Visor and camera providing a parallax-free field-of-view image for a head-mounted eye movement measurement system

A head-mounted, eye-movement, measurement system is provided with an optically flat glass laminated visor through which the observer views the external scene. Mounted so as to be vertically spaced from one side of the eye's optic axis is an eye tracker module for recording the observer's eye-movement relative to the head, principally by measuring the position of the pupil and corneal reflex, by reflecting near infrared light to and from the observer's eye vis-a-vis the front surface of the visor. Mounted so as to be vertically spaced on the opposite side of the eye's optic axis is a field-of-view camera which records the external scene viewed by the observer by reflecting external scene light from the back side of the visor. The distances and angular relationship of the visor, camera and eye are controlled to eliminate parallax from the field-of-view camera while also providing a stable arrangement permitting wide-angle scene viewing and accurate recordal of eye movements.

This invention relates generally to an eye-movement measurement system and 
more particularly to eye monitoring systems adapted to be mounted to the 
observer's head. 
The invention is particularly applicable to an eyemovement measurement 
system which utilizes an eye tracker in combination with a point-of-view 
camera to determine the observer's point of gaze and will be described 
with particular reference thereto. However, it will be appreciated by 
those skilled in the art that the invention may have broader application 
and may be applied in any situation where a picture, preferably a video 
recording, of the external scene actually viewed by the observer is 
desired. Additionally, it will be appreciated by those skilled in the art 
that while the invention has particular application to a headmounted 
system, the arrangement disclosed can easily be adapted for use in a floor 
mounted or remote eye-movement measurement system. 
INCORPORATION BY REFERENCE 
The following documents are incorporated herein by reference as background 
material and form a part hereof: 
(1) An article entitled "Eye-Movement Measurement Techniques" by L. R. 
Young & David Sheena appearing in American Psychologist, Volume 30, No. 3, 
dated March 1975, Pages 315-330; 
(2) Methods & Design - Survey of Eye Movement Recording Methods" by Young & 
Sheena, Behavior Research Methods & Instrumentation, 1975, Vol. 7 (5), 
Pages 397-429; 
(3) "Eye-Trac" Catalog by Applied Science Laboratories, copyright 1982, 
pages 1-31; 
(4) U.S. Pat. No. 4,034,401 to Mann; 
(5) U.S. Pat. No. 3,542,457 to Balding et al; 
(6) European Patent Application Publication No. 0-125-808 dated Nov. 21, 
1984; 
(7) European Patent Application Publication No. 0-157-973 dated Nov. 16, 
1985; 
(8) U.S. Pat. No. 4,755,045 by the present inventors; and 
(9) U.S. Ser. No. 848,154, filed Apr. 4, 1986 and assigned to Applied 
Science Laboratories. 
BACKGROUND 
There are a large number of eye-movement measuring techniques in the art 
and the principal ones are disclosed in the Young & Sheena articles which 
are incorporated by reference herein. This invention relates to those 
eyemovement measuring techniques which use an external light source, 
generally at near infrared wavelength, which is reflected from some 
portion of the eye to obtain a measurement of eye position or fixation. 
Generally, such techniques are classified as corneal reflection per se, 
corneal reflection-pupil center, corneal reflection-double Purkinje image, 
pupil tracking per se, limbus (i.e., the boundary between the iris and the 
sclera) tracking, eyelid tracking and combinations thereof. When used 
throughout this specification, reference to "eye tracker" or "eye tracker 
means" or "eye tracker mechanism" means any and all conventional 
mechanisms which utilize any of the aforementioned tracking techniques 
principally be measuring reflection of light from or over a portion of the 
eye. This is in distinction to electrooculography and contact lens 
eye-movement measurement techniques which do not fall within the 
definition of an eye tracker as used herein. 
Eye trackers of the type to which this invention relates, may be further 
classified as (i) head-mounted, in the sense that the principal 
measurement instruments are secured by a helmet or head band to the 
observer's head or (ii) "remote" or floor mounted in the sense that no 
instruments are applied to the observer's head even though chin rests or 
other devices might be used to immobilize the observer's head movement of 
(iii) a combination of "head-mounted" and "remote" or "hybrid" devices 
which do not exist in a practical, commercial sense, but are present in 
any theoretical consideration. 
