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