Sunset science. III. Visual adaptation and green flashes

Andrew T. YoungAstronomy Department, San Diego State University, San Diego, CA 92182Photographs of green flashes do not preclude a role for physiological effects in these phenomena. While green flashes are certainly not after­images, there is compelling evidence that adaptation in the visual system strongly affects the perceived color of most sunset green flashes. Furthermore, the retinal image of the setting Sun is usually bright enough to bleach most of the red­sensitive photopigment in a few seconds, making the yellow stage of a sunset flash appear green. Even in air so hazy that no green light reaches the eye, a yellow flash may occur and appear green. Many, but not all, visual observations of sunset green flashes are of this yellow flash. The yellow portion of sunset green flashes helps explain their reported durations, which exceed those expected for the appearance of green light alone.

 

There is solid evidence that visual adaptation is important. One could not ask for a more convincing demonstration than C. Vaughan Starr’s report [10]:

“a party of four of us, including two doctors, had discussed the possibility of this phenomenon being due to some effect in one’s eyes and not to refraction; to try this out two of us watched the sun carefully until it had set, while the other two looked eastwards and on a signal from the sun watchers turned quickly just as the sun was disappearing. The two who had watched continuously saw the green flash, while to those who did not look till the very last moment, the sun went downorange­yellow, and no green was seen.’

Conclusion

The setting Sun is so bright that it usually bleaches a large fraction of the red­sensitive pigment from the retina of an attentive observer. To the bleached eye, light that would normally be perceived as yellow appears green. Thus, most sunset green flashes are actually yellow for about a second before any photographable green appears; typically, this is the first half of the visually green sunset flash. Even when the atmospheric aerosol loading is so great that no green light is transmitted at the horizon, a green flash may still be seen during the brighter yellow stage. The retinal bleaching that causes yellow light to be perceived as green helps explain the difficulty of photographing green flashes at sunset.

One should not forget that green flashes are also seen at sunrise, when the eye has not been previously exposed to bright light, and the retina is in its normal, unbleached state. Sunrise flashes are therefore seen more nearly in their intrinsic colors.

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