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F. Schaeffel, H. Kaymak, U. Mester; Rapid Measurement of Longitudinal Chromatic Aberration With Automated Polychromatic Photoretinoscopy. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3992.
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© ARVO (1962-2015); The Authors (2016-present)
To develop and test a white light photorefractor for measuring longitudinal chromatic aberration.
While eccentric photorefraction was automated for infrared light and used in a variety of vertebrates, polychromatic photoretinoscopy was not further explored. We used a USB2 RGB camera and software developed in Visual C++ by FS to record simultaneously the brightness profiles in the pupil for the Red (R), Green (G) and Blue (B) channel, using a USB-controlled bright flash of white LEDs in an eccentric retinoscope. R sampled at around 620 nm, G at 540 nm, and B at 470 nm. Six emmetropic student subjects were individually calibrated with trial lenses in R and G and B.
A major obstacle was the low reflectivity of the fundus in the blue, despite that the white LEDs had a emmission peak at 470 nm. The RGB channels of the camera had to be individually adjusted to provide similar brightness of the photoretinoscopic reflex in the pupil. After this, the factors converting the steepness of the brightness slopes in the pupil into refraction became similar. Surprisingly, the refractions in the three wavelength bands were undistinguishable (average refractions B 0.36D, G 0.29D, R 0.36D) even though the average standard deviations were small (B 0.20D, G 0.13D, R 0.12D). Since measurement noise cannot explain the lack of a change in the refractions with wavelengths, a confounding factor is expected. A possible candidate is the variable penetration depth of light of different wavelengths into the fundal layers. Red light may penetrate deeper, causing an apparently longer (more myopic) eye - which could balance the effects of longitudinal chromatic aberration. To match published chromatic aberration curves, refractions in the blue had to be made 1.05D more myopic than measured and 0.50D in the green when red was taken as baseline. Using this assumption, red light would be reflected from layers about 0.39 mm deeper in the fundus, than green (0.19 mm), and blue (0 mm).
After matching the brightness of the RGB channels of the camera and correcting for the variations in the penetration of light of different wavelength into the fundus, this technique may be a fast and convenient way to measure longitudinal chromatic aberration from a distance in normal subjects and pseudophakic patients.
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