A totally "remote" eye tracker system is produced by Applied Science 
Laboratories, the assignee of the present invention, in its 1996 and 1998 
model lines which are further described in our U.S. Pat. No. 4,755,045, 
U.S. patent application Ser. No. 848,154, and in ASL's Eye Trac Catalog, 
all incorporated by reference herein. In the 1998 model, the position of a 
servo controlled tracking mirror is controlled to maintain the eye image 
within the eye camera field-of-view so that eye line-of-gaze can be 
determined with the pupil center to corneal reflection technique. In this 
manner, rapid, unrestrained movements of the head will not result in loss 
of eye measurement even with as much as one foot of lateral or vertical 
head motion. Because there are no head-mounted instruments nor any other 
distracting instrumentation present to the observer, the 1996 and 1998 
systems are ideal for eye tracking measurements where the observer is 
seated, such as in the cockpit of a flight trainer or in a chair watching 
video commercials, etc. However, there are countless research, industrial 
and military applications where it is desired to accurately see what a 
person is looking at instead of projecting a predetermined scene and 
monitoring the reaction of the observer to the projected scene. Such 
applications typically use headmounted systems to monitor eye-movements. 
As noted, hybrid head-mounted - remote systems exist in the literature. For 
example, in EPC application No. 0157973, the external light source for 
directing the near infrared light for eye measurement purposes is mounted 
in the observation room while the corneal reflection instrument is 
attached to eyeglass frames affixed to the observer, who is viewing a 
scene projected on a screen. In U.S. Pat. No. 4,034,401, both the near 
infrared light source and the eye tracker camera (which is of the limbus 
tracking type) are reflected off a pilot's helmet to locate the eye 
position relative to an externally generated weapons pointing display 
reflected on the windshield of the aircraft. In both applications, the 
observer is seated or stationary and looking at a scene which is projected 
in front of him. To partially mount some of the eye tracker mechanism to 
the head of the observer simply encumbers the observer without presenting 
any enhancement of the system when compared with the ASL 1998 model used 
either in an airplane cockpit environment or in a seated environment for 
viewing artificially projected scenes such as commercials and the like. 
For such reasons, "hybrid" eye measurement systems are not commercially 
practical. 
This then leaves head-mounted systems to satisfy those applications, i.e., 
observer movement and/or real life scene viewing, which cannot be 
addressed by head-free systems. A head-mounted, eye monitoring system as 
thus defined herein requires a field-of-view or scene camera which records 
any external scene as actually viewed by the observer and an eye tracker 
mechanism, both items secured by an appropriate head band or helmet to the 
head of the observer. Different, head-mounted systems have been developed 
in the art for different eye measuring techniques, principally limbus 
tracking and corneal reflection. 
One typical limbus tracking arrangement uses eyeglasses with an infrared 
source of illumination mounted at the bottom of the lens and flanked on 
either side by photo cells which electrically record the light reflected 
to generate an eye image. A field-of-view camera is then added to the 
eyeglasses to obtain a point-of-gaze display. Examples of such 
head-mounted limbus tracking systems may be found in ASL's Eye Trac 
Catalog and in several embodiments disclosed in European Patent 
Application No. 0,125,808, which also discloses use of CCD chips for 
imaging. As noted by Young & Sheena, the eyeglass limbus tracking 
arrangement is suitable for some applications, but is limited with respect 
to vertical eye-movement measurement. Also, the field-of-view camera is 
mounted on one side of the eyeglass frame while the eye position 
measurement instruments are located on the other side and this 
side-by-side mounting arrangement introduces a parallax error, which may 
or may not present a problem. 
To obtain more precise eye measurement over both horizontal and vertical 
eye-movement, corneal reflex cameras have been used in head-mounted eye 
monitoring systems which also employ field-of-view cameras to obtain point 
of gaze information from an observer having freedom of movement. As 
disclosed in the Young & Sheena articles, early head-mounted corneal 
reflex eye monitoring systems used a periscope arrangement with the bottom 
of the scope carrying the infrared light source and scope lenses which 
reflected the infrared image to the top of the scope. The top of the scope 
was mounted on top of the observers head and carried the scene lens and an 
eye tracker camera in combination with a beam splitter prism for 
superimposing the corneal reflection as a spot of light onto the scene 
recorded from the field-of-view camera. Because of difficulties 
encountered in maintaining the infrared light source appropriately 
centered relative to the cornea, this concept has been modified into a 
side-by-side arrangement where the field-of-view camera is mounted on one 
side of the observer's head while the eye tracker mechanism with 
appropriate optics is mounted on the opposite side. Fiber optics have been 
used to lighten the helmet weight. One example of such an arrangement is 
disclosed in U.S. Pat. No. 3,542,457, incorporated by reference herein. As 
best illustrated in U.S. Pat. No. 3,542,457, a dichroic fixed mirror is 
used to reflect light from an infrared lamp to the eye spot or eye track 
camera for subsequent superimposition on the scene viewed by the 
field-of-view camera, the eye also viewing the scene through the dichroic 
mirror which is transparent to visible light. As in the earlier periscope 
version of the helmet, U.S. Pat. No. 3,542,457 uses a complicated optic 
system to reflect the light to the cornea and back to the eye tracker 
camera. 
