Image Reconstruction for the Human Eye
When an optometrist asks a patient to describe their vision, they are presented with a wide array of responses that can include blurred, doubled, ghosted, smeared, distorted, washed out and many variations on these themes. Within these varied descriptions of visual quality, lies significant information about the optical defects of the eye.
The theoretical effects of the common refractive errors such as spherical defocus and astigmatism are well known. Defocus will cause the image to blur in an even, symmetrical fashion, whereas astigmatism will cause blurring along only one meridian. Measurements of the optical characteristics of the eye show that while defocus and astigmatism are the major aberrations of the eye, there are also higher order aberrations such as coma and spherical aberration present in most eyes. It is the unique interaction between these higher and lower order aberrations in an individual eye, which give rise to the distinctive nature of individual reports of vision quality.
 |
 |
Real world scenes are complex, in terms of their constituent spatial frequencies, contrast and the orientation of spatial detail. However, it is possible to break down the complexity of any scene (object) through traditional image processing techniques such as Fourier analysis into its constituent spatial frequency and contrast components. If the wavefront aberrations and the properties of the pupil are known, it is then possible to mathematically simulate the retinal image of the eye in question by combining the object (scene) properties with the complex pupil function of the eye (derived from the wavefront aberration and amplitude pupil functions).
The optical properties of the eye can be characterised by the wavefront error function, which can be described by a polynomial series. The lower-order terms of this polynomial such as prism, defocus and astigmatism are the aberrations that are typically corrected by conventional means
such as spectacles or contact lenses. The higher order terms such as primary coma or primary spherical aberration are more difficult to correct.
The wavefront aberrations of the eye can be measured by a variety of techniques. The most widely used techniques which are currently in use include the aberroscope, the Hartmann-Shack wavefront, psychophysical methods, and double-pass methods. There is growing interest in the effects of these aberrations as attempts are made to correct them for the purpose of improving acuity and better observation of the retina.
The amplitude pupil function, which is required to calculate the simulated retinal image, is represented by both the pupil diameter and the transmittance of light through the various regions of the pupil. Stiles and Crawford (1933) established in their seminal paper, that light entering the eye through the edge of the pupil is less effective in eliciting a visual response, than light entering through the centre of the pupil (the Stiles-Crawford effect). This effect has been shown to result predominantly from the orientation of photoreceptors, which align themselves with the centre of the entrance pupil. To simulate the optical consequences of the directional nature of photoreceptors, the Stiles-Crawford effect can be represented as a filter in the plane of the pupil which is darker towards the edge of the pupil and lighter towards the centre (so called apodization).
The following figures illustrate aspects of the image reconstruction process and the retinal image derived for a subject with astigmatism.
Read More:
Iskander DR, Collins MJ, Davis B, Carney LG. Monochromatic aberrations and characteristics of retinal image quality. Clinical and Experimental Optometry 2000 83(6): 315-322.
 |
 |
Retinal Image Quality, Accommodation and Myopia
The figure shows the effect of higher-order aberrations on the accommodation stimulus/response for a myopic subject. Accommodation errors, visual Strehl ratio (VSOTF) and retinal image reconstructions are shown at various accommodation levels.
Retinal image quality with all aberrations included (left panel), is best at 50-cm distance and gets worse for both further and closer distances. With increasing accommodation levels, retinal image quality deteriorates substantially for only defocus (centre panel). For higher-order aberrations alone (right panel) retinal image quality is better at far and intermediate distances and decreases significantly with increasing accommodation levels (Note that for the 5 D stimulus level, VSOTF is two to ten times better when higher-order aberrations and defocus errors are combined compared with the conditions where either component is excluded).
Read More:
- Collins MJ, Buehren T, Iskander DR. Retinal image quality, reading, and myopia. Vision Research 2005; 46: 196-215. [Email for copy: m.collins@qut.edu.au]
- Buehren T, Collins MJ. Accommodation stimulus-response function and retinal image quality. Vision Research (in press).