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Thread: Color Bullets for the Sun

  1. #31
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    Here is a test image of a piece of white poster board illuminated by the gray sky. The camera is looking "backward" through the binoculars to get a sample of the transmitted light for comparison with the direct light from the board.
    DSCF0110.JPGDSCF0110 neutral with swatch.jpg

    The first image is directly from the camera and shows a bluish cast. The second image has been neutralized in the editing software, and just above and to the left of the binocular objective is a swatch from the binocular light, brightened to closely match the direct view. It has a slight greenish cast. The light passed through a lot of glass, mostly in the Porro prisms.

    Once again, I was not concerned with getting an exact reproduction of the source. I was looking for the brightness and color differences between the center and limb light from the Sun, and this small amount of tinting in the optical system did not have much if any effect on the magnitudes of those differences.

    So far there is not enough snow to make a good clean surface for comparison with the poster board. If we get clear weather tomorrow I will take a shot of the poster board under sunshine with blue sky, to see how it compares with the cloudy skylight.

    Camera: Fuji Finepix S5200, ISO 100, 1/40 sec, white balance set for daylight.

  2. #32
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    Now there is enough snow on the car for a good comparison of the poster board, the snow and the cloudy sky.
    DSCF0112.JPGDSCF0113.JPG

  3. #33
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    Quote Originally Posted by Hornblower View Post
    Now I think I see what you meant with your initial remark about extinction. I somehow interpreted it as asserting that the atmospheric effect made a difference between the center and limb colors when there wasn't any before entering the atmosphere. I now figure that you meant that this extinction brings the bluer center to neutral white while bringing the limb to pale yellow.
    Right. Extinctions are uniformly applied. I think only at the horizon itself will the differences in air mass for that 1/2 degree disk make a difference, and sometimes beautifully so but this is caused by the gradient of air mass itself, not any special properties, of course.

    When looking at that large projected image, I think I was having color constancy along with something analogous which I would call brightness constancy. Those swatches I made show that the limb was much darker than it looked to me as part of the broad gradation from center to limb. The whole thing looked neutral white overall, but I think a tiny composite speck would have looked yellowish white, as does a G star, at least to my eyes.
    Yes, it's surprising how we can be fooled with that much actual contrast. Here is my favorite for this illusion:

    Optical contrast illusion.jpg

    [I added the triangles to better see the tansistion.] The apparent difference between the A and B square is remarkable given that they are the same. The solar disk will always exhibit this CLV effect.
    Last edited by George; 2017-Dec-09 at 08:11 PM.
    We know time flies, we just can't see its wings.

  4. #34
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    Quote Originally Posted by Hornblower View Post
    Here is a test image of a piece of white poster board illuminated by the gray sky. The camera is looking "backward" through the binoculars to get a sample of the transmitted light for comparison with the direct light from the board.
    DSCF0110.JPGDSCF0110 neutral with swatch.jpg

    The first image is directly from the camera and shows a bluish cast. The second image has been neutralized in the editing software, and just above and to the left of the binocular objective is a swatch from the binocular light, brightened to closely match the direct view. It has a slight greenish cast. The light passed through a lot of glass, mostly in the Porro prisms.
    Yes, the green tint is slightly more obvious, I think, when looking at the thumbnail, but it is too slight of a difference to wonder why. Perhaps the greater use of the blue-free cones of the fovea make a tiny difference. [Ok, so I think when the word "why" is used it makes me throw something at it. ]

    Here is today's SOHO image (from space for those who may not know). I floated the mask from the limb to the center. This is what a 1390K temperature gradient does to luminosity.

    Solar CLV.jpg

    So far there is not enough snow to make a good clean surface for comparison with the poster board. If we get clear weather tomorrow I will take a shot of the poster board under sunshine with blue sky, to see how it compares with the cloudy skylight.
    You might want to also take an image of the snow or card in the shade, where only the blue sky is the light source.
    We know time flies, we just can't see its wings.

  5. #35
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    Quote Originally Posted by Hornblower View Post
    Now there is enough snow on the car for a good comparison of the poster board, the snow and the cloudy sky.
    DSCF0112.JPGDSCF0113.JPG
    I will guess the the slight difference in brightness is that your background is closer to the horizon. If the brighter zenith region could be taken simultaneously (don't ask me how) then the two may be very close.

