# Thread: The Dimensions of Colour - a colour theory discussion thread

1. Hi Shindoh, I'd sum it up like this:

Linear = scaled proportional to light energy
Nonlinear = scaled proportional to human perception
Radiance = light energy, scaled linearly
Luminance = light energy, scaled linearly, but with the visual effectiveness of the spectral components factored in (i.e. green wavelengths count for more than blue, because they look brighter to us at the same amount of energy)
Brightness = luminance, scaled nonlinearly
Lightness = brightness of an object relative to the perceived brightness of a white object in the same setting.

However Charles Poynton, whom I cite on the page where you got that formula, noted that so-called R, G and B "brightnesses" are sometimes given in linear units, and sometimes in nonlinear units, usually with no indication of which of the two is being used. "Linear brightness" of R,G or B would be the radiance (OR luminance) of each relative to their maximum radiance (OR luminance).

Today RGB brightness values seem to be dominantly of the nonlinear kind, though I have encountered the linear kind occasionally. In the two tables I used the inverse square law and cosine relationship respectively to get the fall off of light energy, and then converted this to nonlinear brightness (like the B in HSB) using the *0.45 formula.

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Originally Posted by briggsy@ashtons
Today RGB brightness values seem to be dominantly of the nonlinear kind, though I have encountered the linear kind occasionally. In the two tables I used the inverse square law and cosine relationship respectively to get the fall off of light energy, and then converted this to nonlinear brightness (like the B in HSB) using the *0.45 formula.
This is where I feel like I am missing something.
In Your table we have for example 25% Radiance becoming 54% Brightness. (?)
However 25*0.45 = 11.25; and for the next one 11.11*0.45 = 4.9995.

I can't understand how You got to those numbers.

5. OK. It's not mutiplied by 0.45, it's to the power 0.45:

http://www.huevaluechroma.com/092.php

0.25 to the power of 0.45 is 0.5358867.

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## Thank You

Thank You man, I really think You are the best.
I am sorry I am getting all easy now, but I would just like to express how fortunate I feel I am to know You; to have found out about Your knowledge and thinking, and even be able to speak to You.
You are like a lexicon, an encyclopedia of the history, the present, and the future on artists' understanding and use of color in regards to understanding light.
A pioneer for catapulting painting- and drawing artists' understanding of colors and light to a new level, a deeper level.

The more You know, the more You don't just paint and draw from experience, but paint from actually knowing why You use the colors You use, how they make sense with the environment, or how to realistically and believably give a desired effect, while making everything else in the painting look consistent in the context of it. And You feel more and more safe doing what You are doing. Eliminating moments of frustration, where You don't know how to achieve a desired effect, or why, where and how to put a certain color at a certain area.
And if You paint from life, You will be able to observe the laws You learned about at work.
You won't just see different colors next to each other and their relationships, but You start to see a system in it all. A system from which You draw the most important necessary values, like basic colors of surfaces, all possible lightsources, all areas of reflected or bounced light, position of lightsources, level of specular reflection, layering of specular reflection on diffuse reflection, angles of surfaces, effects on color the light has (wavelengths present in the lightsource, and their peaks, wavelength absorbance and reflectance in the surfaces and their peaks), intensity of light with inclination angle, intensity of point light sources with distance, fresnel reflection ... . It's a beautiful thing.

You have given me so much I always wanted to know, and I am sure many others too. And not just that, but You sparked my interest in a lot of other areas again too. Making me go out and research and read on a lot of things I never even thought of before. I really feel my life has been enriched and made more colorful.

Thank YOU, briggsy. From the bottom of my heart.

8. David Briggs is the best art teacher of all time. - I expect a 50% discount off next term for that comment.

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## Light source through colored surface

But Briggsy, there is still one thing I don't quite understand yet.
I asked the question about light shining through a transparent colored cover, like plastic or glass, before; so I know what happens is substractive mixing of the colors of the light and those of the transparent surface.

What I now don't understand, is how in a lot of colored lamps the inside, the center of the light source, or rather, the lightsource itself, can often still be seen in bright white, although behind the colored transparent cover.

What's happening here? Is it so, that our eyes can only see each color to a certain maximum level, and beyond that, the color can't get any brighter? So that, let's say, in the case of a red cover, the spectral reflectance curve is highest on the red value, and becomes lower and lower to the blue side of the spectrum. Now a strong bright light source easily reaches that maximum red level. Now if compared to red it would only reflect let's say 8% of the blue wavelengths the light source emits (if the curve meant relative reflectance in relation to the other colors and there is a maximum possible level of recognition for a colors in our eyes), then at some point we'd have the brightest for our eyes recognizable red, and at some point we would eventually have the brightest possible version of every color, effectively making it appear white, although the actual relation of the colors in the reflectance curve hasn't changed, we just can't recognize them beyond a certain point of amount?

