What decides the colour of light?

[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."

 

[DaveC426913]

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.

It is pretty straightforward. The retinal molecules are not stimulated by wavelength. They can only be stimulated by frequency.

How would a molecular bond be able to detect the wavelength of light? One cannot determine a wavelength unless one knows the speed of light and the length of the wave. Molecules do not know this.
What they know is energy levels.

 

[sophiecentaur]

Suppose a child comes up to you and asks you what the color red is.

But that would be Colourimetry. A child would ask you for an example of something that's red. Imagine that child spoke no English and wanted to know the meaning of the word Red. All you could do would be to give an example of two or three disparate objects whose only common feature was their colour (red). A rational child would (or could) then appreciate that all objects of the same sort of colour were red. Totally subjective and it would not ever involve a source of monochromatic light.
Assuming that they had not been misled by you, there would be no way that they would think that a blue balloon was red. They would already have mapped the various colours of objects around them into groups with their own private names for the colours. Unless they had non-standard colour vision, they would not place bannanas amongst a set of objects that we would all recognise as blue. People seem to suggest that 'one person's red' can be 'another person's yellow' but I have yet to read of anyone for whom that is actually true.

 

[Graeme M]

Yes Vela, that's largely what I was getting at. It was just a simple idle question, very very basic. While I understand that we call something yellow because we all agree on what yellow is (whether we really 'see' yellow or blue) I was curious about how we actually settle on some set wavelengths to define various colours if the property colour is not physical. Somewhere in this thread I think it was mentioned that people can detect a wide range of colours (and here I might mean hues or shades, I don't really know anything about colour), but if all they are perceiving is the responses of rods and cones which is a biological process, it seems unlikely they are all perceiving exactly the same thing. So I am just asking how we can assign a physical correlate to a neural correlate with any precision.

 

[rootone]

. People seem to suggest that 'one person's red' can be 'another person's yellow' but I have yet to read of anyone for whom that is actually true.

Hmm, well I can remember an ex partner described a dress she was wearing as green, but I would have called it blue.
There are some in-between shades which people don't agree on.

 

[sophiecentaur]

nothing to do with perception in the context of colorimetry.

Could we be using the term "colourimetry" at cross purposes?
I have noticed that Chemists and Biologists tend to refer to the measurement of the absorption spectra of solutions etc. as colourimetry. That is a different application of the term and is not aimed at colour reproduction. I guess that, originally, many tests were based on change of colour and would have used colour charts. Now, you can buy colourimeters that do a similar trick but automatically; they are effectively, spectrometers of various qualities.
My experience of colourimetry is to map, as accurately as possible, the perception of colours and to reproduce those colours with a 'Metameric Match'. The chemist's approach is expressly to cut out the subjective bit as much as possible. (to eliminate the 'colour' totally from the exercise)

 

[madness]

It is pretty straightforward. The retinal molecules are not stimulated by wavelength. They can only be stimulated by frequency.

How would a molecular bond be able to detect the wavelength of light? One cannot determine a wavelength unless one knows the speed of light and the length of the wave. Molecules do not know this.
What they know is energy levels.

But it's got nothing to do with energy levels. It's a structural change in a protein (a macromolecule) in a process called photoisomerisation. Given that the wavelength of visible light is around the same length scale as conformal changes in macromolecules, I see no reason why wavelength couldn't be the causally relevant factor.

 

[sophiecentaur]

But it's got nothing to do with energy levels. It's a structural change in a protein (a macromolecule) in a process called photoisomerisation. Given that the wavelength of visible light is around the right length scale for conformal changes in macromolecules, I see no reason why wavelength couldn't be the causally relevant factor.

I googled photoiomerisation and, dang me, if the first diagram on the page didn't have hν on it. ν (greek letter nu) stands for frequency and hν is the photon energy.
How about that? Ol' Dave is not raving mad at all.

 

[vela]

I haven't found a concrete answer to whether it is the wavelength or frequency that matters most, however.

This just doesn't seem like a distinction worth making as wavelength and frequency are not independent of each other. Their product has to equal to the speed of light in the medium, which is established by properties of the medium. If you know the wavelength of light in the eye, you know the frequency and vice versa.

 

[madness]

I googled photoiomerisation and, dang me, if the first diagram on the page didn't have hν on it. ν (greek letter nu) stands for frequency and hν is the photon energy.
How about that? Ol' Dave is not raving mad at all.

If you google retinal cone, every website says that cones are sensitive to the wavelength of light. If you take that as the standard then we're not going to get very far.

