REFRACTOMETRY OF A EYE

From Big Medical Encyclopedia

REFRACTOMETRY OF A EYE (late lat. refractio refraction of Greek metreo to measure, measure) — objective definition of a refraction of an eye by means of special devices — eye refractometers.

The first attempts of objective measurement of a refraction of an eye were made by means of the eye (oftalmoskopichesky) mirror offered by G. Helmholtz in 1851 in a combination to replaceable lenses. Later G. Helmholtz suggested to use the ophtalmoscope to this purpose (see. Oftalmoskopiya ). Further connection of the ophtalmoscope with a subjective optometr (see. Accommodation ) led to emergence of special devices — eye refractometers (optometr).

Any refractometer consists of two optical systems: the optometric, creating the image special brand in space and projecting it on a retina of the studied eye, and oftalmoskopichesky, allowing the doctor to conduct a research under control of sight (in visual refractometers), or by means of the photo-electric receiver (in automatic refractometers).

Fig. 1. Outward of the refractometer of Rodenshtok: 1 — a head rest; 2 — the stationary device for closing of the second eye; 3 — the handle of installation of dioptries; 4 — the handle for movement of an upper part of the device in transverse direction; 5 — the handle of a rheostat; 6 — the network switch; 7 — the lever for movement of an upper part of the device in lengthwise direction; 8 — the handle for fixing of the device; 9 — a socle of a lamp; 10 — an eyepiece; 11 — the device for fixing of a look.
Fig. 2. Elementary optical diagram of the refractometer of Rodenshtok. Optometric system: 1 — the lighting device; 2 — the brand rotating around an optical axis and connected with a scale of corners; 3 — a ring diaphragm; 4 — a lens; 5 — the prism moving along an optical axis and connected with a scale of refractions; 6 — a lens; 7 — the svetodelitelny prism sending to the studied eye a light bunch through a ring zone of its pupil; 8 — the studied eye. Oftalmoskopichesky system: 9 — a lens; 10 — a round diaphragm; 11 — the mobile system which is kinematic connected with a prism (5); 12 — the eyepiece allowing the doctor to observe the image of brands on a retina; 13 — an eye of the doctor. Possible images of brands on a retina of the studied eye in the absence of an astigmatism: and — the image of brand is not combined with a retina — the image of brand is combined with a retina; at an astigmatism: in — the image of brand is not combined with a retina; — the direction of an arrow is combined with one of the main meridians; d — the refraction is measured in the main meridian, perpendicular to the direction of an arrow (the image of brand is combined with a retina).

Principle of operation of all refractometers consists in finding of the plane, the corresponding optical installation of an eye that is reached by movement of the image of brand before combination with this plane. At the same time it is important that the refractometer was located coaxially with the studied eye and at a certain distance from it that is reached at the placement of the sharp image of the shining diaphragm to the center of a pupil of the studied eye. There are two types of refractometers. Refractometers of one type are based on obtaining the sharp image of the shining brand at the bottom of the studied eye. Carry Rodenshtok (fig. 1, 2), Torner's refractometers to them. The automatic refractometer a dioptron belongs to the same type of devices, in Krom infrared radiation is used, but process of measurement is automated, is made consistently in several meridians, and results receive in the form of record with the indication of the sphere, the cylinder and the direction of an axis of the cylinder.

Fig. 3. Outward of the refractometer of Hartinger: 1 — a-degree scale; 2 — an eyepiece of the refractometer; 3 — a measuring scale (diopter); 4 — a tube; 5 — the handles regulating height and the provision of a tube; 6 — a support for a chin of the patient (chin rest); 7 — a head rest.
Fig. 4. Elementary optical diagram of the refractometer of Hartinger. Optometric system: 1 — the lighter; 2 — the brand moving along an optical axis; 3 — a lens; 4 — a pentaprism; 5 — a two-slit diaphragm; 6 — a mirror with an opening in the center; 7 — the lens sending to the studied eye a light bunch through two peripheral zones of its pupil; 8 — the studied eye. Oftalmoskopichesky system: 9 — a round diaphragm; 10 — the lens connected with a scale of corners; and — the eyepiece which is rigidly connected with the moving brand and with a scale of refractions and allowing the doctor to observe the image of brands on a retina of the studied eye; 12 — an eye of the doctor. Possible images of brands on a retina of the studied eye in the absence of an astigmatism: and — the image of brand is not combined with a retina of an eye; the image of brand is combined with a retina of an eye in the presence of an astigmatism; in — the image of brand does not match the main meridians and a retina of an eye; — the direction of strokes of brand corresponds to the direction of the main meridians; d — measurement in the main meridian which direction corresponds to the direction of long strokes is performed.

Refractometers of other type are based on Sheyner's phenomenon (bifurcation of the image projected on a retina through different sites of a pupil), and measurement of a refraction is reached by combination of two images also by means of smooth change of convergence of beams. Carry the earliest, parallax refractometer, Finchem's refractometer to this type. In the USSR the most widespread is Hartinger's (fig. 3, 4) refractometer, the optometric system to-rogo consists of the lighter, the brand moving along an optical axis, a lens, a pentaprism, a two-slit diaphragm, a mirror, the lens sending a light bunch to the studied eye. The Oftalmoskopichesky system consists of a diaphragm, the lens connected with a scale of corners, the eyepiece connected with brand and a scale of a refraction. Operation of the automatic refractometer — an autorefractor 6600 is based on the same principle, in Krom through four peripheral zones of a pupil of the studied eye create four images of brand. The photodetector is also divided into four independently working parts. Installation of coaxiality and necessary removal comes from the studied eye automatically. Dly relaxations of accommodation during measurement show to investigated the pulsing dot object of green color seen by slightly obscured. The result is received in the form of value of the sphere, the cylinder and the direction of an axis. Full automation of operations, a possibility of bystry carrying out a research allow to carry it to the most perfect devices of this sort. Common fault of all automatic refractometers — the small range of measurement.

