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Pyramid Wavefront Sensing for Ophthalmic Applications Investigators: Stéphane Chamot, Liz Daly and Chris Dainty Collaboration: Simone Esposito, Osservatorio Astrofisico di Arcetri, Firenze, Italy Fundings: Science Foundation of Ireland, Sharp-Eye European Research Training Network
Pyramid Wavefront Sensing (PWS) was originally proposed for astronomical applications (Ragazzoni, 1996). Despite a first demonstration of ocular wavefront sensing feasibility (Iglesias, 2001), the potential of wavefront sensing for ophthalmic applications has never been fully investigated.
In the non ideal case where the wavefront in the pupil plane is aberrated, the point spread function (PSF) in the focal Fourier plane becomes distorded and the 4 pupil images show uneven, asymetric light intensity levels. Example: A pure tilt within the pupil would shift the PSF to one facet of the pyramid and the pupil image corresponding to that facet will appear brighter than the 3 others. By recording the 4 pupil images and computing local intensity differences between them, one can then compute two wavefront gradient maps. Thus, the PWS provides the same gradient information as a Shack-Hartman sensor (SHS). one can extend the analogy between these two sensors by saying that We built an adaptive optics systems featuring a PWS for retinal imaging. The other key elements in the system are a deformable mirror (19 piezo-actuators), a fast steering mirror for modulation purposes, a 4 facets refractive pyramid (178 degree vertex angle) and a CCD. Both mirror and CCD are controlled through a Labview interface and the system can operate in closed-loop at a rate of 55 Hz.
In his original paper, Ragazzoni circulates the focal spot around the tip of the pyramid by vibrating it and records the pupil images over a full vibration cycle. This spatial modulation first allows to reduce the diffraction due to the limited spatial extend of the pyramid facets. Secondly and more importantly, it increases the sensing linear range of the sensor. In a similar way, the steering mirror mounted in our system was used to move the focal spot on a circular trajectory around the tip of the pyramid.
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PWS
can be seen as an evolution of the rotating Foucault knife-edge test
originally used by astronomers to assess the quality of telescope
mirrors. By placing the tip of a pyramid refractive element (left figure) in the
focal plane of a telescope and aligning the tip to the optical axis,
the incident focused light splits in 4 sub-beams after the pyramid and 4
independant images of the entrance pupil can be formed in the back conjugated pupil plane.
When
the wavefront passing through the exit pupil plane preceeding the
pyramid is flat, a diffraction limited spot (Airy intensity pattern for
a circular pupil) is formed in the focal plane and the 4 recorded pupil
images show homogeneous light distribution (if not taking into account
diffraction).
