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Description  |
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This invention relates to a visual stimulator which presents a target to
the eye and allows independent or concurrent movement of the target in
three dimensions in order to stimulate or compensate eye movement and
accommodation. That is, using the stimulator, the target can be given the
appearance to the eye of moving horizontally, vertically or toward and
away from the eye, either individually or simultaneously in any
combination.
Detection of defects in the human visual system and defining proper
correction of those defects often require movement of a target in specific
ways to stimulate certain types of eye movements in the subject (patient).
Response of a subject's eye to such target movements is also useful in
developing an understanding of the response of the human visual system to
stimuli and in diagnosing some physiological abnormalities.
Hewitt D. Crane, the present inventor, and Tom N. Cornsweet developed a
visual focus stimulator which moves a target image toward and away from
the subject's eye and thus stimulates focusing. Highly useful for
observing powers of accommodation, this stimulator is described in
"Ocular-Focus Stimulator" by Hewitt D. Crane and Tom N. Cornsweet, JOSA,
v. 60, n. 4 (Apr. 1970), p. 577, "Visual Focus Stimulator Aids in Study of
the Eye's Focusing Action", NASA Tech Brief 70-10568 (Nov. 1970), and also
in connection with FIG. 5 of "Three-Dimensional Visual Stimulus Deflector"
by H. D. Crane and Michael R. Clark, App. Optics, v. 17, n. 5 (Mar. 1,
1978), pp. 706-714 (see specifically pp. 708-709). This focusing
stimulator, however, is only one dimensional. The present three
dimensional stimulator is also described in the Applied Optics article,
and the subject matter of the article is specifically incorporated herein
by reference. The instrument provided by the present invention is the only
visual stimulus deflector known to the inventor at the present time for
stimulating eye movement in three dimensions.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a visual deflector for testing
the human visual system, which deflector can move the visual stimulus
(target) horizontally and vertically as well as in apparent distance.
In visual testing, as in any scientific experiment, it is often important
to be able to change one parameter at a time in order to be able to
determine the effect of the individual parameters. Therefore, it is
another object of the invention to provide such a deflector wherein the
stimulus movements can be made individually and independently in each
dimension.
Further, it is an object of this invention to provide such a deflector
wherein the target image presented to the eye is undistorted by the
deflection optics.
It is still another object of this invention to provide such a deflector
wherein the brightness and visual angle subtended by the visual pattern or
object remain fixed.
In carrying out the invention, a pair of spaced apart, rotatable mirrors is
positioned serially in the path of a target image that is projected to the
eye of a subject, and a nominally undistorted image of the eye, preferably
a unity magnification image, is produced at each mirror, with the center
of rotation of the eye images nominally located at the axes of rotation of
the mirrors. The axes of rotation of the mirrors are oriented to sweep the
image in directions perpendicular to each other at the eye in order to
produce deflection in a plane perpendicular to the optical axes of the
eye. The mirror closest to the target, along with at least part of the
adjacent image forming means and at least an input image of the target, is
moved axially along the path of the projected image between the two spaced
apart, rotatable mirrors to produce image "deflection" along the optical
axis of the eye (accommodation).
The novel features which are believed to be characteristic of the invention
are set forth with particularity in the appended claims.
The invention itself, however, both as to its organization and method of
operation, together with further objects and advantages thereof, may best
be understood by reference to the following description taken in
connection with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1, the single FIGURE, is a perspective view and partially schematic
diagram, illustrating the elements of a visual stimulus deflector
according to a preferred embodiment of the invention.
DESCRIPTION OF PREFERRED EMBODIMENT
The overall three dimensional visual stimulus deflector illustrated is for
the particularly stringent application where it is necessary to move a
target in specific ways to stimulate certain types of eye movements in the
subject (patient). The instrument described here can move the visual
stimulus horizontally and vertically, as well as stimulate accommodation
(focus) by altering the optical distance of the target from the subject,
all without distorting the target image or affecting its brightness
(intensity) or size. Horizontal and vertical movements and focus change
are accomplished independently. Independent, three dimensional stimulation
of both eyes is achieved by two such devices aligned side by side.