I don't know, that's the only way I could explain it to myself.
Which would mean, that any surface with exposure to a strong enough light would appear white. Unless it's black. The same way of thinking also explained to myself, that specular reflection of light sources is usually always 100% the 'lightness' of the light source. Although most surfaces (unless metals) have a generally low level of specular reflection at 0 degrees incident angle (I think about 8 percent, for glass for example). As in: 8 % of the brightness of the sun still looks completely white to us. 8% of other surfaces however, which are not illuminants, is much dimmer.

The second thing I was thinking about, was where the glow around light sources comes from. You know, not the glow created by particles, or gases in the air, of the actual light. This glow around the light disappears, if You for example stand away from the light source, and in Your view to the light, cover the light with Your finger. Just hold a finger in front of You to cover the light source in the distance (not actually covering it, just in Your view).
I thought this probably had to do with the adaptation of the eye between levels of illumination. The light source being in the middle of dimmer areas, and the dimmer area being around the brighter light source, creating this gradient of sensitivity originating in the lightsource and steadily becoming weaker outwards.

Related to that, I was wondering where the sparkle appearance of light sources sometimes comes from. Is it really because of light somehow interacting with the eyelashes?

Thank You

EDIT: It's not easy to find out about something, when You don't know the correct name for it to begin with. lol But through some deeper research and reading between the lines of articles about other related things, I found what I was seeing is called a "glare" (http://en.wikipedia.org/wiki/Glare_(vision)).

And I was partly right with what I was thinking.
Last edited by Shindoh; January 1st, 2012 at 09:37 PM. Reason: Answer

10. ## HDR; light through transparent barrier; questions

bbshrmn, in reply to the question on post #174 about using HDR images as colour references: I wouldn't recommend it. First, there are a dozen different ways of creating HDRs; there isn't a standardised way that's "closer to reality". Second, people often over-do them, leading to artefacts like halos, lack of shadow depth/contrast, and over-saturation.

Shindoh -

Is your question "How come a white light will sometimes shine through as white when behind a coloured translucent barrier, instead of taking on the barrier's colour?"?

If so, as I understand it, it can't be about our eye's exposure range maxing out (like it would be a camera, blowing out the highlights), because even if we look at the sun we can still see colour in it, and that's millions of times brighter than an indoor light. Heck, the sun is so bright that it damages our eyes if we look at it, and we can still see it being yellow.

So, we're imagining a white light source behind a transparent but coloured barrier. The barrier will absorb some of the coloured light. Let's say it's a wall of red jello. Why is it the case that sometimes, the light source looks white (when it's strong), and sometimes it looks red (when it's weak)?

My wild guess -- which might be wrong -- is that this is to do with the fact there's a non-linear relationship between perceived brightness and radiance. (See here.) If you remove 10% of the non-red light photons (radiance) from a dim white light, it will look slightly red, whereas if you remove 10% of non-red light from a bright white light, the result will look more white than the dim one.

For a bright source, losing 10% makes less difference visually than losing 10% when it's dimmer.

Another wild guess is that it's simply colour-correction. We see it as white because it's the whitest thing in our field of view.

The "sparkle appearance of light" you mention in your second question does seem to be from the eyelashes (at least, if I squint and then move my eyelashes out of the way, the effect seems to go away -- hardly scientific but not sure what else it would be).

Briggsy -

I had to go through the whole site a couple of times and make notes to get it, but boy was it worth it. Thank you so much for opening my eyes to this fascinating subject.

My main criticism is that it doesn't always define technical words before using them (or sometimes, at all), so I'd find myself having to guess what you mean. I'll try to write my own simplified take on it, to check understanding and hopefully make some parts more accessible. (And try the sphere exercise, of course.)

Couple questions:

1. What is the origin of the mapping from hues to different greyscale lightnesses?
Some hues, like yellow, are lighter than other hues, like violet-blue. Even the camera exposure meter picks this up (one meters a middle-grey tone for correct exposure, but red and grass-green works too, and yellow would make the image under-exposed). What makes some hues lighter/brighter than others? Do cameras only pick this up because they're made to emulate how we perceive things? (I assume this can't be something objective to do with frequencies, because we don't see frequencies -- metamerism and all that. So.. it's about our eyes? What about them?)

2. How does one learn to bypass one's interpretation of colour constancy? Learn the Munsell system? Learn the situations where colour constancy problems come up? Other tricks like imagining the shadows are actually paint rather than different lighting? All of the above? (I found this to be one of the most amazing ideas mentioned in the site. That's friggin' awesome.)

And, optionally:
3. Does this bypassing of colour constancy affect the qualia ('what it is like' to see a colour), or is it an intellectual thing (i.e. you know the colour is one thing, but it still feels like what colour constancy has it as)?

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The second graph, the black and white one in the original post. I don't know if anyone'll answer this... Oh whatever, at the risk of sounding clinically retarded I want to know, what does the graph itself mean or what format is it in? I do not comprehend the notation.

Everything else is gratuitiously helpful and appreciated it's just I can't understand that one graph.