Edit: see also this http://pubs.rsc.org/en/content/articlelanding/2012/cs/c1cs15179g#!divAbstract, which says "Azobenzene undergoes trans → cisisomerization when irradiated with light tuned to an appropriate wavelength.". I'm not claiming this as evidence that wavelength rather than frequency is the causal factor, I just wanted to show that this is not a good way to settle the issue, since previous studies likely haven't distinguished wavelength and frequency.

 

[madness]

This just doesn't seem like a distinction worth making as wavelength and frequency are not independent of each other. Their product has to equal to the speed of light in the medium, which is established by properties of the medium. If you know the wavelength of light in the eye, you know the frequency and vice versa.

It would be possible to put photoreceptive cells in other media in which light travels at at a different speed in order to decouple the frequency from the wavelength.

 

[DaveC426913]

But it's got nothing to do with energy levels. It's a structural change in a protein (a macromolecule) in a process called photoisomerisation.

Yes. What is it exactly that you think provides the energy to make the change? A wavelength is not energy; it is simply a magnitude of distance.

 

[sophiecentaur]

since previous studies likely haven't distinguished wavelength and frequency.

But they have. That first wiki hit talks about photon energy. I am not aware of anywhere that the wavelength is used directly to give photon energy. Can you? We have already accepted that the wavelength of light is conventionally used in spectroscopy but that's just history.
Your suggestion that wavelength could be relevant, due to the dimensions of the molecules, does sort of have legs, however. The coupling of energy into a structure can be affected by the physical dimension (in a classical system). However, this is a QM situation so I am not sure that it would really be a valid idea. But. of course, the wavelength inside the material would be affected by the bulk properties. But I still stick with E = hν as the clincher for frequency.

 

[madness]

Yes. What is it exactly that you think provides the energy to make the change? A wavelength is not energy; it is simply a magnitude of distance.

I guess if you presume that it takes exactly one photon to induce the structural change, this argument would make sense, but I'm not sure that this is the case. You can get the same amount of energy from light at different frequencies/wavelengths depending on the intensity (i.e., number of photons). I also don't understand by your argument why cells would be tuned to a particular frequency, rather than just a minimum frequency based on an energy threshold. You would expect sigmoidal rather than gaussian sensitivity curves if your explanation is correct.

 

[collinsmark]

I guess if you presume that it takes exactly one photon to induce the structural change, this argument would make sense, but I'm not sure that this is the case. You can get the same amount of energy from light at different frequencies/wavelengths depending on the intensity (i.e., number of photons). I also don't understand by your argument why cells would be tuned to a particular frequency, rather than just a minimum frequency based on an energy threshold. You would expect sigmoidal rather than gaussian sensitivity curves if your explanation is correct.

Yes, it only takes a single photon of the right energy to induce an excited electron state*, if it is absorbed by the photoreceptor.

*[Edit: from one particular state to another particular state. The energy must be enough to cause the state change, but not so much to excite the electron to an even higher state (and certainly not so much as to blow it clean off the photoreceptor molecule).]

Photoreception is a quantum process at its heart. That means only certain energy levels are allowed (and for what it's worth that's even why it's called "quantum mechanics" in the first place, since energy levels are quantized). Going back to the original post, that means frequency governs the interaction through the $E = h \nu$ relationship (from Einstein's photoelectric effect paper circa 1905).

A single photon (even with the correct energy) is not probably enough to register perception in a typical human, but it is enough to excite an electron in the photosensitive cell. Add enough of these photons of the right energy and the human can perceive the light.

You are correct that twice the number of photons at half the frequency (i.e., half the energy per photon) altogether contain the same overall energy, but humans won't see it (because no electrons in the photoreceptors get excited appropriately).** That's the crux of quantum mechanics right there; it's the rabbit's hole if you will (a rabbit hole that goes very deep).

**[Another edit: similarly, half the number of photons at twice the frequency (twice the energy per photon) also altogether contain the same overall energy, but humans won't see that either. However in this latter case bad things can happen such as cellular damage (as a matter of fact, the photons might not even make their way to the retina, due to being scattered by the lens and aqueous humor, etc. as molecules in those tissues are subject to being damaged/destroyed in the process if the energy per photon is great enough). Make sure your sunglasses have UV protection and don't forget to wear your sunscreen!]

 

[Buzz Bloom]

Hi everyone:

I think a concept related to color perception has mostly been ignored so far in this thread. See

https://en.wikipedia.org/wiki/Cone_cell .​

Each of the cones of the eye measures the the intensity of the light's spectrum with respect to the sensitivity spectrum of the cone's pigment. In the normal eye there are three kinds of cones, each with a different pigment. The relative intensities of three cones with different pigments maps onto a perceived color.

