GB2059623A

GB2059623A - Instrument for the subjective determination of the refraction of an eye

Info

Publication number: GB2059623A
Application number: GB8029859A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-10-05
Filing date: 1980-09-16
Publication date: 1981-04-23
Also published as: IT1147758B; DD153323A5; FR2466232B1; GB2059623B; IT8067995A0; NL8005524A; FR2466232A1; CH646322A5

Abstract

The invention relates to an instrument for the subjective determination of refraction of the eye and which comprises a first optical element (3) which forms an image of optotypes (13) in its focal plane on the image side, and a second optical element (4), placed after the former in the optical path, which forms an image of the optotypes on the side towards the eye (7) to be examined. The distance of the object side focal point (F4) of the second element (4) from the image side focal point (F'3) of the first element (3) is variable for the purpose of determining the refraction. The instrument makes it possible to determine the refraction value both for a spherical correction and for a cylindrical correction. <IMAGE>

Description

SPECIFICATION Instrument for the subjective determination of the refraction of an eye The invention relates to an instrument for the subjective determination of refraction of the eye.

Defective vision of the eye, that is to say the refraction, can be determined by objective or subjective means. The objective determination of refraction is carried out with the aid of optical instruments which were developed from the ophthalmoscope. Mental participation of the test person is not necessary when using these. The refractive value of each eye is measured monocularly.

The result of the objective determinatin of refraction is taken as a starting value for the subsequent subjective examination. The decisive point for the prescription of spectacles or contact lenses is always the result of the subjective determination of refraction, since only the latter enables the binocular functions to be tested.

In the simplest case, the subject determination of refraction is carried out by using test spectacles and loose test lenses which are placed one after the other in front of the eye of the test person, in accordance with a defined test system. The subjective examination can be carried out more rapidly and more exactly with the aid of spectacle-specifying instruments (phoropters) in which the test lenses are accommodated in Recos discs. If two Recos discs are placed one behind the other and each of these discs contains a number of n test lenses, a total of n2 correction values can thus be set. ALVAREZ lenses represent a further possibility for carrying out the subjective determination of refraction.

The possibility of subjectively carrying out the determination of refraction with the aid of the granulation on illumination of a diffusely reflecting surface with laser light has not proved successful and has not gained acceptance in practice.

The determination of the astigmatism of the eye by a subjective route is carried out by various refraction methods, using loose test lenses or spectacle-specifying instruments, in which the test lenses are accommodated in Recos discs. For the determination of astigmatism, astigmatic lenses are used (cylindrical lenses or toroidal lenses) which must be rotatable about the optical axis in order to be able to adjust to the different axis directions which occur.

A further possibility for determining the astigmatism of the eye is provided by STOKES' cylindrical lens which consists of two plano-cylinders which are synchronously displaced rotationally relative to one another.

The superposed arrangement of two STOKES' cylindrical lenses, the axes of which enclose an angle of 45 , also enables the astigmatism to be determined, but a calculation procedure must be carried out in order to fix the axis and the size of the resulting cyclinder.

According to the present invention there is provided an instrument for the subjective determination of refraction, which is characterised according to the invention by a first optical element which forms an image of optotypes in its focal plane on the image side and a second optical element, placed after the former in the optical path of rays, which forms an image of optotypes on the side towards the eye to be examined, the distance of the object side focal point of the second element from the image side focal point of the first element being variable.

Thus, using this instrument, rapid refraction is possible with respect to both spherical and astigmatic defects of vision, under conditions of free vision. At the same time, the instrument is also suitable for working out the specification of near-sight spectacles, without putting prisms or other elements in front.

The invention will be more clearly understood from the following description which is given by way of example only with reference to the accompanying drawings in which: Figure 1 shows a first embodiment of the invention in side view; Figure 2 shows a sectional part representation from Fig. 1 along the line Il-Il; Figure 3 shows a side view of a further embodiment of the invention; Figure 4 shows a section along the line IV-IV in Fig. 3; Figure 5 shows a diagrammatic representation of a further embodiment of the invention for the binocular measurement of spherical and astigmatic defects of vision; Figure 6 shows an embodiment which is modified relative to the design shown in Fig.

1; Figure 7 shows a further embodiment; and Figure 8 shows a part, similar to that in Fig.

