Visual Optics

Visual Optics

Adaptive optics

Lenses that alter or correct the optics of the human eye are normally manufactured in plastic or glass. They are then located in front of the eyes in spectacles or contact lenses. But electronic optics are now available that can rapidly change their properties under computer control. These are called adaptive optics and they are typically in the form of flexible mirrors or spatial light modulators (liquid crystals). In the Contact Lens and Visual Optics Laboratory we make use of adaptive optics to test various aspects of vision performance.

Wavefront correction: A deformable mirror corrects the eye’s wavefront aberrations (optical defects)

 

Wavefront induction: The wavefront pattern of optical designs are induced through a liquid-crystal spatial light modulator

 

Wavefronts and image reconstruction for the human eye

In this image, the higher order aberrations from an “average” eye were applied to the image of the moon on the left. The right image is the perfect “diffraction limited” comparison. This gives an idea of the amount of improvement a person could expect over and above their normal spectacles or contact lens correction if the higher order aberrations were corrected perfectly.

When an optometrist asks a patient to describe their vision, they are presented with a wide array of responses which 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 the presence of astigmatism will cause a difference in the amount of blur experienced along each principal meridian of the eye. 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).

Optics of the eye Zernike polynomials are the most common method for mathematically describing the wavefront/power. They break it down into discrete components.

The optical properties of the eye can be characterized 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 technique in use today is the Hartmann-Shack wavefront sensor. The wavefront aberrations of the eye are typically defined by the mathematical function called Zernike polynomials. There is growing interest in the effects of these wavefront 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 (1934) 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).


Further Reading