Is it possible to have a Prophoto RGB monitor

[bjanes]

No, since two of the primaries (green and blue) are not physically realizable.

Eric,

You make an interesting point. Ideally, we would like a monitor to reproduce all visible colors: i.e. the CIE XYZ color space. That also might be impossible, since in their color matching experiments, the investigators had to resort to negative colors. As a compromise, one could shoot to reproduce real world surface colors. These are non-emissive (reflected) colors that are encountered in nature. For an example see Page 10 of Gernot Hofmann's PDF. One attempt to construct such a space is BetaRGB by Bruce Lindbloom. From his CIE plot of this space it looks like the primaries are just within the CIE horseshoe [Red = (0.6888, 0.3112), Green = (0.1986, 0.7551), Blue = (0.1265, 0.0352)]. Would this be realizable on an RGB monitor? Does adding a yellow as in Sharp's new TVs do anything for gamut?

The BetaRGB is larger than Adobe RGB and smaller than ProPhotoRGB, but it doesn't seem to have caught on. ProPhotoRGB works fine if one uses 16 bits and does not create unrealizable colors by overzealous editing. What do you think?

Regards,

Bill

[MarkM]

Bill,

R.W.G. Hunt has a good explanation of the gamut limitations of trichromatic reproduction in The Reproduction of Colour. Reproducing the full gamut of human vision is theoretically impossible with three primaries because of the fact that the sensitivity curves of the different cones overlaps making it impossible to stimulate them independently.

2.5   UNWANTED STIMULATIONS
It is thus clear that the inability of any beams of red, green, and blue light to stimulate the retinal cones separately introduces a basic complication into the whole of trichromatic colour reproduction. If the ρ and β curves did not overlap in the blue-green part of the spectrum, then green light could be found that stimulated the γ-cones on their own; but, since the ρ and β curves do overlap appreciably, the γ-cones cannot be stimulated on their own. For colour vision, this overlapping provides the basis for good detection of changes in hue throughout the spectrum. But, for colour reproduction, it means that simple trichromatic methods cannot achieve correct colour reproduction of all colours.
…Thus the overlapping of the cone sensitivity curves, resulting as it does in the inability to stimulate each type of cone separately, is the reason why correct colour reproduction by simple trichromatic means is impossible to achieve for all colours.

[bjanes]

Bill,

R.W.G. Hunt has a good explanation of the gamut limitations of trichromatic reproduction in The Reproduction of Colour. Reproducing the full gamut of human vision is theoretically impossible with three primaries because of the fact that the sensitivity curves of the different cones overlaps making it impossible to stimulate them independently.

Mark,

That is interesting. What additional color(s) would be needed?

Regards,

Bill

[madmanchan]

For a more geometric/visual way of thinking about the problem, see here:

http://en.wikipedia.org/wiki/CIE_1931_color_space

Observe the horseshoe shape (spectral locus) showing the range of colors. The salient bullet point is the one to the left of the diagrams (quoted verbatim):

It can be seen that, given three real sources, these sources cannot cover the gamut of human vision. Geometrically stated, there are no three points within the gamut that form a triangle that includes the entire gamut; or more simply, the gamut of human vision is not a triangle.

[hjulenissen]

For a more geometric/visual way of thinking about the problem, see here:

http://en.wikipedia.org/wiki/CIE_1931_color_space

Observe the horseshoe shape (spectral locus) showing the range of colors. The salient bullet point is the one to the left of the diagrams (quoted verbatim):

To cover a curve (horseshoe) using vectors within the shape one would need an infinite amount?

-h

[madmanchan]

The amount is not the problem.

The problem is the shape. Additive primaries define a triangle in the space shown in the diagram. There is no way to place 3 primaries within the horseshoe that define a triangle that contains the entire horseshoe.

As a rough analogy, try drawing a circle on a piece of paper. Then pick any 3 dots within the circle. Those 3 dots define a triangle. No matter which 3 dots you pick, there's no way that triangle will contain the entire circle. The only way to make a triangle that contains the entire circle is to put those dots outside the circle.

[hjulenissen]

The amount is not the problem.

The problem is the shape. Additive primaries define a triangle in the space shown in the diagram. There is no way to place 3 primaries within the horseshoe that define a triangle that contains the entire horseshoe.