2, in a modified embodiment.

In the illustrative embodiment shown in Fig.

1, the instrument 1 has a first lens 3 and a second lens 4, which are arranged one above the other in such a way that their optical axes 8, 9 are mutually parallel. Opposite the two lenses 3, 4, a totally reflecting optical element 6 is located, which can be a totally reflecting prism or a totally reflecting mirror system and which reflects the rays emerging from the first lens 3 into the second a totally reflecting mirror system and which reflects the rays emerging from the first lens 3 into the second lens 4. When the third optical element 6 is displaced parallel to the optical axes 8, 9, the optical interval between the lenses 3, 4 is thus varied.

The aperture diaphragm 10 which coincides with the entry pupil is located in the object side focal plane F3 of the first lens 3. At any desired setting of the third optical system 6, the exit pupil 11 of the system is in a constant position and lies, for any setting, in the image side focal plane F'4 of the second lens 4. Between the second lens 4 and the exit pupil 11, a semi-reflecting plane mirror 12 is located, which encloses an angle of 45 with the optical axis 9 of the lens 4. The test type 13 is viewed via a collimator 14. In the illustrative embodiment shown, the path of rays undergoes a deflection, by means of a mirror 15, which is intended merely to shorten the constructional length of the instrument.

If the third optical element 6, which is called the prism 6 in the further text, is adjusted so that the image side focal point F3 of the first lens 3 coincides with the object side focal point F4 of the second lens 4, parallel rays incident on the first lens 3 will leave the system, cor;sisting of the lenses 3 and 4, again parallel. An eye having correct vision clearly sees the image of the test symbol 13, formed at infinity by the collimator 1 4. This setting corresponds to the refraction value of 0.0 dioptre. If the prism 6 is brought closer to the two lenses 3, 4, the rays incident parallel to the axis on the first lens diverge on leaving the second lens 4. At this setting, a distant object is seen in focus by a shortsighted eye of corresponding refraction.The displacement of the prism in the direction of the lenses thus corresponds to short-sighted refraction values, and there is a linear relationship between the position of the prism 6 and the extent of short-sightedness. The ratio of the displacement of the prism to the degree of short-sightedness depends only on the focal length of the second lens 4. Conversely, the setting for long-sighted eyes is obtained by displacing the prism 6 in the opposite direction, that is to say away from the two lenses 3, 4. Parallel rays incident on the system have a convergent course when they leave the system of the two lenses. By calibrating a scale indicating the position of the prism 6, the refraction value determined can be read off directly.

When the first optical element 3 and the second optical element 4 are formed as spherical lenses, a STOKES' cylindrical lens 16 is advantageously provided at the position of the aperture diaphragm 10, in order to correct the astigmatism. For the purpose of determining the position of the axis of astigmatism, this STOKES' cylinder lens is designed to be rotatable about the optical axis. The values obtained by means of the STOKES' cylindrical lens can be directly evaluated if the two lenses 3, 4 have the same focal length. If the focal lengths of the two lenses differ, a correction factor must be applied which is obtained from the quotient of the refractive powers of the two lenses.

In place of the STOKES' cylindrical lens 16 described, it is also possible to use a stationary STOKES' lens combination, the axes of which enclose a constant angle of 45". In this way, the two components of cylinder effects of different magnitude and of any desired directions of the axes can be adjusted. Of course, it would also be possible to determine the astigmatism by means of a normal stem crossed cylinder, in which case the latter should preferably be placed in front in the plane of the aperture diaphragm.

If the measuring range of the spherical effect resulting from the two lenses 3 and 4 is to be widened, additional lenses can be fitted in or close to the plane of the aperture diaphragm 10. Their effect corresponds directly to the correction values if the lenses 3, 4 have the same focal length. Otherwise, a correction factor must be taken into account.

As already stated above, the exit pupil 11 lies, at any setting of the prism 6, in the image side focal plane of the second lens 4.

During the measurement, the pupil (principal plane) of the eye 7 of the test person can coincide with the exit pupil 11. In that case, the so-called principal point refraction is determined. If the pupil of the test person is located at a distance of 16 mm behind the exit pupil 11, the correction values for a distance of 16 mm from the vertex of the spectacle lens are obtained. The distance of the pupil from the exit pupil 11 is thus identical to the vertex distance of the correction lenses. As indicated in Fig. 2, the device has a support 17 which can be formed as a support for the chin or forehead and on which the head of the test person is supported and which can be moved to and fro relative to the housing, in order to adjust the distance of the pupil of the test person from the exit pupil 11.