It should also be noted that in the literature, specifically for one of the 
embodiments disclosed in EPA No. 125-808-A, the concept of using an eye 
tracker camera on the "limbus tracking" eyeglass frame for recording 
corneal reflection without complicated optics is used. However, that 
disclosure fixed the infrared lamp to the bridge of the eyeglass frame and 
would be suspect to the errors and inaccuracies of the earlier corneal 
reflex head-mounted systems which used a light source simply positioned in 
front of the eye. 
In addition, it is known and disclosed in ASL's Eye Trac Catalog and 
discussed in some length by Young & Sheena that any number of different 
sensors, i.e., magnetic head, optic, mechanical, etc., may be applied to 
the observer's head to measure the orientation of the eye in space to 
obtain the point-of-gaze (the angle of gaze relative to a reference point 
in the visual field) relative to ground. 
In summary, the limbus tracking eyeglasses are limited in their ability to 
measure eye-movement and the helmet mounted corneal reflection cameras 
require optics which somewhat tend to distort the spot image projection 
and require extensive calibration and readjustment. More importantly, all 
head-mounted, eye-movement measurement systems heretofore mounted the 
field-of-view or scene camera short distance from the eye (or eyes) whose 
movement was being recorded in a manner which introduced a perspective or 
parallax error. The parallax error could allow the field-of-view camera to 
see an object which is actually hidden and thus not visible to the 
observer. This difference in field-of-view is significantly noticeable at 
short distances and somewhat insignificant at infinity. When an eye 
tracker is used with the field-of-view camera in a head-mounted system, 
the system must be calibrated to the scene distance viewed if accurate 
point-of-gaze data is to be obtained. That is the field-of-view scene 
recorded must be adjusted for parallax for the distance of the particular 
viewed scene and the eye tracker than adjusted relative to the adjusted 
field-of-view scene thus recorded if accurate point-of-gaze information in 
space which is depended on absolute eye position, is to be obtained. 
Heretofore, mechanical and/or optical conflicts have either resulted in 
camera incompatibility with a head-mounted eye tracker or limitations of 
eye tracker performance to a specifically calibrated distance. 
SUMMARY OF THE INVENTION 
Accordingly, it is one of the principal objects of the present invention to 
provide a head-mounted eye-movement monitoring system which provides 
accurate recording of the observer's eye-movement relative to the external 
scene as actually viewed by the observer. 
This feature, along with other features of the invention, is achieved in a 
head-mounted eye-movement system which monitors an observer's view of any 
external scene as seen through at least one of the observer's eyes. The 
system includes a field-of-view camera positioned vertically on one side 
of the optic axis of the eye for recording the scene. An eye tracker 
mechanism which includes an eye tracking source of near infrared light for 
recording the position of the eye as the observer views the scene is 
vertically positioned on the opposite side of the eye's optic axis. A 
visor is vertically positioned at an angle between the field-of-view 
camera and the eye tracker mechanism at the intersection of the eye's 
optic axis with the field-of-view camera's optic axis. The visor is 
transparent to visible light to permit the observer to view the external 
scene while looking through the visor. An optical coating arrangement is 
provided on the visor which reflects the near infrared eye tracker light 
from the eye of the observer to actuate the eye tracker mechanism in 
accordance with standard practice while simultaneously reflecting light 
from the external scene to the field-of-view camera for recording the 
external scene viewed by the observer. The two way, oppositely directed, 
reflective reverse mirror views of the visor (actually a three way 
utilization) permits a stable, vertical mount arrangement with all 
measuring instrumentation positioned over or under the eye whose movement 
is to be recorded. Thus the system can be easily modified to include a 
second field-of-view camera for the other eye in combination with an 
additional eye tracker mechanism so that the movement of both eyes can be 
easily recorded. In such an arrangement the visor would simply be 
laterally extended across the face of the observer. In addition, when the 
eye tracker mechanism utilized any of the corneal reflection techniques, 
the general arrangement described permits a very simple optic system, 
essentially comprising only the visor, to reflect a coaxial infrared light 
source to the eye and back to the eye tracker camera avoiding the 
intricacies of the prior art helmet mounted optics and inherently 
resulting in a clearer eye tracker picture which maintains proper eye 
alignment irrespective of eye-movement. 
