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What causes the green penumbra in this picture?

  1. Dec 16, 2015 #1
    I exposed a black bottle cap by two light sources ( LEDs), one source is white and the other one is red as in the picture.
    I noticed that one penumbra toward the red light source is reddish in color which makes sense because this area is only exposed to the red source. What makes a puzzle is the other penumbra toward the white source is green.
    What is the reason? No green light source was in the room and the screen is a white paper.
     

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  2. jcsd
  3. Dec 16, 2015 #2
    I think this is a product of the materials color reflecting property, white light contains different wave lengths, something that is black usually absorbs light. This material is "glossy" so it reflects some light. My best guess is that the material is reflecting that particular wave length while absorbing the others.

    http://www.physicsclassroom.com/Class/light/U12L2c.cfm
     
  4. Dec 16, 2015 #3
    I took the image and chose a white balance point in the "green" part.
    I IMAG0207balanced.jpg
     
  5. Dec 16, 2015 #4
    I tried with other material and I got the same effect.
     
  6. Dec 16, 2015 #5
    I looked at the first photo and appears to be a green cast off from the brown mug also. I've noticed that LEDS tend to have a different effect then regular incandescent white light bulbs. The green penumbra may just be the product of shadow. Most white LEDS have a blue-ish tint. So this just may be the light source itself.

    I copied this from Wikipedia , white LEDS are.....

    "The existence of blue LEDs and high-efficiency LEDs quickly led to the development of the first white LED, which employed a Y3Al 5O
    12:Ce, or "YAG", phosphor coating to mix down-converted yellow light with blue to produce light that appears white."

    Primary colors yellow and blue make.......green?
     
    Last edited: Dec 16, 2015
  7. Dec 17, 2015 #6
    If the green shadow is due to the light source, the exposed white paper with the white source only would yield a green hue too.
    I exposed a blank white paper with the white LED and I used some apps to roughly measure the color of the area exposed to the white LED and I compared it with the color of the area which appears green in the first experiment. ( see the picture).
    I could not find any green hues anywhere in the paper which is only exposed to the white LED. While the small shadow which appears green in the first experiment gets green color using that apps.
    This means the white LED is not the reason for that green color.
     

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  8. Dec 17, 2015 #7

    sophiecentaur

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    This is not uncommon and it's often only when looking at a photograph that it becomes really obvious. Your eyes tend to compensate when looking at the real thing. Two different light sources, sunlight incadescent/ mfl / led / flash will throw their own shadows. The shadow will be largely filled in by other sources but, if the other source has a different spectrum, the shadow will appear coloured. The unshadowed part reflects a mix of the two lights (appearing to be the correct 'white' colour) whilst the shadow only reflect the light from one. The camera may automatically adjust the colour balance to bring the average of the scene to its chosen 'white' and any of the shadows will appear to be slightly different from the 'white'. Anything that's not the chosen white will appear to have a colour tint.
    Photographers have to try very hard to avoid the effects of mixed illumination. Filling in interior shots with filament lamps can have some really weird effects that people in the room just won't be aware of.
     
  9. Dec 17, 2015 #8
    Thank you for showing me how the camera can modify the colors of the scene.
    But I even noticed that by my naked eyes before using the camera. I thought of a retinal compensation, but this may work in a narrow interface between two areas (something similar to Mach effect) unlike my observation that the whole shadow was green. I am not happy with both explanations.
     
  10. Dec 17, 2015 #9

    sophiecentaur

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    I wasn't referring to transitions between areas. I was referring to the various 'whites' that are visible in your picture and others. There are many optical illusions in which a spot colour will appear different, in the same picture, when presented against two different local backgrounds. The eye does its best and it tends to be a lot better than the auto feature on a camera. It uses context and past knowledge to get the best information it can from a scene.
    But the shadows in your picture are actually illuminated with different light so they will, of course, look different. That difference may show up more on a simple photograph than when viewing the scene by eye. If the background 'should' look the same, in the brain's estimation, then you will probably won't notice the differences.
     
  11. Dec 17, 2015 #10

    sophiecentaur

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    That statement relates to subtractive mixing.
    We are dealing with Additive Colour Mixing in this thread, afaics. The three 'primaries' that are used in additive mixing are Red Green Blue (Hence RGB TV signals). The sort of colour mixing we all learn about first is subtractive, with paints, which absorb wavelengths selectively and the reflected colour is due to different wavelengths being subtracted from the incident white. A yellow pigment can be achieved in many ways but it will usually reflect some reddish light, along with some greenish light. A blue pigment will reflect mostly short wavelengths and absorb reds and yellows but let through some of the green looking wavelengths. So the mixture will look green. The three most useful primary pigments are Cyan, Yellow and Magenta ('minus red', 'minus blue' and 'minus green') and the spectra of the reflected light tends to be broad band; the three spectra will overlap a lot and combinations of these primaries can produce the gamut of visible colours. There is a problem with trying to get all colours with subtractive mixing with just three pigments and that is the resulting colours can look very dark when you want a good saturated colour.
     
  12. Dec 17, 2015 #11
    I know the quote yellow and blue make green is true with paint. With light I'm not so sure. My best guess is the type of light used plays a role in combination with the semi shaded area.

    http://www.bing.com/images/search?q...ac22e840fa9fef47025811252bfdd95fo0&ajaxhist=0

    It seems reflected white light from LEDS produce a blue/green type cast off depending on the angle of reflection. reference above link
     
  13. Dec 17, 2015 #12
    So what do you expect if I use green and white LEDs instead of red and white LEDs? Will the shadow that appeared green in the previous example be green too?
     
