低视力助视器的光学02

低视力助视器的光学,Optical principles of lenses for low vision,提供放大作用的方法,To provide magnification for patients having low vision, the methods used increase the angular subtense of the object for the eyes optical system 4 methods Relative distance magnification Relative size magnification Projection magnification Angular magnification,Methods of providing magnificaiton,Relative distance magnification The magnification that results from decreasing the distance of the object from the eye,Methods of providing magnificaiton,Relative distance magnification To specify the amount of magnification, it is necessary to use an initial standard or reference distance, usually 40 cm,Methods of providing magnificaiton,Relative distance magnification The use of short fixation distances requires large amounts of accommodation If the patients accommodation is insufficient, a clear retinal image can be obtained by the use of a plus lens of sufficient power,Methods of providing magnificaiton,Relative size magnification The magnification obtained by increasing the size of the object at its original position,Methods of providing magnificaiton,Projection magnification The magnification that results from the formation of an enlarged image on a screen, of an opaque or transparent object,Methods of providing magnificaiton,Projection magnification The projection system may be optical or electronic,An example of the latter is CCTV,Methods of providing magnificaiton,Projection magnification Projection magnification can be considered as a special form of relative size magnification May be used in combination with relative distance magnification,Methods of providing magnificaiton,Angular magnification The ratio of the angle subtended by the image of an object formed by the system to the angle subtended by the object when viewed directly,Methods of providing magnificaiton,Angular magnification Angular magnification compares the apparent increase in the size of the object seen through the optical system with the size of the object seen without the optical system in place,远用助视器,望远镜 按光学设计分类 伽利略望远镜 开普勒望远镜 按使用方法分类 手持式望远镜（单筒/双筒） 头戴式望远镜（Full-diameterBioptic）,Afocal telescopes,设计 Contain two optical elements Objective lens（物镜） and the eyepiece（目镜） The objective lens is positive in power The eyepiece is much stronger in power than the objective lens and may be either a positive （Kaplerian）or negative（Galilean）lens,Afocal telescopes,设计 共焦 物镜的第二焦点与目镜的第一焦点重合 提供角放大率但不改变光线的聚散度 平行光线入射，平行光线出射,Afocal telescopes,伽利略望远镜 物镜为正镜，目镜为负镜,Afocal telescopes,The Galilean telescope,Afocal telescopes,The Galilean telescope Characteristics 放大率为正，说明像是正立的 For constant magnification, increasing the power of both the objective and the eyepiece produces a shorter telescopic unit,Afocal telescopes,开普勒望远镜 物镜和目镜都是正的，目镜的度数更高,Afocal telescopes,The Keplerian telescope,Afocal telescopes,伽利略望远镜VS开普勒望远镜 相同放大率情况下，伽利略望远镜比开普勒望远镜短 伽利略望远镜的像是正立的，开普勒望远镜的像是倒立的，因此开普勒望远镜要用于低视力助视器需要an erecting system 伽利略望远镜的设计通常比开普勒望远镜简单 开普勒望远镜通常比伽利略望远镜重些,Afocal telescopes,伽利略望远镜VS开普勒望远镜 通过开普勒望远镜所成的像质量和亮度通常比伽利略望远镜高 开普勒望远镜可提供达10X的放大率，而伽利略望远镜通常低于4X 开普勒望远镜的视野范围通常比伽利略望远镜广,Afocal telescopes,可调式望远镜 In discussing above, it is assumed that they are used with emmetropic eyes 如果配戴者有spherical ametropia，通过调整目镜与物镜之间的距离，可以矫正屈光不正,Afocal telescopes,Adjustable telescopes 如果配戴者是远视眼，镜筒长度要调长 如果是近视眼，镜腿长度要调短 此规律适用于伽利略和开普勒望远镜,Afocal telescopes,Adjustable telescopes Example What tube length is required for an uncorrected -5.00D myope to have clear distance vision through a 4X Galilean telescope having a +5.00D objective lens?,Afocal telescopes,Adjustable telescopes When the adjustable feature to correct ametropia for distant objects is used, the effective magnification is changed For myope, the angular magnification is reduced with a Galilean telescope and increased with a Keplerian telescope For a hyperope, M increased with Galilean and decreased with Keplerian,头戴式望远镜,全尺寸望远镜Full-diameter telescopes Headborne telescopes, mounted in spectacle frames, as full-diameter telescopes, that is, the telescope occupies the entire aperture of the spectacle lens,Headborne telescopic systems,Bioptic telescope A small diameter telescopic unit mounted in the upper part of a carrier lens with the patients prescription provided in the carrier lens,Headborne telescopic systems,Keplerian expanded-field telescopes are available in powers ranging form 2X to 8X Telescopes of Galilean design are available as full-diameter telescopes in magnifying powers ranging from 1.3X to 2.2X and as Bioptic telescopes in powers ranging from 2.2X to 4X,Headborne telescopic systems,缺点 视野显著缩窄 周边像差明显Because of the perceptual problems, full-diameter telescopic spectacles are indicated mainly for relatively sedentary activities rather than for walking and other forms of locomotion,近用助视器,Microscopic lenses Telescopic lenses for near vision Hand magnifiers Stand magnifiers,Microscopic lens,Definition 其中一种最简单的近用助视器 One of the simplest low-vision aids for near vision is the use of high-powered addition in the form of either bifocals or single-vision lenses intended for near vision only（高度数正镜，仅用于视近）,Microscopic lens,Magnification 当物体置于透镜焦距时的放大率,Microscopic lens,Effective magnification 通常以戴镜前物体放置在眼前25cm作为标准参照，戴microscopic lens后放大率为： 此放大率称为 effective magnification,Microscopic lens,Conventional magnification 定义 通过光学系统正好成在标准参考距离（眼前25cm）的像的大小，与不用光学系统，直接看置于标准参考距离的物体的大小，两者之比称为conventional magnification。