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Fig. 2. Schematic representations of the three methods used to study the optical
properties of the lamprey eyes. (A) Photoretinoscopy. The lower half of the
objective of an infrared-sensitive digital camera is covered by a black
occluder holding an array of infrared light-emitting diodes (LEDs). Light
reflected back towards the camera from the fundus of the eye is spatially
filtered by the occluder, which leads to a light upper half of the pupil if
the eye is focused behind the camera and a light lower half of the pupil if
the eye is focused in front of the camera. Multifocal optical systems lead to
alternating light and dark regions in the pupil. (B) Schlieren photography
(setup seen from above). White light is reflected via a beam splitter
onto the lamprey lens. The light is focused by the lamprey lens onto a diffuse
reflector. Reflected light is focused by the lamprey lens onto a small
aperture (pinhole) mounted in front of a digital colour camera. Only light
that has passed through the pinhole can be used to take a photograph of the
lamprey lens. (C) Laser scanning. The beam of a green laser is focused to
reduce beam diameter and scanned through a meridional plane of the lens. Beam
paths are recorded with a digital video camera. Longitudinal spherical
aberration (LSA) is determined from exported frames by a custom-written
program.
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