Sunday, February 26, 2012

Why Is The Sun Yellow?

The images of the Sun below were exposed at ISO 100, f/8 and for 1/100th, 1/200th, 1/400th and 1/800th of a second respectively moving from left to right with the welding filter. The hue and saturation of the Sun appears to change as the exposure decreases.

The two images on the left are yellowish while those on the right are more greenish. Why does the hue change as the exposure is decreased? For the first two images the green portion of the image saturates near the center of the Sun so all the pixel have values of 255. For the first image we have RGB values of (241, 255, 170). Since the red and green values are nearly equal we perceive the color as yellow. The presence of a blue value shifts the color to make it slightly whitish yellow.

One has to be careful to avoid saturation when measuring relative intensities. Saturation can alter the tristimulus values for pixels in an image and alias the color somewhat. Does the same thing happens with the eye? Human perception appears to be rather subjective at times. What we "see" tends to be projected onto the object.

When colors of the spectrum are rendered as RGB colors the wavelength for the peak intensity of sunlight (555 nm) is seen to be green. But the filter may also be altering the color some.

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