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Fig. 3. Diagrams of the optics of the fly facet lens and photoreceptor rhabdomere.
(A) According to geometric optics, the size of the visual field is given by
the spatial angle taken up by the rhabdomere (rh) tip. Due to diffraction at
the facet lens (f1) and the waveguide optics of the rhabdomere, a
Gaussian-shaped angular sensitivity results with halfwidth 
, the
acceptance angle; 
is the incident light angle. (B) When the pigment
granules (pg) in the photoreceptor soma (so) are near the rhabdomere, they
absorb light from the boundary waves of the waveguide modes and thus function
as a light-controlling pupil mechanism. The first mode (1) extends less far
outside the rhabdomere than the second mode (2).
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