As a rough analogy, try drawing a circle on a piece of paper. Then pick any 3 dots within the circle. Those 3 dots define a triangle. No matter which 3 dots you pick, there's no way that triangle will contain the entire circle. The only way to make a triangle that contains the entire circle is to put those dots outside the circle.
My point is that a square is going to fit better within a circle than a triangle. A penta.... is going to fit even better. As the number of corners approach infinity, you basically have a circle.

If the magic of perception and colors allows for primaries that is outside the horseshoe, even better, problem solved?

-h

[digitaldog]

If the magic of perception and colors allows for primaries that is outside the horseshoe, even better, problem solved?

As I said in Post #4, that issue was solved in 1968/2001ish. But only for Dave Bowman <g>

[acgoris]

Yes, it is possible to construct a ProPhotoRGB Monitor that takes in 16-bit ProPhotoRGB Primaries and generates most of the equivalent color on the display.  It requires 5 primaries in the display.   Keep in mind that any color on the CIE 1931 diagram inside the convex hull of your primaries can be produced.   Since two of the ProPhotoRGB primaries have CIE X,Y,Z coordinates outside the CIE gamut, the additional primaries would best be placed around the points where the ProPhotoRGB triangle exits the CIE diagram.

Here's how I would do it:  Start with a LCD that has a very high refresh rate (240Hz) and an RGB backlight (not a white LED backlight).  Replace each of the backlight's RGB LED triplets with a bank of 5 LEDs with wavelengths around 700nm, 550nm, 530nm, 485nm, and 465nm.  We will likely need to increase the number of LEDs to get enough brightness.  While LEDs come in a huge range of wavelengths, there are still some gaps in coverage and the above wavelengths might have to be tweaked.

Next you might need to alter the color filters in the LCD Panel a little.  If you call the filter colors Fr, Fg, Fb, the Fr filter needs to pass the 550nm and 530nm LED light (and reject the others), and the Fb needs to pass the 465nm and 485nm light (and obviously block the primaries on the other side of the CIE diagram).   We now time multiplex between two sets of primaries.  In frame 1, the LCD display turns on the 700, 550 and 485nm primaries and in frame 2 we send across the information for the 700 (redundant, but not necessary), 530nm, and 465nm.   Inside the monitor is a frame buffer to store the ProPhotoRGB image from the computer (because we're going to use it twice), and a gate array or high speed processor that converts the 16-bit ProPhotoRGB primaries to the appropriate 5-color image.   For a 240Hz monitor you would get all the colors every 120/sec, so you shouldn't see any flicker.   This would work best for static images.  Time multiplexed color creates ugly artifacts (IMHO) for moving images.

This approach would have the least change to existing panel design and preserve the existing resolution.

[Bryan Conner]

Here's how I would do it:  Start with a....

Are you planning to do this?

[acgoris]

Bryan asks if I'm planning to do this.

Naw - even a highly leveraged project like this would take 3 or 4 engineers to prototype.  Maybe one of the high-end monitor manufacturers will do something like this some day.   One thing I've noticed in all LCD panels I've ever looked at is a small variation in color and brightness based on the angle you are are viewing.  This is only a minor annoyance in an sRGB workflow, but would be more important in a ProPhotoRGB workflow/monitor, where the discriminating user is depending on being able to see subtle color differences and have those differences be consistent across the screen.

The ultimate way to do a wide-gamut display is an OLED display, with 6 primaries and wide viewing angle.   For the 6th primary, I'd consider a white pixel.   Here's why:  When you synthesize a color in the middle of the CIE diagram with a green primary and  two nearly-monochromatic primaries out near the lower corners of the CIE diagram, it is possible to sense chromatic aberration in the corners if you wear glasses.   I noticed this effect on a LaCie wide gamut monitor.   Here's the experiment:  fill the screen with white text on a black background.  Sit reasonably close to the monitor (typical 18" viewing distance) and look at the text in the center of the screen.  Without moving your head, sense the text in the corners of the screen, and you can see the split of the red and blue channels.   I wear low-dispersion glasses, but easily saw a 1 to 1.5 pixel split between the red and blue signals making up the white characters on the LaCie monitor in the corner.  If I turn my head and look directly at the character the effect immediately goes away.   