The patient views the image of the test type 13, formed at infinity via the collimator 14, by looking, in the viewing direction corresponding to the perpendicular to the plane of the drawing in Fig. 1, at the semi-reflecting mirror 12, which is arranged at an angle of 45 to the optical axis 9. Through the mirror 12, the test person sees the free space simultaneously with the test type 13 which can comprise optotypes or test figures. In this way, the refraction is determined under free vision, wherein the field of vision of the patient is not constricted and interfering effects, such as instrument myopia, are thus eliminated from the outset.

As can be seen from the above, it is possible by means of the instrument described, according to the invention, continuously to adjust the correcting effect by displacing the prism 6. In this way spherical correction values can be determined very rapidly; in the case of spherical aberrations, the full correction can be determined and, in the case of astigmatic aberrations, the so-called best spherical correction can be determined. The tedious changing of the lenses is no longer necessary and, under certain circumstances, the test person himself can carry out the optimum adjustment.

The astigmatic correction effect can also be adjusted continuously from 0 up to a maximum value with the aid of the STOKES' cylindrical lens 16. In this case also, it is possible very rapidly to discover the correct position of the axes and to determine the full correction.

The illustrative embodiment shown in Fig. 3 is used above all for determining the astigmatic correction values. In the instrument shown, the first and second optical elements are formed as cylindrical lenses 18, 19. These are each held in a mount 20, 21, each having a toothed rim on its outer periphery. The mounts are fitted in such a way that, when the mount 20 is rotated, the mount 21 rotates by an equal angle in the opposite direction.

The cylindrical lenses 18, 19 are held in the mounts in such a way that the axial position of a principal plane of both cylindrical lenses are mutually parallel in the manner shown in Fig. 4. In the path of rays, that is to say in front of the first cylindrical lens 18 in the illustrative embodiment shown, a DOVE prism 22 is located. Via a diagrammatically indicated mechanical coupling 24, this prism is rotatably arranged in the path of rays in such a way that its base 23 is always aligned parallel to the axis 25 of the first cylindrical lens 18.In other respects, the prism 6, the mirror 15, the test type 13, the image of which is formed via the collimator 14, and the aperture diaphragm, located at the object side focal distance in the effective principal plane of the cylindrical lens 18, and the entry pupil coinciding therewith are in this embodiment identical to those in Fig. 1 with respect to arrangement and effect and are therefore marked with the same reference numerals.

The exit pupil 11 again lies, independently of the position of the prism 6, in the location of the image side focal length of the principal plane of the cylindrical lens 19. Viewing takes place in the same way as in the first illustrative embodiment, that is to say via a semireflecting mirror 26 which is arranged in such a way that the test person also sees the free space simultaneously with the image of the test type, formed at infinity.

The DOVE prism 22 has the purpose of compensating in each case for the rotation, effected by rotating the cylindrical lenses 18, 19, of the test type to be viewed, so that the test person gains the impression of a test type in a rotionally fixed arrangement.

For refraction, the prism 6 is adjusted such that the image side focal point or the focal line of the first cylindrical lens 18 coincides with the object side focal point or the focal line of the second cylindrical lens 19. If the observer's eye, having normal vision, is in the vicinity of the image side focal point of the second cylindrical lens 19, it can observe distant objects sharply and without distortion from this position. If the eye to be examined has a simple short-sighted astigmatism, the astigmatism can be corrected out by displacing the prism 6 in the direction of the cylindrical lenses 18, 19. By subsequent rotation of the cylindrical lenses 18, 19 by means of the toothed wheels forming the mounts 20, 21, the correct position of the axes is thus set.If the eye to be examined has a simple longsighted astigmatism, the prism 6 must be readjusted away from the cylindrical lenses 18, 19.

