Refractometer for the automatic objective determination of the refractive condition of an eye

The invention contemplates a refractometer that images a test mark on the retina and is automatic in its determination of both refractive-power and astigmatism parameters, for the eye under examination. An imaging lens serves both (1) to project the test mark to the retina and (2) to convey that image for refocus external to the eye, to a scanner having the ability to digitally establish the eye's focal length. In the process of establishing the eye's focal length, the instrument automatically positions the imaging lens to best serve infinity viewing by the eye, and a prism then rotates the test mark while the scanner photoelectrically tracks the refocused image for the angles at which response is at a maximum, thus establishing astigmatism parameters. The instrument provides selective direct indication or print-out of the automatically determined parametric values.

The present invention relates to a refractometer for the automatic 
objective determination of the refractive condition of an eye wherein a 
test mark is focused by an optical image-producing system on the retina of 
the eye to be examined, and wherein light reflected from the retina, after 
passing through the same image-producing system and before reaching the 
test mark, is mirror-deflected toward a photoelectric receiver having a 
scanning device. 
Such a refractometer is known from West German Patent Application No. 
1,955,859. In said refractometer, the signal produced by the photoelectric 
receiver is used directly to vary the refractive power of an optical 
system arranged in front of the eye being examined, until said signal 
disappears. Until the present time, there has been practically no optical 
system whose refractive power can be changed continuously and with the 
necessary precision; therefore, one has been forced, in practice, to have 
recourse to intermittently operating systems, namely so-called phoropters. 
In these systems, lenses of different power of refraction are brought, one 
after the other, in front of the eye being examined; however, this 
technique necessarily means a dark phase upon each change of lens, and 
such dark phases interfere with the accommodation of the eye being 
examined. 
The known refractometer also permits the determination of astigmatism, but 
this is a relatively cumbersome procedure. 
Automatically operating refractometers are also known in which a 
displaceable test mark is focused in an image-forming beam on the retina 
of the eye being examined, and in which the retina image is focused on a 
similar test mark in a separate observation beam. Behind said test mark 
there is arranged a photoelectric receiver, and the marks in the imaging 
and observation beams are displaced synchronously until the receiver 
signal reaches a maximum. In order to be able to measure, in the course of 
a determination of astigmatism, in different principal planes of the eye, 
the two test marks must be turned synchronously. Since the synchronous 
displacement and rotation of elements in separate light beams results in 
large problems of synchronism, the optical and mechanical expense involved 
in such refractometers is very great. 
An object of the present invention is to provide a refractometer for the 
automatic objective determination of the condition of refraction of an 
eye, which makes possible, without great expense, both an undisturbed 
determination of the spherical defect of the eye and of the antigmatism. 
Further, it is an object to provide such a refractometer in which the 
entire pupil of the eye is used for refractioning, and which makes it 
possible to eliminate disturbing reflection signals. 
These objects are achieved, in accordance with the invention, by the 
combination of the following features: 
(a) the image-producing system contains a first lens and a second lens, 
between which there is a parallel-ray path, and these lenses produce a 
first intermediate image of the test mark; in addition, a third lens 
cooperates with the lens of the eye to produce a second intermediate image 
of the test mark on the retina of the eye; 
(b) between the first and second lenses there is provided a partially 
transmitting mirror and, in the light path of the latter, a lens which 
corresponds to the first lens of the image-producing system in such manner 
that light reflected from the retina produces a third intermediate image 
of the test mark outside the illumination beam; 
(c) at the locus of this third intermediate image, there is arranged a 
scanning device which, together with a photoelectric receiver arranged 
behind same, produces a signal which is modulated in accordance with the 
position of the intermediate image; 
(d) using the output of the photoelectric receiver, there is provided 
circuitry which converts the modulated signal into a control signal that 
is proportional to the deviation of the third intermediate image from the 
theoretically proper position; 
(e) one of the lenses of the image-producing system is displaceable by 
means of a positioning motor which is operated by the control signal. 
The eye to be examined is fixed with respect to the direction of view and 
accommodation, as by looking at a vision-testing chart. For the 
measurement itself, infrared light, which is not observed by the eye, is 
used in known fashion. 
