Dion H. Scott

Ocular aberrations in the peripheral visual field.

We modified a commercial Hartmann-Shack aberrometer and used it to measure ocular aberrations twice at each of 38 points across the central 42 degrees horizontal x 32 degrees vertical visual fields of five young emmetropic subjects. Some Zernike aberration coefficients show coefficient field distributions that were similar to the field dependence predicted by Seidel theory (astigmatism, oblique astigmatism, horizontal coma, vertical coma), but defocus did not demonstrate such similarity.

Influence of Stiles–Crawford apodization on visual acuity

The Stiles-Crawford effect (SCE) of the first kind has often been considered to be important to spatial visual performance in that it ameliorates the influence of defocus and aberrations. We investigated the influence of SCE apodization on visual acuity as a function of defocus (out to +/-2 D) in four subjects. We used optical filters, conjugate with the eyes entrance pupil, that neutralized or doubled the existing SCE. With an illiterate-E task, the influence of the SCE was more noticeable for myopic defocus than for hypermetropic defocus, was generally more noticeable for high-contrast than for low-contrast letters, and increased with increase in pupil size. The greatest influence on visual acuity of neutralizing the SCE, across the subjects and range of conditions, was deterioration of 0.06 (4-mm pupil), 0.16 (6-mm pupil), and 0.29 log unit (7.6-mm pupil).

The influence of the Stiles-Crawford peak location on visual performance

We investigated the influence of the Stiles-Crawford peak location on visual acuity, contrast sensitivity and phase transfer with 6 mm diameter pupils in two subjects. Apodising filters were used to move the peak. One subject (SM) had her natural peak 0.9 mm below pupil centre, and visual performance was measured for both this peak position and when the peak was moved to the same distance above pupil centre. The other subject (DAA) had a more centred peak and visual performance was measured for this peak position and when the peak was moved both 2.3 mm temporally and 2.6 mm nasally. Measurements of contrast sensitivity and phase transfer were compared with predictions based on aberration measurements. The peak position had definite influence on performance, but this was mainly noticeable when subjects were defocused e.g. SMs visual acuity was reduced by 0.13 log units under the peak-shifted condition at -2D (hypermetropic) defocus.

Scatter and its implications for the measurement of optical image quality in human eyes.

Purpose. To investigate the effect of scatter on measurements of wavefront aberrations and point-spread functions in a model eye. Methods. The wavefront aberrations of a model eye were measured using Hartmann-Shack wavefront sensing and crossed-cylinder aberroscope techniques and compared with its measured point-spread function in the presence of scattering media of different concentrations. Results. The point-spread functions became broader as the concentration increased. Forward light scatter on both the light path into the eye and the light path out of the eye contributed to this broadening of the point-spread function. Neither the crossed-cylinder aberroscope nor wavefront sensing, which, respectively, measure the ocular wavefront aberrations for light entering the eye and leaving the eye, were affected by the scatter. Conclusion. We predict that by minimizing the contribution of the forward light scatter from one or other of these light paths by manipulating the size of the entrance and exit pupils, it should be possible to objectively assess narrow-angle forward light scatter in the eye by measuring and removing any confounding effect from wavefront aberration.

Influence of Stiles-Crawford effect apodization on spatial visual performance with decentered pupils.

Using theoretical estimates of the optical-transfer function and line-spread function as image-quality criteria, we predicted the influence of the Stiles-Crawford effect (SCE) on both optical performance of the eye and subjective measurements of transverse aberrations when pupils are decentered. The SCE was modeled as a pupil apodization. The SCE appears to improve image quality by providing compensation for aberrations induced by pupil decentration, but this improvement is usually small. When a criterion of the placement of the image is used as the centroid of the line-spread function, an average SCE reduces the influence of pupil decentration on subjective transverse chromatic aberrations (TCAs) for 5-mm-diameter pupils by 30%. This reduction is much less than that obtained by previous experimental studies of TCA, and possible reasons for this discrepancy are discussed. Decentering the SCE produces an appreciable shift in subjective TCA for 5-mm-diameter pupils of 1.4 arc min per 1-mm decentration (at wavelengths 433 and 622 nm).

The Stiles-Crawford effect and subjective measurement of aberrations

We considered the influence that the Stiles-Crawford effect (SCE) has on the measurement of subjective monochromatic and transverse aberration measurements. The SCE was measured with a two channel Maxwellian-viewing system. Transverse aberrations were measured using a vernier alignment technique in three subjects, with the natural SCE operating, with the SCE neutralised by filters optically conjugate with the eyes pupil, and for one subject with filters that shifted the SCE by more than 2 mm. As pupil diameter increased from 1 to 5 mm diameter, without the filters the slope of the transverse aberration versus position in the pupil decreased, e.g. for chromatic aberration this decreased by approximately 90%. The filters had little influence on transverse aberration. The results indicate that subjects do not use the centroid of the image of a blurred line target for alignment, but may rely very much on other cues.

Theoretical analysis of peripheral imaging after excimer laser corneal refractive surgery for myopia

Purpose: To explore theoretically the retinal point images in the peripheral fields of eyes that have had excimer laser refractive surgery. Setting: University research laboratory. Methods: Model eyes were based on Navarros finite schematic eye, the eyes being made myopic by an increase in axial length. To simulate photorefractive keratectomy (PRK), the anterior shape and thickness of the cornea were modified. Variables included pupil size, ablation zone size, preexisting refractive error, and the addition of a blending zone. Image‐quality criteria for each retinal point image were its size and the angular separation of the centroids of those parts of the image produced by rays passing through ablated and unablated corneal zones. Results: In the peripheral visual field, the boundary between the ablated and unablated cornea caused a separation of the retinal image of a single point into 2 parts. The separation increased with the preexisting refractive error. Image quality was correspondingly reduced by ablation. As pupil size increased, the field angle at which the retinal image doubling first occurred decreased. Increasing the diameter of the ablation zone or using a blending zone increased the angle at which the doubling first occurred, and the blending zone improved image quality considerably. Chromatic effects appeared to be relatively unimportant. Conclusions: This analysis provides further evidence of the disadvantages of small central ablation zones in excimer laser refractive surgery and of the advantages of well‐designed blending zones in improving postsurgical peripheral image quality. Image quality in the peripheral field of the pseudoemmetropic post‐PRK eye is generally worse than in a naturally emmetropic eye, even though the axial image quality may be similar.

Description of a method for neutralising the Stiles-Crawford effect.

The influence of the Stiles–Crawford effect on visual performance can be investigated by filters based on the apodisation model of the Stiles–Crawford effect. We describe the development of practical filters to achieve neutralisation. We present some results of the Stiles–Crawford function showing that the filters work well for expected errors in aligning filters in front of the eye.