There is also a very unusual color perception phenomenon discovered by Edwin Land. See

http://www.greatreality.com/Color2Color.htm .​

I remember seeing a demonstration. I saw a picture projected on a screen for a brief time as having a normal range of colors. As I recall, the image was an American flag. The projection was made using two black and white transparencies, each projected with a slightly different yellow light. The time of projection was kept short because a longer projection would have spoiled the effect.

Regards,
Buzz

 

[FactChecker]

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 have actually been banned from one of these discussions for saying that. There is a "perception is all that counts" mind set here.

 

[evan-e-cent]

A doctor's perspective. I think we should be able to answer this by referring to the chemical process in the eye. The retina produces a chemical called retinal which can exist as two isomers. The cis isomer of the double bond has a higher energy state than the trans isomer due to steric interference between the adjacent parts of the molecule. The cells in the retina expend energy to convert the trans isomer into the cis isomer which is only semi-stable.

When the cis isomer gets hit by a photon the added energy causes it to flip back into the trans form. The molecule is housed inside a large protein called rhodopsin and with the change to trans configuration the protein also changes its shape. This sets off a chain reaction which causes a nerve signal to be sent to the brain. There are three types of cone cells detecting three colors of light red, green and blue. Each responds with a bell shaped curve and the eye is most sensitive to green as our surroundings are dominated by green foliage (unless we live in a town).

So the question becomes "Is it the wave length or the frequency of light that determines whether it causes the structure of retinal to change". I think the answer is: it is the energy of the photon that is important and that will not vary as the photon travels through different media so I think it is frequency that is the deciding factor not wavelength.

PS Have you ever wondered why water is perfectly clear?

 

[madness]

Yes, it only takes a single photon of the right energy to induce an excited electron state*, if it is absorbed by the photoreceptor.

*[Edit: from one particular state to another particular state. The energy must be enough to cause the state change, but not so much to excite the electron to an even higher state (and certainly not so much as to blow it clean off the photoreceptor molecule).]

Photoreception is a quantum process at its heart. That means only certain energy levels are allowed (and for what it's worth that's even why it's called "quantum mechanics" in the first place, since energy levels are quantized). Going back to the original post, that means frequency governs the interaction through the $E = h \nu$ relationship (from Einstein's photoelectric effect paper circa 1905).

A single photon (even with the correct energy) is not probably enough to register perception in a typical human, but it is enough to excite an electron in the photosensitive cell. Add enough of these photons of the right energy and the human can perceive the light.

You are correct that twice the number of photons at half the frequency (i.e., half the energy per photon) altogether contain the same overall energy, but humans won't see it (because no electrons in the photoreceptors get excited appropriately).** That's the crux of quantum mechanics right there; it's the rabbit's hole if you will (a rabbit hole that goes very deep).

**[Another edit: similarly, half the number of photons at twice the frequency (twice the energy per photon) also altogether contain the same overall energy, but humans won't see that either. However in this latter case bad things can happen such as cellular damage (as a matter of fact, the photons might not even make their way to the retina, due to being scattered by the lens and aqueous humor, etc. as molecules in those tissues are subject to being damaged/destroyed in the process if the energy per photon is great enough). Make sure your sunglasses have UV protection and don't forget to wear your sunscreen!]

Maybe this is just because my knowledge of chemistry is too rusty, but I'm not making the link between the excitation of an electron when it absorbs a photon that you are disucssing, and the conformal change in the structure of a macromolecule that underlies phototransduction in the retina. It seems a perfectly reasonable hypothesis to me that this conformal change would only occur when a wavelength of the right length scale to excite the molecular structure in the correct way would be present. Moreover, this would explain the Gaussian sensitivity curves to wavelength/frequency observed in the retina, whereas your mechanism would predict a monotonically increasing response curve with increasing frequency, in contrast to what is actually observed.

 

[collinsmark]

Maybe this is just because my knowledge of chemistry is too rusty, but I'm not making the link between the excitation of an electron when it absorbs a photon that you are disucssing, and the conformal change in the structure of a macromolecule that underlies phototransduction in the retina.

It's still based on quantum principles. That means only photons of a relatively narrow range of energy values (seen more below) will be absorbed that would cause the proper excitation of the electron, which is the trigger that ultimately leads to the brain perceiving the stimulus (after a very complicated process; @evan-e-cent gave a more detailed description in post #67). Any more energy or less energy in the photon, and the excitation does not happen.