In the new refractometer, all elements necessary for the formation of an 
image are common to the formation of the image and to the observation. For 
measurement, one of the lenses, preferably the second lens of the 
image-producing system, is displaced by the motor until the control signal 
fed to the motor achieves a value of zero. At this moment, the second 
intermediate image of the test mark is produced on the retina of the eye, 
and the third image which has been segregated is conjugate to the test 
mark. The new refractometer thus operates in accordance with the very 
precise and error-free principle of null-finding. 
The new refractometer permits of a simple determination of astigmatism. For 
this purpose, a reversing prism, preferably a Dove prism, is mounted for 
rotation about the optical axis in the parallel beam between the first and 
second lenses of the image-producing system. Upon rotation of this prism, 
the intermediate image of the test mark is rotated. Since the observation 
beam also passes through the prism, the position of the test image at the 
place of the scanning device remains unchanged, regardless of the angle of 
rotation of the prism. 
During rotation of the reversing prism, and if astigmatism is present, the 
motor (actuated by the control signal) will endeavor constantly to 
displace the lens of the image-producing system in such manner that a 
state of balance is maintained. When the point of reversion in the 
displacement of the lens is reached, this point corresponds to the 
principal plane. A reading is made (a) of diopters and (b) of the axial 
position indicated by the angle of rotation of the prism. Thereupon, the 
split image is turned 90.degree. by the prism, and the measurement values 
determined in this position are read off. It is, of course, also possible 
to operate automatically with a continuously rotating prism and to use 
electronic-circuitry to identify (a) the angle at which said reversion 
occurs and (b) the angle which is 90.degree. displaced therefrom. 
With the new refractometer, it is advisable to provide, between the 
deflection mirror (which serves to single-out the reflected light) and the 
photoelectric receiver, a scanning device which consists of two 
motor-driven rotating semi-circular screen disks, which are axially spaced 
apart from each other and displaced 180.degree. with respect to each 
other, the screen disks being so arranged that in the balanced condition 
the third intermediate image of the test mark lies at the axial midpoint 
between the two screen disks. When a slit-shaped test image is used, each 
screen disk is advisedly so devised that it contains screen slits whose 
sides correspond to that of the third intermediate image and which follow 
each other in a 1:1 scanning ratio. The reflection coming from the cornea 
of the eye being examined then extends, on each screen disk, over a 
plurality of screen slits which, together with the solid (unslitted) 
portions in between, form a 50% filter so that light modulation 
attributable to this disturbing reflection is practically zero. The same 
is true also of other disturbing reflections from the optical beam. The 
resultant noise signal may be readily separated from the periodic useful 
signal by electronic means, and filters may be provided as needed to 
remove harmonics of the disturbing signal. 
In the new refractometer, only extremely little light is reflected by the 
retina of the eye being examined, so that it is essentially a question of 
excluding, in addition to the corneal reflection, also the reflections 
produced on the surfaces of the optical elements in the ray path. For this 
purpose, it is advantageous to arrange the second and third lenses of the 
illuminating beam at an inclination to the optical axis. If these lenses 
form an angle of, for instance, 8.degree. with the optical axis of the 
refractometer, then the reflections produced on the lens surfaces will be 
deflected out of the ray path and will no longer form a disturbing signal. 
Refraction values determined by the new refractometer may be used for the 
direct or indirect control of a subsequent device for the subjective 
determination of spectacle lenses, for example, a phoropter. Such a 
digitally controllable phoropter can therefore be connected "on-line" with 
the new refractometer, whereby automatically determined values are 
automatically set. It is also possible to operate "off-line", in which 
case the refractometer may deliver the automatically determined measured 
values via a data-bearing device, for instance a punched card, which is 
introduced into the phoropter before the subsequent subjective 
determination of the initial adjustment.

In FIG. 1, 1 is a source of light which, via a condenser 2, illuminates a 
test mark 3 in the form of a slit. A filter 4 arranged in front of the 
test mark 3 permits the passage only of infrared light. The test mark 3 is 
focused at infinity by a collimating lens 5; and a second such lens 6 
produces a first intermediate image 3' of the test mark. In the 
parallel-light region between the lenses 5 and 6, a reversing or Dove 
prism 7 is positioned for rotation about the optical axis, and a partially 
transmitting deflection mirror 13 is shown between lens 5 and prism 7. 