Lastly, if I was doing this for real (e.g. if I worked at Samsung, LG, LaCie, Sony, etc), I probably would just go ahead and spread the primaries out to completely cover CIEXYZ and use CIELAB directly as the signal rather than ProPhotoRGB.  The CMM has to be aware of of the CIEXYZ and CIELAB color spaces anyway since these are the Profile Connection Space for converting one gamut to another, so if you have a monitor that for all practical purposes covers the entire CIEXYZ space, why not just send the signals that way rather than constraining them to strange triangle shapes.   In the past, the real-time conversion would have been too compute intensive for the 200+Mpixel rates needed, but modern electronics could now feasibly put this in the graphics card or the monitor.   We may still need a little faster CPUs before Adobe uses CIELAB or better yet a hyperspectral internal working space :-).

Lastly, we have to remember that real-world surfaces rarely have reflectances out near the edges of the CIE diagram, and today's displays, printers, cameras, and photo software are already capable of very pleasing reproductions without ultra-wide gamut capability, so the market may be limited.   But oooooooh, would I love a display that could reproduce the Neon Light Tunnel at Chicago's O'Hare International Airport (between Terminals B and C)!!, or the structural (diffractive) color of a peacock tail, or Scarlet Tanager.  Gabriel Lippmann would have been proud :-).

[digitaldog]

Since two of the ProPhotoRGB primaries have CIE X,Y,Z coordinates outside the CIE gamut, the additional primaries would best be placed around the points where the ProPhotoRGB triangle exits the CIE diagram.

Since that actual primaries falls outside the gamut of human vision, what good would it do us? And if you move them into the coordinates, how can we call it ProPhoto RGB?

[WombatHorror]

Hi,

We had a recent discussion on this forum about using Adobe RGB vs. sRGB on monitors.

As I guess we all know there are visible colors outside Adobe RGB and a digital camera has a significantly larger gamut than Adobe RGB. Similarly, a printer can print some colors outside Adobe RGB. So weather sRGB or Adobe RGB we will not able some of the colors we actually have in images and also in prints.

For the time being I have the impression that Prophoto RGB, or some derivation thereof, is the best working color space available today. Will we see monitors some day, supporting Prophoto RGB?

Would that be possible at all? The primaries are outside the "horseshoe" of the CIE 1931 xy diagram, meaning that they would be more saturated than spectral colors! Could such a monitor be built, and would it have real benefits?

Best regards
Erik

Well many wide gamut monitors already show a decent amount of colors beyond AdobeRGB (although almost all also fall a little short near the extreme yellow greens) even if well short of Prophoto. Most of my photos fit into AdobeRGB but I do have plenty that look different on my monitor if I convert them down from ProphotoRGB to AdobeRGB, almost all of my deep purple flower photos are one such case.

I really don't know enough about materials, filters, backlight light sources and their properties to say what could reasonable be accomplished. I'm sure they could make them with much larger gamuts right now but maybe they would wear out too quickly or cost too much to put into commercial sets for now.

[WombatHorror]

Is there a reason that a large-gamut monitor has to adhere to some sRGB/aRGB/... standard only? Does it not make more sense that these specialized monitors come with a precise color profile that lets the color management software take advantage of whatever it offers?

-h

that's exactly what does happen
hardly any standard gamut monitor is really sRGB and perhaps none of the wide gamuts are AdobeRGB. Most both extends past and fall short of the standard ideal gamuts that they are closest too. You use a probe and measure them and then the color aware software knows how to handle the native gamut of the monitor.

[WombatHorror]

No, since two of the primaries (green and blue) are not physically realizable.

well you could make what would make them so in a sense they are physically realizable but since human eye wouldn't response to them properly it wouldn't do any good and it wouldn't work out so yeah I guess it would not be possible to ever make a prophotorgb monitor, with a very complex array of primaries you might be able to make one that covers all the visible parts of it though which is all that matters

[acgoris]

We can call it ProPhotoRGB because the monitor could reproduce every color within the ProPhoto RGB color space that represents a color that the human visual system can perceive.  It would be useful because the monitor could display many colors that humans can see and print that are not displayable by monitors that have only 3 primaries.   In some ways it is loosely analogous to printers which use more than 3 inks to achieve a wider gamut.

