Y. Le Grand, Form and Space Vision, rev. ed. (Indiana University Press, Bloomington, Ind., 1967), for an interesting review of early techniques. Recently, optometers have been described by M. J. Allen and J. H. Carter, Am. J. Optom. 37, 403 (1960); F. W. Campbell and J. G. Robson, J. Opt. Soc. Am. 49, 268 (1959); J. G. Carter, Arch. Soc. Am. Ophthal. Opt. 4, 137 (1962); N. Roth, Rev. Sci. Instr. 36, 1936 (1965); J. Warshawsky, J. Opt. Soc. Am. 54, 375 (1964). All of these instruments operate on the same underlying principle. The one described here most closely resembles Campbell and Robson's optometer.
This is a consequence of the fact that the eye is in the focal plane of L2. The relationship is derived, e.g., in G. Westheimer, Vision Res. 6, 669 (1966).
We will use the convention, here, of stating the refractive power of the eye relative to its power when accommodated for infinity. That is, 0.0 diopters of refractive power means that the retina is conjugate with_infinity; at 2.0 diopters, the retina is conjugate with a plane 1/2.0 m away, etc. The absolute refractive power of a normal eye accommodated for infinity is about 60 diopters (i.e., its equivalent focal length is about 17 mm).
We are using a quadrant cell, Electro Nuclear Labs (Menlo Park, Calif.) model 640A, in which the two members of each vertical pair are connected in parallel.
The dc component of the sum is directly proportional to the area of the subject's natural pupil, and may be monitored if pupil size is of interest.
If there were inhomogeneities of transmittance across the entrance pupil of the eye, the relative amounts of light transmitted to the retina from the two source positions might vary with eye movements. The resulting artifact could be avoided if the sum signal were used continuously and automatically to control the relative intensities of the sources.
A. Ivanoff, Les abérrations de l'oeil (Editions de la Revue d'optique, Paris, 1953).
R. A. Weale, J. Physiol. (London) 186, 175 (1966).
The optical display system used in these experiments is described in Crane and Cornsweet, J. Opt. Soc. Am. 60, 577 (1970).