[Atomic-S (OP)]

Somewhere recently amongst these posts, which I cannot currently locate, I read some comment to the effect that the human eye sees color through a system of 3 kinds of receptors: "red", "green", and "blue", the response of the "red" and "green" type overlapping significantly so that if a wavelength falls other than on the edges of their combined response, what is seen is neither red nor green, but some other color such as orange or yellow. Then the blue overlaps to some extent but it is more isolated in the spectrum. However, the implication is that ultimately the eye is not all that different from a color TV camera. the implications of that of course are that every color the human eye can see, ought to be reproduceable upon a color TV screen.

Question: Has anyone ever witnessed the color indigo upon a color TV screen?

Question: Ultraviolet lamps produce a certain amount of purplish visible glow due to the imperfection of their filters. To the eye this looks violet. When viewed through a color TV hookup, however, I understand it looks blue, not violet. Why is that?

[eric l]

You may be referring to one of my replies, and when it concerns colour perception I usually add a link to Wikipedia (http://en.wikipedia.org/wiki/Colour_perception) for the simple reason that I can not explain it any better.
Anyhow, some additional remarks.

• The human eye and the colour television camera may be a close match, but they are not a perfect fit !
• If you have any experience with colour photography, you will have noticed that different brands of film will give a different rendering of the same scenery.  With colour slides Kodachrome will be warmer (actually reddish), Ektachrome will be colder (blueish) and Fujichrome will be greenish.  You have similar differences between different types of cathode ray tubes (or probably also with different plasma screens or LCD screens
• Some of the information may simply get lost between the tv-camera and your screen

So : is standard color theory correct ?  Well, it is as close as can be provided with the current state of the art. 

[another_someone]

I believe that some of the Fujifilm products actually give a 4^th colour to their films:

http://www.fujifilm.com/products/consumer_film/technology.html

Fujifilm’s patented 4th Color Layer Technology “sees” color in the same way as the human eye. Film emulsions using the traditional three color-sensitive layers (red, green and blue) don’t always render certain hues accurately -- a weakness that is particularly noticeable towards the green end of the spectrum.
By adding a proprietary fourth color-sensitive layer, Fujifilm realizes exceptionally faithful color throughout the entire color spectrum. In fact, 4th Color Layer Technology films offer natural color even under fluorescent lighting unlike conventional films which tend to produce prints with a greenish cast.

[Carol-A]

Many animals only have 2 types of receptor in their eyes, and so have limited colour vision... but there are some who have 4, and presumably see colour much better than us.

[another_someone]

Many animals only have 2 types of receptor in their eyes, and so have limited colour vision... but there are some who have 4, and presumably see colour much better than us.

I have heard speculation that some women might have more than 3 colour receptors insofar as no two people have exactly the same peek frequencies for their receptors, and these are X-linked, and women can have X chromosomes from both parents, and so could have slightly different colour vision from one parent than the other.

[lightarrow]

Somewhere recently amongst these posts, which I cannot currently locate, I read some comment to the effect that the human eye sees color through a system of 3 kinds of receptors: "red", "green", and "blue", the response of the "red" and "green" type overlapping significantly so that if a wavelength falls other than on the edges of their combined response, what is seen is neither red nor green, but some other color such as orange or yellow. Then the blue overlaps to some extent but it is more isolated in the spectrum. However, the implication is that ultimately the eye is not all that different from a color TV camera. the implications of that of course are that every color the human eye can see, ought to be reproduceable upon a color TV screen.
Question: Has anyone ever witnessed the color indigo upon a color TV screen?

And emerald green? TV screens or monitors cannot reproduce all colours of the chromaticity diagram using only 3 primary colours: with 3 colours you can only take all the colours inside a triangle which vertex are the 3 primaries, but the chromaticity diagram is not a triangle, so some colours remains outside. Choose 3 different colours as points in the border of the following diagram and then draw a triangle connecting the 3 points; you will notice that there isn't a choice that takes all the colours of that diagram. There is however a choice that allow you to take most of them; that choice is called "primary colours".
http://en.wikipedia.org/wiki/Image:CIExy1931.png
(Of course, in this image you cannot see all the colours that you would see in a printed image)

Question: Ultraviolet lamps produce a certain amount of purplish visible glow due to the imperfection of their filters. To the eye this looks violet. When viewed through a color TV hookup, however, I understand it looks blue, not violet. Why is that?

