Fig. 7. Fraunhofer diffraction patterns of facet annuli for the diurnal moth P. tristifica, where the facet lens diameter is 24 µm, for monochromatic light with wavelength 530 nm. The light flux density at the corneal level is 1 W µm^–2. (A) The light distribution in the focal plane of the facet lens–crystalline cone system, assuming a focal distance of 200 µm. The irradiance due to the single, central facet is low and spread out. The peak irradiance resulting from the sets of 6 facets with numbers 2, 3, 4 and 6 is 36 times higher than that of the central facet, and the peak irradiance due to light from the set of 12 facets with number 5 is 144 times higher. (B) Normalized light patterns plotted as a function of the angle. The angle is the lateral distance of a divided by the focal distance. The width of the light distribution patterns reduces with increasing radius of the annuli.