In FIG. 1, the focusing elements have been shown as simple lenses for ease 
of illustration. Actually, they are lens systems, so that the word "lens" 
as used below should be understood to additionally include lens systems. 
In front of the eye 10 to be examined, a lens 8 is arranged, fixed in 
space, in such a manner that its image-side focal point F' coincides with 
the object-side principal plane of the eye lens 9. This principal plane, 
in the case of an eye which is accommodated to infinity, lies about 2 mm 
behind the corneal vertex. In the case of an eye with normal vision, the 
lens 8, in cooperation with the eye lens 9, produces a second intermediate 
image 3" of the test mark on the retina of the eye. 
In front of the eye 10 there is arranged a partially transmitting mirror 11 
via which light in the visible wavelength region is reflected into the eye 
from a target, for instance, a vision-testing chart. The eye is thus fixed 
with respect to the direction of view, as is indicated by the arrow 12; at 
the same time, accommodation of the eye is achieved by the target which is 
mirrored therein. 
Between lenses 5 and 6 of the image-producing system, the partially 
transmitting mirror 13 and an associated lens 5' (corresponding to the 
lens 5) focus light reflected from the retina of the eye 10 at a third 
intermediate test-mark image 3"', outside the illumination beam. At the 
locus of intermediate image 3"', there is provided a scanning device 
consisting of two semicircular screening disks 14-15 which are axially 
spaced apart and angularly offset 180.degree. with respect to each other; 
disks 14-15 are rotated by motor means 16 and are so positioned that in 
balanced condition the intermediate image 3"' lies at the axial midpoint 
between the two screening disks 14-15. Light passing through these 
screening disks falls on a photoelectric receiver 17. 
As can be noted from FIG. 2, each screening disk 14-15 consists of a 
transmission screen in the form of screen slits which extend, in the 
example shown, over half a circle. The other half of the circle of each 
screening disk is fully transparent. It is also possible to develop each 
screening disk such that the transmission screen extends only over a 
quarter of a circle and to make the remaining quarter of a circle opaque. 
In this way, dark intervals are obtained in interlace with scanning 
intervals, and the dark intervals may be used in the electronic section 
for accommodation of a signal or for like purposes. 
When the eye 10 which is to be examined is not of normal vision, the 
intermediate image 3" is not produced on the retina, and the third 
intermediate image 3"' does not lie precisely at the midpoint between the 
two screening disks 14-15, in which case the signal supplied by the 
photoreceiver 17 is as illustratively shown in FIG. 3a. Such a signal is 
characterized by a frequency f.sub.2 which is determined by the screen 
constant and the rotational speed of the disks 14-15. This signal is 
further characterized by amplitude-modulation at the frequency f.sub.1, 
wherein the signal section 14', for instance, is produced when light is 
interrupted by the screening disk 14, and wherein the signal section 15' 
is produced when light is interrupted by the screening disk 15. The signal 
of FIG. 3a will thus be seen to be modulated in accordance with the 
instantaneous position of the intermediate image 3"', and to the extent 
that such position is axially offset from the midpoint between disks 
14-15. The signal output of the photoreceiver 17 is supplied to an 
amplifier 18 and thence to a filter 19 which passes only the frequency 
f.sub.1. This actual measurement signal is then phase-sensitively 
rectified at 20, using the synchronizing or phase-reference signal from a 
timing-pulse generator 24, and is thus converted into a control signal 
which is proportional to the instantaneous deviation of the third 
intermediate image 3"' from the theoretically proper midpoint position. 
This control signal is amplified at 21 and is then supplied to a 
positioning motor 22 which displaces the second lens 6 of the 
image-producing system in the direction of the optical axis. In the form 
shown, a digital generator 23 connected to the shaft of motor 22 produces 
a digital signal which is proportional to the instantaneous position of 
lens 6. 
Depending upon the optical arrangement selected, the position of lens 6 is 
proportional to the diopter value of the eye 10. With a lens 8 of focal 
length f=50 mm, a diopter range of .+-.20 diopters can be covered by the 
lens 6. 
