Chris Peterson wrote: I disagree. The Sun definitely has a warm cast when viewed directly through a neutral density filter, and it definitely has a yellowish cast when projected, as long as the projection is viewed in an area shielded from skylight. I see this from my observatory at 3000m elevation with very clean, dry air. I also remember it from back when I worked at Big Bear Solar Observatory, again under good atmospheric conditions.
This is interesting since it is contrary to all that I’ve seen so far. I would enjoy learning what you would observe if you did a simple pinhole projection onto a white sheet of paper, especially from 3000m.
There is a slight chance you are experiencing yellow induction due to the blue sky. [In the 1700s, Nocolas de Beguelin claimed the yellow light from a candle’s flame would produce a bluish color of any shadow cast upon a white paper. He claimed the blue sky was the reason. Goethe, however, took it further by doing the demonstration in a non-blue room on an overcast day. He showed that blue and yellow are complementary so one color can induce another.]
I am curious if Big Bear uses filters for their Solar projections, or could there be some lens coating issue. I wouldn’t think the extra humidity would be an issue, or is this a possibility?
There are pretty good neutral density filters over the visible range. In fact, Baader material (and aluminized Mylar in general) is actually quite flat.
Transmittance info would be real handy about now. I have looked in the past for this info but gave up for the reasons below. I can say my Baader filter does produce a yellow Sun….
Here is one:
Solar projections at solar observatories – though not always true at Big Bear and perhaps not at others, but at least at Kitt Peak - appear as solid white and without any hint of yellow. Here is an image taken with the help of Dr. Drew Potter and Roy Lorenz at Kitt-Peak’s McMath-Pierce telescope. [The color pieces, of course, were added for color calibration assistance.]
This is also how it appears in person.
There are two important points worth discussing: filtration and CLV.
I verified that they were not using any filters for this projected image, so WYSIWYG, and what we get is all white.
The center to limb variation (CLV) also did not alter the image’s color. This is significant because the CLV is almost 1400K (6390K at the center, 5000K at the limb). If an unfiltered image of the cooler limb region is white then it seems highly unlikely that any yellow sun argument can hold much ground.
However, color is subjective, so you and others may simply see yellow where others see white. If so, it would be interesting to find out what variables contribute to this variability.
Not related. You can duplicate the effect easily in a lab with a monochromator. A change in intensity of a single spectral band, or a mix of bands, will produce an apparent shift in hue, separate from the obvious change in color.
I have never done this, but I must assume that the hue itself does not change but our perception of the colour does based on changes in brightness and saturation. I also assume any variation of a monochrome light, say from a green laser, will never produce a red, orange, yellow, or blue projection without some other major factor like induction, though I doubt induction or anything else is powerful enough to make a monochromatic green beam appear red. A green laser will always look green though a yellowish-tint might be observed as the intensity changes. Is this what you are saying?
[quote="Chris Peterson"] I disagree. The Sun definitely has a warm cast when viewed directly through a neutral density filter, and it definitely has a yellowish cast when projected, as long as the projection is viewed in an area shielded from skylight. I see this from my observatory at 3000m elevation with very clean, dry air. I also remember it from back when I worked at Big Bear Solar Observatory, again under good atmospheric conditions. [/quote] This is interesting since it is contrary to all that I’ve seen so far. I would enjoy learning what you would observe if you did a simple pinhole projection onto a white sheet of paper, especially from 3000m.
There is a slight chance you are experiencing yellow induction due to the blue sky. [In the 1700s, Nocolas de Beguelin claimed the yellow light from a candle’s flame would produce a bluish color of any shadow cast upon a white paper. He claimed the blue sky was the reason. Goethe, however, took it further by doing the demonstration in a non-blue room on an overcast day. He showed that blue and yellow are complementary so one color can induce another.]
I am curious if Big Bear uses filters for their Solar projections, or could there be some lens coating issue. I wouldn’t think the extra humidity would be an issue, or is this a possibility?
[quote] There are pretty good neutral density filters over the visible range. In fact, Baader material (and aluminized Mylar in general) is actually quite flat.[/quote] Transmittance info would be real handy about now. I have looked in the past for this info but gave up for the reasons below. I can say my Baader filter does produce a yellow Sun….
Here is one:
[url=http://img231.imageshack.us/i/sunvia8inchsct.jpg/][img]http://img231.imageshack.us/img231/3883/sunvia8inchsct.jpg[/img][/url]
Solar projections at solar observatories – though not always true at Big Bear and perhaps not at others, but at least at Kitt Peak - appear as solid white and without any hint of yellow. Here is an image taken with the help of Dr. Drew Potter and Roy Lorenz at Kitt-Peak’s McMath-Pierce telescope. [The color pieces, of course, were added for color calibration assistance.]
[url=http://img188.imageshack.us/i/sunscolorsmall.jpg/][img]http://img188.imageshack.us/img188/7577/sunscolorsmall.th.jpg[/img][/url]
This is also how it appears in person.
There are two important points worth discussing: filtration and CLV.
I verified that they were not using any filters for this projected image, so WYSIWYG, and what we get is all white.
The center to limb variation (CLV) also did not alter the image’s color. This is significant because the CLV is almost 1400K (6390K at the center, 5000K at the limb). If an unfiltered image of the cooler limb region is white then it seems highly unlikely that any yellow sun argument can hold much ground.
However, color is subjective, so you and others may simply see yellow where others see white. If so, it would be interesting to find out what variables contribute to this variability.
[quote] Not related. You can duplicate the effect easily in a lab with a monochromator. A change in intensity of a single spectral band, or a mix of bands, will produce an apparent shift in hue, separate from the obvious change in color.[/quote] I have never done this, but I must assume that the hue itself does not change but our perception of the colour does based on changes in brightness and saturation. I also assume any variation of a monochrome light, say from a green laser, will never produce a red, orange, yellow, or blue projection without some other major factor like induction, though I doubt induction or anything else is powerful enough to make a monochromatic green beam appear red. A green laser will always look green though a yellowish-tint might be observed as the intensity changes. Is this what you are saying?