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Embargoed until 2020-06-01
Copyright: Nguyen, Nicholas
Embargoed until 2020-06-01
Copyright: Nguyen, Nicholas
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Abstract
Subjective refraction is part of an optometrist’s daily routine to elicit and correct the refractive error of
patients. Refraction is often carried out in the clinic using test charts at 4 m or 6 m distances. Often in small
rooms, practitioners use mirrors to extend the distance at which the chart is presented. Room illumination,
chart luminance, testing distance and letter arrangements/layout will, therefore, vary between clinics and
locations. The conditions when testing vision and measuring refractive error subjectively thus vary among
locations, and this affects the results of the tests when performed on a patient at different locations.
Furthermore, different projector charts with multiple letter configurations are still in mainstream use, but
letter contrast may also differ between visits as the projector bulb or lens may attract dust and dirt, reducing
the contrast over time.
Holograms are unique because they literally ‘freeze time’ by capturing the original scene and storing it in a
glass plate. Through a process known as ‘reverse phase-conjugation’, clinicians are able to ‘reverse time’ to
recreate the same initial scene with all 3D features. In this thesis, three transmission phase holograms were
successfully recorded that were useful for vision testing. The vision and spherical refractive errors of some
subjects were measured using these holograms and compared with the results from conventional methods
currently used in optometry clinics. The results showed that the holographic method is a suitable alternative to
conventional method when used to measure spherical refractive error. However, as expected when using
monochromatic illumination, vision measured using the hologram was about 0.50 logMAR greater (worse
vision) than vision measured through conventional methods.
Many practitioners are aware that some patients (especially young myopic subjects with their high
accommodative amplitude) tend to accommodate when seated behind a refractor or when looking through
instruments such as microscopes. This research revealed that holograms possess the useful ability to inhibit
reflex accommodation. It was fascinating to observe young subjects having difficulty exercising
accommodation and focusing on the closer targets in the hologram. The research also revealed that there is a
medium correlation between these subjects and myopic progression, suggesting a possible predictor for
myopic progression.