WO1983002716A1

WO1983002716A1 - Apparatus and method for measuring the refraction

Info

Publication number: WO1983002716A1
Application number: PCT/DE1983/000025
Authority: WO
Inventors: Helmut Krueger
Original assignee: Helmut Krueger
Priority date: 1982-02-12
Filing date: 1983-02-10
Publication date: 1983-08-18
Also published as: DE3204876A1; DE3204876C2; JPS59500163A; US4637700A; JPH0350530B2; US4730917A; EP0101474A1

Abstract

Ophtalmologic method useful for measuring the eye refringency with an enhanced time resolution. In order to proceed to the measurement, the image of an object is reproduced on the retina. The sharpness of said object image is controlled according to the light reflected by the retina, as a function of the distance between the eye and the object. The measurements are effected with narrow band invisible infrared light, which does not upset the sight and reduces the influence of the chromatic aberration of the eye. With said method, what is aimed at is not the distance between the eye and the object, at which distance the image of said object on the retina is sharper, but two distances at which the image is equally unsharp or fuzzy. By using said method, a differential signal is obtained with a very good signal/noise ratio allowing to determine the refringency of the eye. The apparatus and method are particularly appropriate for the automatic and objective measurement of the reflection, particularly the accomodation, timewise, of the human eye in relation to distances when the latter are modified, in other words they may be used not only to measure the static functions but also the dynamic functions.

Description

Method and device for determining the refraction

The invention relates to a method and a device for carrying out this method for determining the refraction, in particular the temporal course of the distance setting of the human eye, briefly the refraction state, in which an object located in the object space via an optometer lens system, briefly an optometer lens, on the The retina of the eye is reproduced and in which the retinal image of this object is in turn reproduced via the optometer lens into the object space back into an image plane conjugated to the retina and determined by the refractive condition of the eye and the refractive power of the optometer lens, the distance from the optometer lens being a measure of the refractive condition , the beam coming from the retinal image being analyzed at different distances from the image plane to determine the distance.

In known automatic refraction methods (see FIG. 1), an image plane conjugated to the retina of the examined eye is sought. In known methods (DE-AS 2937891, 3.1 / 10576, 3102 ^ 50), an object, which can be designed as a grid, as a point or as a slit, is imaged on the retina with an optometer lens system. The image of this retinal image which is thrown back into the object space via the optometer lens and a beam splitter mirror is analyzed there with another measurement object. For this purpose, the luminous flux transmitted by this measurement object is measured with a photosensitive element. For this purpose, both objects must correspond in shape and dimension. To find the conjugate image plane, the two objects are moved together with the beam splitter along the ODti see axis or the refractive power of the Optometer lens system changed. A maximum space or, depending on the version, a minimum of the luminous flux delivered by the photoelectric element behind the measurement object is measured when the position of the conjugate plane is reached. The refractive power of the eye must not change during the shift along the optical axis. With such a method, a change in the refractive power of the eye over time cannot be followed easily.

An improvement can be achieved according to DE-AS 2262885 if the object arrangement is shifted periodically at high frequency only a small distance along the optical axis. This results in a periodic change in the photo signal that is synchronous with the shift. From the phase shift between the movement of the object arrangement and the photo signal, an electrical photo signal can be obtained at a sufficiently high oscillation frequency, which indicates the direction in which the object arrangement must be shifted so that the conjugate plane lies in the middle between the reversal points of the periodic shift. The disadvantage of this method is the necessarily high mechanical vibration frequencies. In addition, the phase determination causes difficulties due to the low signal-to-noise ratio of the photo signal. The process according to DE-AS 2654608 therefore means progress in mechanical vibrations, in which the continuous vibration is replaced by a measurement in two points alone. Here too, however, the signal for shifting the object arrangement along the optical axis must be determined indirectly from the phase relationship between the mechanical position and the photo signal.

A completely different possibility of automatic refraction is offered by the retinoscopy described below. The invention has for its object to avoid the disadvantages of the prior art. In particular, the invention is intended to provide a low-noise photo signal which allows the conjugate level to be determined quickly and easily. To achieve this object, the invention provides the measures of the characterizing part of claims 1, 2 and 3. Preferred embodiments of the invention can be found in the further claims. Further advantages and details of the invention result from the description of exemplary embodiments with reference to the drawing. The drawing shows:

Figure 1 is an illustration of the measuring method according to the prior art.

