Ratio of blue to red light: a brief increase following sunset.
Visible spectra of solar radiation were recorded during sunset. With the development of twilight, there was an expected decrease in the blue-to-red ratio; but directly upon setting of the sun there was a sharp rise in this ratio (due to the predominance of sky-light) which attenuated rapidly to follow the pattern that it had previously taken.
Johnson
T B
TB
Salisbury
F B
FB
Connor
G I
GI