I carefully chose my words in the first sentence - "color that the human visiual system can perceive".   The explation quoted from Hunt earlier in this thread explains it well, but let me try and rephrase and see if that helps.  The eye can "see" or "detect" light in the electromagnetic spectrum between about 400nm and 700nm.   The huge amount of  information in this spectrum gets compressed down to only 3 numbers through our S (blue) cones, M (green) cones, and L (red) cones.  To reproduce the color sensation of any arbitrary collection of light from the 400-700nm spectrum, we don't need to reproduce that whole spectrum (like in Lippmann Photography), but we do need to stimulate the SML cones the same way as the original light.   If there existed 3 unique colors of light that could independantly stimulate the S,M, and L cones, we'd be there - these three lights (primaries) could be combined in different amounts to reproduce any color we can perceive.   However, since the spectral responses of the S,M and L cones overlap, it is physically impossible to have a light (or a primary color in a display) that individually stimulates the S,M,L cones.   Since we can't independantly stimulate the SML cones, we have to come up with some affordable number of lights (primaries), less than infinity,  that can produce the same SML response as real world spectrals produce.   If we pick 3 primaries that are close to the peak response of the S,M, and L cones, we can cover a good chunk of what we can perceive.   That is the basis of NTSC, sRGB, AdobeRGB, and many other color spaces.  But we can't cover everything we can perceive, as explained earlier.  As we add primaries, we can cover more and more of the possible sensations, or perceivable colors, because we have more control over the response we want from the SML cones.

Here is the concept that takes some time to absorb:  even though we can't *reproduce* any arbitrary color using only 3 real-world primary lights, we can still *describe* any perceivable color with only 3 numbers.   This makes sense since we only have 3 types of cones.   Describing all perceivable colors with only 3 numbers is what CIEXYZ does with the X,Y,Z axes.  Two of the ProPhotoRGB primaries are imaginary colors - they do not represent colors we can perceive, but these numbers still serve a purpose - they allow us to describe more colors than color spaces based on perceivable primaries.

And this is why we can call it a ProPhoto RGB monitor.

[MarkM]

Since that actual primaries falls outside the gamut of human vision, what good would it do us? And if you move them into the coordinates, how can we call it ProPhoto RGB?

I think what acgoris is suggesting is a device that reproduces the visible subset of ProPhotoRGB by using five primaries as in the attached image.

I'm not sure where current tech is on affordably producing pure colors, but it seems you would need to create bright spectral (or near spectral) colors for each of the primaries for this to work. 

[acgoris]

Nice diagram Mark!
LEDs can be pretty close to monochromatic.  Attached is a picture of a bunch of LEDs on a chart showing their locations on the CIE diagram.  This is from a NIST site.

One other thing to note is that the large missing green space isn't as big as it looks.  The CIE XYZ space has many nice properties in describing color, but it is not perceptually uniform -  the same spacing in the upper left is not as easy to tell apart as lower in the chart.
-Andy

[digitaldog]

I think what acgoris is suggesting is a device that reproduces the visible subset of ProPhotoRGB by using five primaries as in the attached image.

Ah, OK but that then would not be ProPhoto RGB. Call it acgRGB if you want. Primaries that fall outside human vision are not colors. ProPhoto RGB defines such primaries. As Eric wrote, due to the shape, it has to. That doesn’t mean any real world device should or can mimic this theoretical color space.

[acgoris]

This is just semantics.
The monitor could do two things: 
  (1) It would accept pixels in a ProPhoto RGB format.
  (2) It would produce colors on the display for all regions of the ProPhoto RGB space which have meaning. 
It is irrelevant how it performs these feats.
What more could anyone want from a monitor that claims to support ProPhoto RGB?

I agree that primaries that fall outside human vision (i.e., the CIE diagram) are not colors.  They have no physical or perceptual meaning.  I agree that ProPhoto RGB defines such primaries.

Claiming a monitor supports ProPhoto RGB is not claiming it can produce something meaningful for all the mathematical combinations of numbers within the ProPhotoRGB space, because not all combinations have meaning.
Nor is it claiming that it has *physical ProPhoto RGB primaries (which is impossible)* any more than AdobeRGB monitors contain actual, physical AdobeRGB primaries - they don't.  But they use the primaries they do have to cover the majority of the Adobe RGB space.