FIG. 2 shows an illustration of the principle of finding the conjugate plane according to the measuring method according to FIG. 1;

Fig. 3 shows a first embodiment;

Fig. Ty a second embodiment;

Fig. 5 shows a third embodiment;

FIG. 6 shows an illustration of the principle of finding the conjugate plane according to the method according to FIG. 5;

7 shows a fourth preferred exemplary embodiment of the invention;

Fig. 8 shows the experimentally determined course of the photo signals when moving the movable

Object arrangement according to the method of FIG. 7; 9a, 9b a fifth particularly preferred embodiment of the invention;

10a, 10b a modification of the embodiment of FIGS. 9a, 9b

1, as already mentioned above, illustrates a measuring method according to the prior art. In which an image plane 4 or 4 ′ conjugated to the retina 2 of an eye 3 is sought at a distance 9 from an optometer lens systera 1. The distance 9 is a measure of the refraction state of the eye 3. For this purpose, an object 6 is imaged on the retina 3, which can be designed as a grid, as a point or as a slit. The image of a retinal image 2 which is thrown back into the object space 5 via the optometer lens system 1 (optometer lens 1) and a beam splitter mirror 7 is analyzed there with a second object, the measurement object 6 '. For this purpose, the luminous flux transmitted by this measurement object 6 'is measured with a photosensitive element 8. For this purpose, both objects 6, 6 'must correspond in shape and dimension. To find the conjugate image plane 4 or 4 ', the object arrangement 19 consisting of the two objects 6, 6' and the beam splitter 7 is displaced along the optical axis 18, the beam splitter 7 also being able to maintain its position, or the refractive power of the optometer lens system 1 changed. The situation is explained in FIG. 2. The imaging beam for an image point 61 '(61) of the image of the retinal image 2 is entered there in the conjugate plane 4' (4). As soon as the measurement object 6 ', here a pinhole for simplification, in front of the conjugate plane 4' z.3. in position 31 ', a portion of the beam 30 corresponding to the defocusing is intercepted by the measurement object 6', the portion with increasing distance of the measurement object 6 'from the conjugate plane 4' increases. The same applies analogously to all positions 31 "of the test object 6 'behind the conjugate plane 4'. Only if the test object 6 'is in the conjugate plane 4' will ideally all of the light energy contained in the beam 30 pass through the test object 6 '. In this case, a maximum or, in other exemplary embodiments, a minimum of the luminous flux A supplied by the photoeleraent 8 in Fig. 1 is measured behind the measurement object 6 ', which indicates the position of the conjugate plane 4' or 4. The refractive power of the eye 0 may do not change during the displacement along the optical axis 18. With such a method, a temporal change in the refractive power of the eye cannot easily be tracked.

Unless otherwise agreed, the same reference numerals as in FIGS. 1 and 2 are used in the following figures for the same elements.

3 shows an improvement according to the invention with regard to the electrical photo signal evaluation. In this case, measurements are carried out at the same time in front of and behind the conjugate plane 4. For this purpose, a test object 6 'is arranged in front of the conjugate plane 4' and a second test object 6 "is arranged at a fixed distance from 6 'behind the conjugate plane 4". This can be achieved, for example, by a further beam splitter 7 '. A photo element 8, 8 'is arranged behind each measurement object 6', 6 ''. If the object arrangement 19 is displaced along the optical axis 18, the luminous flux of the imaging bundle of rays 30 transmitted by the measurement objects 6 'and 6 "in accordance with FIG. 2 will decrease steadily with increasing distance from the associated conjugate planes 4' and 4". Since the two measuring objects 6 'and 6 "together in fixed distance from each other, cannot both lie in the conjugate plane at the same time. The photo signals A and B will therefore reach their maximum space at different positions of the object arrangement 19. The photo element of the measurement object closer to the conjugate plane delivers the larger photo signal. Only in the event that both measuring objects 6 ', 6 "are equidistant from the associated conjugate planes 4', 4", ie the image of the retina image on both measuring objects 6 ', 6 "is equally defocused and the conjugate plane lies between the objects , The photo signals A and B are the same size. Otherwise, the difference A - B provides a photo signal for a controlled displacement of the object arrangement 19 with the elements 7, 7 '; 6, 6', 6 "; 8, 8 'In the direction of the symmetrical position with respect to the planes 4, 4', 4 ". The information for the direction of displacement is now no longer contained in the phase, but advantageously directly in the amplitude of the photo signals A and B. The gain in signal Noise-to-noise ratio, however, cannot be fully used due to the need for the additional beam splitter 7 '.

