The principle of diffractive lenses, recently often applied to intraocular lenses (IOLs), is described. A diffractive IOL is constructed using a normal reflective lens, upon which is pasted a diffractive surface with many concentric thin microprisms. A light ray that enters the diffractive surface splits into two rays; the lens then becomes bifocal, with the focal points close to each other and the phase differences of the exit rays well controlled. The energy distribution of the exit rays varies with the wavelength, since the dispersion of the prism material is not equivalent to the dispersion of the diffractive phenomenon itself.
When we observe an object at a constant distance, refraction closely swings rhythmically. This is called accommodative microfluctuation. Accommodatifve microfluctuation is divided into a low frequency component of less than 0.6Hz and a high frequency component (HFC) of 1.0–2.3Hz. HFC occurs because of quiver of the ciliary muscle, and increases when excessive load is imposed on the ciliary muscle. In normal ciliary muscle, even if accommodative load is increased slightly, HFC does not rise. However in tired ciliary muscle, HFC rises with slight accommodative load. HFC level rises after having performed video game and falls at rest. In the eye whose HFC is rising to a high level, HFC falls significantly with the wearing of progressive addition lens glasses and multifocal contact lenses. HFC decreases with intake of the health supplement (astaxanthine), which is thought to be effective for asthenopia. There are many reports on myopic progression. And there are reports that the content of those studies is mutually contradictory. However, I can find relevance common to all reports if I focus on accommodative microfluctuation. I suggest the hypothesis (mechanical stress hypothesis) that accommodative microfluctuation plays a role in myopic progression. Observation of the accommodative microfluctuation is useful for diagnosis and treatment of asthenopia. To offer comfortable correction, it is necessary to study accommodative function before studying refraction.
To quantify the visual acuity of patients with ultra low vision, we examined the visual acuity of 12 subjects, who had no ocular disease other than ametropia, with using the Berkeley Rudimentary Vision Test (BRVT) and a custom-built vision test with a computer display (PC TEST), in which BRVT charts were displayed on the PC monitor. Visual acuity was decreased with an occluder (logMAR≧1.4). The Bland-Altman plot showed that visual acuity value and variability were almost the same in the two groups. Additionally, the luminance was significantly affected by the illuminance in BRVT, but remained almost the same in the PC TEST in all conditions. The PC TEST can be a more accurate and reproducible method, if the display conditions are kept constant.
We investigated the vection (visually induced self-motion perception) that occurred when 2 optical flows expanded at different speeds in the same space. In the experiments, we set the difference in speed between 2 optical flows as the speed condition, and varied the ratio of the numbers of random dots composing the optical flows. Subjects estimated vection speed produced by 2 optical flows. The results showed that when the speed difference was small, the estimated vection speed varied linearly with the ratio of the random dots. When the speed difference was large and the 2 optical flows were perceived to be separate, the results were classified into 2 groups. In one group, vection speed tended to depend upon the slower optical flow, whereas in the other group, vection speed depended upon both optical flows. Even in the latter case, however, the slower optical flow contributed more to the vection speed. To explain these characteristics of vection speed, we developed a model in which vection speed was predicted by the summation of the speeds of 2 optical flows, weighted by the ratio of the random dots of each optical flow.