It is to be understood that this particular application is shown and
described precisely because it has most exacting requirements. Many other
applications of the principles involved are extant. For example, in some
cases it is desirable to have an image or target (stimulus) follow the eye
movements, and this deflector in cooperation with an eye tracker will
provide such deflections. The use of basic elements and principles of the
stimulus deflector is found in a beam stabilizer which forms part of a
stabilized visual system and is disclosed and claimed in a copending
patent application, Ser. No. 14,746, entitled STABILIZED VISUAL SYSTEM,
filed Feb. 23, 1979 in the name of the present inventor and assigned to
the assignee of the present application.
Referring specifically to the single figure, the subject's eye 10 is
positioned in front of the visual deflector system to receive the target
light 11, which is projected through the deflector from any two or three
dimensional target, such as the illustrated target station or screen 12.
The target image station 12, for the application illustrated here, remains
fixed, and the apparent target movement is accomplished by the deflector
system. Vertical and horizontal deflection of the image (as presented by
the beam 11) at the eye 10 is based upon forming an essentially
undistorted image (here a nominally unity magnification image) of the eye
10 with its center of rotation located nominally on the horizontal axis of
rotation of a rotatably mounted vertical position deflection mirror 14 and
another on the vertical axis of rotation of a rotatably mounted horizontal
position deflection mirror 16. Note that the images of the eye 10 are
shown in phantom at both the vertical and horizontal position deflection
mirrors 14 and 16, respectively. Rotation of the vertical position
deflection mirror 14 about its horizontal axis produces vertical movement
of the visual field at the eye 10 without causing image distortion.
Rotation of the horizontal position deflection mirror about its vertical
axis produces horizontal movement of the visual field at the eye 10, again
without image distortion.
The image of the eye 10 at vertical position deflection mirror 14 is formed
by a pair of relay lenses 18 and 20, separated by the sum of their focal
lengths and located nominally a focal length from the eye 10 and the axis
of rotation of the vertical position deflection mirror 14, respectively.
For the present application, the relay lenses 18 and 20 actually used are
identical (50 mm), and this is preferred for practical reasons. A pair of
identical lenses, separated by the sum of their focal lengths and
similarly positioned, leads to a second optically conjugate, nominally
undistorted unity magnification image. In order to redirect the beam 11 by
90.degree. and give it a convenient direction relative to the eye 10, a
stationary folding mirror 22 is provided between the relay lens pair 18
and 22 (at a 45.degree. angle). The location of the folding mirror,
although placed approximately a focal length from each of the relay lenses
18 and 22, is not critical and does not alter the optical characteristics
of the system.
The undistorted unity magnification optically conjugate image of the eye 10
formed at the horizontal position deflection mirror 16 is provided by
including a second pair of identical relay lenses 24 and 26, separated by
the sum of their focal lengths and respectively located nominally a focal
length from the axis of rotation of vertical position deflection mirror 14
and horizontal position deflection mirror 16. Thus, the unity
magnification image of eye 10, which has its center of rotation nominally
coincident with the axis of rotation of vertical position deflection
mirror 14, is "relayed" to a corresponding position on the horizontal
position deflection mirror 16, and the axes of rotation of both mirrors
are nominally conjugate to the center of rotation of the eye 10.
The image from the target station 12 enters the deflection system by way of
an input image deflecting mirror 28 which redirects the input image beam
11 by 90.degree. and presents the image to the horizontal position
deflection mirror 16, and consequently, to the eye 10 of the subject. Note
here that the input image deflecting mirror 28, horizontal position
deflection mirror 16 and the next closest lens 26 (nominally a focal
length away) of the second relay lens pair (24 and 26) are all mounted on
a focusing, or accommodation, platform 30 which is movable along the axis
of the beam 11 from the target station. The reason for this arrangement is
explained below in connection with accommodation, or optical focus. First,
however, consider deflection of the visual field.