This disadvantage can be remedied (FIG. 4) if the object 6 and the measurement object 6 'have a depth dimension in the direction of the optical axis (18), so that at least two parallel sub-objects 61, 62 and 61', 62 'are present. If, for example, as shown in FIG. 4, a photo element 8, 8 'is positioned behind each partial object 61' and 62 'of the measuring object 6', which measures the luminous flux A and B passing through the partial objects 61 ', 62' , the conjugate plane 4, 4 'can again be sought with the clearly directed difference signal A - B, as in the exemplary embodiment in FIG. 3, by moving the object arrangement 19 along the optical axis 18. Then she is reached when both photo elements 8, 8 'deliver the same photo signals A and B. Despite the photo signal improvement achieved in this way, the signal-to-noise ratio is unfavorable for a continuous registration of the refraction state.

A fundamentally different method for the automatic measurement of the refraction is that of the cutting method (skiascopy), e.g. DE-AS 2315135, 2951897 and 3020804 can be used for automatic refractometry. This method does not control the sharpness of the image, but rather determines the width of a bundle of steel in the most general sense, which must be minimal in the conjugate plane. In these methods, the width of the imaging beam is measured successively at different distances from an optometer lens perpendicular to the optical axis and a phase-dependent variable is calculated from the difference signal, with which a position of the smallest or the same beam diameter is sought. This is achieved when the conjugate plane lies in the middle between the two measuring points.

In a certain sense, the cutting method is used in the exemplary embodiment in FIG. 5. In contrast to the methods of the prior art, the cutting edges are not shifted perpendicular to the beam direction, but in the direction of the imaging beams. In the illustrative Fig. 6, the imaging beam 30 becomes a pixel in the conjugate plane 4 'of Fig. 2 by a plane 32' spanned by imaging rays 32 through the pixel 61 'in two partial beams 30a (hatched) and 30b divided above and below this level 32 '. The plane 32 'coincides with the beam 32 in the two-dimensional representation. As shown in Fig. 6, that Beams 30a (30b) in front of and behind the image plane have a different luminous flux. The difference becomes greater the closer the plane 32 'or the beam 32 approaches the edge beams 32 or 33 of the beam 30. The change in the light flux of at least one of these partial beams 30a and 30b at the location of the conjugate plane 4 'is used to locate the conjugate plane 4'. For this purpose, the luminous flux of at least one of the partial beams 30a, 30b along the beam 32 must be measured. For this purpose, as in the exemplary embodiment in FIG. 5, the optical axis of the eye 17 and that of the device 18 can be offset parallel to one another. With two photo elements 8, 8 ', at least one light-dark 61 and one dark-light 62 transition, the luminous flux A or B left by the measurement object 6' is measured. This results in an improvement in the electrical signal-to-noise ratio, which is not measured successively in time, but rather simultaneously, and thus the information is not contained in the phase but in the amplitude of the photo signals A and B. Measurements of the difference signal A - B of the photo elements 8, 8 'when the object arrangement 19 is displaced along the optical axis 18 result in an easily usable difference signal for finding the conjugate plane 4 or 4' for this method. However, the local change in the difference signal, ie. sensitivity, low in the vicinity of the conjugate plane.

A further improvement can be achieved if the method of assessing the sharpness of FIG. 4 is used together with the last-mentioned one of FIG. 5 (FIG. 7). Due to the parallel displacement of the optical axes 17 and 18, the conditions in FIG. 5 and those in FIG. 4 due to the inclined position of the object 6 are realized simultaneously. 8 shows the associated course of the photo signals A and 3 of the two Photo elements 8, 8 '. The photo element 8 mainly provides a photo signal A if the associated object edge 61 'is in front of the conjugate plane 4', and the photo element 8 'conversely then a photo signal B if it is behind the conjugate plane 4'. The overlap C (see FIG. 8) of both photo signals, which causes a steeper zero crossing of the difference D of the photo signals A and B, is brought about by the simultaneous implementation of the additional focus criterion according to FIG. 4.

A further improvement in the signal-to-noise ratio can be achieved both in the method according to FIG. 7 and in those according to FIGS. 3, 4 and 5, even if the beam splitter 7 and the object 6 itself are dispensed with, as in the two The following guide examples (Fig. 7 and Fig. 10) executed as a mirror and thus object 6 and object 6 'coincide in one object. This simultaneously avoids all adjustment problems of the object 6 and the measurement object 6 'which would otherwise arise due to the high demands on the geometric correspondence and the optically correct positioning of the objects 6 and 6'.

