Glasses are optical systems used to correct refractive errors, accommodation, and impaired function of the eye muscles.
Fig. 1. The course of light rays after refraction by the lens: a - collective, b - - scattering; M - imaginary image.
Eyeglasses consist of eyeglasses, or lenses (collective or diffuse), and a frame. Collective spectacle glasses (Fig. 1, a) are denoted by a + (plus) sign, scattering (Fig. 1, b) by a sign - (minus). Both collective and scattering glasses are spherical, having the same refractive power in all directions of the glass, and cylindrical, having different refractive power in different directions.
The unit of measurement of the optical power of the spectacle glass is the diopter (D) - see. Diopter . Collective spherical lenses are prescribed to correct farsightedness (see), scattering - myopia (see), cylindrical spectacle glasses - to correct astigmatism (see. Astigmatism of the eye). In cases where frequent change of glasses is inconvenient, it is advisable to assign glasses with bifocal glasses, which are a combination of two lenses of different refractive power.Go
Fig. 2. "Universal" rim for the selection of points.
For the selection of glasses use sets of spectacle glasses and a "universal" frame (Fig. 2), adapted to put in front of the eye several lenses, rotate them, as well as move horizontally and vertically. For the reference position of the axis of the cylindrical glass on the rim of the rim marked division in degrees. Selection of points for distance is made from a distance of 5 m from the table to determine visual acuity under standard lighting conditions (see Visual Acuity , Rota Apparatus ). Each eye is examined separately, placing an opaque shield in front of the second eye. If visual acuity is less than 1.0, then an eyeglass is installed in front of the eye to be examined + 1.0 D. In the case of improved visual acuity (this usually indicates the presence of hyperopia), the strength of the optical glass is increased until the highest visual acuity is obtained. In case of deterioration of visual acuity, when viewing a table with optical glass +1.0 D, a lens is installed in front of the eye to be examined —1.0 D. Improving vision indicates the presence of myopia. Increasing the power of scattering lpnz, note the highest visual acuity. If the visual acuity does not increase from spherical glasses, resort to the selection of cylindrical glasses (c ± 1.0 D) at different positions of the glass axis. Having determined the power of the optical glass for each eye, they are installed respectively in front of both eyes. If the patient does not have unpleasant sensations (ghosting, feelings of sharpness of the image, etc.), glasses are written out for the corresponding strength. The size of the frame is determined by the distance between the pupils. It is measured using a standard ruler with millimeter divisions, taking into account the horizontal distance from the outer edge of the cornea of one eye to the inner edge of the other eye. A diopter is required at each point of release points (a device for measuring the main characteristics of eyeglass lenses).
Selection of glasses for children produces an oculist after dilating pupils by instillation of 1% atropine sulfate solution into the eyes. With age-related changes in accommodation, the so-called presbyopia (see), pick up glasses for reading and working at close distances, taking into account the age and refraction of the eye .
Fig. 3. Contact lenses: 1 - spherical with scleral rim; 2 - made by an individual form of the eye; 3 - corneal.
If ordinary glasses do not increase visual acuity, prescribed telescopic glasses. They represent an optical magnifying system consisting of a collective and scattering lenses, mounted in a single frame. Telescopic glasses significantly narrow the field of view and should be used only for work. A special type of glasses are contact lenses (Fig. 3), applied directly to the anterior segment of the eyeball. Depending on individual tolerance, these lenses can be worn for 4 to 16 hours in a row; they are prescribed with a decrease in vision due to irregularities in the curvature of the cornea, with a large difference in eye refraction, etc. For muscular asthenopia (weakness of the eye muscles), prismatic glasses are prescribed, consisting of two prisms. These glasses provide binocular vision without much strain on the eye muscles. Isaiconic glasses are intended to correct aniseikonia, that is, the phenomenon of unequal size of the image of objects on the retina of each eye. With the help of these points is achieved by trimming the size of images Mirror glasses, which increase the field of view, is prescribed when half of the field of view falls out (see Hemianopsia). With the help of a small flat mirror attached to the nose half of the spectacle frame, the images of objects are transferred to the functioning half of the retina.
Goggles (Fig. 4) are designed to protect the eyes from mechanical and chemical damage, as well as from the harmful effects of radiant energy. They can have optical and ordinary glass. Safety glasses should not reduce visual acuity and narrow the field of view.
Glasses are simple or complex optical systems used to correct refractive errors and accommodation disorders, correct deficiencies in the muscular system of the eye, and protect the eyes from mechanical damage and the harmful effects of certain types of radiant energy.
Corrective glasses are used to correct refractive errors and accommodation disorders using optical glasses or lenses that have the ability to refract rays of light, that is, to deflect them from the original direction. The refractive power (D), or refraction, of a glass is measured by the amount opposite to its main focal length (F):
D = 1 / F
Thus, the larger the main focal length, the smaller the refractive power of the glass, and vice versa. The unit of measurement of the refractive power of glass is diopter (see). There are main and vertex refraction of glass. When determining the main refraction, the focal length is measured from the main planes of the glass, and when determining the vertex refraction, from its surfaces. In spectacle optics, it is customary to designate lenses by their posterior apical refraction, that is, to measure the principal focal distance from the posterior surface of the glass.
