# Does anyone know how to measure the color of a Post-It note?

#### sophiecentaur

Gold Member
Yes, lighting. Back in the 60's I was at a party where different parts of the room were illuminated with different colours of lights. My girlfriend had red eyes under a green light and green eyes under a red light. Complicated, indeed!
The secret of good paint matching (fabrics and paper is to get a match under widely varying light conditions. This is only possible if the spectrum of reflected light is wide band (not narrow band peaks) so that the eye's analysis gets an input from all wavelengths and then it can some with more varied illuminants. A peak in one narrow range of wavelengths can miss some of the wavelength bands that you can get with CFLs and that will give lunatic results.
Those CFLs were a temporary blip in the history of colourimetry and we are well rid of them, imo.

#### James Demers

The RGB values assigned to a color are highly relevant to your question, because they're designed to be relevant to RGB monitors, which in turn are intended to serve as input devices for the human eye. I would photograph the two pads side-by-side, under any reasonable lighting conditions - the flash from a digital camera or cellphone ought to do nicely. (The light source affects appearances, as notedd by others, but any reasonably white light won't much affect the relative "redness" of two similar colors.) Open the image on a Mac using Preview, and use the color picker as shown above to get the RGB values. (On a PC, Photoshop or the like will do the trick.) Let's call the results r, g and b.
If you can agree that the "redness" of a color can be measured by its distance from pure red (the lower left corner of the above color cube), you have a re-statement of your question that can be answered quantitatively. The distance from pure red (255, 0, 0) is the square root of [(255-r)2 + g2 + b2].
In the example above, all points on the front plane of the cube are fully red (r = 255), but the selected point is removed from the pure red corner by g=129 and b=139. Per the formula, the distance from pure red is the square root of [02 + 1292 + 1392], or about 189.6. (Your colors are likely to have values of R that are other than 255, meaning that they're somewhere within the volume of the color cube, but only the surfaces are visible in the image of the cube I'm using as an example.)

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#### sophiecentaur

Gold Member
View attachment 220738
The RGB values assigned to a color are very relevant to your question, . . . . . . . . .
This is ok as far as it goes but you are concentrating on Colour Synthesis. The really important process is the Analysis that your eye does and, without getting onto Primaries at all, there are many different spectral distributions that will give any chosen 'colour', as appreciated by the brain. Matching to a given paint sample, using RGB values from an analyser will not tell you the spectrum that's being reflected off that paint. Whilst the Colour TV approach is very successful and definitely near enough for many purposes, it is an approximation. A spectral analysis of the light from a surface plus the spectral analysis of the illuminant will tell you much more and it will allow you to produce a 'perfect' match under any other illuminant.
I just mean that it ain't that simple.

#### James Demers

A spectral analysis of the light from a surface plus the spectral analysis of the illuminant will tell you much more and it will allow you to produce a 'perfect' match under any other illuminant.
I just mean that it ain't that simple.
Agree 100%, but the question of "which is redder" is a question of perception, at least to my way of thinking. To the extent that one agrees on the use of distance in RGB color space as the measure of "redness", I think my solution answers the OP's question.

For example, when asking which of two colors is "more yellow", it doesn't matter whether a perceived "yellow" is yellow light, or the equally-yellow result of red and green light. Spectral analysis can tell you this, but it doesn't help answer the question.

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#### sophiecentaur

Gold Member
Agree 100%, but the question of "which is redder" is a question of perception, at least to my way of thinking. To the extent that one agrees on the use of distance in RGB color space as the measure of "redness", I think my solution answers the OP's question.

For example, when asking which of two colors is "more yellow", it doesn't matter whether a perceived "yellow" is yellow light, or the equally-yellow result of red and green light. Spectral analysis can tell you this, but it doesn't help answer the question.
No problem with that except that your RGB space relates to particular primaries, I think. The tristimulus values X,Y (and possibly Z) would not be 'primary specific'.

#### Sherwood Botsford

Put out a spectrum of post-it notes.

Using even illumination, preferably direct sunlight, take a picture with a digital camera.

Using the eyedropper tool in most photo editing apps, place it's cursor over each Post-it in turn. On most when you mouse over you will get an ordered triple (r,g,b) with the first number corresponding to red, the second green, and the third blue.

Consider however is 2,0,0 is 100% red. But it has only 1/50 of the red of 100,150,200 So you need to compare the amount of red to the total luminance. This is usually calculated = 0.299 R + 0.587 G + 0.114 B as your eye is most sensitive to green and least to blue, so it takes a lot of extra blue to appear brighter.

#### sophiecentaur

Gold Member
Consider however is 2,0,0 is 100% red
I have to ask "Which Red?" Will it be the same red that there lab down the road is using?

#### lekh2003

Gold Member
You would be looking for a spectrometer. Such at this:
http://ikancorp.com/productdetail.php?id=1975&model=mk350d

This is not something that your high school is likely to have.
But the right technical college may be willing to demo one for you.
Some State labs might have one - and they are often open to demo their equipment to high schoolers.
Some private schools have them, surprisingly. I attended a private school for a few years and they had one. We even found electron guns in dusty cardboard boxes and other unused university-level equipment.

As for this crisis, I really don't think you can quantify color, especially in an art class. It's all open to interpretation. There is no universal standard for what is the reddest color, at least if it concerns artists. Wavelengths won't be able to satisfy any art student.

#### sophiecentaur

Gold Member
It's all open to interpretation.
Of course it is. But it is still possible to 'measure' two sources of light (light sources or illuminated objects) and to predict whether or not most people will say that they match. So a quantitative approach to this subject is very much worth while and allows colour TV and printing to work very well.
It is not necessary for a computer to say to itself " ah yes, that is pale pink" for it to select a set of pigments to mix you up a pot of (matching) pale pink paint.

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#### Gapster

Hi all! I initially came here to peruse the threads about Vacuums but stumbled across this topic. Having worked in the print and reprographic industry ( specifically colour correction from computer to paper) I can offer my twopenceworth.

Taking the initial OP on a lighter note (scuse the pun)...

If it is simply a question of personal colour bias then you would both have to agree on a method of determining the colour measurement of the particular post-it that has fuelled the debate. Given that the paper colour is likely to be generated withing the print and paper industry I would suggest going to a local offset printing company and asking them if they have a colour densitometer as a benchmark (and simple) way of determining colour separation/qualities.

"Does anyone know how to measure the color of a Post-It note?"

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