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Additive Color Mixing - red colored beams on a screen

  1. Jul 2, 2016 #1
    Hello Forum,

    Awhite background reflects all colors (i.e. all incident wavelengths). A black screen, being black, absorbs all incident types of light (i.e. absorbs all incident wavelengths).

    a)Let's consider a red monochromatic beam projected on a black screen inside a dark room. Would the screen still look black (since it absorbs the red light) where the beam is projected or does it depend on the intensity of the red light?
    b) What if the room was illuminated by the presence of white sunlight? Would the screen region illuminated by the red beam still look black because all the incident light, regardless of its color, will

    By the way, black is not really a color in the sense that the "black" color is produced when no photons reach our eye. Grey is a brighter black. A grey surface absorbs all incident wavelengths but not as much as a black surface.

    Thanks!
     
  2. jcsd
  3. Jul 2, 2016 #2

    DaveC426913

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    What do you think?
     
  4. Jul 2, 2016 #3
    Well,
    I think a red beam on a black screen will look black in a dark room. And a red beam on a black screen in a lit room should also look black since the screen absorbs all incident wavelengths.
     
  5. Jul 2, 2016 #4

    berkeman

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    I think you are correct. But keep in mind it is pretty hard to make a completely non-reflective black surface. you can come close, but for practical surfaces, you will usually get a little reflection.

    Fun questions though. What is the context? Are you working on a new display technology?
     
  6. Jul 2, 2016 #5
    Thanks bekerman,

    No, I wish. Just fun saturday afternoon thoughts thinking about colors. The physiology of color perception is quite interesting.

    I can see how a perfectly absorbing screen is hard to make in practice. More scenarios here :) A white screen in a dark room, if illuminated with a red beam, will look red. A white screen in a lit room, if illuminated with a red beam, will look pink (which is white and red mixed together, i.e. a white with a strong red component). The color brown, I think, is simply red at low brightness (same as grey is white as low brightness).

    As far as colors go, we know that white is the coolest (temperature wise) and black is the warmest (since it absorbs all incident light). A black car gets boiling hot in the sun. What is the next coolest color after white? Can we pick such a color?
     
  7. Jul 2, 2016 #6

    DaveC426913

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    If it's Vantablack, that little reflection will be about 0.035% (that's 35 one thousandths of one percent). :smile:
     
  8. Jul 2, 2016 #7

    DaveC426913

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    As you note, black and white are not spectral colours. Absorption is not dependent on the specific colour but on the amount of light absorbed.

    A pastel - or tint - of any colour will be cooler than a shade of any other colour.

    That being said, typically, yellow is the brightest colour.
     
  9. Jul 3, 2016 #8
    "....That being said, typically, yellow is the brightest colour..."

    What do you exactly mean by that? I know human eyes are the most sensitive to green (555 nm). I can see how, from a heat standpoint, a light shade of any color will be cooler than the deeper shade. I agree that the degree of coolness or hotness, temperature wise, depends on the amount of light absorbed. I guess I was asking if certain colored pigments absorb more incident light than others...

     
  10. Jul 3, 2016 #9
    The molar absorptivity will be a function of structure. Organic pigments tend to be highly absorbing due to resonance over the pigment structure. Diarylide yellows are intense yellows and oranges, quinacridones and perylenes are intense reds and violets, anthraquinones can vary in color. Phthalocyanine blues and greens are some of the most intense pigments in the market. Dyes are strongly absorbing as well. Inorganic pigments such as iron oxides, mixed metal oxides, mercury cadmium sulfides, and lead chromates, and ultramarine blues do not absorb as strongly as the organics. They offer great opacity and heat stability, but not as much chromaticity.
     
  11. Jul 3, 2016 #10
    Also, bear in mind, that monochromatic beams of light are additive. If you cross beams of red, blue and yellow light, the intesection of the beams will appear like white light. When adding pigments and dyes to a substance, the effect is subtractive, i.e. the reflected color is darker (or dirtier in color lingo). To control color you need titanium dioxide for lightness, carbon black for darkness, and pigments to control the red-green axis and the blue-yellow axis.
     
  12. Jul 3, 2016 #11

    DaveC426913

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    This is fascinating ,thank you KM.
     
  13. Jul 3, 2016 #12
    Your welcome. Pigment chemistry is fascinating. So is polymer compounding. The key to success in formulating is knowing which pigments are heat stable enough for your process, dispersability, and which pigments are light stable enough for the intended application. Solubility is also an issue, as not all colorants are soluble in all polymers. Bleed out is a common problem due to colorant/polymer incompatibility.
     
  14. Jul 3, 2016 #13

    DaveC426913

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    I'm an artist, and studied photography in college and continued on into photographic processing, so I know a fair bit about the theory of colour, but not so much about the mechanics and application.

    Have you read Color: A Natural History of thePalette? Follows the discovery/invention of pigments through time and around the world.
     
  15. Jul 6, 2016 #14
    I can see this material becoming the new ASTM reference material for calibrating color spectrophotometers. The goal is to tell the machine what 100% and 0% reflectance looks like. This is done with standard white and black tiles.
     
  16. Jul 6, 2016 #15

    DaveC426913

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    If they're looking for an absolute black, as opposed to some relative black, I fear to think what they might use for absolute white. An exploding hydrogen bomb pressed against the lens?
     
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