Optical characteristics of eyeglasses and precautions for prescription glasses
Date:2022.11.14 Views:464
Refractive errors include myopia, hyperopia and astigmatism. At present, the correction of refractive error is divided into three methods: optical correction, medication and surgical correction, of which optical correction is divided into frame glasses correction and corneal contact lens correction (contact lenses). Today, we will first introduce the optical characteristics of frame glasses and the considerations of prescription glasses.
1. Optical characteristics of spectacle lenses
(1) The size and shape of the lens
From the appearance of the lens, there are round and oval, etc. The size and shape of the lens varies according to each person's preference and face shape. Lenses for children should be larger to maintain a sufficient field of view and prevent the wearer from looking outside the lens. A major advantage of making the lens of a pillar lens ovoid is that it prevents the lens from rotating inside the frame. Depending on the curvature fit of the two surfaces of the lens, the lens can be broadly classified as symmetrical, asymmetrical, plano-convex, perimetric, crescent or bulbous.
To get the best optical results, both eyes must remain fixed in the center of the lens. When the wearer wants to see a lateral object, he or she must turn the entire head toward the object to be seen. If the glasses are fixed in front of the eyes and the eyes want to see objects in the peripheral field of vision, a perimetric lens is better.
(2) The disadvantages of spectacle lenses
① spherical difference: the application of any form of lens, are inevitable because of the different degrees of incident light deflection and different degrees of spherical difference. Spherical aberration increases with the skewness of the light. In practice, the flat or concave side of the lens should be placed close to the eye, and the convex side should be placed against the incident light to produce the smallest spherical aberration, and a crescent-shaped lens is preferable.
Retinal image change: After wearing corrective glasses, the light reflected by external objects must first pass through the glasses and then through the refractive system of the eye image in the fundus, the size of the retinal image in the fundus is different from the size of the retinal image when not wearing glasses, which is the magnification or reduction of corrective glasses. This effect is related to the lens position, diopter and lens shape. It is common to see some patients wearing glasses for more than ten years or even decades, once they change their glasses, even if they have the same mirror degree, they often report discomfort, in which the change of lens form, refractive index, thickness, etc. leads to the change of image size is an important reason.
③ Prismatic effect: when dispensing glasses, the optical center of the lens must be in line with the eye's visual axis, this is called the positive optical center of the lens. A lens can be seen as composed of numerous prisms. If the light does not pass through the optical center, a prismatic effect is formed. This can cause visual disturbance symptoms, such as displacement of the object. Dispersion, when the wearer looks at a distant white object through the edge of the lens, the object is often surrounded by color. As a result of dispersion, blue and red overlap on the image spot, and the sharpness of the image is naturally reduced. Clinically, there is a color interference around the periphery of the lens when wearing ultra-thin lenses for the first time, but it will adapt over time. Like jumping, wearers of bifocal lenses often have a sudden jumping upward complaint, is because the human eye in the process of looking from far to near, with the point of view and the main film optical center distance increases, resulting in gradual prismatic effect, and the visual axis over the sub-lens upper edge, the sub-lens under the luminosity and the sub-lens near the optical center distance and determine the prismatic effect of the sub-lens. The algebraic sum of the two becomes the total prismatic effect at this time.
Circular blind area: when wearing a positive lens, the eye flat in front of the object, the light from the most edge of the lens and directly into the human eye between the outermost light area, the light reflected by the object because of the prism effect excessive tilt and can not enter the pupil, that is, can not be seen by the eye, is the so-called blind area. When the head is turned, the object in the blind area will reappear in the human field of vision, and this kind of intermediate objects appear suddenly and inconveniently.
⑤ Image distortion: It is one of the most common causes of visual disturbance in eyeglass wearers, and is caused by the magnification or reduction of the peripheral part of the lens. When the peripheral field of vision is compared to the central field of vision, the long straight lines become curved. When viewing an object with the peripheral part of the glasses, it can produce an object movement like head movement. This is a disadvantage that cannot be avoided and can only be slowly accustomed to through exercise in application.
2. Precautions for prescription glasses
(1) When you first wear glasses, you may have the following feelings about glasses because your original habits have changed
(1) Near-sighted glasses: there is a narrowing of vision, dizziness, afraid to walk, especially bright; high myopia even have nausea, vomiting feeling.
②Farsighted glasses: there is a magnification of vision, headache, dizziness, and over time, there is also soreness between the eyebrows.
Astigmatism: in addition to the above reactions, there are also visual distortion.
Clinically, such as wearing the appropriate degree, low refractive error generally will not appear in the above situation, such as the appropriate downward adjustment of glasses, check the astigmatism axial, such as accurate, the patient really need, can gradually increase the wearing time, one or two weeks will adapt.
(2) old eyesight glasses, only near use, not far use, in reading and writing distance of one foot for the standard.
(3) optical center of the horizontal deviation: the human eye's visual axis in the positive and parallel, the distance between the two pupil centers is called the pupillary distance, usually need to align the optical center of the lens to the pupil center, otherwise it will produce prismatic effect.
(4) Selection of frame should pay attention to the characteristics of the face, children's eye orbits have not yet developed in place, the size of the nose bridge should pay particular attention to the seemingly small frame, often the size of the adult nose bridge, such glasses often can not be well fixed, affecting the improvement of vision.
(5) Because the frame glasses lens and the corneal apex there is a certain distance, the height of the number of lenses there is a magnification problem, especially refractive partials due to the difference in the magnification of the two eyes and difficult to adapt, can be solved by other methods.
(6) glasses should be wiped often to keep clean, do not place the lens convex surface on the desktop to avoid sassafras hair.
(7) resin lenses must be used for special trial cloth. Lenses with dust and sand, should first rinse with water before wiping, otherwise the lens is easy to be worn.
(8) Wearing glasses should be taken off with both hands, not from the head set, otherwise the legs are easily deformed, forming an angle. If the frame becomes loose, the optical center is outside the center of the pupil, which affects the visual effect.
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