As previously pointed out, the axes of rotation of horizontal and vertical
position deflecting mirrors 16 and 14, respectively, are nominally
conjugate to the center of rotation of the eye 16. Thus, pure horizontal
and vertical movement of the visual field (image 11) beam at the eye 16 is
achieved without translation artifact, image distortion, or image size
change by rotating the mirrors about their respective axes. In the
embodiment illustrated, a pair of servo motors 32 and 34 is provided to
rotate the respective vertical and horizontal position deflection mirrors
about their axes, although adjustments may be made manually. As shown
here, a cursor 36 having a joy stick 38 controls the vertical and
horizontal driving servo motors 32 and 34, respectively. A pure up and
down movement of the joy stick 38 (up and down as drawn) produces an
output on vertical drive servo motor 32 via the cirucit to position the
image field vertically. In similar fashion, the image field is positioned
horizontally by horizontal deflection mirror 16 in response to a signal
produced at its driving servo motor 34 via its circuit 42 when the joy
stick 38 is moved sideways (in drawing). Since the unity magnification
images of the eye 10 at vertical and horizontal position deflection
mirrors 14 and 16 are optically conjugate, the pupil plane of the eye 10
has a conjugate plane (labeled AP for artificial pupil) at both of the
position deflection mirrors 14 and 16. If a stop (not shown) smaller than
the natural pupil is placed in the artificial pupil plane AP, it becomes
the limiting aperture of the system; consequently, the effects of natural
pupil changes are eliminated. Cylindrical and spherical correction lenses
for each subject can be placed in a trial lens holder (also not shown)
located in front of the aperture of the stop, and since plane AP is
conjugate with the pupil plane of the eye, correction lenses placed near
the artificial pupil plane AP have the same visual effect as if placed
directly at the spectacle plane.
Simultaneous axial movement of the second lens 26 (of second relay lens
pair 24 and 26), horizontal deflection mirror 16 and image deflecting
mirror 28 adjusts the spherical power of the system without change in
image position, size or brightness. This movement is accomplished by
moving focus adjusting platform 30, to which all of these elements are
affixed. Spherical power, in diopters, is linearly related to the axial
position of the moveable carriage, which can be adjusted manually or, as
illustrated, driven by a third (focus adjust) servo motor 44.
In the drawing the focus adjust servo motor 44, like the horizontal and
vertical deflection servo motors 34 and 32, respectively, is driven from
the ouptut of cursor 36. In this case, vertical pressure on the joy stick
38 of the cursor 36 produces a signal on its "focus adjust" output circuit
46. Thus, by proper movement of the joy stick 38, the image field can be
moved independently or simultaneously in any one of three dimensions. It
will also be appreciated that other arrangements can be made for driving
the image field. For example, by a simple modification the stimulus
pattern can be programmed by computer to provide any sequence of
deflections.
In practice, it has been found desirable to use good quality (e.g., camera
quality) lenses. It is believed that optical proofs and derivations are
out of character here; they are found, however, in the Crane and Clark
Applied Optics paper, op. cit.
It will be recognized that the objects of this invention have been carried
out by considering the foregoing discussion along with the following
characteristics of a system which has been built using the principles
described. That is, a three dimensional visual stimulus deflector has been
built which allows a subject to view any stimulus pattern or object
through it, and the pattern (up to 25.degree. in diameter) can be moved
over a range of 40.degree. horizontally and 30.degree. vertically. The
optical distance of the object being viewed can be changed over a 15
diopter range while the brightness and visual angle subtended by the
object remain fixed. Further, the observer can view the object through a
pupil of any desired shape and transmittance. Horizontal and vertical
movements are independent, with time delays of 1 msec and a response range
from dc to 200 Hz. Focus change is independent of lateral field motion and
has a time delay of 12 msec and a maximum slewing rate of approximately 40
diopters/sec.
While particular embodiments of the invention have been shown and described
here, it will, of course, be understood that the invention is not limited
to these particular arrangements, since many modifications, both in the
circuit arrangements and in the instrumentalities employed, may be made.
For example, the order of the vertical and horizontal deflection mirrors
14 and 16 can be reversed, and other arrangements for driving the mirrors
and focus adjust station may be readily made without departing from the
scope of this invention. It is contemplated that the appended claims will
cover any such modifications as fall within the true spirit and scope of
this invention.
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