If the DOVE prism 22 were not used, the observed image would also rotate when the cylindrical lenses are rotated. In this case, it would be possible to use a fixed line figure 13 for testing the astigmatism. On rotation of the cylindrical lenses 18, 19, the image of the line figure would then in each case move into the position in which it is seen sharply by the test person. If rotation of the image is to be omitted, this effect is achieved by providing a DOVE prism or another suitable reversion element which rotates the image.

If, using the illustrative embodiment shown, spherical correction values are also to be determined additionally, spherical lenses are inserted in the entry pupil or aperture diaphragm 10 in place of the STOKES' cylindrical lenses described in the first illustrative embodiment. Using the embodiment described, it is possible, in a particularly rapid and simple manner and with continuous adjustment, to determine the astigmatism and its position in an eye having defective vision.

Advantageously, the two cylindrical lenses 18, 19 in Fig. 3 are formed as plane cylinders which have the same. focal length. This has the advantage that, depending on the setting of the prism 6, always only the cylinder effect in a principal plane changes but, in the principal plane perpendicular thereto, there is no optical effect. The axial position of the effective principal plane is adjusted in the manner described by rotating the cylindrical lenses.

This ensures that the spherical effect, which always varies in known processes and devices for determining astigmatism, when the astigmatic value is altered, remains unchanged and repeated further correction of the spherical effect on alteration of the astigmatic value can be omitted.

In principle it can be appropriate to omit the DOVE prism 22 and then to utilise the rotation of the image, which results on rotation of the cylindrical lenses 18, 19, for determining the refraction. To find the principal planes, it would then not be necessary to use rotatable test objects. It would then be necessary, however, to use special optotypes, since normal optotypes form oblique images if the position of the axes is oblique.

In the embodiment of the instrument according to the invention, shown in Fig. 5, two symmetrical paths of rays 37, 38 are provided for a binocular determination of refraction.

The two paths of rays 37, 38 contain exactly the same elements so that only one of them is described. The elements in the path of rays 37, corresponding to those in Figs. 1 to 4, are each marked with the same reference numerals, and the elements in the second path of rays, corresponding to the elements in the first path 37 of rays, are characterised by corresponding reference numerals with a dash.

The path of rays, as viewed from the test type, initially coincides fully with the path of rays shown in Fig. 3 and differs only in that a completely reflecting plane mirror 27 is provided instead of the semi-reflecting mirror 26.

The part, described by reference to Figs. 1 and 2 and consisting of the aperture diaphragm 10, mirror 15, first lens 3, displaceable prism 46, second lens 4, semi-reflecting mirror 12 and support 17, of the axially symmetrical system described there is placed after the first part of the instrument, corresponding to the embodiment shown in Fig. 3.

The second part is here arranged in such a way that the aperture diaphragm 10 of the axially symmetrical system of Fig. 1 coincides with the exit pupil 11 of the first anamorphotic system which is identical to the representation in Fig. 3. This combined system makes it possible, continuously for any values and under conditions of free vision, to set the spherical correction values and the astigmatic correction values and to determine the position of the axes in a simple and rapid manner without inserting any additional lenses.

In the embodiment shown in Fig. 5, two collimators 14, 14' are provided. It is also possible, however, to use a collimator with a concave mirror of such a size that an image of the test type is formed simultaneously for both paths of rays and hence for both eyes.

Using the instrument shown in Fig. 5, it is possible to check the binocular refraction. The semi-reflecting mirrors 28, 28', which correspond to the semi-reflecting mirror 12 in their arrangement and function, are arranged to be rotatable about vertical axes 30, 30'. Any desired convergent and divergent positions of the pair of eyes can be realised by rotating these mirrors about the vertical axis. These adjustments correspond to prism effects.

Thus, a check for heterophoria can be carried out without placing prisms in front. This has not only the advantage of simplifying refraction, but there is also no interference with the observation due to coloured fringes and other aberrations of a prism which would otherwise be placed in front.

The fact that the semi-reflecting mirrors 28, 28' can be swivelled also makes it possible to obtain the near-sight spectacle specification without placing prisms or other elements in front. The requisite convergent position is effected in front of the eyes by rotating the two mirrors 28, 28'. Accommodation results from the adjustment of the displaceable prisms 46, 46'. The near-sight test is thus not fixed at a defined observation distance but, rather, it can be carried out at any desired near-sight distances.

