WO1993008496A1

WO1993008496A1 - Biocular head-up display with separated fields

Info

Publication number: WO1993008496A1
Application number: PCT/US1992/008970
Authority: WO
Inventors: Oded Arnon
Original assignee: Arnon, Tamar
Priority date: 1991-10-24
Filing date: 1992-10-22
Publication date: 1993-04-29
Also published as: AU2896792A

Abstract

A simple system for head-up display based on the separation of the ocular fields. When the right eye's (RE, 22) field of view (RFOV) is separated from that of the left eye, (LE, 24) the possibility of parallactic splitting of the displayed image is eliminated. Such a configuration is particularly useful where a two-dimensional display image is superimposed on a three-dimensional view having considerable depth, so that disturbances due to parallax effects are unavoidable. Preliminary design for automotive applications suggests an instantaneous display field consisting of two 12 mrad vertical strips, symmetrically located ± 2.2 degrees about the meridional plane. Such a field allows free head movement within a frame of 40.0 mm up and down, and side to side. For observation of at least half of the field, that is, viewing at least one strip or the other, the horizontal span of head movement would be expanded to about 195 mm.

Description

Title: BIOCULAR HEAD-UP DISPLAY WITH SEPARATED FIELDS

BACKGROUND OF THE INVENTION 1. Field of the invention

This invention relates to a head-up display (HUD) for use in vehicles and similar applications and, more particularly, to a head-up display which can be used in applications wherein the user is reguired to observe a three-dimensional scene while receiving the HUD information and where it is thus important to eliminate parallactic disturbances or effects.

2. Description of the prior art

Head-up displays (HUD) for fighter aircraft applications are well developed. HUD systems were initially developed as an aircraft targeting system. In operation, the system projects to the pilot's eyes the sight symbol and other essential information, forming them as a virtually "floating" image which coincides with the outside view seen in the distance. These two images, the outside view and the projected symbols, are combined and superimposed into one integrated image by means of a semi-transparent mirror, called a combiner, which is mounted onto or next to the cockpit windshield.

A head-up display system used in aircraft typically features a source of the display, usually a CRT tube, mounted so as to project the symbols to the front focal plane of a collimating lens. The collimated light is partially reflected by a combiner to the eyes of the pilot. Light emanating from the outside view passes through the combiner and to the eyes of the pilot.

Head-up displays have traditionally been built as biocular systems, creating one image which is seen jointly by both eyes. When the display was expanded to incorporate additional displays beyond sighting information, biocular viewing became the entrenched and accepted design norm.

Achieving good biocular performance places a heavy burden on the optical design of the system and on its production.

The system must meet the rigorous demands for high display accuracy characteristic of any targeting system. This is the accuracy with which the displayed symbols appear in the field of view relative to the outside view. In addition, use of a biocular system entails problems of binocular disparities resulting from parallax errors. Binocular disparities are deviations from collimation as determined from the different positions of the two eyes.

These deviations appear as horizontal errors, both converging and diverging, and also as vertical errors, causing the eyes to squint inward, outward and vertically, as appropriate, to prevent splitting the image in the field-of- view that is shared by both eyes. It should be pointed out that these parallax errors, even if mild and correctable by the viewer's visual perception system, can have a harmful effect. Such errors bring about fatigue owing to the mental and physiological effort involved in preventing the image from splitting into two. More details regarding the parallax errors will be discussed below.

The functional demand for high display accuracy, and the eyes' great sensitivity to parallax errors require tight tolerances in the production of a head-up displays. In a well corrected 25 degree total field of view collimator, the display accuracy error, at the worst case of field/pupil position, can be controlled to less than 1.0 mrad. In such a system the disparity is reduced to about 2.5, 1.0, and 1.0 mrad for the converging, diverging and the vertical errors, respectively. This level of performance can be achieved through the use of costly multi-element lenses, whose use, because of the great cost involved, is virtually restricted to costly fighter aircraft. Attempts have been made in recent years to adopt head-up display systems for more down to earth purposes, most notably, for automotive use. The need to incorporate information currently displayed on the car dashboard display into the outside view seen through the windshield is quit apparent. What is not often commonly realized is that despite the fact that automobiles travel at considerably lower speeds than do aircraft, the near outside view of the road, i.e., the view of objects from a few meters to a few dozen meters from the driver, passes the driver more rapidly, in terms of angular speed, than does the outside view perceived by a pilot of an aircraft flying high in the sky. Thus, the need for head-up displays in a motor vehicle seems no less important than that of a typical military aircraft which is not specifically designed for air-to-air dogfight. Combining the view of the road with displayed dashboard symbols would constitute a significant enhancement in road safety. At present, with the dashboard and the outside view being direc ionally separated from each other, every look at the dashboard, even the briefest glance, means losing eye contact with the road. On the other hand, too much Concentration on the outside view, without occasional glances at the dashboard, delays or even prevents the receiving of important data which may be urgently needed.

