eye could calibrate itself. I also previously mentioned the poor colour memory in a darkened cinema, compared with viewing TV in the home.

but does it follow that every human retina responds to that in exactly the same proportion?

This has also been mentioned before. Is it not well known that the colour sense varies a lot from person to person? (And animal to animal) What is far more important is the discrimination between adjacent areas - revealing patterns and shapes with predators and prey are against a similar coloured background and the slight blush of embarrassment or pleasure on another person's face. We are very very good at that.

We still have to agree on what spectral yellow is before we can assign a wavelength to THAT colour.

I can't imagine a 'Scientist' going to a lot of trouble to name the colour of a spectral line in an experiment - except in very broad terms. The whole point of assigning a wavelength to a spectral line is to make it possible to refer to it with precision. Otoh, an artist, who would never be dealing with spectral lines (there may be exceptions to that statement but it wouldn't involve pigments) will be using an entirely different way of referring to the colours (see the 'Colour Wheel" system) which doesn't refer to wavelength at all. If you look at the CIE diagram, it is surely pretty obvious that the majority of colours in that colour space do not lie on the spectral arch, over the top.
There is no disagreement between the Colour and Wavlength descriptions. They are just appropriate in different contexts. The only thing is to avoid using them for the same thing.
P.S.

if we want to create spectral yellow

. . .we wouldn't use a reflective surface or a filter. We would use a light emitter - probably a sodium discharge lamp. Nothing else would give a totally pure match. (Google colour synthesis in TV and read about the principles behind it). PF can only do so much.

 

[Graeme M]

Thanks sophiecentaur. That still doesn't quite answer my question so I'll assume my question indicates a basic misunderstanding on my part. Oh well, I run into that a LOT! :)

 

[vela]

What I am more getting at is that if we consider yellow which is evoked by way of light stimulating the L and M cones in some proportion, and we have some 6 million cone cells in total, there seems to me to be a fair amount of potential latitude in the exact numbers of cone cells stimulated. That is, how likely is it that for arguments sake exactly 1.8 million Ls and 1.65 million Ms are stimulated in each person's retina.

It's not the number of cones that are stimulated that matters in perceiving color.

As colour is not a real thing how is it that we can agree on what spectral yellow looks like? Yes it might be light at 570nm, but does it follow that every human retina responds to that in exactly the same proportion? Or does it not matter in that near enough is good enough? Or do we derive the agreement on what spectral yellow looks like by statistical sampling? That is, if we want to create spectral yellow we can't simply produce something that reflects at 570nm, we have to judge by eye surely?

You were likely taught as a child that bananas are yellow. In your brain, you perceived some color and assigned it the name yellow. For all you know, the color you perceive as yellow in your brain is what my brain correlates with the color red. There's no way to know what each person actually perceives. But we all agree that whatever color we see that a banana has is called yellow.

 

[Graeme M]

Vela, I think that's exactly what I was getting at. As I understand it, colour is not a property of the physical world. It's an internal representation. Light has the physical properties of wavelength, frequency and so on which we can measure instrumentally, but colour is not a measurable property, or so I thought. A spectrometer will show us the relevant physical properties but it doesn't tell us which wavelengths are which colour, so that must be a subjective judgement?

How then do we settle on 570nm as being the wavelength that represents spectral yellow rather than 580 or 560? If it's being perceived according to an organic perceptual system there must be biases and variability between individuals, so it just seems unlikely to me that every person agrees that a particular wavelength represents a pure colour. So on what basis do we conclude that spectral yellow is 570nm.

I did do a little research/googling but none of the references I found talked about that, they all just operated from the basis that a particular colour has a particular wavelength. The implication seems to be that colour is a physical property and variability between people's perception is just a subjective interpretation of an objective property, but I had thought that colour is not an objective property.

So perhaps I just misunderstand what is meant by colour being represented internally.

Note: I know very little about light, EM radiation, spectroscopy etc so my use of terminology might be a bit (or a lot!) suspect. The question itself is simply a conceptual one about the perception of colour.

 

[sophiecentaur]

For all you know, the color you perceive as yellow in your brain is what my brain correlates with the color red. There's no way to know what each person actually perceives.

IF what you say is true then colour printing and TV displays couldn't work at all. Whilst it is true that the fine detail of peoples' perceptions of colour have a spread, it has been found that people agree, largely with which synthesised colour matches a given original colour. If what you suggest were true then there could be no, (well established) CIE colour space diagram. You would need to turn bits of it inside out, according to who was using it.
I know that people quote colour names associated with sea, sky, blood etc etc differ a lot between cultures but that could well be because the average actual colours actually are different in different climates and lattidudes. Also, skin colours are very different in different places, so the appearance of blood will also be different. It is wrong to confuse the 'names' of colours with how they can be matched to certain mixes of primaries. The latter is a pretty well established bit of psycho-engineering. (Nikon, Cannon and Pentax sell the same cameras throughout the world and they don't need to be tweaked to fit the users in each country.