In the illustrative embodiment shown, the semi-reflecting mirrors 28, 28' are arranged to be rotatable both about their vertical axes 30, 30' and additionally about a horizontal axis, by means of a suspension on gimbals. In this way, it is possible to check not only for heterophoria with respect to a lateral deviation but also with respect to a possible deviation in the height of the axes of the eyes.

In the illustrative embodiments described above, the change of the optical interval between the first and second optical elements 3, 4; 1 8, 1 9 is effected via respective prisms 6, 46 arranged in the path of rays. In principle, it is also possible, however, to omit the deflection prisms 6, 46 and, instead, to arrange the lenses one behind the other and to make them movable relative to one another. This has, however, constructional disadvantages with respect to the length of the instrument and it entails the problem that the position of the exit pupil or of the aperture diaphragm is not constant.

A widening of the range of measurement is also possible in the embodiment shown in Fig. 5 by arranging appropriate lenses in the exit pupil 11 which coincides with the entry pupil of the following spherical system and with the focal point of the lens 3.

In principle, the prisms 6, 6', 46, 46' can be shifted by manual displacement, it being possible to read off the associated correction value via a correspondingly calibrated scale from the position of the prism and to read off the position of the axes from the rotational position of the lenses 18, 1 9. As shown in Figs. 1 and 5, the prisms 6, 6', 46, 46' are adjusted via a carriage 36, 40 and a motor 35, 39. As a function of the position, an output signal is passed from the motor via appropriate lines to a computer 33 which, as a function of the position of the prisms, determines the corresponding dioptre values and indicates these on a display 34. In the same way, a signal corresponding to the rotational position of the lenses 1 8, 1 9 can also be fed in via an appropriate line, so that the position of the axes can also be indicated by the display. Of course, the two symmetrical paths of rays 37, 38 have corresponding drives and signal lines. For simplification, however, these were only shown in the path of rays 37. Additionally, the angular position of the mirrors 28, 28' can also be transmitted to the computers via appropriate lines, so that, by means of a corresponding programme, the computer can also indicate a requisite prismatic correction which may exist.

In the embodiment according to Fig, 3, a drive by a motor is not shown. This can, however, be provided in the same way as in the embodiment according to Fig. 1 or according to Fig. 5.

The basic construction of the embodiment shown in Fig. 6 corresponds to that of the instrument shown in Fig. 1, mutually corresponding parts being marked with the same respective reference numerals.

A STOKES' cylindrical lens 16 is located in the aperture diaphragm 10. In this STOKES' cylindrical lens, consisting of two opposed identical individual cylindrical lenses, a spherical effect is additionally generated in each case in a known manner when a cylindrical effect is set. To compensate this spherical component, thus generated, the STOKES' cylindrical lens 16 and the lens 3 are mechanically coupled to one another in such a way that they can at the same time be displaced to and fro by the same distance parallel to the optical axis 8 in the direction of the arrow 53.

For this purpose, the lenses are fitted in mounts which are not shown and which are fixed to a displacement device which is not shown.

As in the device shown in Fig. 1, the spherical correction value is first determined by displacing the prism 6 to and fro. Subsequently, the cylinder value is determined by means of the STOKES' cylindrical lens 1 6. If, for example, a negative cylinder effect is set, a positive spherical component, amounting to half the cylinder effect, necessarily results. To compensate for the latter, the STOKES' cylindrical lens 16 and the lens 3 are now displaced in the direction of the prism 6 by a corresponding distance. Due to the shortening of the light path between the lenses 3 and 4, the positive spherical component is compensated. The simultaneous displacement of the STOKES' cylindrical lens 16 is necessary to ensure that the latter always remains in the aperture diaphragm 10.

As can be seen from Fig. 6, the cylinder value resulting from the setting of the STOKES' cylindrical lens and the axis, resulting from the angular position of the STOKES' cylindrical lens, of the set astigmatic value and, if appropriate, a displacement of the STOKES' cylindrical lens 16 together with the lens 3 are passed via an appropriate line to the computer 33 which indicates the thus resulting correction values on the display 34.

