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Fig. 4. Acceptance angle (
) of fly photoreceptors as a function of
wavelength for different states of adaptation of the pupil. (A) Distal
rhabdomere diameter Dr=1.4 µm; (B)
Dr=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.
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