FIGS. 9 and 10 each show particularly preferred exemplary embodiments of the invention in which it is possible, while improving the signal-to-noise ratio, to measure the refraction state automatically and continuously by measuring the light reflected on the retina while simultaneously reducing the energetic stress on the eye to be documented using a printer and / or writer. The two exemplary embodiments differ only in the interchanging of

Imaging system Fig. 9a, 10a and measuring system Fig. 9b,

10b. The description can therefore refer to FIG. 9 restrict and be transferred accordingly to Fig. 10.

The optical arrangement of the method consists of an imaging system (in FIGS. 9a and 10a: 1, 60, 61, 62, 10, 11) and a measuring system shown separately for a better overview (in FIGS. 9b, 10b: 1, 6, 61, 62, 12, 13, 8, 8 '), which correspond in the two essential components optometer lens 1 and object 6. For simplification, the parallel displacement of the optical axes of the eye and the exemplary embodiment has been omitted. It is important that the same object is used in both the imaging and the measuring system. For the measurement, only the optometer lens maintains a fixed distance from the eye, while the other bordered components 19 are moved together in the direction of the optical axis or the rear focal length of the optometer lens T is changed accordingly. To determine the refraction of the eye in different main sections, the arrangement can be rotated about the optical axis of the system or, when the method according to FIG. 7 is implemented, about a parallel one. The front main plane 14 of the eye coincides with the front focal plane of the optometer lens 1.

Imaging beam path (FIG. 9a): In the refractometer according to the invention, an object 6, which is at an angle to the optical axis 18, is illuminated from the front with a high-frequency modulated light having a small aperture, with two parallel light-dark 61 and dark-light edges 62. The lighting can be realized in the focal point of a condenser 10 with a light-emitting diode 11, which emits light with a wavelength of 820 nm, for example. The use of light with a small aperture leads via the optometer lens 1 to a small, artificial entrance pupil όes the imaging beam path in the front main plane 14 of the eye. This is also the the subject's view as a faint red glowing slit, which is practically seen sharply in a wide range of vision distances. The accommodation mechanism is therefore not disturbed by the imaging system. To reduce interfering reflections, the optometer lens 1 can be inclined against the optical axis 18 or polarized light can be used for imaging the object on the retina, which is depolarized on the retina, but largely retains its direction of polarization when reflected on reflecting surfaces. These reflections can then be separated from the retinal reflection light in the measuring beam path by a crossed analyzer.

Measuring beam path (FIG. 9b): The retinal image 2 of the object 6 with the parallel edges 61, 62 is reimaged by means of the light reflected on the retina 3 via the optical arrangement of the imaging system, the optometer lens 1, onto the object 6 producing the retinal image 2. At the edges, a portion of the light corresponding to the defocusing and parallel displacement of the optical axes to the width of the beam will not run back into the imaging beam path, but will reach the measuring beam path 12, 13, 8, 8 '(hatched beam bundle). For the measurement of the hidden, defocused portion of the high-frequency modulated energy reflected on the retina 3, each of the two object edges 61, 62 is imaged with a lens 13 via a field lens 12 onto one of the two photo-elements 8, 8 '. Since the front main plane 14 of the eye (approximately the pupil plane) coincides with the focal plane of the optometer lens 1 and the lens 13 lies in the rear focal plane of the lens 12, the lenses 1 and 12 form a telecentric system which is independent of the position of the movable object arrangement 19 the front major plane 14 of the eye maps onto the lens 13. As a result, there is the possibility, in front of the lens 13, of suppressing reflections from the imaging system in the front eye media which may be disturbing. This is not necessary if the optical eye and device axes are offset in parallel. When using polarized light, a crossed analyzer can be interposed between the object 6 and the photo elements 8, 8 ', which filters out the on reflecting lens surfaces.

Determination of the conjugate plane: If the movable arrangement 19, which is characterized by a border, is moved along the optical axis 18 while the distance setting of the eye (accommodation) is fixed, the portion of the reflected light separated at each edge 61, 62 of the object 6 becomes , which leads to the difference signal D of the photo elements 8, 8 ', starting from the optometer lens 1 with increasing distance from it, according to the position of the plane 4 conjugated to the retina 3, first decrease, go through zero in level 4 and then continue drop (see Fig. 8 difference D). The difference signal D passes through the zero value precisely when both object edges 61, 62 are symmetrically equally far away from the conjugate plane 4. So there is a relatively interference-free zero method. The aim of the method is to position the movable arrangement 19 in such a way that both object locations 61 and 62 lie symmetrically to plane 4. For this purpose, the displaceable part with respect to the lens 1 only has to be positioned in a feedback control loop in such a way that the difference signal becomes zero (see FIG. 8).