In accordance with another principal feature of the invention, a helmet or 
head band arrangement is provided which precisely mounts the field-of-view 
camera in a fixed relationship to the eye of the observer. More 
particularly, adjustment mechanisms associated with the visor, 
field-of-view camera and the helmet space the visor at equal distances 
between the eye and the field-of-view camera. This distance is measured 
from the intersection point of the eye's optic axis with the field-of-view 
camera's optic axis. Importantly, the attitude or plane of inclination of 
the visor is adjusted so that the angle between the visor and the 
field-of-view camera axis is equal to the angle between the visor and the 
eye's optic axis, both angles being measured from the same side of the 
visor. This geometric relationship minimizes or eliminates parallax and 
permits the field-of-view camera to accurately record the external scene 
as visualized or as actually seen through the observer's eye. In concept, 
the use of the visor which is at least partially transparent to visible 
light to the observer's eye so that the observer can view the external 
scene while also partially reflective of the external light to a 
field-of-view camera where the relationship between the eye, visor and 
field-of-camera is such to minimize, if not totally eliminate, parallax is 
a feature of the invention which has utility or usefulness in eye studies 
with or without an eye tracker camera. When this feature of the invention 
is used in combination with an eye tracker camera using the corneal 
reflection technique, the optic axis of the eye tracker camera is 
preferably parallel to that of the field-of-view camera's optic axis so 
that an accurate and precise positioning and reflection of the near 
infrared eye tracker camera light source occurs without the position 
inaccuracies associated either with the mount or the optic adjustments 
which afflicted the prior art systems. 
In accordance with another feature of the invention, the visor is a flat 
glass laminate having an infrared reflective coating positioned on one 
side of a glass substrate while a polycarbonate substrate is positioned on 
the opposite side of the glass substrate by means of an adhesive. The 
visor, as thus defined, uses an infrared coating which is a conventional 
or dichroic or "heat mirror". In the head-mounted eye-monitoring system of 
the present invention, the outer surface of the infrared reflective 
coating reflects the eye tracker's infrared source light while the 
opposite surface of the infrared coating reflects the near infrared light 
transmitted from the external scene to a monochromatic (IR) field-of-view 
camera. In accordance with the present invention, the conventional type 
heat mirror thus described is easily modified by mounting a polarizing 
film on the back surface of the polycarbonate substrate by means of 
removable clips secured to the ends of the visor. A metallic reflecting 
film or coating is applied to the opposite side of the polarizing film and 
a thin non-detrimental air space exists between the polarized layer and 
the polycarbonate substrate. As thus modified, visible light from the 
scene is partially reflected by the metallic reflecting film or coating on 
the polarized film to the field-of-view camera which is now a color (or 
monochromatic black and white) camera. An infrared cut filter and a 
polaroid filter is fitted over the lens of the field-of-view camera. In 
this manner, distinct, conventional video pictures can be obtained on 
which can be superimposed the eye tracker spot representative of the 
observer's point of gaze. Importantly, the clips permit the infrared visor 
to be easily converted to a visible light visor and vice versa. The visor 
laminates described have minimal thickness of approximately 3 mm for the 
infrared light visor and approximately 4.5 mm for the visible light visor. 
As thus constructed, both visors have minimal coloration and visual 
distortion. 
In accordance with other features of the invention, miniaturized imaging 
sensors such as a charge-coupled semiconductor device (CCD Chip Camera) 
are employed or alternatively a vidicon tube with optic fiber connections 
may be employed to permit a rigid, lightweight easily mounted and 
adjustable head band or helmet for the observer. In addition, any of the 
conventional head sensing devices can be attached to the helmet to locate 
the position of the head relative to space or ground which data can then 
be combined with the eye tracker data locating the position of the eye 
relative to the head to locate the position of the eye in space or, in 
effect, relative to the world, throughout a 360.degree. field-of-view. 
In accordance with yet another feature of the invention, the visor is 
dimensionally sized to easily span the observer's face in an unobtrusive 
manner while permitting maximum eye measurement movement. Significantly, 
the visor is sized to permit a relatively large field-of-view or scene 
range of typically 55.degree. horizontal.times.45.degree. vertical, and 
this range can be adjustably biased in a vertical direction if required 
because of the particular scene application being studied. 
It is thus an object of the invention to provide a head-mounted 
eye-movement measurement system which accurately records the field-of-view 
seen by the observer by correcting or minimizing parallax. 
It is another object of the invention to provide a head-mounted 
eye-movement measurement system which uses instruments vertically mounted 
relative to the face of the observer to produce a simple optic system. 
Another object of the invention is to utilize a flat, glass visor laminate 
in a head-mounted, eye-movement monitoring system which has minimal visual 
distortion and coloration artifacts. 
Another object of the invention is to utilize a flat reflective-transparent 
visor in a head-mounted eye monitoring system which produces a vertically 
adjustable, large or wide field-of-view for tracking point of gaze 
information. 
Still another object of the present invention is to provide a head-mounted, 
eye-movement measurement system which uses visible light for recording 
field-of-view images. 
Still yet another object of the invention is to provide in a head-mounted, 
eye-movement monitoring system a visor configuration which reflects near 
infrared wave light from two different directions or near infrared wave 
light from one direction and visible light in the opposite direction. 
