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Fig. 5. (A) Achromatic contrast between four stimuli (the white evening primrose,
the red hindwing of D. elpenor, the yellow flower, and the blue
flower) and a green leaf background. Positive contrast indicates that the
object is brighter than the background. (B) Chromatic contrast, defined as the
distance between the relative quantum catches of the stimuli and the leaf
background. (C) Chromatic contrast assuming that D. elpenor has von
Kries color constancy and is adapted to a background of green leaves under
each illuminant. (D) Coefficients of variation of the achromatic and chromatic
contrasts of the four stimuli when viewed under nautical twilight, moonlight
and starlight.