Electronic photo signal processing (Fig. 9b, 10b): The measuring light is used to improve the signal-to-noise ratio and to filter out extraneous light

(for example the light of a light-emitting LED) modulated at high frequency. The difference signal d of the photo-elements 8, 8 'is amplified in a phase-controlled manner and rectified (lock-in amplifier 20). The differential signal D amplified in this way is fed to a window Schmitt trigger 21 with two trigger thresholds T1 and T2 which can be adjusted symmetrically to 0 volts (see FIG. 8). At the output, this provides signals for standstill, counterclockwise rotation and clockwise rotation of the motor drive 22 of the movable arrangement 19. If the difference signal D lies between the two trigger thresholds T1 and T2, the position remains unchanged. If, on the other hand, both thresholds T1, T2 are exceeded or undershot, the movable arrangement 19 is shifted along the optical axis 18 until the difference signal D lies between the two thresholds T1, T2. The light source 11 is controlled by a function generator 24 which simultaneously controls the lock-in amplifier 20 via a phase shifter 23. The distance 5 (see FIG. 7) from the optometer lens 1 is the measured variable of the refraction value of the eye which arises when a visual object is fixed at a predetermined distance from the eye. The measurement of this distance 5 can be done, for example, when using a stepping motor via the number of steps or can also be measured using a tracking potentiometer.

Observation marks: For the measurement of the refraction state it is known that the examined person has to be offered corresponding observation marks, which stimulate the accommodation, to fix the direction of sight and to adjust the distance of the eye to the respective measurement target (refraction, accommodation course). These are faded in with an achromatic or better a dichroic splitter mirror 15 between eye 0 and optometer lens 1.

Measured value recording: The refraction values are on Output of the electronics as analog or digital voltage values. They are recorded, printed out and charged on using standard recorders. By rotating the arrangement (1, 19) around the optical axis of the eye, when using linear objects 6 (slit or grid), the refraction values for various main sections result, from which the astigmatism with its axis position is then determined. This can be printed out numerically in the form of customary characteristic values or can be output in graphic form because of the good temporal resolution of the method according to the invention, which in addition to the characteristic values also allows a statement about the regularity of an astigmatism. Because of the good temporal resolving power of the method according to the invention, the temporal course of the distance adjustment of the eye can be recorded parallel to the course of the distance of the visual object (refraction time curves) and thereby disturbances of the accommodation system can be found before they are ignored when measuring static equilibrium settings of time.

Claims

1 .

Method for determining the refraction, in particular the time course of the distance setting of the human eye (0), briefly the refraction zastany, in which an object (6) located in the object space (5) is directed onto an optometer lens system (1), briefly an optometer lens Retina (3) of the eye (0) is imaged and in which the retinal image (2) of this object (6) in turn via the optometer lens (1) in the object tract (5) back into a conjugate to the retina (3) of the refractive condition of the eye (0) and the refractive power of the optometer lens (1) certain image plane (4, 4 ', 4 ") is reproduced, the distance (9) from the optometer lens (1) is a measure of the refraction state, with the determination of the distance (9 ) the beam of rays coming from the retinal image (2) is analyzed at a different distance from the image plane (3) with a measurement object (6 ', 6 "),

characterized in that the image sharpness in the bundle of rays is at the same time at at least two locally different positions (31 ', 31, 31 ") displaced in the direction of the optical axis (18) in the object area (5) (see FIGS. 2, 3 , 4).

Second

Method for determining the refraction, in particular the time course of the distance setting of the human eye (0), briefly the refraction state, in which an object (6) located in the object space (5) is directed onto an optometer lens system (1), briefly an optometer lens Retina (3) of the eye (0) is imaged and in which the retinal image (2) of this object (6) in turn via the optometer lens (1) in the object tract (5) back into a conjugate to the retina (3) of the refractive condition of the eye (0) and the Refractive power of the optometer lens (1), certain image plane (4, 4 ', 4 ") is reproduced, the distance (9) of which from the optometer lens (1) is a measure of the refractive condition, the distance (9) from the retinal image ( 2) coming bundles of rays are examined at different distances from the 3ilde'oene (3) with a measurement object (6 ', 6 "),

characterized in that the imaging ray bundle (40) of the image (2) reflected on the retina (3) by a surface (32) spanned by rays (32) of the ray bundle (40) passing through an image point (41) are divided into two light components (30a, 30b), of which at least the light energy of one component (30a or 30b) is measured at different positions (31 ', 31, 31 ") in the direction of the optical axis (see Fig. 5, 6).

