Fig. 4. Acceptance angle () of fly photoreceptors as a function of wavelength for different states of adaptation of the pupil. (A) Distal rhabdomere diameter D_r=1.4 µm; (B) D_r=2.0 µm. The rhabdomere tip is assumed to be localized in the focal plane of a 25 µm facet lens with F=2.2. The rhabdomere, length 250 µm, tapers parabolic to a proximal value of 1.0 µm. The increases slightly in the UV with increasing sensitizing pigment. The numbers near the curves indicate the distance, h (see inset), of the front line of pupil granules to the rhabdomere border. With the pupil in the fully dark-adapted state (h=), approximates _r, the acceptance angle predicted by geometric optics. In the fully light-adapted state, given by h=0 µm, where the higher modes are completely suppressed, approaches _l, the halfwidth of the angular diffraction function. The angular sensitivity is broadened due to the non-negligible diameter of the rhabdomere.