Sunglasses - optical systems used to correct the anomalies of refraction, accommodation and disadvantages of the functions of the eye muscles.

Fig. 1. The speed of light rays after refractive lens and collectively, b - - scattering; M - virtual image.

Glasses consist of spectacle glasses or lenses (collective or diffuse), and structure. The collective of the glass (Fig. 1, a) denote the + (plus)sign, scattering (Fig. 1, b) the sign of the - (minus). As a collective, and the scattering of glass are spherical, with the same refractive power in all areas of glass, and cylindrical, with in different directions different refractive power.
The unit of measurement of the optical power of the glass is the diopter (D) - see Diopter. Generalized spherical lenses prescribed for correction of presbyopia (see), the scattering - myopia (see), cylindrical spectacles glass - to correct astigmatism (see Astigmatism eye). In cases where frequent change points inconvenient, it is reasonable to prescribe glasses with bifocal glasses, which represent a combination of two lenses of different refractive power.

Fig. 2. Universal frame for the selection of glasses.

For the selection of glasses have a bunch of spectacles glasses and universal rim (Fig. 2)adapted in order to put in front of the eye a few lenses, rotate them, and move horizontally and vertically. For reference of the axis of the cylindrical glass on the rim rim caused division in degrees. The selection of glasses for Dali is made from a distance of 5 m from the table for the determination of visual acuity at standard conditions (see visual Acuity, Rota apparatus). Each eye is examined separately, by setting before the second eye opaque shield. If visual acuity of less than 1.0, then the investigational eye install glass + 1,0 D. In case of improvement of visual acuity (this usually indicates the presence of hyperopia) increases the strength of optical glass to higher visual acuity. In case of worsening of vision when looking at table with optical glass +1,0 D before investigational eye set the lens -1,0 D. Improvement of view indicates the presence of myopia. Increasing the power of the scattering LPNs, marks the highest visual acuity. If the spherical glasses visual acuity does not increase, resort to picking cylindrical glass (with ±1,0 D) at different positions of the axis of glass. Defining the power of optical glass for each eye, install them accordingly before both eyes. If the patient does not arise unpleasant sensations (double vision, sense of sharpness etc.), prescribe appropriate power points. The size of the frame is determined by the distance between the pupils. The measurement of its produce normal line with millimetric divisions, account for the horizontal distance from the outside edge of the cornea of an eye to the inner edge of one eye. Each item vacation points required dioptries (a device for measuring the main characteristics of spectacle lenses).
Selection of glasses children produces the doctor-ophthalmologist after the expansion of the pupils instillation in the eye of 1% solution of atropine sulphate. Age changes of accommodation, the so-called presbyopia (see), pick up reading glasses and work at a close distance with respect to the age and refraction of the eye.

contact lenses
Fig. 3. Contact lenses: 1 - spherical with scleral rim; 2 - made in individual form eyes; 3 - corneal.

If regular glasses do not improve visual acuity, appoint telescopic glasses. They represent optical magnifying system consisting of collecting and scattering lenses, fortified in one frame. Telescopic glasses significantly narrow field of view and they should be used only for work. A special kind of glasses are contact lenses (Fig. 3)imposed directly on the front part of the eyeball. Depending on individual endurance these lenses can be worn from 4 to 16 hours in a row; designate them with a decrease in vision due to irregular curvature of the cornea, when a big difference eye refraction and other muscular asthenopia (weakness of the eye muscles) designate prismatic glasses, consisting of two prisms. These points provide binocular vision without particular strain of the eye muscles. Zakonchennie glasses are used to correct aniseikonia, i.e., the phenomenon of unequal size images of objects on the retina of each eye. With these points is reached, the trim size of the images. Mirror glasses, which increases the field of vision prescribed in the loss of half of the field of view (see Hemianopsia). Using a small flat mirror attached to the bow half a spectacle frame, images of objects transferred to the functioning half of the retina.
Protective eyewear (Fig. 4) are designed to protect the eyes from mechanical and chemical damage and the harmful effects of the radiant energy. They can have optical and ordinary glass. Safety glasses should not be lower visual acuity and narrowing Nola of view.
protective spectacles

Fig. 4. Various kinds of protective points: 1 - sunglasses-grid; 2 - scaly; 3 glasses Dolganov; 4 - driver; 5 - the dust; 6 - points in hermetic rubber casing; 7 - open type; 8 - opened in nylon frame.