In case of an eye 10 with defective vision, a control signal is produced 
which displaces the lens 6 until the intermediate image 3" is produced on 
the retina of the eye. At this moment, the third intermediate image 3"' is 
produced at the midpoint between the two screening disks 14-15, and the 
signal produced by the photoreceiver 17 has the shape shown in FIG. 3b. As 
can be noted, the control signal of frequency f.sub.1 has disappeared, and 
motor 22 thus comes to rest. The corresponding digital signal (at 23) is 
now fed via a counter 34 and a memory device 25 to an output unit 26 which 
serves to indicate, print out, or deliver an encoded data-bearing device, 
such as a suitably punched card. 
The reversing prism 7 is rotatable about the optical axis by gear means 
27-28, the same being driven by a motor 29 having a digital position 
generator 30 coupled thereto. Generator 30 produces a signal which is 
proportional to the instantaneous rotary position of prism 7; when a 
balance position is achieved, the digital output of generator 30 is fed 
via a counter 31 and a memory device 32 to the output unit 26. 
For the determination of refraction, the reversing prism 7 is first of all 
held in its starting position. As soon as the lens 6 has come to rest, a 
logic circuit 33 is caused, via counter 34, to operate the motor 29, thus 
rotating reversing prism 7. If the eye 10 to be examined suffers from 
astigmatism, the balancing electronic system endeavors, upon rotation of 
prism 7, to retain the condition of balance, which is effected by 
corresponding movement of the lens 6. In this connection, the diopter 
reading changes continuously, and the logic circuit 33 checks when the 
point of reversal of this reading is reached. This position corresponds to 
a principal plane. Under control from the logic circuit 33, the axial 
position is then indicated via counter 31, and the diopter value is 
indicated by counter 34. Thereupon, the slit image 3' is displaced 
90.degree. by the prism 7, and lens 6 is driven by means 17-20-22 until a 
new balance is obtained. The values for axis and diopter obtained at this 
new balance position are also included in the output reading at 26. 
Instead of the step-by-step manner of operation described, when examining 
for astigmatism, it is also possible to use a continuously rotating prism 
7, output readings being keyed by operation of logic circuit 33, upon 
noted occurrence of reversal of the direction of diopter change. In this 
case diopter value and axial position (of lens 6) are included in the 
output indication at 26, for each occurrence of a point of reversal of the 
diopter indication. 
With the refractometer shown, it is important to eliminate the 
optical-element reflections produced in the ray path, e.g., surface 
reflections between optical elements. For this purpose, the lenses 6 and 8 
are shown inclined to the optical axis of the illuminating system. These 
lenses are so designed that, despite their oblique position, errors in 
imaging are avoided. In the case of the oblique position shown for lens 8, 
its vertical axis 8' forms with the optical axis an angle .alpha. which 
has a value of, for instance, 8.degree.. Thus, the result is obtained that 
reflections which are produced on the surfaces of the lens 8 are deflected 
outwardly, i.e., such reflections cannot be passed to the screening disks 
14-15. 
However, reflection from the cornea of the eye 11 does reach the screening 
disks 14-15. But such reflection has such a position and size that, as 
already indicated above, it produces only an easily separated d-c 
component of the light reaching disks 14-15. 
FIG. 1 indicates another way of separating the useful signal from the 
reflection signals of the optical system. As shown in dashed lines, an 
interrupter disk 36 is provided for this purpose, and is driven by a motor 
35 to periodically interrupt the light which arrives at the eye. The 
frequency of interruption is selected to be greater than the scanning 
frequency which characterizes the screening disks 14-15. One thus obtains 
an alternating signal of high frequency which comes from the eye 10 and 
which is superimposed on the low-frequency scanning signal. It is very 
simple, by electrical means, for instance by a second frequency filter 
interposed behind the frequency filter 19, to filter out the 
high-frequency measurement signal and thus to obtain a control signal 
which is associated with the retinal image and has but one undesired 
component, namely that associated with corneal reflection. The corneal 
reflection, as already mentioned, may be eliminated by selecting the 
periodicity of the screens 14-15 in accordance with the size of the 
reflection image.