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H.S. Ginis, N.G. Karyotakis, S. Plainis, A. Pallikaris; The Effect of Light Intensity on Angular Resolution of the Human Eye as Determined by a Two–Point Discrimination Task . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1222.
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© ARVO (1962-2015); The Authors (2016-present)
The Point spread function (PSF) of the eye defines the intensity distribution of a point source imaged on the retinal plane. This distribution is moreover broadened by other factors such as intraocular scatter and retinal diffusion. The perceived point source shows wider angular dimensions as the intensity of the point source increases. The purpose of this study was to measure the correlation of the luminous intensity of a monochromatic point source with its apparent angular dimensions and the associated loss of angular resolution at increased luminous intensities.
Seven subjects (age 24 – 36 years, mean: 29.4) were enrolled in the measurements. The stimulus involved two equally bright point sources with variable overall luminous intensity. Beams emerging from two pinhole diaphragms (30 µm in diameter) back illuminated by He–Ne lasers (=0.633 µm) were combined using a beam splitter and observed monocularly with the best–corrected refraction. Overall transmitted power was controlled with a pair of crossed polarizers. A range of angular separations (0 to 4 arc minutes) in equal steps was tested in random order at six different source luminous intensities. Each condition involved 20 recordings. Angular separation was achieved by the lateral displacement along the horizontal direction of one of the sources. For each combination of intensity and angular separation the subject reported whether the sources were distinguishable or not. The radiant power reaching the eye ranged from 0.072 to 3.9 nW corresponding to source luminous intensities ranged from 0.025 10–3 to 1.3 10–3 cd, respectively. For each luminous intensity, the minimum angle of resolution (MAR) was defined as the 50% discrimination threshold of the fitted phychometric function.
At the lowest intensity the population average MAR was determined to be 2.27 arcmin (1.58 to 2.74 SD= 0.42) while at the highest intensity the average MAR was 3.24 arcmin (2.50 to 3.92 SD=0.53). A statistically significant (P=0.001) positive correlation of measured MAR with luminous intensity of the source was found. The MAR correlated well with the horizontal dimensions of the PSF derived by the wavefront aberration of the measured eyes.
The apparent size of the point source increases as the tails of the retinal distribution become visible while the peak is suppressed due to the non–linear response of the visual system. This effect should be considered when evaluating visual performance at relatively high light levels.
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