In addition to what I wrote above, probably the violet we see is also due to the fluorescence of the eyes hit by UV.

[another_someone]

Many animals only have 2 types of receptor in their eyes, and so have limited colour vision... but there are some who have 4, and presumably see colour much better than us.

Even this is simplistic, because you are only talking about colour receptors - humans have 3 types of colour receptors, but they still have rods, which have a different spectrum altogether, and I imagine could cause colour shifts in low light (where cones and rods may overlap their sensitivity).  This is why simulated night time shots are shot with a blue filter - since that is the colour the rods predominantly see - but it is a simulation only.

[lyner]

The reproduction of colour pictures involves two processes - analysis and synthesis.
A colour tv display is easier to explain than  colour film.
The CIE diagram shows how  any colour  that you can see is represented as a point on the surface of the coloured shape. This is the way your eye analyses colours. A camera or film emulsion tries to mimic this analysis but there are limitations to the range and accuracy - basically it is pretty difficult to include all available colours in a simple analysis. 
Assuming you have done your best to 'map' the colour of the scene on this diagram you then need to display, or synthesise  it.
The points of the black triangle represent the colours of the sets of red, green and blue phosphor dots on the face of the tube. It is possible, by using the appropriate proportions of the three phosphors, to produce a 'metemeric match' with any colour inside the triangle i.e. your eye will percieve that colour. (Remember - colour is only in your brain!) White and de-saturated colours are in the middle and saturated colours are near the sides.
You cannot reproduce any colours outside this range (for a given set of phosphors) - and, of course, you will not see these  outer colours on the diagram, which is being displayed on a conventional  tv display. Any colour that lies outside  this triangle will be limited to its outer edge. You can often see crowd scenes where many people appear to be wearing the same bright  coloured clothing - they are not really - it's just that they are all limited to the same bit of the edge of the triangle.
Indigo lies outside the triangle - so you can't see it as different from a whole set of bluey purple  original colours when on a tv screen.
These days it is, no doubt, possible to analyse and reproduce a bigger range of colours (phosphor  and sensor technology has improved a lot) but, to be compatible with existing systems, the system still uses the same basic analysis.

[another_someone]

As you say, it is about synthesis and analysis.

The synthesis is easy, but the analysis is highly variable.  I would question if any three phosphors, if they create a certain mix of emissions, whether all people would even perceive those emissions as representing exactly the same colour (and this is even if we discount the possibility that some people may be detecting more than 3 colours - and ofcourse, colour blind people, who only have two cones, will have a completely different spectrum).

[lightarrow]

As you say, it is about synthesis and analysis.
The synthesis is easy, but the analysis is highly variable.  I would question if any three phosphors, if they create a certain mix of emissions, whether all people would even perceive those emissions as representing exactly the same colour (and this is even if we discount the possibility that some people may be detecting more than 3 colours - and ofcourse, colour blind people, who only have two cones, will have a completely different spectrum).

Please, explain it better.

[another_someone]

Nominal colour perception is:



In other words, nominally we have cones that peek at 498nm/534nm/564nm.

But what is by natural variation, Bob has spectral peeks for his cones at 510nm/520nm/580nm, whereas Alice has spectral peeks at 490nm/545nm//555nm.  Clearly, Bob will see monochromatic light at 534nm wavelength differently to Alice.

In most everyday purposes, this will not matter too much - Bob will see an object with a 534nm emission and call it green, and Alice will see the same emission, and also learn to call it green, and it does not matter that they have a slightly different mix of cone stimulus that they each call green.

The problem happens when you try and synthesise the colour by mixing other colours, because Bob will see 534nm as slightly closer to his red, while Alice will see the colour as slightly closer to her blue.

