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Brian Vohnsen, Alessandra Carmichael, Najnin Sharmin, Salihah Qaysi, Denise Valente; Direct Integration of the Stiles-Crawford Effect of the First Kind using Pupil Flicker Methodology. Invest. Ophthalmol. Vis. Sci. 2017;58(8):4236.
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© ARVO (1962-2015); The Authors (2016-present)
The Stiles-Crawford effect of the first kind (SCE-I) describes the psychophysically-determined visibility dependence on angle of incidence at the retina and is normally determined sequentially by scanning a Maxwellian secondary source of light across the pupil. The purpose of this study is to analyze the validity of pupil integration in normal non-Maxwellian viewing situations of a back-illuminated letter chart for healthy subjects.
A motorized pupil size flicker system has been constructed that alternate between a small (1.4 mm) reference and increasingly large test pupils (up to 7.4 mm) with flicker frequency set at 0.5 Hz. The motorized pupil is mounted in a conjugate plane to the eye pupil. Subjects adjust brightness of the light using a programmable neutral density filter until a satisfactory match has been found between the reference and test lights. The color of the light is adjusted with a tunable bandpass filter. Effective visibility is determined by the power ratio between the test and reference versus pupil size for five healthy emmetropic and sligthly myopic subjects at 6 wavelengths across the visible spectrum.
The subjective results confirm validity of the integrated SCE-I function using a characteristic directionality parameter, ρ, in the range of 0.04 - 0.10/mm2. Although some wavelength variations are present these are minor. In turn, deviations for small and large pupils are present and may be a result of blur or inaccuracies in the standard Gaussian SCE-I function. We compare the results with those of an integrated Airy disc model based on model by one of the authors (Vohnsen, Biomedical Optics Express, 2014) and find correspondingly good agreement with SCE-I integration. Plots of the measured integrated SCE-I functions are used to determine the effective directionality parameter.
The experimental results show that the integrated SCE-I function is approximately valid for normal viewing situations although care needs to be taken for extreme pupil sizes where variations appear. These may have consequences for improved refractive optics designs that incorporate the SCE-I as a pupil apodization. An impact of the flicker frequency is also possible and is the subject of further ongoing research.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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