Wavelength dependence of the Brückner red eye effect

Abstract

 

Purpose

 

We investigated the wavelength dependency of the Brückner red eye effect with a continuous changing angle of horizontal and vertical eccentric gaze.

 

Methods

 

Four subjects, aged 21, with a vision deficiency within 3 DPT followed the fixation object that oscillated horizontally and vertically along a straight trajectory with fixed speed in front of the camera from ± 8°. The eyes were illuminated with LED’s of 470 (0.4W), 525 (0.5W), 640 (0.13W) and 870 (5W) nm. The LED’s were coaxially placed with a Proscilica GC2450 CCD camera at 1 m, using a small aluminum mirror. 700 images were made in 47s. The eccentric gaze in degrees was determined, using the Purkinje image and Hirschberg ratio. OCT scans were made to compare the reflection profile with the geometry of the fundus of the eye at the fovea.

 

Results

 

The graph shows the left eye data of a subject, while the fixation object is moved horizontally. the eccentricity in degrees with respect to the pupillary axis is displayed against the pupil brightness (logarithmic scale). The graphs show similar reflection profiles for different wavelengths. The IR-LED result is displayed in pink. Scaling the data for quantum-efficiency of the CCD chip and photon intenstity of the LED’s has yet to be taken into account, but the power input of the IR-LED was nearly a factor 10 higher than the other LED’s explaining the high intensity of the infrared light. The missing points reflect areas where the pupil detection algorithm failed by lack of contrast between the pupil and the iris.

 

Conclusions

 

The different colors seem to give approximately the same shape of curve. With respect to an interpretation of this finding, a combination of several mechanisms must be considered, such as: 1. The Bruckner test is not using a conjugate system, making the point-spread-function crucially important. A conjugate system is obtained when the subject perfectly focuses on the fixation object. The point-spread-function is at the visible light sources around 1 degree of arc, at the IR wavelength this is substantially more. 2. The thickness of the retina and choroid are not constant, causing changes in the point-spread-function. 3. The dispersion back of short wavelengths being relatively high (lens turbidity). The reflection at these wavelengths comes from the retina and the surface of the lens. 4. The macula pigment absorbs short wavelengths, lowering the reflection at the retina of such wavelengths.

 

 

Keywords: 723 strabismus: diagnosis and detection • 630 optical properties