TL;DR: Can additive RGB color gamuts produce the perception of light that has a shorter wavelength than the Blue component? If not, does shortening the wavelength of the shortest component of a 3-channel-color system (e.g., "Blue") increase the range of human-perceptible colors producible by the gamut? Is that not done due to technical obstacles?

Digital photography and light projectors typically use an RGB (Red + Green + Blue) color model, which at least superficially makes sense because it approximately corresponds to the peak responses of the three human color receptors:

Human photoreceptor spectral response curves

But humans can perceive light with higher frequency than "blue." Spectrally we refer to that higher-frequency color region as "violet" or purple:

Visible spectrum

It is not physically possible to produce light with a frequency higher than the Blue component through any combination of RGB color projectors.

We know that a standard RGB color gamut does not cover the range of perceptible frequencies. Here is a standard gamut coverage chart, with the gray area indicating "colors" that cannot be produced by any combination of the RGB source:

RGB color gamut

Considering the violet region: There is a region in the RGB gamut we refer to as "violet," which is a combination of red and blue. But referring back to the photoreceptor response curves it is distinguishable from true violet because true violet barely stimulates the red photoreceptors.

If we increase the frequency of the highest-frequency component of a 3-color projector – i.e., we increase the frequency of "blue," pushing it towards the "violet" limit of human color perception – do we not increase the coverage of the projector gamut?

I suspect the answer is "Yes. But: the blue photoreceptors are not excited as strongly by the violet frequency. The RGB frequencies were picked to correspond with the peaks response frequency of each photoreceptor. If you shifted the blue frequency towards violet (call this an RGV gamut) then your projector would have to be able to output more violet light than the red or green channels in order to cover the rest of the unshifted gamut." If that's correct then this is a technical issue, and not one that seems particularly challenging. For back-lit projectors, which produce color by filtering a white source, the red and green filters would have to be recalibrated to cut more light than the violet filter.

However the answer might be, "No: See that region between blue and green? No matter how you boost your boosted violet channel it just can't reach into it as far as a centered blue channel." (I think the only way to reach this answer would be to have complete parameters of the photoreceptor response curves to spend some time linear programming.)

  • What is the problem, in relation to photography, that needs to be solved? – Alaska Man Jul 11 '20 at 18:58
  • @AlaskaMan The "problem" is maximizing the gamut of visible colors that can be captured and reproduced using a three-color-channel system (which is the basis of the vast majority of digital photography equipment in current use). One could argue that color reproduction is a foundation of photography. – feetwet Jul 11 '20 at 19:05
  • Perhaps you could state that as the goal in the text of the question. I.E. "How to maximize the gamut of visible colors that can be captured and reproduced using a three-color-channel system in digital photography" I did not know WHY you wanted to know if one could shift a RGB channel to increase color gamut into the violet range? – Alaska Man Jul 11 '20 at 20:03
  • "One could argue that color reproduction is a foundation of photography." Perhaps you mean,one could argue color reproduction is a foundation of color photography. Much of my photography does not have any color, nor is it digital. I May shift the tones of black or gray buy using a filter on my lens to make a color a different shade of grey. – Alaska Man Jul 11 '20 at 20:03
  • one can say that the low sensitivity to borderline uv is a "technical issue" but it is also a "physiological issue", and the one you can't really solve without implanting a different kind of eyes. we can't realistically make our screens emit 1000W of UV in hope that 0,1% of that will be capture by our eyes, producing desired results. – szulat Jul 12 '20 at 9:23

Yes but you do not even need to.

The color gamut indicated by the triangle is the coverage of linear combinations of the three RGB primaries. By moving the primaries you can expand or contract the color gamut. This can be seen if you compare the sRGB and Adobe RGB color spaces, both are RGB with slightly different primaries.

Theoretically, you can move the primaries anywhere. The reason though they are not simply moved really wide apart is to avoid banding due to quantization. While mathematically any color within the color-space is part of it, when using fixed color-depth, not all exact colors are representable. So a wider color space has larger steps in between colors and therefore is more likely to show banding and other color artifacts.

Given modern advances, one can use deeper pixels and even floating point respresentation which allows for a wider color-space while minimizing quantization errors. Take a look at the difference with ProPhoto RGB that does something similar to what you suggest:

ProPhoto RGB

Source Photography Life

Once colors can be represented by non-integers, the next step is to allow negative values. This is easy to store in floating point numbers but is also used with fixed-point numbers. The sRGB64 color-space, now renamed scRGB, does exactly this and achieves an extremely wide color-gamut that covers nearly the entire visible light spectrum because each component can from around -0.5 to 7.5 as a factor multiplying a primary. See the diagram in the Wikipedia articled.

  • the question itself is the better answer to the question than this answer – szulat Jul 12 '20 at 9:05
  • 2
    WTH? Seriously. I have demonstrated that moving the primaries is (1) possible and (2) has been done before and (3) it results in a wider color gamut, thus answering the original question. Plus, I pointed to an option expanding the gamut that does not move the primaries by extending the range. If you have a comment on who to improve the answer, go ahead, otherwise this downvote is not constructive at all. – Itai Jul 12 '20 at 14:28
  • I found this answer helpful. But I find it interesting that none of the gamuts shown reach into the sub-450nm "violet" region. Which supports the possibility that in practice there may be technical obstacles to covering that region. – feetwet Jul 12 '20 at 16:15
  • Did you follow the link for scRGB? Using negative numbers they actually cover a huge color-space and it looks to me as to include violet. The main issue with most color space limits is practical as inventors rarely want to waste bits representing things not visible to the human eye. – Itai Jul 12 '20 at 16:25
  • @Itai it's worse than that, any color space with primaries outside the CIE space can't be realized with physical materials. Imaginary color spaces might be convenient for math manipulation but they're no help for cameras, displays, and printers. – Mark Ransom May 4 at 22:43

Gamut is much different than sensor spectral sensitivity. The title of your question infers gamut so to start there, digital cameras do not have gamuts.

Color gamuts are applied to sensor data when pixels are made and a container color space is chosen. So the gamuts shown are limited by the RGB color space not the camera's sensor.

So to achieve the largest gamut from any sensor data I would recommend using ProPhotoRGB as the destination color space in your raw file conversion process. Change the destination color space to ProPhotoRGB a significantly larger gamut than what is in your graphs.

3Profile Gamut comparisons

How does this happen? Well The ICC profile is a container space defined in XYZ and converted to CIELab. RGB is static and all RGB color values are limited to 0-255 for each channel. The code values though are determined by the limits of the container space in CIELab.

If the first graph is an indication of direction towards spectral sensitivities, then those are determined at the time of sensor creation. The RGB filters used over the sensor attempt to get as close to human cone responses as possible in the world of filter technology.

So if you are attempting to maximize something different than gamut, I would have to ask for what purpose? This is an important question because there are many solutions beyond the standard sensor technology and capture systems, but they are all solution specific.

Photography is art and is sold as art. So much of what we can do technically can either enhance that art, detract from it, or make it cost-prohibitive.

Multispectral Prism cameras are still part of the latter group.

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