3rd

Method for determining the refraction, in particular the temporal course of the distance setting of the human eye (0), briefly the refraction state, in which an object (6) located in the object space (5) via an optometer lens system (1), briefly an optometer lens, onto which Retina (3) of the eye (0) is imaged and in which the retinal image (2) of this object (6) in turn via the optometer lens (1) in the object tract (5) back into a conjugate to the retina (3) of the refractive condition of the eye (0) and the refractive power of the optometer lens (1) certain image plane (4, 4 ', 4 ") is reproduced, the distance (9) from the optometer lens (1) is a measure of the refraction state, with the determination of the distance (9 ) the beam of rays coming from the retinal image (2) is analyzed at a different distance from the image plane (3) with a measurement object (6 ', 6 "), characterized in that the image sharpness in the bundle of rays is measured simultaneously in at least two locally different positions (31 ', 31, 31 ") displaced in the direction of the optical axis 18 in the object space (5) and in addition the imaging bundle of rays (40) on the retina (3) reflected image (2) is divided into two light components (30a, 30b) by a surface (32 ') that is spanned by rays (32) of the beam (40) that pass through a pixel (41), of which at least the light energy of a portion (30a or 30b) is measured at different positions (31 ', 31, 31 ") in the direction of the optical axis (see FIGS. 5, 7, 9, 10).

4th

Method according to claim 2, 3 or 4, characterized in that the optical axes of the arrangement (1, 19) and the examined eye (0) are offset parallel to one another.

5th

Method according to Claim 2, 3, 4 or 5, characterized in that both light components (30a, 30b) are measured simultaneously in the reflected beam (40).

6th

Method according to Claim 2 or 3, characterized in that in the reflected beam (40) both light components (30a, 30b) are preferably measured simultaneously at two different positions (31 ', 31, 31 ") at a constant distance from one another.

7.

Method according to Claim 2, 3, 4, 5 or 6, characterized in that the surface (32 ') separating the two light components (30a, 30b) is laid such that the surface is as close as possible to the edge (33 or 34) of the . Lies (30) and thus one of the two light components (30a, 30b) is negligibly small compared to the other.

8th.

Method according to one or more of the preceding claims, characterized in that along the optical axis

(18) the difference between the two light components (30a, 30b) of the beam (30) is formed and the measurement object

(6) is shifted along the optical axis (18) until the difference becomes zero.

9th

Method according to one or more of the preceding claims, characterized in that the light components of the beam of rays separated from edges (61, 62) of the measurement object (6 ') are measured separately.

10th

Method according to one or more of the preceding claims, characterized in that the difference between the light components separated at the different object edges (61, 62) is calculated to form a difference signal (A - B).

11th

Method according to one or more of the preceding claims, characterized in that the difference signal (A - B) supplies a photo signal for automatically finding the image plane (4).

12th

Method according to one or more of the preceding

Claims characterized in that the measuring system (1,

19) mechanically or optically around the optical axis of the

Measuring system (17) or a parallel to it can be rotated.

13th

Device for carrying out the method according to one or more of the preceding claims, characterized in that the object (6) has at least one light-dark and one dark-light edge, which are used separately for the measurement.

14th

Device for carrying out the method according to one or more of the preceding claims, characterized in that the edges (61, 62) of the object (6) are parallel.

15th

Device for carrying out the method according to one or more of the preceding claims, characterized in that the object can consist of mirrors.

16th

Device for carrying out the method according to one or more of the preceding claims, characterized in that the object (6) and the measurement object (6 ') coincide (Fig. 9, 10)

17th

Device for carrying out the method according to one or more of the preceding claims, characterized in that high-frequency modulated light is used to image the object (6) on the retina (3).

18th

Device for carrying out the method according to one or more of the preceding claims, characterized in that for the imaging of the object (6) preferably infrared light is used.

19th

Device for carrying out the method according to one or more of the preceding claims, characterized in that the aperture of the imaging system is small.

20th

Device for carrying out the method according to one or more of the preceding claims, characterized in that the differential signal (A - B) supplied by the photoelements (8, 8 ') is amplified with low noise using a lock-in amplifier.

21st

Device for carrying out the method according to one or more of the preceding claims, characterized in that the object (6) can consist of photosensitive elements.

22.

Device for carrying out the method according to one or more of the preceding claims, characterized in that the object (6) has a depth dimension in the direction of the optical axis (18).