 

[sophiecentaur]

How then do we settle on 570nm as being the wavelength that represents spectral yellow rather than 580 or 560?

There are many different "spectral yellows". All that is necessary is that sit on the spectral curve and viewers assess them as 'yellow'. You can be more precise and call it 'Sodium Yellow", which nails it to the narrow pair of sodium emission lines. But what you have written implies to me that you are only considering the colours of monochromatic light. I don't know how many times I have to make it clear that most colours are not formed of monochrimatic light. All wavelengths can be assigned a colour but that doesn't imply that all colours can be assigned to a wavelength. If you haven't read statements to that effect then you have not been reading publications about colourimetry. Many (otherwise well informed) people are incredibly sloppy about this issue.

 

[sophiecentaur]

That still doesn't quite answer my question

Which question is the one that's not been answered? What have you done your homework on, so far? Have you seen a CIE colour chart? Have you seen how colours (points) on that chart can be matched with combinations of other points (primaries)? I suspect that you are trying to get your understanding from this PF thread alone. It can't work that way.

 

[DaveC426913]

For all you know, the color you perceive as yellow in your brain is what my brain correlates with the color red. There's no way to know what each person actually perceives. But we all agree that whatever color we see that a banana has is called yellow.

I think we can know a little more than that. All colours are not made equal. For example, when we produce colours by means that are controllable (in terms of brightness, saturation, etc.) we still all agree that a 'standard' yellow is a brighter colour than the other colours. And we agree that the 'standard' blue is darker. Likewise, combining them, colours that combine with yellow still produce lighter than average intermediate colours, etc.

If you were seeing red when I was seeing yellow, there should be a discrepancy between how we rate what we are seeing.

Yet we all agree that yellow is the preferred colour to paint signs that need to catch the eye at night, that red is not as visible as yellow, and that blue would be a poor choice because it's so dark. We will also spot a banana out of a field of neutral grey noise fast than we will spot a purple/blue eggplant.

 

[vela]

Yet we all agree that yellow is the preferred colour to paint signs that need to catch the eye at night, that red is not as visible as yellow, and that blue would be a poor choice because it's so dark. We will also spot a banana out of a field of neutral grey noise fast than we will spot a purple/blue eggplant.

Sure, but that's because the eye responds to certain wavelengths of light more strongly than other. That's independent of how the brain interprets those signals.

 

[Ravyan Asro]

The beam of light should disperse the colours with different wavelengths and the colours will deviate according to their wavelength.

 

[sophiecentaur]

This thread keeps drifting away from the path of righteosness, I'm afraid. People are not sticking to the principles of PF and they are quoting personal views rather than finding out the actual facts and figures of colourimetry. There are numerous links that give the standard models of colour vision and they are based on a lot of measurements and statistics. If it hadn't been sorted out pretty well, then TV and colour printing would never be as good as it is for nearly everyone (proof of the pudding again). This link is full of good stuff and this wiki article is worth getting into and doing more than just skimming.
It is so easy to get the wrong idea about this topic and it ought to be treated in the same way that 'regular Physics' is treated, with a certain amount of reverence for the established theories. It should not be assumed to be an easy chatty topic.

 

[madness]

It seems to me that the basic question of this thread should boil down to whether retinal cone cells are sensitive to the wavelength or the frequency of light. This wiki page goes into the details of how phototransduction in the retina works - https://en.wikipedia.org/wiki/Visual_phototransduction. I haven't found a concrete answer to whether it is the wavelength or frequency that matters most, however.

 

[vela]

This thread keeps drifting away from the path of righteosness, I'm afraid. People are not sticking to the principles of PF and they are quoting personal views rather than finding out the actual facts and figures of colourimetry.

I think you're just missing the point of Graeme's question, which has nothing to do with perception in the context of colorimetry. Any time you start talking about perception, there's a subjective element, and this subjectiveness is what's at the root of Graeme's question and confusion. Suppose a child comes up to you and asks you what the color red is. You'd be hard-pressed to explain what that color is other than showing him a red object and saying "this color is red." The child perceives something and associates it with the color red. If the child is red-green colorblind, he's likely not seeing the same thing you see, yet he still has some notion of "red."

So Graeme's question arises because he has it backwards. We can't define "spectral yellow" in terms of perception and then figure out what frequency of light it corresponds to, because we don't know what another person sees in their mind's eye. For all I know, what you see in your mind's eye would look like a picture from a clown college in mine, and vice versa. Instead, we define that spectral yellow as light of a certain frequency, and we associate that our individual perception with the name "spectral yellow."