Ofcourse, all of this is without taking into account the sensitivity of the rods in the 420nm region, or the slight upturn in sensitivity of the red cones on the lower side of 450nm.

[lyner]

Yes. Colour is all in your mind.  It's a bit akin to stereo sound. Everyone gets some sensation of three dimensionality in a good stereo system (just two channels) but surround sound - using more channels, produces better agreement between listeners about the sound image layout. I am sure that a multiple phosphor system would have a similar advantage with portraying colours better.
But my point about seeing indigo on TV (in the original posting)   is correct; you can get various subjective sensations of colours within the gamut of the display phosphors but you can't produce a colour outside it.
I am surprised that no one seems to have marketed a system with a bigger gamut of colours; it could be stunning -  a bit like the original tv was stunning to monochrome  tv owners.   

[another_someone]

Yes. Colour is all in your mind.  It's a bit akin to stereo sound. Everyone gets some sensation of three dimensionality in a good stereo system (just two channels) but surround sound - using more channels, produces better agreement between listeners about the sound image layout. I am sure that a multiple phosphor system would have a similar advantage with portraying colours better.
But my point about seeing indigo on TV (in the original posting)   is correct; you can get various subjective sensations of colours within the gamut of the display phosphors but you can't produce a colour outside it.
I am surprised that no one seems to have marketed a system with a bigger gamut of colours; it could be stunning -  a bit like the original tv was stunning to monochrome  tv owners. 

I agree, I believe a 6 colour TV would be a significant improvement over the present 3 colour TV.

The big problem is that colour TV is transmitted in 3 colours; DVD's are recorded in 3 colours; images are stored as RGB.  We simply do not presently have the software to support a 6 colour TV - so it would need to be fairly specialised usage until we get the software to support it.

[Atomic-S (OP)]

I observe, in the diagram from another_someone, that the response for the red cones has a second peak near the short end of the spectrum. That agrees with data found in the CRC Handbook of Chemistry and Physics , where the red response for  the standard observer is shown with two distinct peaks.

I opine that this fact and the fact that the responses of the receptors are all broad and overlapping, explains why the visual response diagram is not a strict triangle. Because of overlapping response, one cannot, in general, turn on any one of the cone types using one wavelength (with the possible exception of the red, using a long wavelength). Therefore the task becomes trying to find the wavelengths where each type of receptor is, to the maximum possible extent, isolated. But because this isolation is never perfect, 3 primary wavelengths will fall somewhat short of harnessing the entire capability of the eye.

Do I not recall that a certain type of color photo printer uses 6 different inks? Of course, the behaviors of mixed inks may themselves be less than ideal, another reason why more than 3 are advised for really good reproduction.

And in hardware stores, it appears that paints are mixed to specification using considerably more than 6 different pigments. Here, I would assume that the main obstacle is the non-ideal behavior of paints, in that the mixing of paints is not physically identical to the mixing of spectra, due to the way light can be reflected, refracted, or transmitted in numerous different ways among the particles of pigment.

[another_someone]

Do I not recall that a certain type of color photo printer uses 6 different inks? Of course, the behaviors of mixed inks may themselves be less than ideal, another reason why more than 3 are advised for really good reproduction.

You can get photo printers with up to 10 inks, but the data they are using is still 3 colours.

The problem is mostly because of the fact that printed output uses reflected light, and that the colour is subtractive rather than additive, and that it takes the white from the underlying paper rather than from colour mixing.  The colours are usually no more than black (which can poorly be simulated by using a mix of the other inks, since the colours are subtractive - but it is an imperfect black), and then lighter shades of the other colours (i.e. light magenta, and light cyan) that gives a better effect than relying on the white of the paper to show through a thin cover of ink.  Sometimes the printer will have 2 different blacks, one that is designed to mix better with the other colours (to darken them), and another that works better when you only want the black on its own (e.g. for pure black text). None of this is really intended to change the underlying colour curves, but merely to get the printer to better reproduce the colour curves that are natural to a 3 colour VDU.

[lightarrow]

Do I not recall that a certain type of color photo printer uses 6 different inks? Of course, the behaviors of mixed inks may themselves be less than ideal, another reason why more than 3 are advised for really good reproduction.

