Now, if I draw two tiny spots with red and blue paint on paper and they are so close (but they do not cover each other) to each other that human eyes cannot identify there are two spots. To the color that our eyes can perceive, I come across two statement on the web.

The first is the subtractive principle. It says that, in that small area, the red paint absorbs light other than red and the blue paint absorbs light other than blue. As a result, Longwave and Shortwave are absorbed and there is almost no light come into our eyes and we see black.

The second says in that small area, red lights are reflected from red paint spot and blue lights are reflected from blue paint spot. Thus these two lights, shoulder to shoulder, come into our eyes. These two lights are so close to each other and almost one light for our eyes’ ability. So when the light come into our eyes, it is a mixture of red and blue lights, just magenta.

I have made some experiments with my scanner and printer which told me the result of the mixture is black but not magenta. But I really do not know how to refute the second statement, so I come here for help.

Your helpful answers and comments inspired me that the answer is purple and the first statement is wrong because Subtractive color only happens when two inks overlaps. Here is a image I made to show the result (Interpolation is used to compress it, so zoom in will not show you the original pixels). enter image description here


closed as unclear what you're asking by Kyle Kanos, sammy gerbil, Jon Custer, Sebastian Riese, Cosmas Zachos Jul 7 '18 at 15:13

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    $\begingroup$ I'm voting to close this question as off-topic because it's about the human's ability to perceive colors and not physics. $\endgroup$ – Kyle Kanos Jun 21 '18 at 12:21
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    $\begingroup$ @KyleKanos I know it is a cross field question, but I think as no colorimetry exists on stackexchange, physics is most suitable for my question. $\endgroup$ – Page David Jun 21 '18 at 12:32
  • $\begingroup$ You are evidently conflicted about half-tone: why? $\endgroup$ – Cosmas Zachos Jun 21 '18 at 14:54

Here is the color perception map:


A similar exists in the corresponding wikipedia article.

Note there is no black, black is the absence of radiation, and cannot appear in our perception by the addition of two frequencies.

It says that, in that small area, the red paint absorbs light other than red and the blue paint absorbs light other than blue.As a result, Longwave and Shortwave are absorbed and there is almost no light come into our eyes and we see black.

wherever you read this , it is wrong. We see light, which is composed by zillions of elementary particles called photons, by reflection, and light hitting a red spot to be seen as red is reflected to our retina, the same for blue. The absorbed parts are absorbed by the atoms and molecules in the paint of the spot. What was reflected when at a distance, is reflected when adjacent.

Even if adjacent, the overlap of the reflected light( the dots are assumed adjacent) at most will give a color from the color map above, not the absence of color. My main point is that the behavior of reflected frequencies of the spots is not a function of the distance between them. If they reflect red and blue apart, they will do the same when adjacent. If they overlap it is another story, which will depend on the chemistry and transparency or not of the spots.

  • $\begingroup$ I agree with Anna that the subtractive principle you are quoting is plain wrong, and, specifically, "in that small area". It does hold when the dots overlap, and the bleeding of the printer ink in your experiments may be responsible for that. Try decreasing the distance between your red and blue lines gradually, to see where the magenta-to-black transition occurs... $\endgroup$ – Cosmas Zachos Jun 21 '18 at 15:16
  • $\begingroup$ @CosmasZachos In the half tone wiki you linked previously and the materials I have read on printing that we produce four color patterns (CMYK) individually from original picture, but never consider the situation that color may overlap in the generation process. As you stated that there will be a transition between magenta to black as the distance between dots narrows, why can we ignore them? $\endgroup$ – Page David Jun 21 '18 at 15:53
  • $\begingroup$ I think (know) your question is addressed directly in WP. $\endgroup$ – Cosmas Zachos Jun 21 '18 at 15:58
  • $\begingroup$ @CosmasZachos WP says tiny dots of each primary color are printed in a pattern small enough that humans perceive a solid color in Halftoning, but what is exactly the theory behind them? How does that process faked our eyes and what is the different from LED screens? $\endgroup$ – Page David Jun 21 '18 at 16:16
  • $\begingroup$ @anna v Sorry that I misunderstand your statement in my previous comment, so maybe what I put forward here on black is something near a dark gray. $\endgroup$ – Page David Jun 21 '18 at 16:21

I come across two statement on the web...

It is important to understand whether each of those statements is talking about paint or, talking about ink.

Ink is transparent. In color printing, the ink doesn't reflect any light at all. It's the white paper under the ink that does the reflecting. Red ink transmits only red light, and blue ink transmits only blue light. Any place on the paper where red overlays blue will appear black, because all light wavelengths will be absorbed by either one or both of the two different inks before and after being reflected off the paper.

Paint usually is opaque. Red paint reflects red light, and blue paint reflects blue light. None of the light (if the paint is thick enough) ever even reaches the paper. If you overlay red paint on top of blue, the red layer will reflect red light, and no light will make it through the red layer to the blue layer underneath.

Either way though...

If you have tiny red dots side-by-side with tiny blue dots, then the pattern will look purple from a distance. If the dots are side-by-side (as opposed to being on top of each other), then the light reflected by the red dots is unaffected by the nearby blue dots and vice versa. If you are far enough away that you can not make out the individual dots, then all you will see is paper that reflects both red and blue light. That combination looks "purple."


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