Image Pre-compensation for Ocular Aberrations
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Published: Mon, 11 Sep 2017
On-screen image pre-compensation has good prospect with the increasing usage of various display screen devices in our daily life. Comparing to glasses, contact glasses and ocular surgery, on-screen image pre-compensation can be easily carried out by computer calculation without any irreversible change in the eyes, as long as the ocular aberration is known. Further, since neither contact lenses nor glasses are advised to be worn all of the time, on screen pre-compensation could even supplement glasses and contact lens use. It is known that compensation for higher aberrations can lead to ‘super-sight’, which is the neural limit of human eye. On-screen compensation also has the prospect of achieving this with customized screens in the foreseeable future.
- Image Processing Theories
- Human Visual System
The human visual system is the combination of the optical system of the eye, and the neural processing of the light information received [Roorda (2011)], in which the latter is out of the concern of this research. The optical system of the eye is an intricate construction including the pupil, cornea, retina and lens (see Fig.1). The light come through the pupil is refracted by the lens and make an inverse image on the retina. During this process, any deficit would cause aberrations. For instance, myopia may result from the lens that the refraction is too high or that the distance from the lens and retina is too long.
Fig.1 Cross-section of eye structure
There is a limit resolution dominated by the neural receptor on the retina, which is below the diffraction limit. Although even for normal emmetropic eyes the sight is below neural limit and diffraction limit due to the minor deficit of eye structure. [Austin (2011)] For eyes with refractive issues, caused by cornea or lens from an ideal spherical shape, the aberrations would significantly dominate over this limit. Thus, in the following research, we shall omit the neural limitation. To increase the efficiency in the following, we can simply model the eye structure as such: a lens (regarding the cornea and the lens as a whole) with an adjustable size (pupil size) and an image plane (retina).
- Point Spread Function and image quality
As is stated in the previous section the aberrations would come from any deficit of eye structure. In order to quantify the distortion in mathematical means, we introduce the Point Spread Function (PSF). Fundamentally, the PSF is defined as a function describes the response of an imaging system to a point source or point object. Note that the loss of light would not be considered in the PSF. Then, if we consider the PSF does not change across the field of view, which applies to the central 1-2Â° of visual angle [Reference!!!], the image can be expressed by the convolution of the PSF and the object in this area.
Where denotes the convolution algorithm. Note that the deconvolution method is based on the inverse operation of Eq.1, which will be introduce in Section 1.2.4.