You can get photo printers with up to 10 inks, but the data they are using is still 3 colours.

The problem is mostly because of the fact that printed output uses reflected light, and that the colour is subtractive rather than additive, and that it takes the white from the underlying paper rather than from colour mixing.  The colours are usually no more than black (which can poorly be simulated by using a mix of the other inks, since the colours are subtractive - but it is an imperfect black), and then lighter shades of the other colours (i.e. light magenta, and light cyan) that gives a better effect than relying on the white of the paper to show through a thin cover of ink.  Sometimes the printer will have 2 different blacks, one that is designed to mix better with the other colours (to darken them), and another that works better when you only want the black on its own (e.g. for pure black text). None of this is really intended to change the underlying colour curves, but merely to get the printer to better reproduce the colour curves that are natural to a 3 colour VDU.

Yes. To get a specific nuance, the printer should mix the inks perfectly and in the exact proportions, but this is not clearly possible, even because every ink's drop has a finite volume and it spread on the paper on a finite area (even if the drop had negligible dimensions). So, with 3 only inks you could have, e.g., a specific nuance of a face's colour and the closest to it which is quite different, so a picture of that face wouldn't be as good as with more inks.

[lightarrow]

Yes. Colour is all in your mind.  It's a bit akin to stereo sound. Everyone gets some sensation of three dimensionality in a good stereo system (just two channels) but surround sound - using more channels, produces better agreement between listeners about the sound image layout. I am sure that a multiple phosphor system would have a similar advantage with portraying colours better.
But my point about seeing indigo on TV (in the original posting)   is correct; you can get various subjective sensations of colours within the gamut of the display phosphors but you can't produce a colour outside it.
I am surprised that no one seems to have marketed a system with a bigger gamut of colours; it could be stunning -  a bit like the original tv was stunning to monochrome  tv owners.

As I've already written, green's field also is not well reproduced by monitors.

[Atomic-S (OP)]

I suspect the present color TV system exists not because it is the best possible, but was the best that could be achieved with reasonable technology at the time when color TV was being developed. When color TV was first being developed, it was difficult and a number of cumbersome systems were devised, eventually something like the present system emerged. The present system does a good job on most common scenes, and at that time it was arguably not worth the problems to try to do anything better.

[lightarrow]

I suspect the present color TV system exists not because it is the best possible, but was the best that could be achieved with reasonable technology at the time when color TV was being developed. When color TV was first being developed, it was difficult and a number of cumbersome systems were devised, eventually something like the present system emerged. The present system does a good job on most common scenes, and at that time it was arguably not worth the problems to try to do anything better.

I hope you are joking! If some company had the idea of the slightest improvement on monitor's working, they would immediately produce such a new monitor advertising it aloud to the world!

[another_someone]

I hope you are joking! If some company had the idea of the slightest improvement on monitor's working, they would immediately produce such a new monitor advertising it aloud to the world!

There are two issues with regard to this.

Aside from the old story about VHS and Betamax, and most people think that the inferior standard won out; but in more recent times - SACD and other high quality audio storage systems have failed to find a commercially viable market; while MP3, which is highly utilitarian, but inferior quality, audio recording has been highly successful as a commercial standard.

Secondly, any limitation of the colour system for visual reproduction is first and foremost a limitation of the media (recorded or broadcast data) before it is one of the display technology.  All the improvements in display technology in the world will make not the least difference if the extra data is missing from the recording/broadcast transmission.  This is also a problem with audio data (and was certainly an inhibition for SACD), but was less of a problem with MP3, because this was a format that users were encouraged to record at home and not be reliant on commercially available recordings using that media (again, this was made easier because MP3 does not try and introduce extra quality, but only extra utility).

The only way that we could be in any position of developing a system that recorded and displayed 4 or more colours is to market a high end total system solution, but this is unlikely to be initially as a display standard but rather (if we are able to significantly improve print quality) a recording and print standard (since a professional is not interested in what is on his display, as he cannot sell that, but he is interested in what he can print, since that is a saleable item).