Research On The Measurement Of Lens' Power Based On Moiré Deflection Technology

Power is one of the most important parameters of ophthalmic lenses, and it is a universal unit that presents the refractive ability of ophthalmic lenses in the world. As this parameter will affect the patient's corrected visual ability, measurement of lens'power has high status in ophthalmic optics. With the tendency of lens'free and individual designs, all kinds of new lenses come out, and the distribution of lens'power becomes more and more complicated. By far, in most countries, to the measurement of lens'power, opticians and optometrists still depend on multiple point focimeters. However, focimeters are obviously unfit for the measurement of lenses with complicated power distribution. Therefore, it is necessary to find new ways and develop new instruments to solve this problem.Moirédeflection technology (MDT) is the combination of Talbot imaging effect and the amplificatory character of moiréfringes. Using MDT we can get a non-contact optical measuring method. Its basic principle is measuring the features of the objects according to the offset of moiréfringes caused by the test object itself. It can be used to measure the phase objects and reflective surfaces according to moiréeffect, and provide the corresponding distorted moiréfringes which are different from lag in phase. There is no need to know the actual phase of the object. Furthermore, MDT only has requirement of spatial coherence and has no requirement of temporal coherence. Therefore, mechanical stability of the experimental setup is not as strict as interferometry, as makes the measuring equipment simple and cheap.Based on many documents, the conception of lens'power is introduced. To the character of lens'power distribution, the corresponding measuring methods and measuring principles are analyzed. Comparison of these measuring methods is also given, and method of MDT for the measurement of lenses'power is proposed.Theories about MDT such as MoiréEffect and Talbot Effect are introduced. Relationship between moiréfringes'tilt angle and the test lens'power is deduced respectively according to the combination of Talbot effect and principle of grating shadow and Fresnel-Kirchhoff theory. A conclusion that lens'power is linear with the tangent of moiréfringes'tilting angle is obtained. Meanwhile, some factors that will affect measuring error of lens'power are discussed, and proper ranges of these parameters are discussed too.Optical path of the experiment is constructed. Moiréfringes during the measuring process of single lens, aspherical lens and PAL are given. Digital image processing of Moiréfringes are discussed, and process of preprocessing, noise removing, threshold sectioning and thinning processing are given. Curve fitting and numerical analysis of the thinned moiréfringes are also provided. In the end, distribution of lens's power is gained. Thus, measurement of lens's power is realized. Meanwhile, measuring results of several single lenses with different power are provided, and comparison with the measuring results using focimeter is given. Measuring results of a PAL are also provided, and graphs of the PAL's power distribution are given too. Two power distribution lines through the PAL's corridor obtained respectively using MDT and by focimeter are given, and the two lines are nearly uniform. It can be verified that one-time measurement of lenses'power using MDT, especially to those lenses with complicated power distribution, can be implemented and has high measuring precision.In the end of the paper, numerical relationship between power distribution and sagittal distribution of the lens is discussed. Power distribution in the corridor of a PAL is deduced. During the process of calculating, an idea is put forward. That is, curve in the PAL's corridor can be considered as the connection of a series of short arcs with their radius varying gradually and each arc has the same projected length along the tangent line of the optical center of lens, and the link of each arc's initial point and its center of circle pass the previous arc's center of circle. The result shows that this solution means is practically reasonable for the calculation of sagittal distribution in the corridor of a PAL.