In principle, for compensating the spherical component generated, it would also be possible correspondingly to displace the prism 6 or even the lens 4, whereby the displacement of the STOKES' cylindrical lens would become superfluous. A displacement of the lens 4 would, however, change the location of the exit pupil, and this is undesirable. A displacement of the prism 6 would make it more difficult to read off the spherical correction values, since a conversion would be necessary. Using the embodiment shown in Fig. 6, the position of the prism 6 gives an unambiguous measure of the spherical correction va lute.

The embodiment shown in Fig. 7 is in part identical to the construction shown in Fig. 5, mutually corresponding elements being provided with the same respective reference numerals. The two paths of rays are formed to be symmetrical to one another so that, for simplification, reference is again made only to one of the paths of rays. An image of a test type 13 is formed at infinity via a collimator 14 and a mirror 15. As in the embodiment of Fig. 5, a first optical element 3, a second optical element 4 and a prism 46 are provided in the path of rays. The arrangement of these optical elements and of all the remaining parts is provided in the same way as in the embodiment shown in Fig. 5.The second optical element 4 is again a lens, whilst the first optical element in this embodiment is assembled from two identical cylindrical lenses 300, 301 which have a collecting action and the axes of which form a fixed angle of 90 relative to one another. The two lenses 300, 301 are located as closely together as possible and at the same point as the lens 3 in Fig. 5. Together, the two cylindrical lenses 300, 301 thus have a spherical effect. The two cylindrical lenses 300, 301 can be displaced relative to one another, individually or together, along the optical axis by means of a device which is not shown.

When the lens 301 is displaced towards the prism, a negative astigmatic effect is produced (minus cylinder). When the lens 300 is displaced away from the prism, a positive astigmatic effect is produced (plus cylinder).

The magnitude of the displacement depends in each case on the power of the cylindrical lenses.

When the instrument is used, the first and second optical elements 3, 4 and the prism 46 are again adjusted such that the focal points F'3 and F4 coincide, as in Fig. 1. By means of displacing the prism 46, the spherical correction is determined which, depending on the location of the prism 46, can be read off on the display 34. Subsequently, depending on the preferred method of refraction, in which either minus cylinders or plus cylinders are used, the cylinder lens 301 is displaced towards the prism 46 or the cylindrical lens 300 is displaced away from the prism 46 until any astigmatism which may be present has been determined. The displacement of the lenses is fed via a signal line to the computer 33, so that the determined astigmatic value can be read off on the display 34.The cylindrical lenses 300, 301 are mounted in a holder, which is not shown, in such a way that, in a rotationally fixed mutual position, they can be rotated together about the optical axis, so that the cylinder which has been adjusted by the displacement can be rotated into the axial position corresponding to the defective vision. The rotational position is also fed via the signal line to the computer 33, so tha the axial position of the cylinder can also be read off on the display 34.

During the above-described displacement of either the cylindrical lens 301 or the cylindrical lens 300 alone, a purely astigmatic component is generated without an additional spherical component, so that, as distinct from the embodiment shown in Fig. 6, an additional correction is not required. If, however, the two cylindrical lenses 300. 301 are synchronously displaced towards one another, a spherical component is generated, as in the example of a STOKES' cylindrical lens shown by reference to Fig. 6, and this would have to be compensated by displacing the light path between the first optical element 3 and the second optical element 4.

As in the illustrative embodiments described above, the cylindrical lenses 300, 301 in the illustrative embodiment described by reference to Fig. 7 are formed, according to the invention, as plano-cylindrical lenses which have a zero action in one vertex and a cylinder action in the vertex perpendicular thereto.

As can be seen from the above, the embodiment shown in Fig. 7 is very particularly advantageous since the construction is substantially simpler than in the embodiment shown in Fig. 5. Since there is no rotation of the image when adjusting for the astigmatism, the use of additional DOVE prisms, in particular, is also unnecessary.

As stated above, the position where the eye which is to be examined must be located, depends on the focal length of the second optical element 4, since the pupil of the eye should be located at the position of the exit pupil or at a distance from the latter, corresponding to the vertex distance. The focal length of the lens 4 becomes shorter when the measuring range is widened. Since the semi-reflecting mirror 28 also must still be arranged between the lens 4 and the exit pupil, it can happen that the eye must move undesirably close to the mirror 28.To increase this distance, it is advantageous, according to the embodiment shown in Fig. 8, to provide an afocal system consisting of two collecting lenses 50, 51 between the lens 4 and the semi-reflecting mirror 12 of the embodiment shown in Fig. 2 or between the lens 4 4 and the semi-reflecting mirror 28 of the embodiment shown in Fig. 7. The two collecting lenses have the same focal length. The lens 50 is located in the imge side focal plane F'4 and the object side focal point F51 coincides with the image side focal point F'50.

