Note: Descriptions are shown in the official language in which they were submitted.
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EYE-POSITION SENSOR
This invention rela-tes to an eye-position sensor and more
particularly to one that may be used in an eye-activated optical
transducer which Functions as a keyboard emulator for controlling such
devices as a printer, monitor, or telephone modified with an
eye-activated dialler and voice synthesizer.
Back~round of -the Invention
Certain severely handicapped people can only
reliably communicate through their eye movements. To facilitate
this communication, typewri-ting devices have been developed which
utilize eye-position control. An example of such a system is
described in an article entitled "Eye-Controlled Communication Aids"
by J.H. ten Kate et al., Medical Progress Through Technology 8:1-21,
Springer-Verlag 1980. Eye tracking or positioning devices have
also been described in United States Patent No. 3,507,988 entitled
"Narrow-Band, Single-Observer, Television Apparatus" by
~illiam S. ~lolmes, issued April 21, 1970i United States Patent
No. 3,72~,932 enti-tled "Eye Tracker And Method" by Tom N.Cornsweet et al
issued April 3, 1973. Reference to eye tracking devices is also made in
United States Patent No. 4,513,317 entitled "Retinally Stabilized
Differential Resolution Television Display" by Carl F. Ruoff, issued
April 23, 1985. In general, these devices use a photo-sensor which
detects the change in diffused or specular infrared reflections from the
eye as it moves. A limiting factor in the detection of eye movements for
communication purposes is the non-linearity of the transducer. As a
result, eye tracking calibration has often been diFficult, time consuming
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and inconsistent.
S temen-t of the Invention_
It has been discovered tha-t a substantially linear analog
signal output can be obtained From an eye-position sensor by utilizing
optical componentry including a point source of light and a spatial
filter between the eye and a quadrantal array of contiguous sensors so as
to cast a substantially rectangular pattern of light on the sensors -from
the specularly reflected infrared light. With such an arrangement the
eye acts as a convex mirror to reflect an image o-f the point source onto
the sensor rather than an image of the eye itself.
In accordance with the present invention there is
provided an eye-position sensor which provides signals representative
of the X and Y coordinates of light reflected from the cornea of the
eye. The sensor comprises a substantially point source of light for
illuminating the eye and a light sensor which includes a quadrantal array
of contiguous photo detectors each of which generates a signal which is
proportional to its illuminated area. Also included is a lens for
collimating the light from the point source of light reflected from the
cornea of the eye onto the quadrantal array of contiguous photo
detectors. The sensor also includes a spatial filter for shaping the
distribution of light reflected from the eye to provide a rectangular
pattern of light on the array. The spatial filter is oriented so that
each corner of the rectangular pattern lies in a different quadrant of
the array. The filter is disposed so that movement of the eye changes
the position of the pattern illuminating the array. In a particular
embodiment each of the photodetectors has a substantially square sensing
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surFace, the size of wh-ich is pre-ferably the same as that of the pattern
being cast on the array.
Brief Description of the Drawings
An example embodiment of the invention will now be
described with reference to the accompanying drawings in which:
Figure 1 illustrates an eye-position sensor in
accordance with the present invention;
Figure 2 illustrates an enlarged portion of the
eye-position sensor illustrated in Figure 1;
Figure 3 illustrates a light sensor which forms part of
the eye-position sensor illustrated in Figure 1;
Figure 4 illustrates a translucent character display and a
spatial filter which forms part of the eye-position sensor illustrated in
Figure 1; and
Figure 5 illustrates in further detail control
circuitry which forms part of the eye-position sensor illustrated in
Figure 1.