    Regarding any color issues, snow has a slight variation in spectral reflectivity based on the size of the particles. I posted some differences a good while back, but snow is typically about 90% or better in spectral reflectance in the visible spectrum. It's whiteness is another argument for a non-yellow Sun and the example of yellow snow adds some graphical kick to it when mentioned, not that I would ever do that, of course.
    Last edited by George; 2017-Dec-09 at 08:17 PM.
    We know time flies, we just can't see its wings.

  6. #36
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    Quote Originally Posted by George View Post
    I will guess the the slight difference in brightness is that your background is closer to the horizon. If the brighter zenith region could be taken simultaneously (don't ask me how) then the two may be very close.

    Regarding any color issues, snow has a slight variation in spectral reflectivity based on the size of the particles. I posted some differences a good while back, but snow is typically about 90% or better in spectral reflectance in the visible spectrum. It's whiteness is another argument for a non-yellow Sun and the example of yellow snow adds some graphical kick to it when mentioned, not that I would ever do that, of course.
    Let me clarify what is in those photos. The left one is the poster board and some adjoining snow on the rear deck of my car. The right one is the sky near the zenith. All I am looking for now is seeing how nearly neutral the snow and the poster board are as reflectors. Then I can evaluate the hue of the combined Sun and blue sky as a light source illuminating the board, as compared with the projected image of the Sun in the darkened room. I can allow for the slight green tint of the binoculars in doing that. While I am at it I will put the board in the shade to get a look at the hue from the blue sky alone. In each case I am manually setting the exposure to get close to the same luminance on each image, for consistency in analyzing the color in the software.

  7. #37
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    That makes sense.

    It almost looks as if there is a tiny extra tint of blue for the snow. However, I took your left image and floated a mask from just below the bottom edge of the board and placed it upon the board. The juxtaposed snow is within the circle, but a Waldo challenge would be much easier to find. Here it is:

    snow and board A.jpg

    snow and board B.jpg
    We know time flies, we just can't see its wings.

  8. #38
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    Here are some images I have prepared of the Sun as projected in the blacked out room.

    1. Projected on white poster board, corrected for greenish tint from binoculars. Time 10:42 EST December 7; altitude 26 degrees.

    2. Same image, with background filled with composite daylight at 12 noon on same board, that is, Sun and blue sky. That background almost perfectly matches neutral tone from software. Note how much more apparent the limb darkening is.

    3. Same image, but adjusted in computer to match center to board illuminated by Sun at 3:56 EST, altitude 7.5 degrees. Same background as #2. That is probably similar to an M star at the zenith.
    Sun image 2017-12-07 correct for bino.jpgSun and composite daylight.jpgSun 3h56m EST.jpg

    If someone had showed me those first two images and I had never heard of the Sun being considered yellow, and they asked me what color I thought it was, I would have said "white" in a heartbeat. Upon looking closely at the limb I might have noticed the brownish tinge.

  9. #39
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    Quote Originally Posted by Hornblower View Post
    Here are some images I have prepared of the Sun as projected in the blacked out room.

    1. Projected on white poster board, corrected for greenish tint from binoculars. Time 10:42 EST December 7; altitude 26 degrees.
    26 degrees is an air mass of 2.28. I too would call it white, but, looking closer, the dirty yellow is not hard to see especially along the limb. I would have expected, however, a lower elevation for this amount of yellowing. Perhaps the Sunspotter adjusted for low elevations will make a fair comparison.

    Any idea if your particle counts are higher than usual?

    2. Same image, with background filled with composite daylight at 12 noon on same board, that is, Sun and blue sky. That background almost perfectly matches neutral tone from software. Note how much more apparent the limb darkening is.
    What I cool-looking image! It makes one see the Sun in daylight and has the 3D touch. Nice!

    The dark limb region is not something I would expect, however. Why would the limb be much darker than the light outside of it. [Warning: The IR level is still too intense to stare at it.]
    This seems very unlikely since, with scattering, how can "the sum of some of the parts be greater than the whole?"

    3. Same image, but adjusted in computer to match center to board illuminated by Sun at 3:56 EST, altitude 7.5 degrees. Same background as #2. That is probably similar to an M star at the zenith.
    7.5 deg. gives an air mass of about 7.5 (7.41 deg = AM7.41; Rozenberg formula). This low elevation should give some distinct yellow results. Perhaps your level of attenuation makes the color stand out more than a normal projection. It seems more yellow than it should be, but perhaps not.

