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Fig. 6. Optical path length difference and coherence length of the diurnal moth
P. tristifica and the nocturnal moth E. kühniella. The
numbers adjacent to the large symbols indicate facets in a meridional row
(Fig. 5). The optical path
length difference between rays passing through the facets marked by the number
2 and a central ray passing through facet 1, is smaller than the coherence
length of light for the green-sensitive photoreceptors
(lc=2.8 µm). This also holds between rays passing
through facet pairs 7 and 8, 13 and 14, and 16 and 17.