Still yet another object of the invention is to provide an improved 
head-mounted eye monitoring system which utilizes a two way light 
reflecting visor to produce a small size, lightweight head-mounted 
arrangement. 
Still yet a more detailed object of the invention is to provide a visor 
which can be easily converted from a device used to reflect scene light to 
an infrared field-of-view camera to a device used to reflect visible scene 
light to a field-of-view camera. 
Another general object of the invention is to provide a reflective, 
transparent visor having any of the aforementioned characteristics for use 
with any eye-movement measurement system which may be remote and not 
head-mounted. 
Still yet another object of the invention is to provide an improved 
head-mounted, eye-movement, measurement system which utilized a visor and 
a particular geometric configuration therewith which permits a side 
variety of eye tracking mechanisms using different eye-movement measuring 
techniques to be employed therewith. 
Still another object of the invention is to provide an improved 
head-mounted, eye monitoring system which is relative simple and 
inexpensive. 
These and other objects of the present invention will become apparent to 
those skilled in the art upon a reading and understanding of the 
specifications.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings wherein the showing are for the purpose of 
illustrating a preferred embodiment of the invention only and not for the 
purpose of limiting the same, there is shown a general arrangement in FIG. 
1 where an observer 10 is focusing eye 12 on an external scene 14. The 
optic axis 16 of eye 12 is shown as a dot-dash line in FIG. 1 and observer 
10's line of gaze for viewing external scene 14 is directed through a flat 
laminated visor 20. Visor 20 is a laminated glass which extends laterally 
in the z-z direction across the face of observer 10 and will be discussed 
in detail hereafter. Laminated visor 20 is one element of a head-mounted, 
eye-movement measurement system. The other components of the system 
include a field-of-view camera 22, an eye-tracker module 24 an a helmet or 
head band 26 for mounting and adjusting the various positions of visor 20, 
field-of-view camera 22 and eye tracker module 24 relative to the 
observer's eye 12. A variety of helmet or head band arrangements 26 exist 
in the prior art and any particular design or configuration of helmet or 
head band 26 is believed readily obvious to those skilled in the art and 
is thus not shown or described in detail herein. What is important is that 
a helmet or head band 26 contain any number of conventional adjustment 
mechanisms to permit the proper orientation and placement of field-of-view 
camera 22, visor 20 and eye tracker module 24 in a manner to be set forth 
in detail hereafter. 
More specifically, FIG. 1 shows a head-mounted system in an x-y plane and a 
third dimensional z-z axis is defined as a line projecting into or out of 
the plane of the paper. 
Generally, field-of view camera 22 is mounted in an adjustable, swivelable 
manner to an articulated arm 28 which in turn is mounted to head band 26 
by means of a lockable mounting arm 27. The particular articulated arm 28 
shown is pivotally pinned at three joints, 31, 32, 33, to permit 
articulated arm 28 to bend and twist three-dimensionally in a 
non-telescoping manner and thus properly position field-of-view camera 22 
at any desired point in space. Eye-tracker optics module 24 is mounted as 
a unit to headband 24 and slides in a z-z direction on a dovetailed track 
(not shown). For drawing position and illustration purposes only, 
eye-tracker optics module 24 is shown mounted to a fixed arm 34. In 
practice, a conventional eye-tracker module 24 is simply mounted to 
headband 26 in a z-z movable direction and contains the necessary position 
elements or optics to adjust the eye-tracker camera 20 within the module 
in a conventional manner. A pair of telescoping visor arms 29 (only one is 
shown) spaced apart from one another in the z-z direction, is pivotally 
mounted at one end 35 to the helmet or headband 26 and pivotally mounted 
at the other end 36 to visor 20. This arrangement permits easy adjustment 
of the distance of visor 20 from observer's eye 12 as well as the angle of 
visor 20 relative to eye optic axis 16. 
Preferably, field-of-view camera 22 is positioned vertically below 
observer's eye 12 while eye-tracker optics module 24 is positioned 
vertically above observer's eye 12. The positions, however, can be 
reversed. Also, the invention will be described with reference to 
monitoring the position of one eye 12 of observer 10. In such an 
application, visor 20 need not extend across the face of observer 10. When 
both eyes of observer 10 are to be monitored, visor 20 must extend across 
the face of observer 10. Also, a second field-of-view camera 22 mounted on 
a second articulated arm 18 is positioned on the opposite side of headband 
26 and a second eye tracker module 24 must be similarly applied to helmet 
or head band 26. 
As noted above and as diagrammatically indicated in FIG. 6, light from 
external scene 14 is viewed by observer's eye 12 through visor 20 which is 
transparent to visible light. Additionally, light form external scene 24 
is reflected by visor 20 into field-of-view camera 22. Also, light for eye 
tracking purposes from the observer's eye 12 is reflected by visor 20 into 
eye tracker module 24. 