Thus, the exit pupil 11 of the overall system is generated at twice the focal length 2f50 of the collecting lens 51. The working distance thus increases as a function of the selection of the focal lengths of the lenses 50, 51.

Although the invention has been described with reference to specific example embodiments, it is to be understood, that it is intended to cover all modifications and equivalents within the scope of the appended claims.

Claims

1. An instrument for the subjective determination of refraction of the eye, such instrument having an optical system comprising a first optical element which forms an image of optotypes in its focal plane on the image side and a second optical element, placed after to the first element in the optical path, the second element forming an image of the optotypes on the side adjacent an eye to be tested, the distance of the object side focal point of the second element from the image side focal point of the first element being variable.

2. An instrument according to claim 1, including a third optical element to provide such variation in distance and located in the optical path between the first and the second optical elements.

3. An instrument according to claim 2 wherein the first and second optical elements are in a mutually adjacent arrangement with approximately coincident principal planes, and the third optical element deflects the optical path leading from the first optical element towards the second optical element.

4. An instrument according to claim 1, 2 or 3, wherein a semi-reflecting mirror, inclined at an angle to the optical axis, is arranged between the second optical element and the exit pupil of the instrument.

5. An instrument according to claim 4, wherein the semi-reflecting mirror is arranged to swivel about a vertical axis.

6. An instrument according to any one of claims 1 to 5, wherein the first and second optical elements are cylindrical lenses.

7. An instrument according to claim 6, wherein the cylindrical lenses are coupled to one another such that when one rotates, the other rotates by the same amount in the opposite direction, there being a rotational condition in which their principal planes are mutually parallel.

8. An instrument according to any one of claims 1 to 7 and having two such optical systems arranged in series in the optical path.

9. An instrument according to claim 8, wherein the entry pupil of the second system coincides with the exit pupil of the first system.

10. An instrument according to claim 8 or 9, wherein the optical elements in the first system are cylindrical lenses and the optical elements in the second system are spherical lenses.

11. An instrument according to any one of claims 1 to 5, wherein the first or the second optical element is formed from two cylindrical lenses which are arranged to be displaceable relative to one another on the common optical axis.

12. An instrument according to claim 11, wherein the two cylindrical lenses are in a fixed rotational position relative to one another and are mounted to be rotatable together about the optical axis.

13. An instrument according to claim 11 or 12, wherein the axes of the cylindrical lenses are mutually offset by 90 .

14. An instrument according to any one of claims 1 to 5, wherein in the aperture diaphragm of the optical system thus formed, a a Stokes cylindrical lens is located which is coupled to the first, second or third optical element in such a way that, on setting a cylinder value by means of the Stokes cylindrical lens the light path between the first and second optical elements is varied in order to compensate for the spherical component thus generated.

15. An instrument according to claim 14, wherein the Stokes cylindrical lens and the first optical element are displaceable together on the optical axis, as a function of the setting of the Stokes cylindrical lens.

16. An instrument according to any one of claims 1 to 15, wherein on the image side of the second optical element, an afocal system of two lenses is provided, of which a first lens adjacent the second optical element is located in the image side focal plane of the latter.

1 7. An instrument according to any preceding claim in which there are cylindrical lenses and wherein the cylindrical lenses are plano-cylindrical.

18. An instrument according to any preceding claim wherein, for a binocular determination of refraction, a second identical optical system is provided.

19. An instrument according to any preceding claim, wherein a housing surrounding the optical elements has an adjustable support to be contacted by a patient such that the eye has a predetermined vertex distance from the exit pupil of the overall system.

20. An instrument for the subjective determination of refraction of the eye, such instrument being substantially as hereinbefore described with reference to and as illustrated in Figs. 1 and 2. Figs. 3 and 4, Fig. 5, Fig. 6, or Fig. 7 of the accompanying drawings, or modified substantially as hereinbefore described with reference to and as illustrated in Fig. 8 of the accompanying drawings.