12. Originally Posted by Jarlix
The second graph, the black and white one in the original post. I don't know if anyone'll answer this... Oh whatever, at the risk of sounding clinically retarded I want to know, what does the graph itself mean or what format is it in? I do not comprehend the notation.

Everything else is gratuitiously helpful and appreciated it's just I can't understand that one graph.
I'm sure you're not the only one, Jarlix. It's all explained here:
http://www.huevaluechroma.com/012.php

13. Originally Posted by stevejjd
David Briggs is the best art teacher of all time. - I expect a 50% discount off next term for that comment.
I've organized your discount, but they're bringing in a 100% surcharge for smartarses this year, so it'll work out the same. See you next week!

14. Thanks very much for the praise, Shindoh. Very nice to be appreciated!

Originally Posted by Shindoh
What I now don't understand, is how in a lot of colored lamps the inside, the center of the light source, or rather, the lightsource itself, can often still be seen in bright white, although behind the colored transparent cover.
It seems to me that with a strongly coloured filter the colour is still visible even using the strongest artificial light I have handy (I haven't tried it with the sun, and neither should you!). But I agree that there is some reduction in apparent saturation, or in other words, some approach to the appearance of white light. I think your explanation applies exactly to why the light could appear white in a photograph, and is kind of right for the eye as well, except that here it is not just the instantaneous response to the light that is involved. You are looking at the light with a desensitized patch of retina, caused by localized bleaching of the photopigment, which also causes the persistent afterimage that you see when you look away (dark against a light background, light against a dark background).

Originally Posted by Shindoh
The second thing I was thinking about, was where the glow around light sources comes from. You know, not the glow created by particles, or gases in the air, of the actual light.
.....
EDIT: It's not easy to find out about something, when You don't know the correct name for it to begin with. lol But through some deeper research and reading between the lines of articles about other related things, I found what I was seeing is called a "glare" (http://en.wikipedia.org/wiki/Glare_(vision)).

And I was partly right with what I was thinking.
As I understand it the main factors are scattering within the media of the eye, and at their interfaces, plus scattering within the retinal epithelium, and stray light bouncing around within the eye. Plus the effect of your eyelashes if your eye are half shut. I think twinkling is caused by turbulent movement of air masses in the atmosphere, though.

15. Originally Posted by Lulie
1. What is the origin of the mapping from hues to different greyscale lightnesses?
Some hues, like yellow, are lighter than other hues, like violet-blue. Even the camera exposure meter picks this up (one meters a middle-grey tone for correct exposure, but red and grass-green works too, and yellow would make the image under-exposed). What makes some hues lighter/brighter than others? Do cameras only pick this up because they're made to emulate how we perceive things? (I assume this can't be something objective to do with frequencies, because we don't see frequencies -- metamerism and all that. So.. it's about our eyes? What about them?)
The maximum chroma version of yellow is lighter than that of all other hues partly because to be bright yellow an object needs to strongly reflect a particularly large band of the spectrum (ROYG), partly because many yellow materials approach this ideal quite closely (more than cyan and magenta materials do), and partly because the ROYG band of the spectrum spans the part of the spectrum that looks brightest to us (i.e. that our cone cells respond most strongly to), which peaks in yellow-green. Cameras pick this up because they measure luminance, which is light energy weighted according to the effect of each spectral band on the human visual system (no sense in responding to ultraviolet or infrared radiation, right?)

Originally Posted by Lulie
2. How does one learn to bypass one's interpretation of colour constancy? Learn the Munsell system? Learn the situations where colour constancy problems come up? Other tricks like imagining the shadows are actually paint rather than different lighting? All of the above?
Yes, excellent summary! I'd especially stress the second one for people who worry that they will never learn to see through these illusions. Even if you can't avoid making a mistake initially, if you know about these effects you can get it right on the second or third attempt, instead of going around in circles.

Originally Posted by Lulie
And, optionally:
3. Does this bypassing of colour constancy affect the qualia ('what it is like' to see a colour), or is it an intellectual thing (i.e. you know the colour is one thing, but it still feels like what colour constancy has it as)?
Very, very good question. Without doubt some of the tricks for bypassing colour constancy affect the qualia, but unsurprisingly, since they involve a substantial change in the visual stimulus (e.g squinting, or using a reduction screen with two apertures). Certainly it can be a purely intellectual recognition, involving a conscious switch between seeing the colour of an object in an image, and judging the image colour. Most interestingly, a dramatic change in the qualia can sometimes be effected simply by changing between global and attentive viewing, which can bring about the latter switch without us necessarily knowing consciously that this is what we are doing.

17. ## What causes hue lightness

Originally Posted by briggsy@ashtons
The maximum chroma version of yellow is lighter than that of all other hues partly because to be bright yellow an object needs to strongly reflect a particularly large band of the spectrum (ROYG)
Not sure how this could contribute, because: What about a yellow laser? It would only emit one frequency, but we still see it as yellow and still see it as lighter than other hues.

Surely whether a hue looks light or not can't be different for the pure spectral hue and non-pure spectral frequency version of the hue, because metamerism says they look the same?

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