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Yun Chen, Frank Schaeffel; Factors affecting the calibration of white light eccentric photorefraction. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):526.
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© ARVO (1962-2015); The Authors (2016-present)
Infrared eccentric photorefraction is widely used to measure refractive errors in animal models and humans(Schaeffel et al. 1993).If photorefraction is performed by a flash of white light LEDs,a separate analysis of the R G and B channel of the video camera could provide longitudinal chromatic aberration.It is unknown how variations in fundal reflectance due to macular pigment,as well as higher order aberrations affect the calibration of photorefraction.
Calibration of white light photorefraction was done simultaneously in the R G and B by trial lenses (-4D~+4D) in 12 eyes of young Caucasian subjects.Pupil size was controlled (5mm).Calibration occured under 3 conditions:1)in the fovea, 2)10 degrees off-axis in the temporal retina,3) in the fovea but after 10mins of previous exposure to bright light.The luminance to pixel response of the video camera was determined in white light.Higher order aberrations were measured with WASCA wavefront Analyzer (Carl Zeiss Meditec AG,Germany).MPOD was measured by the MPOD Tinsley(UK).Ocular parameters were correlated with the calibration factors (converting the brightness slope in the pupil into refractive error;Schaeffel et al 1993).Paired t-tests were used to comparing differences in calibration factors.
The luminance to pixel response function of the video system could not explain the interocular variance of the conversion factors.As previously found,calibration factors were highly variable among subjects and even much more variable than what is typical in infrared light, from 3.8 to 13.8 in condtion1, 2.3-8.0 in 2, and 7.6-16.0 in 3. There were a number of significant correlations:calibration factors fovea to temporal retina,calibration factors to spherical aberration,and even to the foveal refractive error.Furthermore,calibration factors were highly correlated to MPOD in G and B in the fovea but not at 10 degree temporal retina. Exposure to bright light increase conversion factors.
The following ocular parameters determined the calibration of white light photorefraction:MPOD, refractive error, previous history of light exposure and spherical aberration. While the effect of the first 3 correlations may ultimately trace back to variations in fundal reflectance, the correlation to spherical aberration is unexpected and requires further studies. These factors have to be controlled before polychromatic photorefraction can be used to measure LCA.
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