    Last edited: Dec 17, 2015
  14. Dec 17, 2015 #13

    sophiecentaur

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    The best way to predict what you get with additive colour mixing is by reference to the CIE chromaticy chart. It describes the human 'colour space'. Any colour you care to think of is at a point on that chart. See this link and many others. If you want to predict what an additive mixture of colours will look like, you draw a line between the points you chose and the perceived colour will lie at a point along the line. The position depends upon the relative weightings and, for equal weights, the colour will be at the mid point. If you take a point in the yellow region and a point in the blue region, it will be greenISH but, as it lies very near the centre (white), it will be a very de-saturated green or even white with a hint of green. With RG and B primaries at the vertices of a triangle, you can produce any colour within the sides of the triangle. Good TV systems use the best primaries available at the time, which are as bright as they can be made and as near the curved (spectral) edge of the chart as possible.
     
  15. Dec 17, 2015 #14

    sophiecentaur

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    In a region of shadow with zero contribution from one of the light sources, you will just see the other one reflected. There is no reason to expect anything other than that - unless the 'context' in the image suggest to your brain that the Red + White contributions are, in fact the 'correct 'white illuminant', in which case the shadow will look (very desaturated) green. Likewise for the green and blue LEDs, the shadow may be interpreted as (very desaturated) red. A camera or spectrometer will tell you the 'truth' of the situation, which may not be what you actually 'see'.
    There is really no such thing as White. It can lie anywhere near the centre of that CIE Chromaticity diagram.. What you see will depend on a combination of the illuminant and the reflecting surface. The White that's used in TV displays can vary, according to the taste of the viewer, the characteristics of the display and the camera. There are agreed standards, which programme producers use.

    The colour of light from any source can easily be far from isotropic; that's no surprise. It will depend on the quality of the source. You would hope that the relative intensities would'nt vary too much around the axis. Many times it won't matter though.
     
  16. Dec 18, 2015 #15
    That is exactly what I observed when I used a green and white LEDs ( see the picture).
    The shadow toward the white LED (on the right side of the picture) appears red as intuitively expected.
    This is clear by both naked eyes and by the camera.
    If the brain interprets the mix of the green and white laying on the screen as white, then why the shadow appears red? Are you saying that the brain has a similar compensation like modern cameras that adjust the color and saturation of hues according to the average colors of the scene? If so, what is the name of this phenomena? I am a doctor but I did not recall that I have studied such thing before. I only know Mach effect which appears at edges between areas of different illuminations because of retina compensation.
    Why do we have to consider this as optical illusion? If I have to study the spectrum of the light in this red area, will not I find the light comes toward the red spectrum?
     

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    Last edited: Dec 18, 2015
  17. Dec 18, 2015 #16

    sophiecentaur

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    Yes - but our perception is pretty complex, of course. In a limited area, we decide on what is 'white' and anything else is interpreted as 'colour' - albeit very desaturated. However, there are some scenes in which one area with given RGB values and which appears 'coloured' and yet another area, elsewher, with the same RGB values can be see as white, and the background in the other area can appear to be coloured.
    You don't have to look at colours to get this sort of effect. There are many monochrome illusions in which you interpret areas with the same surface brightness as, in one place, pale grey and in other places, as dark grey. See this link.
    The poor old brain is doing its best to interpret what the eyes are telling it but can often get it wrong.
     
  18. Dec 18, 2015 #17

    sophiecentaur

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    That's one way of saying it but all three of the 'whites' that are seen - (the main area and the two penumbra) would all be considered to be white because the source LEDs are used as a white illuminant. What we perceive is not 'an illusion' but we can be presented with deliberate illusions that can fool us. As with many of our abilities, it's when they actually fail to work that we can begin to find out how they operate.
    Let's face it, when you go outside in the winter, you would call the light 'white' and when you go indoors into a brightly lit room, you would also call the lighting 'white'. Likewise in a kitchen or office with cheapo flu tubes. None of those 'whites' are the true white because there is not one. Some are bluish, orangeish, greenish and, after five minutes with one illumination, you are quite happy to assess and match the colour of some clothing or paint. Within limits, you can usually get it right, too.
    We needed to evolve with the ability to compensate for a variety of white illuminants but, during our evolution, we were never presented with LEDs, CFLs or mercury arcs so we never evolved to cope with them.
     
  19. Dec 18, 2015 #18
    If the illumination of the background of the white screen by a mix of white and green is interpreted as "white" and other shadows as colored, then this effect should disappear when this background screen is hidden. Like when I cute another piece of black paper and fit it on the white screen but leaving an area for the shadow then see what its color.
     
  20. Dec 18, 2015 #19

    sophiecentaur

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    Do you conduct the experiment in total darkness, except for the visible bit of the screen, lit by just the LED?
    Whatever you 'see' is psychological and the result of the actual spectrum of the light reaching your eye. What 'colour' is the patch of light to you? If it changes with and without the other light source present then some of that light is getting to the patch of screen. Nothing in this sort of experiment can actually alter the wavelengths of the light so it just has to be down to perception, comparison and memory. There are loads of papers about colour vision. The basic tristimulus theory that's applied to Colour TV just has to be pretty credible because 'everyone' who sees colour TV pictures, these days, agrees that they are pretty damn good.
    Land's Retinex theory is interesting and you can find about it in this link, http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.475.6201&rep=rep1&type=pdf [Broken] and many others.
     
    Last edited by a moderator: May 7, 2017
  21. Dec 18, 2015 #20
    Yes. It was in total darkness.
     
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