In an attempt to focus on the road ahead and at the approximately the same time receive vital information regarding the performance of the automobile, the driver is forced to frequently shift his eyes and attention from the outside view to the dashboard and vice versa. The transition between the two images takes up significant periods of time. The eyes have to compensate for the different distances, as well as for the different levels of luminance between the two images. Furthermore, even after the driver's glance returns to the outside view, he is unable to immediately perceive and correctly interpret the events occurring in the outside view. Careful driving depends on the driver's ability to perceive. in a timely fashion, every event on the road which may potentially develop into a danger. ^• Such perception requires continuous scanning of the road to establish in the brain of the viewer a picture of those features which are changing, and which need to be monitored more frequently. Turning one's gaze away from the road to the dashboard damages the continuity of this scanning process, and additional time is required to update with the new picture, so that attention can be properly focused again on the process of identifying new potential dangers. These physiological and mental transitions take a second or two, especially under conditions of tension and fatigue, and during this period of time the driver has real contact with neither the road, nor with the dashboard. Various attempts have been made to implement HUD in automotive applications. The efforts have been largely directed toward creating improved holographic images. U.S. Pat. No. 4,613,200 to Hartman describes a simplified HUD employing a pair of identically constructed reflective holograms, wherein one of the holograms is in the field of view of the observer and serves to correct dispersion and provide the virtual image of the display source within the observer's field of view. "HOE: Out of the Lab and into the Car," SPIE's International Technical Workincr Group Newsletter HOLOGRAPHY. July 1991, pp. 4-5, describes the efforts of Volkswagen to implement a holographic HUD in its automobiles. Other attempts have been made. For example, PCT Application WO 91/00674 discloses a video program source producing images which are reflected to only one eye of the driver via a small mirror on the windshield. No attempt is made to reflect images to both eyes of the driver simultaneously thereby limiting the amount of information which can be transmitted to the driver.

Certain factors act to make the implementation of head-up displays in cars difficult. To appreciate these difficulties it is useful to compare the differences between a display for aircraft use with that which would be appropriate for automotive use. The fact that the view from an aircraft cockpit is of far off objects gives the aircraft view a two- dimensional character. Thus, the symbol display which is projected to the pilot's eyes and which is also of a two- dimensional character, is said to coincide with the outside view as long as it too is focused at infinity. The display of symbols focused at other than infinity would lead to parallax errors. Parallax errors may appear in two interrelated phenomena — (1) monocular parallax error, which causes near objects to shift relative to distant objects when the single viewing eye is moved, and (2) biocular parallax error, which makes a near object appear doubled when two eyes are focused on a distant scene.

In contrast with the monocular parallax error which is tolerable for many civilian applications, but not in certain combat aircraft applications where it obviously adversely affects the performance of head-up displays as a targeting system, the biocular parallax error is intolerable and must be closely controlled.

Biocular parallax error can be demonstrated by viewing a distant scene using both eyes with an object (say, the viewer's finger) interposed between the eyes and the scene near the line of sight. The near object appears doubled. Of course, translation of the eyes leads to relative motion of the split image relative to the distant scene, which demonstrates the combined effect of the monocular and biocular parallax errors. In contrast with the two-dimensional view seen from an aircraft, the scene from an automobile is characterized by its great depth. The automobile driver perceives a three- dimensional picture with depth ranging from the front end of the car (approximately 2 meters) to optical infinity. This raises a difficulty for the designer of head-up display systems for automotive applications. The designer could focus the image display at some convenient distance from the driver which will, hopefully, coincide with the most relevant distance in terms of activities which will impact on the driver's ability to drive safely and which the driver should focus on. For example, the display images could be focused at approximately six meters from the driver (about 4 meters in front of the car) . However, it is clear that whenever the driver focuses on objects located a shorter or longer distance away, there will be significant parallax errors which will cause the image to split, thereby dramatically reducing the image's clarity and effectiveness, or worse, actually distracting and confusing the driver and reducing his driving efficiency. A fused image is an image which, despite some splitting, is still comprehensible to the viewer. If we define a fused image as an image with a parallactic split smaller than a certain value, say 2.0 mrad, then, we can calculate the depth of field, i.e., the range of distances from the eye, in which objects will be in tolerably accurate focus to the viewer. Assuming a typical interpupillary distance of 65 mm, calculations show that, for an image focused at 6 meters from the eyes, a fused image will be obtained in the range of from about 5 meters to about 7.5 meters. Outside of this range the viewer will clearly perceive the parallactic splitting.