Description of the Preferred Embodiment
Referring to Figures l and 2, the eye-position sensor
comprises a translucent alpha-numeric character display pad 10 in which
the characters are illuminated e.g. by ambient light. The entire
structure which is contained in a housing (not shown) can be positioned
so that the eye 11 of a user such as a severely handicapped person, is
focussed on the display pad 10 through a 34 millimeter diameter plastic
25 lens 12 which is positioned a distance approximately equal to its focal
length of 37.5 millimeters From the eye 11, less the virtual image
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distance behind its cornea 15. This distance ensures that the reflected
rays which have passed -I:hrough the lens 12 emerge substantially
collateral. A light emi-tting diode (LED) 13 transmits 10 microwatts of
inFrarerl light at an 820 nanometer wavelength. The infrared light is
coupled From the light-emitting diode (LED) 13 through the lens 12 by an
optical fiber 14, where it is transmitted towards the eye 11. The
diameter of the illurninated area of the eye 11 from the infrared
light from the fiber 14 is restricted by the beam spread to about
6 millimeters. This aids the sensitivity of the system.
When the eye 11 of the user is focussed on one of the
characters of the character display pad 10, light from the infrared
source 13 is reflected from the cornea 15 of the eye 11 through the lens
12 to a spatial filter 20 having a square annular shape with overall
dimensions of 3 millimeters and an annular thickness of 1 millimeter as
shown in more detail in Figure 4. The infrared light reflected From the
eye 11 which passes -through this filter 20 is then coupled through a 6
millimeter focal length plastic lens 21. AFter passing through a window
aperture 22 which is used to exclude extraneous light, the reflected
light is detected by a light sensor 23.
The lens 21 is used to concentrate the light from the
filter 20 onto the sensor 23. The aperture 22 may be used to attaenuate
unwanted reflections. If sufficient reflected light is available the
elements 21 and 22 may be eliminated so that the light passing through
the filter 20 falls directly on the sensor 23. Also the relative
position of the filter 20 and lens 21 can be reversed. The filter 20
provides two significant advantages. By restricting the light
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illuminating the sensor 23 to d square pattern, its response to eye
movement is substantially linear. Also the filter 20 attenuates diffuse
reflec-tions from o-ther parts of -the eye 11. In some instances such
reflections may be focussed in the center of the pattern. By utilizing
an annular shaped filter, this center portion is blocked thereby further
improving the response characteristics.
As shown in Figure 3, the light sensor 23 comprises a
quadrantal array of con-tiguous photo detectors A, B, C, and D, each of
which generates a signal current which is propurtional to the illuminated
area. The distance between the lens 21 and the surface of the sensor 23
is such that a defocussed square annulus of light 24 from the spatial
filter 20 is cast thereon. Because the illuminated area 24 is square and
oriented in the same direction as the axes of the sensors 23, a
horizontal X or vertical Y movement of the image will result in a
substantially linear change in the output signal current from each of the
photo detectors A, B, C, and D. The signals from the four quadrants A,
B, C, and D of the sensor 23 are coupled to control circuitry 25 which
processes the signal so as to generate standard ASCII characters for
controlling a printer 26. Concurrently, the control circuitry 25 powers
a selected one 28 of an 8x8 matrix of LEDs 27. The light from the one
illuminated LED 28 is reflected from a beam splitter 29, disposed between
the display panel 10 and the lens 12, to the eye llo The matrix of LEDs
27 is disposed so that the selected LED 28 provides virtual highlighting
of the alpha-numeric character on the display pad 10 on which the eye 11
is currently focussed.
Referring to Figure 5, infrared light is coupled from the
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LED 13 through the optical -fiber 1~ where it is transmitted so dS to
to illuminate the eye Il. The LED 13 is remote from the light sensor 23
-to prevent electrical interference from the modulated drive curren-t
genera-ted by a LED driver 30 which swi-tches the LED on and off at a 1 kHz
rate under control of a 1 kHz oscillator 31 in the control circuit 25.
The use of a modulated signal permits -the use of a high gain low noise
amplifier at the input to the control circuit 25, and substantial freedom
From room lighting interference.