    When the Sun is very low, the attenuation is often enough to make it easy on the eyes to look at and see the very yellow, or sometimes, orange disk. Imaging at this point would allow visual comparison to see how well the image color fits.

    Very interesting. I'm delighted you are doing this. [I am running hard this week but may have time this weekend to do some Sunspotter imaging.]
    Last edited by George; 2017-Dec-12 at 03:30 PM.
    We know time flies, we just can't see its wings.

  10. #40
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    Imaging by this camera obscura method is difficult except around noon because the low Sun is behind trees in this heavily wooded neighborhood. My next project is to play with my eclipse photos through the solar filter which made the Sun look strongly yellow. I will neutralize it in the software and see if I still get the center-to-limb color change. That may give me a baseline for experimenting with images at low elevation. The eclipse images were near the zenith, where the atmospheric yellowing is minimal.

    The limb looks so dark against that bright background because it is that dark. When looking at the image on a black background, so there is a smooth gradation from the center, it appears that my visual process has a partial brightness constancy function.

  11. #41
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    Quote Originally Posted by Hornblower View Post
    Imaging by this camera obscura method is difficult except around noon because the low Sun is behind trees in this heavily wooded neighborhood. My next project is to play with my eclipse photos through the solar filter which made the Sun look strongly yellow. I will neutralize it in the software and see if I still get the center-to-limb color change. That may give me a baseline for experimenting with images at low elevation. The eclipse images were near the zenith, where the atmospheric yellowing is minimal.
    I look forward in seeing your work.

    The limb looks so dark against that bright background because it is that dark.
    But wouldn't the actual surface brightness at the limb exceed that of only scattered light as seen outside the limb? I'm puzzled.
    We know time flies, we just can't see its wings.

  12. #42
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    Quote Originally Posted by George View Post
    I look forward in seeing your work.

    But wouldn't the actual surface brightness at the limb exceed that of only scattered light as seen outside the limb? I'm puzzled.
    That bright background view is a simulation. I started with the image on the black background and replaced the black with composite daylight in the photo-editing software. There is no scattered light.

  13. #43
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    Let me assure you that my images are not exaggerating the limb darkening. From the published center and limb effective temperatures, my Planck law calculation gives the limb a bolometric luminosity of only about 40% of the center, and visually it will be somewhat less because of being shifted more into the infrared. That is in good agreement with my calibration of the luminance of my monitor over the range of RGB values in the photoediting software. I did that with a professional grade exposure meter.

  14. #44
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    Here are some cleaned up images, eliminating possible excessively dark edges on the Sun when seen on the light background.

    Noon sun on black.jpgNoon sun on composite daylight.jpg1546 sun on black.jpg1546 sun on composite daylight.jpg

    From left to right:
    1. 11 AM, December 7, elevation about 26 degrees, black background.
    2. Same image on light background.
    3. 3:46 PM, December 11, elevation about 7.5 degrees, black background.
    4. Same image on light background.

    My attempt at neutralizing the color of the filtered images I took in August were not satisfactory. The yellow tint was just too saturated to deal with, and besides the image scale is much smaller. I will stick with my camera obscura method about 11 AM, when the Sun is clear of the trees. It gives a nice big image, and I can monitor the yellowing at lower altitudes with the illuminated poster board as a reference.

  15. #45
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    More images, with date, time and altitude in degrees:

    b0737.jpg 12/13 7:37 2.5
    b0748.jpg 12/13 7:48 4.3
    b0807.jpg 12.13 8:07 7.2
    b0822.jpg 12/13 8:22 9.4
    b1045.jpg 12/07 10:45 26.1

    To be continued in next post.

  16. #46
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    Same images with light backgrounds. Note how much more striking the limb darkening appears this way.

    w0737.jpgw0748.jpgw0807.jpgw0822.jpgw1045.jpg

    The sky was clear and as free of particulates as it ever gets around here.

    As noted earlier, I have no satisfactory means of imaging the Sun except through that south-facing window for a couple of hours just before noon. I got the early morning colors by photographing the illumination of my white poster board at the stated times, and then adjusting copies of the last image until they matched the board. When I get a chance I will fill in the gap in the altitude range and extend it as the Sun returns north in the next 6 months. It will be interesting to see how much blue shift I get at higher altitudes.

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