At measurement of a refraction (see. Refraction of an eye ) by means of Hartinger's refractometer rays of light from the device send to the studied eye and receive on his retina the image of a test sign in the form of three vertical and two horizontal strips.

Optical system eyes (see) carries this image in the focal plane of the refractometer, edge at the initial position of optics of the device (the index of a measuring scale on zero) it is accompanied by a further point of clear sight of an emmetropichesky eye (see. Further point of clear sight ). The test sign is visible to the researcher through an eyepiece of the refractometer. At emmetropias (see) both semi-images of vertical and horizontal strips are combined, at a hypermetropia (see. Far-sightedness ) and myopias (see. Short-sightedness ) disperse. Rotation of the ring located near an eyepiece of the device try to obtain combination of strips and determine a look and size of a refraction of an eye by a scale of the device. At an astigmatism (see. Astigmatism of an eye ) horizontal strips are displaced also down. Eliminate with turn of a tube of the device around a horizontal axis discrepancy of strips down and establish these a tube of the device in one of the main meridians. Determine by the way described above a refraction of this meridian, and then, having turned a tube of the device on 90 °, other meridian. Measurement ranges of the refractometer of Hartinger from — 20,0 to +20,0 dptr, the accuracy of measurement are up to 0,25 dptr. Hartinger's refractometer is especially convenient for measurement of an astigmatism of an eye at not weakened accommodation; the spherical ametropia is defined on it a little shifted towards a myopia.

At a research of a refraction of an eye automatic refractometers on a bottom of the studied eye project invisible (in infrared beams) brand and carry out the automatic electron-optical analysis of its image. The role of the eye investigating is carried out by photosensors, system of strengthening of a signal and the computing device turning this signal into record of a refraction of the studied eye.

Fig. 5. The schedule of a clinical refraction of an eye in various meridians of optical system of an eye received on the automatic refractometer — the oftalmetrena: on a vertical axis — meridians of optical system of an eye in degrees; on a horizontal axis — a clinical refraction in dioptries. Sinusoidal character of a curve demonstrates existence of an astigmatism; shooters specified maximum (+ 6 dptr on a meridian 10 °) and minimum (+ 2 dptr on a meridian 10 0 °) values of a refraction.

In one of automatic refractometers — the oftalmetrena — record of a refraction is made in the form of the schedule, on a vertical axis to-rogo meridians of optical system of an eye in degrees, and on a horizontal axis — a wedge, a refraction in dioptries in each of these meridians (fig. 5) are postponed. At an emmetropia or a spherical ametropia the schedule has an appearance of a straight line parallel to a vertical axis, at an astigmatism — sinusoids with the period 180 °. Its highest point corresponds to a refraction of the main meridian with stronger refraction, the lowest point — refractions of the main meridian with the minimum refraction. By means of an oftalmetron it is especially convenient to investigate patients with difficult disturbances of optical system of an eye, e.g., after operations on a crystalline lens and a cornea.

Refractometers — a dioptron or an autorefractor are more convenient for mass researches. During the definition of a refraction by means of a dioptron the brand in infrared light is projected on a bottom of the studied eye. Its image gets on the sensor of the analyzing system through a narrow crack, to-ruyu serially orient in each of meridians of optical system of an eye. The analyzer is ready for search of situation, in Krom the image is most contrast. This search is carried out movement of a lens of mobile optical system of the device. The provision of a lens, at Krom contrast the greatest, specifies a refraction of an eye in this meridian. By means of the computer of the device receive these researches on the special form. The dioptron allows to measure a spherical ametropia from — 10,0 to + 15,0 dptr with an astigmatism to 5,0 dptr, the accuracy of measurement to 0,01 dptr.

See also Optical techniques of a research .



Bibliography: Avetisov E. S. and Rozenblyum Yu. 3. Optical correction of sight, M., 1981; V. V. Wolves, about r and N A. I. and D and l and and sh in or O. A. Klinicheskaya vizo-and refractometry, L., 1976; Krasnovidov V. S., Baranovsky Ya. M. and Ilyin V. V. Use of the infrared diagnostic unit for a research of eyes, Vestn. oftalm., No. 3, page 32, 1966; Soft T. N., Crolles A. U. and Etko L. I. Pribor for measurement of a refraction of an eye by an objective method, Is new. medical tekhn., century 2, page 43, 1977; D u k e - E 1 d e r W. S. Duke-Elder's practice of refraction p. 126, Edinburgh a. o., 1978; Knoll H. A. a. Mohrman R. The ophthalmetron, principles and operation, Amer. J. Optom., v. 49, p. 122,1972; Me Devitt H. I. Automatic retinoscopy, Optician, v. 173, No. 4485, p. 33, 1977; Poise D. A. a. Kerr K. E. An automatic objective optometer, Arch. Ophthal., v. 93, p. 225, 1975.


E. S. Avetisov; T. H. Soft (tekhn.).

Яндекс.Метрика