,Microscopic lens,Conventional magnification,Microscopic lens,Conventional magnification,Microscopic lens,Problems associated with microscopic lenses 像差Aberrations Lenses of the very high powers required for microscopic aids often produce severe aberrations Aberrations may be controlled by using aspheric surfaces or multiple elements, which adds considerably to the expense,Microscopic lens,Problems associated with microscopic lenses 阅读时的光线Illumination When reading material is held very close to the eyes, ti is difficult to provide adequate illumination Because much ambient light is blocked by the patients head,Microscopic lens,Problems associated with microscopic lenses 距离太近时只能单侧配镜 When a reading distance closer than 10cm is necessary, binocularity becomes impractical-only a lens for the better eye should be prescribed,Microscopic lens,Problems associated with microscopic lenses Maintenance of the proper viewing distance Many patients have difficulty adapting to the very short reading distance Holding the reading material at the correct viewing distance and keeping the facial plane parallel to the page is often a most difficult task,Telescopic lenses for near vision,概述 如果不想把物体拿得很近，可以选择望远镜 但直接通过望远镜视近是几乎不可能的，即使眼睛有很强的调节力，但由于望远镜会产生聚散度放大作用vergence amplification effect的缘故,Telescopic lenses for near vision,概述 Vergence amplification effect可以通过在望远镜前面或后面加适当的正镜矫正 如果加在物镜之前，放大作用还没发生，所加正镜的度数较少，而如果加在目镜之后，则所加正镜的度数要大得多 因此，通常是在望远镜的物镜前加一reading cap,Telescopic lenses for near vision,概述 The reading cap is easily removable and can be carried by the patient and worn when needed for near vision,Telescopic lenses for near vision,Power of the reading cap Reading cap的度数由阅读距离决定 For a reading distance of 40cm, the reading cap must have a power of +2.50D The combination of an afocal telescope and a reading cap is known as a telemicroscope,Telescopic lenses for near vision,Equivalent power of a telemicroscope 比较不同的视近助视器的效果，常用的指标是该助视器的等效屈光力equivalent power，或称equivalent viewing power (EVP) 对于telemicroscope，FE通过以下公式计算,Telescopic lenses for near vision,通过FE，我们可以判断： 如果几个telemicroscope的FE相同，则配戴后产生的放大作用相同 Example 以下四种助视器的FE相同，都为24D,Telescopic lenses for near vision,Equivalent power of a telemicroscope,Telescopic lenses for near vision,Equivalent power of a telemicroscope Example分析 前三种是telemicroscope，第四种是microscopic lens，它们的FE相同，说明它们的放大效果相同 只是这四种助视器的阅读距离不同，使用telemicroscope可以在相同放大效果的前提下，增加阅读距离 因此，telemicroscope与microscopic lens相比有优点，也有缺点，可供低视力者选择的空间,Telescopic lenses for near vision,Microscopic lens VS telemicroscope With the magnifying power provided by the afocal telescope, for a given amount of magnification, a patient wearing a telescope with a reading cap can read at a greater distance than that possible if only a microscopic lens is worn,Telescopic lenses for near vision,Microscopic lens VS telemicroscope An afocal lens with a reading cap has the disadvantage of a smaller field of view than is present if only a microscopic lens is worn,思考,我们怎么知道病人需要多少FE的助视器？ 在实际工作中，常利用病人所需的equivalent viewing distance(EVD)去计算FE举例： 一低视力病人，检查近视力，在25cm处可以看到6M视标，他要求看报纸的1M大小的字体，所需的EVD为？需要多少FE的助视器？,手持式放大镜,概述 Many hand magnifiers are available at relatively low cost The magnifiers are particularly useful for short-term visual tasks, but they can also be used for prolonged reading,手持式放大镜,概述,Hand magnifiers,Magnification of hand magnifier 手持放大镜的放大效果并不是一个固定的值 因为放大效果不仅与手持放大镜的屈光力有关，还与使用放大镜的方法有关 眼放大镜的距离和放大镜物体的距离都会影响手持放大镜的放大效果,Hand magnifiers,Magnification of hand magnifier Normally, the magnification of a hand magnifier is specified by the manufacturer based on the formula:,Hand magnifiers,Magnification of hand magnifier Use of this formula, assumes that the standard reference distance for unaided vision is 25cm and that the object is located at the primary focal point of the lens,Hand magnifiers,Magnification of hand magnifier When the object is positioned at a distance closer to the magnifier than the primary focal length, the image falls behind the magnifier, closer than infinity,Hand magnifiers,Magnification of hand magnifier To see the image clearly, an emmetropic observer must resort either to accommodation or to the use of a reading addition,Hand magnifiers,Magnification of hand magnifier In this event, the equivalent power of the system is equal to:FE=the equivalent power FM=the power of the magnifier FA=the amount of accommodation or power of add d=the distance from the magnifier to the spectacle plane,Hand magnifiers,Magnification of hand magnifier 当放大镜眼（眼镜平面）的距离等于放大镜的焦距时，系统的FE为：,Hand magnifiers,Magnification of hand magnifier With the result that the equivalent power of the optical system is equal solely to the power of the magnifier It is independent of the amount of accommodation or of the power of the reading addition,Hand magnifiers,Magnification of hand magnifier When the distance d is less than the focal length of the magnifier, FE is greater than FM Reading a maximum value when the magnifier is placed in the spectacle plane,Hand magnifiers,Magnification of hand magnifier On the other hand, when the distance d is greater than the focal length of the magnifier, the resultant FE is less than FM The greater the separation, t