Eye tracker module 24 can be any conventional eye tracking mechanism used 
in the art to record eye-movement and can employ any of the measurement 
techniques discussed above. It is, however, preferred that eye tracker 
module 24 used in the invention be of the type which measures eyemovement 
by the pupil center to corneal reflection technique which is generally 
illustrated in FIG. 5. The reader is referred to FIG. 28 of Young & 
Sheena's survey article (incorporated herein by reference) for a more 
definitive explanation than that presented herein. Briefly, a light source 
90 in eye tracker module 24 generates a generally infrared beam coaxial 
with the optic axis of an eye tracker camera 30 in eye tracker module 24. 
Light source 90 is directed against the cornea of observer's eye 12. 
Because the cornea has a different radius of curvature than the eyeball, 
the relative motion of the corneal reflection and pupil center by angle 
and distance, is indicative of the eye line of gaze as the eye views 
external scene 14. The displacement consists of two parts, a displacement 
resulting from eyeball rotation relative to light source and a lineal 
displacement of the center of rotation of the eye normal to the incident 
light beam. Reference should be had to Young & Sheena's article for a more 
complete description of the measurement technique than that provided 
herein. 
Eye track camera 30 as well as field-of-view camera 22 are preferably CCD 
chip cameras to minimize the weight of the system. Alternatively, vidicon 
tubes with fiber optics as described in the prior art may be used. Also, 
the eye tracker light illuminator 90 as used in the preferred embodiment 
is of the conventional near infrared type which produces an 
unobjectionable, barely visible red spot to the eye of the observer, 
although, in theory, any light source of any wavelength suitable for 
tracking may be employed. 
Referring now to FIG. 2, there is shown the laminar construction of a visor 
20 which is suitable for use with a monochromatic (IR) field-of-view 
camera 22 along with an eye tracker module 24 also using source of 
illumination 90 at near infrared wavelength. The laminar construction of 
visor 10 shown in FIG. 2 includes a glass substrate 40 having a front 
surface facing observer's eye 12 upon which is coated (typically by vacuum 
deposition) an infrared reflective, "heat mirror" coating which is 
identical to that used in many of the commerically available "heat 
mirrors", for example, the Calflex-C heat mirror supplied by the Balzers 
Optical Group. Secured to the backside of tempered glass substrate 41 by 
means of a conventional, optically flat glass adhesive 43 is a 
polycarbonate substrate 44. Polycarbonate sheeting 44 and tempered glass 
substrate 41 provide impact strength and a safer breakup structure to 
visor 20. In addition, heat mirror reflective coating 40 is carefully 
selected and applied to glass substrate 41 so that the coating maximally 
transmits visible light (at least 95%), maximally reflects near infrared 
light, does not introduce coloration artifacts and is not critically 
sensitive to a 45.degree. angle of incidence for optimal performance. 
Glass substrate 41 on which heat mirror coating 40 is applied is optically 
flat and thin to minimize distortions and double reflections. The 
thickness of visor 20 shown in FIG. 2 is only approximately 3 mm. In 
addition, since field-of-view camera 22 is a CCD chip camera which is 
maximally sensitive to light and generates a normal black and white image 
of external scene 14, field-of-view camera 22 is made sensitive only to 
near infrared light by placing in front of the lens of field-of-view 
camera 22 an IR-Pass filter 47 so that only infrared or near infrared 
light is recorded by field-of-view camera 22. Similarly, if eye tracker 
camera 30 is also a CCD chip camera, a similar IR-Pass filter would be 
employed. Of course, the retina of observer's eye 12 is not responsive to 
near infrared light. 
As illustrated in FIG. 2, visible light from external scene 14 as shown by 
arrow 49 is transmitted in a transparent manner through visor 20, as shown 
by arrow 49a to the observer's eye 12 with minimal reflection distortion 
and without the introduction of coloration artifacts. Also, that portion 
of external scene light 49, which is of infrared or near infrared 
wavelength, is reflected by the back surface of double mirror coating 40, 
as shown by arrow 49b, and transmitted as a reversed mirror image through 
IR-Pass filter 47 to the lens of field-of-view camera 22 where the scene 
is recorded. Additionally, the near infrared light source 90 which is 
reflected from the front surface of heat mirror coating 44 to observer's 
eye 12 is similarly reflected off the front surface of heat mirror coating 
40 as shown by arrow 51. The cosntruction of visor 20, as shown in FIG. 2, 
is similar to and may even, in fact, be identical to some "heat mirrors" 
conventionally manufactured. However, the use of the same visor 20 for 
both IR eye-tracking and IR scene image (or as explained below, visible 
light scene image) is believed new and unique and, more specifically, IR 
eye-tracking and IR scene imaging achieved by reflecting off the front and 
back surfaces of heat mirror coating 40 different light sources as mirror 
reversed images in diametrically opposite directions is believed new and 
unique. 