There is thus a fundamental problem in implementing HUD for automotive applications which comes from the inability to make a three-dimensional view coincide with a two dimensional display. Such a difficulty cannot be overcome using conventional approaches or through use of sophisticated lenses.

In addition to the principal difficulty mentioned above, another reason for the delay in implementing head-up display systems in automobiles is the high cost involved. Although head-up display for automobiles is obviously free of the rigorous demands of military targeting, the automotive system must still overcome parallax errors, which arise from the lens residual aberrations. Satisfying tolerances within 2.0 mrad for converging, diverging or vertical disparities is currently too much of an obstacle for this popular and essential purpose.

HUD systems to date, including those proposed for automotive use, have been biocular, requiring use of both eyes of the viewers. Such systems, especially in automotive and similar uses, where the scene to be viewed is three- dimensional, lead to disturbing parallactic errors, of which the doubling of the projected image is the most serious. There is thus a widely recognized need for, and it would be highly advantageous to have, a simple and inexpensive head-up display system for use in automotive and similar applications, which will eliminate biocular parallactic error and will produce a single clear image regardless of where the viewer's eyes are focused.

SUMMARY OF THE INVENTION According to the present invention is presented a head-up display system, for communicating information to a viewer's eyes, comprising a display source capable of transmitting light images containing the information; a lens capable of forming the light images; directing means for directing the light images to the eyes of the viewer; and an aperture stop for creating separate images to be viewed separately by each eye of the viewer.

The system can be used in a variety of applications, including microsurgery, video games, simulators, aircraft and in automobiles and trucks.

Further embodiments of the invention call for a lens or a means for forming and directing the images to the eyes of the viewer which are separated by a typical interpupillary distance of about 65 mm. Alternatively, in case the invention is applied in a robotic artificial vision, the artificial eyes are separated by a different interpupillary distance, as required, for example, for obtaining a three-dimensional view, or for any other reason. In the description herein use is made of the human interpupillary distance. It is intended that the present invention be applicable also to non-human viewers having interpupillary distances different from that commonly found in humans. In both cases the light may be directed to the eyes via a combiner or a mirror.

The light images may be projected in an upward or in a downward direction, and the images may be transmitted to the viewer from directly ahead or sideways from an angle of up to approximately 45 degrees from the forward looking direction. In vehicular applications the system may be mounted on the windshield or it may be mounted independently.

The present invention successfully addresses the shortcomings of the presently known configurations by providing an inexpensive yet highly effective system for head- up display in automotive and similar applications capable of producing a single clear image regardless of at what distance the viewer's eyes are focused.

The field of .view of a head-up display system is composed of the right eye field-of-view (RFOV) and the left eye field- of-view (LFOV), with the central area of the overall field being shared by both eyes. This central area is the overlap

FOV. It is in the overlap FOV where a double-image may be caused by the parallactic split. In conventional systems the overlap field-of-view makes up about a third of the total field-of-view, and in advanced holographic systems, which effect optical conjugation between the entrance and exit pupils of the system, the RFOV and LFOV overlap almost entirely.

Contrary to the accepted trend of maximizing the size of the overlap field for the purpose of increasing the instantaneous field-of-vision (IFOV) to the dimensions of the total field-of-view (TFOV), the head-up display system according to the present invention is based on doing just the opposite. According to the present invention it is desired to separate the right eye FOV from the left eye FOV. The absence of overlap between the RFOV and LFOV prevents any possibility of parallactic splitting of the displaced image. This separation may be accomplished in a number of ways.

One method is by limiting the extent of the aperture stop of the system along the direction parallel to the imaginary line connecting the two eyes — the interpupillary line, usually along the horizontal direction, regardless of where such a line is located in the system, whether at one of the lens surfaces, at the combiner or at any other surface, not necessarily attached to any physical optical element.

