Role of Mandelbaum-like effect in the differentiation of hyperopes and myopes using a hologram.

Recently, we tested the vision of spectacle corrected subjects using a special hologram (http://dx.doi.org/10.1364/BOE.3.001173). A subject viewing through this hologram sees the images of various numbers at different distances from the eye. Each of these images subtends an angle of 50' at the eye. Each image corresponds to a different amount of divergence or convergence at the eye. The limit of convergence (positive blur) with which a subject can recognize the numbers was measured. Hyperopes could recognize numbers with 0.9 D more of positive blur in comparison with myopes. This difference was not obtained with a standard test chart under white light illumination using positive lenses to provide the blur. The observed difference with the hologram is then attributed to the multivergence nature of the target in the hologram and/or the monochromaticity of the illumination used. Using a hologram of a logMAR chart at a single distance of infinity we have now found that the observed difference is induced by the multivergence target and is due to an effect that is similar to the Mandelbaum effect. We present the details of this experiment and compare the results obtained with our earlier experiment.

An experimental investigation of the vision of hyperopes and myopes using a hologram.

A hologram of a specially designed multivergence target which displays real and virtual objects (numbers) simultaneously has been used to test the vision of various spectacle corrected subjects. Through the hologram, the subjects see standard '60-meter' numbers that have different amounts of blur. It is found that there is a difference between myopes and hyperopes in the amount of positive blur with which they can recognize numbers seen through the hologram and this difference is statistically significant. A similar study was then conducted in white light illumination using the '60-meter' numbers of a standard test chart at 6 meter distance and positive lenses to provide the blur at the eye. This study showed no difference between the refractive groups. Our results indicate that hyperopes may be relaxing their accommodation more than myopes in viewing through the hologram.

Simple method to measure the visual axis of the human eye.

We propose a simple method to measure the angle "α" that the visual axis (VA) makes with the optical axis of the human eye. In this method, we capture the images of the pupil from three different angles, one along the VA and the other two along angles that are symmetrical with respect to the VA. These views of the pupil are captured simultaneously by a camera in a single photograph using a pair of plane mirrors. Assuming a model eye and with the help of finite ray tracing, we compute the value of α from the measured diameters of the three images of the pupil. Having a simple means to measure the value of α can be useful for optical modeling and analysis of the human eye. Measurement of the VA may also be useful in monitoring progressive myopia.

Performance of the holographic multivergence target in the subjective measurement of spherical refractive error and amplitude of accommodation of the human eye.

We recently suggested the use of a holographic multivergence target to measure the spherical refractive error and the amplitude of accommodation of the human eye [K. V. Avudainayagam and C. S. Avudainayagam, Opt. Lett.28, 123 (2003)]. In this paper we report the performance of the holographic target in measuring real eyes. The holographic technique compared well with subjective refraction and autorefraction in the measurement of spherical refractive error. The performance of the holographic technique in measuring the amplitude of accommodation was similar to that of the minus lens to blur method and that of the push-up method. These results promote holography as a promising technique for testing human vision.

Holographic multivergence target for subjective measurement of the astigmatic error of the human eye.

In subjective refraction the spherical and cylindrical errors of the human eye are measured. Recently, we reported a technique by which the spherical error can be measured in one step using a hologram [K. V. Avudainayagam and C. S. Avudainayagam, Opt. Lett.28, 123 (2003)]. We have now developed a holographic target consisting of sunburst patterns to measure the astigmatic error of the human eye as well. Apart from being simple, quick, and low cost, holography offers the advantage of being an open field refraction system that is devoid of the problem of proximal accommodation. We describe the holographic target, its working principles, the measurement procedure, and the initial results obtained.

Holographic multivergence target for subjective measurement of the spherical power error of the human eye.

We have fabricated a single hologram with which to determine the spherical refractive error of the human eye in the range 5 to -2.5 diopters (D) with an accuracy of +/-0.25 D in a single step. The amplitude of accommodation is determined at the same time. The hologram is a record of 16 targets, each of which has a different image vergence in the range 5 to -2.5 D in steps of 0.5 D. This idea can be applied readily to any dioptric range. The measurement method is simple, fast, and reliable.

Lau fringes and subjective refraction.

Lau fringes formed in the far field of a pair of gratings illuminated by spatially incoherent light are known to disappear when one grating is rotated slightly (+/-3 degrees ) with respect to the other. We observed that a cylindrical lens that was appropriately rotated could restore the high-contrast Lau fringes by bringing together the rays that are coherent with respect to each other. We have developed a new refractometer based on this phenomenon, the details of which are presented. Measurement of the cylinder power and axis of the human eye is possible with good accuracy and repeatability with this instrument.