For those applications where it is necessary to use a color camera, or a 
monochromatic camera using visible light as opposed to infrared light 
(i.e. a black and white camera) visor 20, as shown in FIG. 2, must be 
modified as shown in FIG. 3 so that visor 20 reflects not only infrared or 
near infrared light to eye tracker camera 30, but also reflects the full 
color visible light to the camera. Visor 20, as shown in FIG. 3, comprises 
heat mirror coating 40 applied to the front surface of glass substrate 41 
and the back surface of glass substrate 41 has a polycarbonate substrate 
44 secured thereto by means of adhesive 43 identical to that shown and 
described for visor 20 of FIG. 2. The modified visor 20 of FIG. 3 
additionally has a light-reflecting film or coating 60 applied to one side 
of a sheet of polarizing film 62 while the opposite side of polarizing 
film 62 is secured to the back surface of polycarbonate substrate 44. 
Preferably, polarizing film 62 is removable secured to polycarbonate 
substrate 44 by means of a pair of spring C-shaped clips 50 (FIG. 1) which 
merely clamp polarizing film 62 to the visor 20 of FIG. 2 to form the 
visor 20 of FIG. 3, there being a slight or thin air space 63 between 
polarizing film 62 and polycarbonate substrate 44. This permits easy 
construction of visor 20 of FIG. 3. Alternatively, polarizing film 62 
could be permanently secured as by adhesive to polycarbonate substrate 44. 
Polarizing film 62 which is a conventional polarizing sheet is selected to 
maximally transmit visible light from external scene 14 to the observer's 
eye 12. As noted, polarizing film 62 is coated with a metallic film 60 so 
that a portion of the visible light from external scene 14 can be 
reflected to field-of-view camera 22. The metallic coating could be 
aluminum vacuum-deposited onto the surface with its thickness carefully 
controlled so as not to materially impede the transparency of the light 
from external scene 14 to observer's eye 12. To control haze and other 
infrared optical artifacts, the lens of field-of-view camera 22 is now 
fitted with an IR-Cut filter 66 blocking out transmission of excessive 
infrared light from external scene 14 and eye-tracker optics 24 to 
field-of-view camera 22 and a polarizing filter 67 to control haze. 
Polaroid filter 67 is adjusted so as to block the view seen through visor 
22. The same considerations of optical flatness, etc., commented on with 
respect to the visor shown in FIG. 2 apply to visor 20 of FIG. 3 and the 
thickness of visor 20 in FIG. 3 is controlled to approximately 4.5 mm. The 
operation of visor 20 in FIG. 3 is similar to that discussed with respect 
to the visor of FIG. 2. External scene light 49 is, in part, reflected by 
metallic reflecting coating 60 as shown by arrow 49b to the color 
field-of-view camera 22 while the remaining portion, which is polarized by 
film 62, is transmitted as visible light 49a to the observer's eye 12. At 
the same time, heat mirror coating 40 is reflecting eye tracker light 90 
to observer's eye 12 and back as an eye tracker light 51 to eye tracker 
camera 30. 
As thus far defined, the eye-measurement, head-mounted system is 
functional. With the helmet or head band 26 placed on the observer's head, 
the field-of-view camera 22 can be activated to project a field-of-view 
picture and the field-of-view camera 22 and visor 20 geometrical 
relationship adjusted until the observer indicates that the field-of-view 
camera picture is what he is actually observing through visor 20. The eye 
tracker module 24 can then be activated and eye tracker camera 20 adjusted 
relative visor 20 which is now fixed to generate a point-of-gaze spot 
appearing on the field-of-view picture an the electronic recording 
equipment calibrated so that the point-of-gaze spot correlates to that 
portion of the field-of-view picture or scene which the observer is 
focusing on. As so used, the optics achieved by visor 20 result in a very 
stable point-of-gaze spot reflected by eye tracker camera 30 relative to 
the field-of-view scene as recorded by field-of-view camera 22 and as such 
is a substantial improvement over prior art systems in terms of both the 
measurement of the corneal reflection and maintaining the reflected 
point-of-view spot properly positioned relative to the field-of-view 
picture. 