It is important to note that separation of the two fields does not, in itself, reduce or is intended to reduce the lens aberrations which may cause parallactic errors. The separation between the ocular fields is intended merely to eliminate any possibility of parallactic errors to occur which otherwise would normally result at the overlapping field of a biocular vision system. In other words, when symbols shown to the right eye are separated from symbols shown to the left eye, the reasons which normally cause parallax errors will only cause a slight difference in the already large separation distance between these two groups of symbols, but visual perception is almost completely insensitive to these changes. With the separated ocular fields, the depth of field of the entire display, for both eyes, will be the same as the depth of field of a single eye, viz.,+/-0.3 diopter. Thus, the entire display appears sharp to both eyes while focusing them at virtually all possible distances in the outside view, beyond a certain minimum distance on the order of two meters. Furthermore, any chromatic aberrations will be virtually unnoticed since, while symbols of different colors will be focused at somewhat different distances, the extraordinary depth of field of single-eye vision will make such an error imperceptible to the human eye and thus of no practical significance. Similarly, while symbols of different colors will be focused at somewhat laterally shifted positions, such an error is not significant since the HUD of the present invention is tolerant of general errors of display accuracy.

The head-up display according to the preset invention can be used for automotive and similar applications where it is desired to combine a display with an outside view which has depth. The system according to the present invention is designed in a biocular or binocular configuration with zero overlap between the ocular fields. The system, which originally is ree of constraints due to display accuracy, is seen to overcome the problem caused by parallactic splitting of the image.

More specifically, the system according to the present invention, supplies separate images to each of the viewer's eyes. Because only one eye is viewing a particular symbol, that symbol will be clearly perceived regardless of where the eye happens to be focused. This is because a single eye perceives images which have a considerable depth of field. Furthermore, since only one eye is perceiving each displayed symbol, that symbol will be seen as a single entity, without the splitting which occurs when two eyes are used and when the eyes are focused at distances widely different form the distance at which the symbol is located.

The system according to the present invention could be installed in an automobile or in other applications. It could be adjusted to accommodate drivers of various heights, and lateral head positions. The system would enable the driver to focus his eyes on various portions of the scene in front of him, just as he would at present, while at the same time receiving clear sharp images communicating to him vital information, such as car speed, time, fuel consumption, engine status, etc., without the need to avert his eyes from the road and to study the dashboard displays.

BSIEF DESCRIPTION OF THE DRAWINGS The above and other embodiments of the present invention may be more fully understood from the following detailed description when taken together with the accompanying drawing wherein similar reference characters refer to similar elements throughout and in which: FIG. 1 is a schematic side view of a conventional head-up display system with upward projection of the light images;

FIG. 1a is a schematic side view of a typical head-up display system with downward projection of the light images;

FIG. 2 is a schematic top view of a configuration according to the present invention, unfolded in the horizontal plane, with the combiner and display source removed for ease of presentation;

FIG. 3 is a schematic depiction of the head-up display according to the present invention for use in the determination of the Head Motion Box and the display field-of- view;

FIG. 4 is a plot showing the Horizontal Head Freedom (HHF) (in mm) as a function of the symbol angular size (in mrad) for a typical case, where IP = 65 mm, d_eye = 5.0 mm and ER = 830 mm;

FIG. 5 is a depiction of the field of view of a HUD according to the present invention.

FIG. 6 is a schematic plan view showing a perpendicular projection of the light images to the viewer's eyes; FIG. 7 is a schematic plan view showing a sideways projection of the light images to the viewer's eyes. DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is of a head-up display system for use in automotive or similar applications which can be inexpensively installed and which effectively and clearly communicates to the viewer/driver vital information.

Conventional head-up display systems operate as shown schematically in Figure 1. A display source 10, usually a CRT tube, is mounted so as to project light through a lens 12. Light beams such as the one designated by 14, containing the information to be communicated to the viewer, are projected from display source 10 through lens 12. Light passing through lens 12 proceeds toward a means for directing the light images to the eyes of the viewer, typically a combiner 16. Part of the light reaching combiner 16 passes through and is lost while a portion of the light is reflected by combiner 16 to the eyes 18 of the viewer. Light from the outside view in front of the viewer passes through combiner 16 and is combined with the light emanating from display source 10 which is reflected by combiner T2. In this way the viewer simultaneously receives light from the outside view in front of him as well as from display source 10. Combiner 16 thus serves to "combine" the light from the two different sources and to direct the combined image to the eyes of the viewer. One possible embodiment of the present invention is depicted in Figure 2. The system is shown from above, unfolded in the horizontal plane. For clarity of presentation, the combiner and display source have been ' omitted.