It seems a perfectly reasonable hypothesis to me that this conformal change would only occur when a wavelength of the right length scale to excite the molecular structure in the correct way would be present. Moreover, this would explain the Gaussian sensitivity curves to wavelength/frequency observed in the retina,

It's the photon energy that is critical here. (And the photon energy is directly proportional to its frequency, so if you had to pick between frequency and wavelength here, frequency would be the one to choose, given its proportionality to energy).

whereas your mechanism would predict a monotonically increasing response curve with increasing frequency, in contrast to what is actually observed.

That's not correct. It is certainly not what I meant convey anyway.

“Red” photoreceptor cones are not particularly sensitive to blue light even though blue light has a higher photon energy. And that is completely consistent with the underlying quantum nature at its core. Just because a photon's energy is significantly greater than the allowed change of electron energy states it does not mean that the electron will necessarily absorb some or all of that energy. Instead, quantum theory predicts that that the electron state will likely not be impacted by the photon at all.

(Light frequencies anywhere from near-infrared all the way through ultraviolet will be absorbed by the eye in one way or another. It’s just that a particular color cone type is only sensitive to photon energies within a limited range. And that is not inconsistent with quantum theory.)

Let me illustrate a more simple example.

Consider a light source with a uniform spectrum of light (with no gaps in the spectrum, and let’s assume the spectrum spans at least the visible band, if not the infrared and ultraviolet too). Shine that light through a cold gas (any gas will work, but hydrogen or neon might make good choices), then observe the resulting spectrum. You'll notice that now there are very narrow gaps (called spectral lines) in the resulting spectrum (after passing through the gas)! Photon energies higher than a given spectral line passed right through the gas as did photons of lower energies. Only certain energies are absorbed.*

*(In this simple gas case, the spectral lines are not infinitesimally thin, but do contain a very small bandwidth, which can be explained by the Doppler effect of the moving gas molecules. It's way more complex for [non-gaseous] huge molecules working together as a tissue.)

The molecular structures of human photoreceptors are far, far more complex than a simple gas (not to mention not being in a gaseous state). There’s more complex mechanisms for absorption and thusly the absorption bandwidths are larger than the simple gas example. Yet extraordinarily more complex as they may be, they still follow the same quantum rules.

 

[collinsmark]

Regarding the wider bandwidths of the human color cone photoreceptors compared to the very narrow bandwidths of the simple gas example, let me present another analogy. (This is just an analogy; the human eye is far more complex than a photo-diode, but I think it's still a useful analogy).

I'm sure you are familiar with light emitting diodes (LEDs). One can design a circuit to use an LED in reverse, so to speak, so it detects light rather than emitting it. And the sensitivity does have a bell-curve shape to it larger than the corresponding simple gas example that I used in the last post. as a matter of fact, the LED sensitivity bandwidth is not too terribly different than the photoreceptors in the human eye.

I use this analogy because the sensitivity bandwidths of LEDs (used as photo-diodes) can be predicted with great precision using quantum mechanics.

The larger bandwidth comes from the fact that atoms in the diode form a well defined structure. And when that happens the allowed electron energies form "bands" rather than single values*. Each energy band has a "thickness" (in terms of energy) so to speak. The bands are discrete from one another, but unlike the simple gas example, they are not single values of energy.* So for a photon to be absorbed, it must have the energy which is no less than the smallest energy difference between the bands and no greater than the largest difference. The result produces a bell-like curve shape for sensitivity.

I highly speculate that the human eye has even other mechanisms of widening the sensitivity bandwidth. This comparison to photo-diodes and LEDs is just an analogy, but I think it's a pertinent one.

*(the learned reader will recognize that when I say "single value" of allowed energy for a particular state of an electron in an atom, I am ignoring such things such as fine and hyperfine splitting, as well as Doppler shifting. I don't want to get into that stuff here since it's not really relevant.)

[Edit: This link explains what I was getting at involving the energy band theory:
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/band.html
In order for the material to absorb a photon, the photon energy must be greater than the bandgap but less than the combined widths of the bandgap plus valance state bandwidth plus conduction state bandwidth. The nonzero bandwidths of the conduction band and the valance bands, combined with statistical properties of electrons within those bands, lead to a bell-like shaped curve of absorption sensitivity as a function of photon energy.
Of course the photoreceptor proteins in the eye area far more complex to analyze than what is discussed in this link regarding simple solids. But the same sort of ideas apply, at least in principle (and at least in part).]