Based On Wavefront Aberration Of White Space Like Modulation

Based on wavefront aberration technology as well as measurement of contrast sensitivity function (CSF), the visual procedure from retina to brain and the impact of ocular aberration on the whole-eye CSF are studied in this dissertation. Firstly, the principle of Hartmann-Shack wavefront sensor has been reviewed, and the presentation of ocular aberration with Zernike polynomial function series has been presented. In addition, a CSF measurement system based on cathode ray tube (CRT) display has been set up, and a calibration system has been employed to ensure that the luminance of each pixel in the optotype represents the intended value. Furthermore, a novel method to measure the retinal aerial image modulation (AIM) in white-light is proposed. Finally, the impact of various aberrations on CSF is described.Presently, visual acuity (VA) and CSF are the main parameters for the ophthalmologist to evaluate the performance of visual system. Compared with the VA, CSF can provide more information for some ocular disease including glaucoma, lens opacity and diseases of the retina and optic nerve. However, the CSF testing device in clinic is faced with a critical drawbacks, that the testing spatial frequency is limited and not high enough to show the contrast sensitivity of fine details. A CSF measurement system has been developed based on CRT display with testing spatial frequency in a range of 2 to 48 (c/deg). Due to the high testing spatial frequency, the aliasing phenomenon is demonstrated. The GUI (Graphic User Interface) software can change testing frequency conveniently, adjust testing frequency with desired steps, plot the CSF curve, and indicate the Nyquist limit. The measured CSF shows a typical band-pass shape peaking at around 10 c/deg with sensitivity dropping off either side of the peak, which agrees with the channel theory based on lateral inhibition within the retinal ganglion cells.The visual function of human eye can be broken down into two cascading processes. One is the optics of the eye which forms an image on the retina; the other is the retina-brain course which consists of perception by the photoreceptor, neural interactions in the retina and subsequent visual pathways. CSF reflects the visual performance of complete eye. Once CSF curve fluctuates abnormally, it is difficult to determine whether the dioptrics or the retina-brain process should account for the peculiarity. AIM represents the contrast threshold that the photoreceptor of retina can detect at different spatial frequencies, which reveals the character of retina, neural system and visual centre. Measurement of the AIM of human retina has been made by the well-known interference fringe technique. However, the AIM acquired with the method mentioned above corresponds to monochromatic light. For real-world vision, human eye perceives white light rather than monochromatic, so it is meaningful to attain retinal AIM in natural light. Wavefront aberrations are measured using Hartmann-Shack wavefront aberrometer. Given dependence of the wavelength and wave-front aberration, we calculate the modulation transfer function (MTF) of white light by summing the weighed optical transfer function (OTF) at each individual wavelength. On the other hand, the characteristics of complete eye can be acquired with CSF testing device based on CRT. According to the relationship between CSF and MTF, the retinal AIM of white light is obtained. It is showed that AIM varies slightly at lower and intermediate spatial frequencies among different eyes; at the higher frequencies AIM is the predominant factor for VA when the wavefront aberration is not significant.The impact of various aberrations including defocus, astigmatism, spherical and coma on CSF is calculated using individual white-light AIM. Through manipulating the Zernike coefficients which represent corresponding desired ocular aberration, the affected CSF is predicted by employing the white-light AIM. Compared with the method that measures CSF directly with intended aberration which is introduced by spherical-cylinder lens or adaptive optics component, the alternative way we propose avoids the uncertainty of subjective response of observer and measurement. Contrast to diffraction limited eye model, the optimal CSF of living eye is achieved when some amount of defocus exist. The effect of the amount of astigmatism on CSF is much noticeable than the axis. It is also found that coma aberration mainly influences CSF at higher spatial frequency and spherical aberration affects CSF in the whole spatial frequency range non-selectively. Additionally, it is spherical aberration rather than coma that impacts the CSF more substantially. Furthermore, the maximum value of area under CSF (AUCSF) is obtained without full correction of higher-order aberration, which indicates that there is compensatory mechanism among aberrations.