However, as noted and discussed above, it is in all instances desirable and 
in fact mandatory in many applications to have field-of-view camera 22 
accurately record what is actually seen by observer's eye 12 no matter 
what the focal distance is of external scene 14. Accordingly, the general 
arrrangement of the components of the head-mounted system of the present 
invention permits the system to be adjusted in a manner which compensates 
for parallax at all focal distances. As best shown in FIG. 1, the distance 
from eye 12 to visor 20 D.sub.e and the distance from the lens aperture of 
field-of-view camera 22 to visor 20 E.sub.c are adjusted to be equal to 
one another. More precisely, eyevisor distance D.sub.e is measured from 
eye 12 to the pint 71 where eye optic axis 16 is intersected by 
field-of-view camera's optic axis 70. Eye-camera distance D.sub.c is 
measured from intersection point 71 to the lens aperture of field-of-view 
camera 22. Next, the camera angle C.sub.a is made equal to theoptic angle 
O.sub.a. Camera angle C.sub.a is defined as the angle formed between 
field-of-view camera' s optic axis 70 and the back side of visor 20. Eye 
optic angle E.sub.1 is defined as the angle between eye optic axis 16 and 
the back side of visor 20. It will be appreciated that the angular 
relationship of field-of-view camera 22, visor 20 and observer's eye 12 
could be expressed differently, but the angular relationship thus defined 
in combination with the defined distances is critical to the operation of 
the invention in the sense that parallax or perspective errors are 
eliminated or minimized to result in an external field-of-view scene 
recorded by field-of-view camera 22 which actually represents what is seen 
by observer's eye 12. It is also noted the optic axis 72 of eye tracker 
camera 30 is preferably parallel to optic axis 70 and in this connection, 
it should be apparent to one skilled in the art that the adjustment 
mechanism for the helmet or headband could be modified to automatically 
maintain optic axes 70 and 72 parallel to one another. 
In the preferred embodiment and for practical reasons, it has been found 
that the largest convenient field-of-view has been obtained with the 
field-of-view camera 22 having a focal length lens of 8 mm. Thus, with the 
aforementioned distance E.sub.d and C.sub.d set at about 40 mm and the 
field-of-view camera focal length set at 8 mm, the size of visor 20 is 
established at approximately 4 inches in height or vertical distance and 7 
inches in length in the z-z direction (although the 7 inch length need 
only be approximately half that distance if the movement of only one eye 
is being measured). A lens/visor of these specifications will yield a 
55.degree. horizontal.times.45.degree. vertical field-of-view. It has been 
found empirically that if observer 10 wishes to look beyond this 
field-of-view there is a high probability that observer 10 will move his 
head towards the new object of interest rather than exercise the limit of 
his eye's field-of-view in eyeball rotation. A 55.times.45.degree. imposed 
field-of-view is not so wide that the equipment becomes obtrusively bulky 
and not so small that the eye is usually looking outside that "window". In 
addition, the adjustment mechanism (described above) permits visor 20 to 
be positioned in such a manner that the 45.degree. vertical field-of-view 
can be biased upwards or downwards. For certain application where an eye 
tends to predominantly look to a higher or lower region of the scene, this 
adjustment is beneficial. 
With visor 20 properly adjusted, conventional circuitry and electronics 
such as disclosed in ASL's Eye Trac Catalog can be used to accurately 
monitor a field-of-view scene 80 (FIG. 4) actually seen by observer's eye 
12 with a point-of-gaze spot 82 shown on field-of-view scene 80 to obtain 
position data relative to the head of observer 10. To obtain data on the 
absolute position of field-of-view picture 80 and point of gaze 82, any 
conventional head tracker device 85, as shown in FIG. 6 and as discussed 
above, is applied to helmet or head band 26 of observer 10 to locate the 
position of the observer's head relative to ground. The output from head 
tracker device 85, field-of-view camera 22 and eye tracker module 24 can 
then be inputted to a central control device such as the computer EVM 
control, diagrammatically indicated at 87, available and described in 
ASL's catalog. The output from EVM control 87 can then accurately record 
an absolute point-of-gaze 82 relative to a 360.degree. scan in both 
horizontal and vertical directions and record such positions through an 
appropriate position time control 89. 
The invention has been described with reference to a preferred embodiment. 
Obviously, modifications and alterations will become apparent upon a 
reading and understanding of the specifications. For example, the 
invention has been described for use with a head mounted system in that 
the visor, eye tracker and field-of-view camera are all secured to a 
headband or helmet. This is, in all practicality, where the invention will 
have the most beneficial application. The relationship, however, between 
visor, eye-tracker, field-of-view camera and the observer's eye, as taught 
herein, and in a broader sense the simple use of the visor disclosed 
herein, could easily occur in an eye measurement system which did not 
utilize a helmet or headband. For example, the visor, eye-tracker and 
field-of-view camera could be easily mounted to a floor stand and a chin 
rest be used for the observer so that the invention can be used in a 
"remote" eye-measurement system. It is our intention to include all such 
modifications and alterations insofar as they come within the scope of the 
invention. 
It is thus the essence of the invention to provide a new and improved 
head-mounted, eye position measurement system which utilizes a unique 
visor to record an actually observed field-of-view scene while inherently 
providing improved eye tracker performance.