Glasses - simple or complex optical system used for the correction of refractive errors and disorders of accommodation, correction of defects of the eye muscle system and to protect the eyes from damage and the harmful effects of some types of radiant energy.
Corrective glasses are used to correct refractive errors and disorders of accommodation with the help of optical glasses or lenses that have the property to refract the rays of light, that is to reject their course. Refractive power of (D), or refraction, glass measured by the inverse of its main focal length (F):
Thus, the more important focal length, the narrower the refractive power of the Windows, and Vice versa. The unit of measurement of optical power of the glass is the diopter (see). There are main and vertex refraction glass. When determining the main refraction focal distance is measured from the main planes of glass, the definition of vertex - from the surface. In ophthalmologic optics to denote the lenses on their back vertex refraction, i.e. to measure primary focal length from the back surface of the glass.
Depending on the position of the main focus spectacle lenses are divided into collective (convex-glass), denoted by + (plus)and dissipative (konkav-glass), denoted by the - (minus). The collective of glasses main focus (counting from the light source is behind glass, scattering - ahead of him. Collective glass give a parallel rays converging (convergent) direction, scattering glass - divergent (divergent). There are spherical glass, in which the refractive power of all meridians are the same, and cylindrical and toroidal glass, having two major mutually perpendicular Meridian. In one of them lens has the lowest refracting force in the other - the greatest. In the interim meridians refractive power of the glass takes the values that lie between these two values. Generalized spherical lenses are used to correct hypermetropia and presbyopia, scattering for the correction of myopia. Cylindrical and toroidal glass is used for the optical correction of astigmatism (see Astigmatism eye).

the form of spherical glass
Fig. 1. The form of spherical glass: 1 - biconvex; 2 - biconcave; 3 - convex; 4 - - concave; 5 - meniscus, acting as a collective glass; in the meniscus, acting as the scattering glass.

In the form of refractive surfaces spherical glass are bi-forms (both the surface of the glass convex or concave), plan-forms (one surface of the flat, and the other concave or convex) and menisci (Fig. 1). The last one surface of the glass convex and the other concave. The positive meniscus (acts as a convex glass) smaller radius of curvature has a convex surface, negative meniscus (acts as a concave glass) is concave. Menisci in turn are divided into periscopic glass, in which the refractive power of one surface is always equal to 1.25 D, and actually menisci - glass with a refractive power of one surface 6.0 D.
By the nature of the surfaces of cylindrical glass can be: bi-cylindrical (both the surface of the glass cylindrical), plan-cylindrical (one surface of the glass is flat, the other cylindrical) and sphere-cylindrical (one surface of the glass spherical, other cylindrical). The first and third types of cylindrical glasses are used to correct a mixed and complex astigmatism, the second, for the simple correction of astigmatism.
The glass, especially large refractive power, have on sight a number of undesirable side effects: influence on the size of the image on the retina, impede the proper perception of the spatial perspective, distort the shape of the objects. As of wearing glasses and getting used to them these effects usually disappear. When assigning and manufacture of glasses has to be considered, and if possible to eliminate some of the optical lens disadvantages, mainly astigmatism slanting beams. The essence of it is that the rays of light coming from one source, after passing through the Central and peripheral parts of glass are refracted differently and are connected in more than one focal point. Consequently spherical glass at a sideways glance acquire the properties of cylindrical and cylindrical glasses changes dramatically the difference between the refractive power of the main meridians. Astigmatism of slanting beams significantly expressed in glass bi-forms, which is why now is not made. More perfect in optical glass plan-forms and especially panastigmaticec meniscus lens shape or aspherical (punchline) lenses, which astigmatism slanting beams is practically absent. When astigmatism correction to address this shortcoming instead cylindrical appoint toric glass.

bifocal spectacles glass
Fig. 2. Bifocal spectacles glass

In some cases, refractive errors and disorders of accommodation (presbyopia in combination with hyperopia, a high degree of myopia and other) necessary to use two pairs of glasses for distance and at near - or, what is much more convenient, one pair of glasses with bifocal glasses different refractive power. The upper part of such glass is used in order to look into the distance, the lower - to work at a close distance (Fig. 2). There are and mnogofotonnye (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 plastic masses and put directly on the front part of the eyeball. They favorably differ from usual glasses, and cosmetic effect, as almost not visible to the eye. If the direction of the eyes contact lens repeats all of the movement of the eyeball. Contact lens (Fig. 3) consists of a Central, more convex, optical parts, situated in front of the cornea, and peripheral, flatter, haptic part, adjacent to the sclera and hold the lens on the eyeball. Between the inner surface of the Central part of the lens and cornea surface there is a narrow space, filled with a liquid with a refractive index equal to the index of refraction of the cornea. Contact lenses can be worn in a row from 4 to 16 hours, depending on individual sensitivity. Better tolerated so-called corneal contact lens without scleral basic parts, which are held by a thin layer lacrimal liquid directly on the cornea. Proposed mikrosomalna lenses and contact lenses of elastic material (gel); dressing on the eyeball they take its shape. Assign the contact glass by professional artists, athletes, miners and others) and optical indications (high myopia, anisometropia, unilateral
aphakia, and others). Among the optical indications most significant is the decrease in visual acuity in connection with the irregular curvature of the cornea (keratoconus, wrong corneal astigmatism), under which regular glasses. useless.