As explained previously, the infrared light reflected
from the cornea 15 of the eye 11 generates four electrical signal
currents A, B, C3 and D, on the four quadrant light sensor 23. These
signal currents are proportional to the incident light on them. The
electrical signal currents from the sensor 23 are amplified and processed
in an analog signal processor 32 which generates horizontal X and
vertical Y coordinate voltages such that X = C+D-A-B and Y = A-~C-B-D.
These two signals X and Y are then coupled to a multiplexing A/D
converter 34 through synchronous demodulators 33 and low pass filters
~0. The demodulators 33 are driven by the 1 Khz oscillator so that their
output is in synchronization with that of the outpu-t from the LED 13 to
produce output signals which are then digitized in the 8-bit
analog-to-digital (A/D) converter 34. The 3 Hz low pass filters 40
ensure that the control circuit 25 will respond only to eye movements.
The output of the converter 3~ is fed to a microprocessor 35 which
generates the standard 8-bit ASCII coded signals which are fed to the
printer 26. Through an LED driver 36 the selected character is given
virtual highlighting through the 8x8 ~ED matrix 27.
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One fea-ture oF the present system is the coaxiali-ty of the
illuminating radiation From the Fiber 14 end, the axis of the photosensor
array 23 and the axis of the display pad 10. When -the eye 11 is gazing
at the center of the display pad 10, the incident and reflected radiation
are coaxial. This permits the system to respond in comparable fashion to
both horizontal X and vertical Y movements. Non-coaxial systems as
described in the prior art, react very differently to the two directions
of movement. One example of these systems is described in "An Ocular
Control Device For Use By The Severely Handicapped", by G.A.Rinard, and
D.E.Rugg, 1976 Conference on Systems and Devices for the Diabled, Boston,
Mass. June 1976. Another example is described by K.C. Anderson et al in
"An Eye Position Controlled Typewriter" Digest 11th International
Conference Med. Biol. Eng. 29:410-411, 1976.
Utilizing the LED matrix 27, the present system provides
immediate feedback to the user as to where the eye is focussed as
determined by the system. When the display pad 10 is illuminated by
ambient light from the front or back, black characters on a translucent
white background are seen. The confirmatory illumination from the LED
matrix 27 is a colour (typically red or green) so that the user can
readily adapt to the system when the latter is not accurate in its
selection. For example, if the illuminated character is one space to the
right of the character being focussed on, the user may shift his gaze by
one space to the left thus achieving his desired choice. The angular
distance between characters is approximately 4 degrees and both positions
are well within the visual span of the static eye.
Character selection is completed by the expiration of a
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dwell time period typically in the order of one second. After the dwell
-time has expired, control circuitry within the microprocessor 35 produces
a sound on a loudspeaker or buzzer (not shown) to indicate the need for a
next character selection. Alternatively, when the user has some
elemen-tary muscle control, he may be able to activate a switch to
complete the selection.
The display pad 10 contains d rest space in lts center.
When the eye 11 gazes at this space the system takes no action and the
user may ponder his next move without interference. Once the system is
installed, electronic calibration under control of the microprocessor 35,
is achieved by the user gazing at a sequence of flashing lights located
in the four corners of the display pad 10. The processed signal voltages
modify the digital output of the microprocessor 35 to match the
characteristics of the operator's eye and his exact eye position in
reference to the optical sensor 23.
In an alternative embodiment, the LED matrix 27 may be
located directly behind the display pad 10 while the infrared source,
from the output of the fiber 14, may be in the position shown in the
drawings for the LED matrix 27. This may be altered further so that the
output end of the Fiber 14 and the detectors 23 are interchanged. In
each case however the spacial filter 20 is utilized to provide the square
pattern of light on the photo detectors 23. With either of these
alternative embodiments, unwanted reflections of infrared light from the
lens 12 may occur. These may be reduced by angling the axis oF the lens
12 about 6 degrees or by adding anti-reflective coating to the lens. In
all embodimen-ts however, it is important that the housing for the
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structure be held s-teacly with respect to the eye such as by using a
glasses -frame and/or elast-ic strap, in conjunction with the br-idge of the
nose.
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