Depending on the position of the main focus, the spectacle lenses are divided into collective (convex glass), denoted by the + (plus) sign, and diffusing (concad glass), denoted by the - (minus) sign. In collecting glasses, the main focus (counting from the light source) lies behind the glass, in diffusing glasses it lies ahead of it. Collecting glasses give parallel rays a convergent (convergent) direction, scattering glasses - divergent (divergent) direction. There are spherical glasses, in which the refracting force is the same across all meridians, and cylindrical and toric glasses, having two main mutually perpendicular meridians. In one of them the lens has the smallest refractive power, in the other - the greatest. In the intermediate meridians, the refractive power of glass takes the values lying between these two quantities. Collective spherical lenses are used to correct hyperopia and presbyopia, while scattering lenses are used to correct myopia. Cylindrical and toric glasses are used for the optical correction of astigmatism (see Astigmatism of the eye).
Fig. 1. Forms of spherical glasses: 1 - biconvex; 2 - biconcave; 3 - flat convex; 4 - flat bent; 5 - meniscus acting as a collecting glass; c - meniscus acting as a diffusing glass.
According to the shape of the refracting surfaces, spherical glasses are bi-shapes (both surfaces are convex or concave), plan-shapes (one of the surfaces is flat, the other is convex or concave) and menisci (Fig. 1). In the latter, one surface of the glass is convex, the other is concave. In a positive meniscus (acts as a convex glass), a smaller radius of curvature has a convex surface, in a negative meniscus (acts as a concave glass) - concave. Menisci, in turn, are divided into periscopic glasses, in which the refractive power of one of the surfaces is always 1.25 D, and the menisci itself is glass with the refractive power of one of the surfaces of 6.0 D.
By the nature of the surfaces, cylindrical glasses can be: bi-cylindrical (both surfaces are cylindrical glass), plan-cylindrical (one glass surface is flat, the other cylindrical) and sphero-cylindrical (one glass surface is spherical, the other is cylindrical). The first and third varieties of cylindrical glasses are used for the correction of mixed and complex astigmatism, the second - for the correction of simple astigmatism.
Eyeglasses, especially large refractive power, have a number of undesirable side effects on the eye: they affect the size of the image on the retina, impede the correct perception of spatial perspective, and distort the shape of objects. As you wear glasses and get used to them, these phenomena usually disappear. In the designation and manufacture of glasses, it is necessary to take into account and, if possible, eliminate some optical defects of the lenses, mainly astigmatism of oblique beams. Its essence lies in the fact that the rays of light emanating from the same source, after passing through the central and peripheral parts of the glass, are refracted differently and are not connected at a single focal point. As a result, spherical glasses in the lateral directions of sight acquire the properties of cylindrical, and for cylindrical glasses the difference between the refractive power of the main meridians changes dramatically. The astigmatism of oblique beams is significantly pronounced in bi-shaped glasses, which are therefore no longer manufactured. Optically perfect plan-shaped glass and especially anastigmatic lenses of meniscus or aspherical (punctual) lenses, in which the astigmatism of oblique beams is almost absent. When correcting astigmatism, toric glasses are used instead of cylindrical to eliminate this deficiency.
Fig. 2. Bifocal glasses
In some cases, anomalies of refraction and accommodation disorders (presbyopia combined with hyperopia, high degrees of myopia, etc.) have to use two pairs of glasses - for distance and for near - or, much more conveniently, one pair of glasses with bifocal glasses of different refractive power. The upper part of such glasses is used to look into the distance, the lower - to work at close range (Fig. 2). There are also multifocal (for example, trifocal) spectacle lenses, consisting of several parts, each of which is designed for visual work at a certain distance.
In the optical sense, the most perfect contact lenses made from plastics and worn directly on the anterior segment of the eyeball. They compare favorably with ordinary glasses and in cosmetic terms, since they are almost invisible to the eye. When you change the direction of view of the contact lens repeats all the movements of the eyeball. A contact lens (Fig. 3) consists of a central, more convex, optical part located in front of the cornea, and a peripheral, flatter, haptic part, adjacent to the sclera and holding the lens on the eyeball. Between the inner surface of the central part of the lens and the surface of the cornea there is a narrow space filled with a liquid with a refractive index equal to the refractive index of the cornea. Contact lenses can be worn in a row from 4 to 16 hours depending on individual sensitivity. Better transferred so-called corneal contact lenses without a scleral support part, which are held by a thin layer of tear fluid directly on the cornea. Microcorneal lenses and contact lenses made of elastic material (gel) are proposed; when put on the eyeball, they take on its shape. Assign contact glasses for professional (artists, athletes, miners, etc.) and optical indications (high myopia, anisometropia, one-sided
aphakia, etc.). Among the optical indications, the most significant is a decrease in visual acuity due to irregularities of the curvature of the cornea (keratoconus, irregular corneal astigmatism), in which ordinary glasses. are useless.