May 1965 L E T T E R S T O T H E E D I T O R 573

Absolute Spectral Reflectance of the Fundus Oculi

J . J . VOS, A. A. MUNNIK, AND J . BOOGAARD Institute for Perception RVO-TNO, Soesterberg, The Netherlands

(Received 21 December 1964)

IN studying phenomena of entoptic scatter and in particular the scattering by the fundus, we wanted to know the absolute

spectral reflectance of the fundus oculi. Most data published on the fundus reflectance relate to changes in the reflectance rather than to its absolute value. We know only the data of Brindley and Willmer1 and of Alpern and Campbell.2

We replotted their results in Fig. 1 in terms of an "equivalent reflectance." By this we mean that the results were interpreted as if the fundus were a diffusely reflecting surface, seen through completely transparent eye media. As can be seen, the results of these two investigations show large differences both in absolute level and spectral dependence. In order to provide some more data and to get some feeling for the difficulties apparently involved in these measurements, we did a few experiments ourselves.

Our experimental arrangement was essentially built like the device of Rushton et al.3 (Fig. 2). Light from a monochromator M

FIG. 1. Equivalent reflectance as a function of wavelength according to Brindley and Willmer,1 Alpern and Campbell.2 (a) Brindley and Willmer: macular region; field size 1°. (b) same as (a) at 10° eccentricity; field size 1°. (c) Alpern and Campbell: 27° eccentricity; field size unknown, but larger than 1°.

FIG. 2. Experimental arrangement.

was collimated by L1 and the parallel beam was split by the glass plate G which was mounted so as to reflect at Brewster's angle. The reflected, completely polarized part is focused at the pupil of the eye to be investigated. The observer views the lens L2 uniformly illuminated in Maxwellian view. The diffuse fundus reflection is focused by L2 and L3 on the cathode of the photo-multiplier tube T. The light transmitted by G at first passage, passes through a small variable aperture D1 and reaches the photocathode via the metal mirrors M1, M2, and M3. By means of the rotating sheet polarizer P1, the light is alternatingly directed into the multiplier via the eye and via the comparison light path. The polarizer P2 is orientated in such a way that vertically polarized light cannot enter the multiplier. This assures that the beamsplitting by G is really complementary. Moreover, P2 inter­cepts residual vertically polarized light from the measuring beam, such as stray light from G and reflections from L2 and the cornea.

The size of the part of the fundus reflecting light to the photo-multiplier is controlled by the field stop D2 which is conjugate to the fundus. The photomultiplier is used as a null instrument. The experimenter adjusts D1 until the oscilloscope reading shows no residual ac component. Then both intensities are equal and the size of the aperture D1 gives the values of the fundus reflectance. At first it appeared that birefringence of the glass parts produced a small phase shift between the beams so that a complete zero adjustment could not be attained. Therefore, a quarter-wave retardation plate was inserted at C in the comparison light path. By proper orientation of it we could eliminate the phase shift.

The electrical circuit between multiplier and oscilloscope con­tained a narrow bandpass filter (bandwidth 1 cps) with peak at the alternation frequency of the light system (28 cps). This greatly increased the signal-to-noise ratio and thus improved the reading accuracy.

The arrangement as such gives the spectral reflectance on only a relative scale. To calibrate it on an absolute scale, we replaced the eye by an artificial eye consisting of a 68 D biconvex lens with a frosted-paper "fundus." The specifications of the artificial eye are: focal length 15 mm, entrance aperture 5-mm diam, reflectance of the "fundus" 70%, independent of wavelength.

For the human eye we assumed a focal length of 23 mm and a pupil size of 8 mm (approximate value for the dilated pupils of our subjects). We checked that the light paths had no appreciable differences in transmittance for different wavelengths by running a complete spectral traverse on the artificial eye. The reflectance of the artificial fundus measured in this way showed a flat maxi­mum at 580 nm which was only 7% higher than at the ends of the spectral range used.

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FIG. 3. Equivalent reflectance as a function of wavelength for various sizes of the field of view φ with central fixation, Δ— — —Δ first series; ○—○ second series; bottom curve: mean of all measurements (subject J.B.).

Results. Results obtained are shown in Figs. 3,4, and 5. Figure 3 shows results and their reproducibility for one subject with various sizes of the fundus field. At the bottom the average curve for all of the closely agreeing experiments is indicated. Figure 4 gives the average curve for another subject; the accuracy is about the same as in Fig. 3.

Figure 5 finally gives for the same subject the spectral reflec­tance for various locations on the fundus. That for the central region (●) coincides with that at 18° to the temporal side ( Δ ) . The curve measured at 18° to the nasal side ( × ) at the location of the optical disk is distinctly higher. Though we doubt that it has any significant relation to the results, we mention for the sake of completeness that J. B. has normal color vision, whereas J. J. V. is protanomalous.

Discussion. The main purpose for publication was to provide supplementary data on a little-studied subject. As to the inter­pretation, we restrict ourselves to a few general remarks.

(a) The level of the fundus reflectance found by us roughly agrees with that found by Alpern and Campbell and is distinctly higher than that found by Brindley and Willmer.

FIG. 4. Equivalent reflectance as a function of wavelength for another subject (J. J. V.), again with central fixation. Average curve for various field sizes φ.

FIG. 5. Equivalent reflectance for φ =6.5° at the macular region (central) ●—●. at the blind spot (18° nasal) ×—×, and at the contralateral region of the retina (18° temporal) ∆— — —∆ (subject J. J. V.).

(b) We did not find a notable difference between the reflectance in the macular region and outside—apart from an understandably different behavior of the optical disk. The difference found by Brindley and Willmer was attributed by them to absorption by the macular pigment. That we did not find an influence of the macular pigment might be due to the large fundus field used (6°.5 diam compared with Brindley and Willmer's 1°). The subject J. J. V. has no difficulty seeing the Haidinger brushes so that there is no evidence of an exceptionally small macular pigmentation.

(c) The accuracy of the conversion to absolute reflectance is not great. The artificial eye is not an exact replica of the human eye, if only for the reason that we do not know exactly the data for the subjects' eyes. This uncertainty does not affect the con­clusions about the order of magnitude, however.

1 G. S. Brindley and E. N. Willmer, J. Physiol. 116, 350 (1952). 2 M. Alpern and F. W. Campbell, J. Physiol. 164, 478 (1962). 3 W. A. H. Rushton, F. W. Campbell, W. A. Hagins, and G. S. Brindley, Opt. Acta 1, 183 (1955).