The device according to the present invention includes an aperture stop. The aperture stop may take on a variety of forms, such as the clear width of a lens or the clear width of the combiner or mirror used to direct light images to the eyes of the viewer. Clear width being defined in a direction coinciding with the interpupillary line. In yet another alternative, the aperture stop may be created by a stop located between the image source and the lens, or between the lens and the combiner or mirror, or even between the combiner or mirror and the eyes of the viewer.

For purposes of illustration, the principles of the present invention are described in terms of an aperture stop defined by the clear width of the lens. For the case where the aperture stop is attached to the lens, a rectangular lens 20 having a horizontal aperture D_H is located at distance ER in front of the viewer's eyes, with the right eye 22 and the left eye 24 being separated from each other by an interpupillary (IP) distance of, say, 65 mm. Lens 20 creates a virtual image set into the landscape at a distance T from eyes 22 and 24.

The size of the horizontal RFOV, like that of the LFOV, is equal to the angle subtended by the width D_H of the aperture stop, as seen from the eye. Moreover, for a given value of the interpupillary distance, IP, and the desired image distance T, the width D_H can be narrowed till separation between the two ocular fields is obtained. The larger the separation the smaller is the size of the fields. Thus, an optimal value can be reached where the two fields just touch each other with zero overlapping. The point "0" at the center of the image plane indicates the point of contiguity.

It should be pointed out that the idea of separating the ocular fields from each other can be realized in another way. In another embodiment of the present invention, one can adopt the binocular approach. In the binocular embodiment, two entirely different channels are used, one for the right eye and the other for the left eye. The channels can be parallel or substantially parallel, with the right channel for the right eye and the left channel for the left eye. In such a case the principle of separation between the ocular fields is honored by preventing any intrusion of light from one channel to the other eye. In what follows the description is limited to the primary embodiment. The full width required for the aperture stop to cover the maximum field with no ocular overlapping can be readily calculated. From the geometry of Figure 2, the following relationship is obtained:

Rearranging, the desired lens width is:

(2) - D_H = IP (T - ER)/T

It is readily seen that if the image is projected to a long distance, the width of the lens will be the same as the IP distance, typically 65 mm, whereas projecting the image to a nearer distance will require a narrower lens width.

For example, for a projection distance, T, of 3300 mm, with ER = 830 mm, a lens of 48.7 mm width is required:

(3) D_H = 65 (3300 - 830)/3300 =^'48.7 mm

For automotive applications a projection distance of 3.3 m seems to be a proper choice for a number of reasons. First, thanks to the eye's +/-0.3 diopter depth of field, the whole practical range of distances up to infinity is fully covered. Second, at a larger distance the illusion of a "floating" image can be upset when the displayed symbols will not be hidden behind closer objects in the view. Such an effect may lead to an unnecessary disturbance. Third, with a shorter projection distance a combiner of a smaller width is required to minimize obstruction effects on the outside view. Finally, the closer the projection distance is, the more immune is the system from focusing the sun directly on the projection board.

The instantaneous-field-of-vi.ew in the horizontal direction of both eyes can also be calculated. In Figure 2 this field is indicated as FOV_H and is equivalent to: (4) F0V_H = 2 ζ.

Since angled is external to the triangle Δ(0,M,RE), it equals:

(5) = RFOV

Substituting the following:

(6) £ = (ip/2)/τ

(7) RFOV =^* D_H/ER

we obtain:

(8) FOV_H =" IP/T + 2 (D_H/ER)

Putting D_H from equation (2) into equation (8), yields:

(9) FOV_H = (IP/ER) {2 - (ER/T)}

IP and ER have fixed values. The interpupillary distance, IP, is an anthropometric constant, while ER is geometrically constrained by the size and shape of the vehicle. Only T, the display distance, is to some extent flexible.

Following the numerical example above, the size of FOV_H can be calculated to be:

(10) FOV_H = (65/830) {2 - (830/3300)} = 136.9 mrad = 7.85 deg

This is the gross horizontal field which also includes the dark frame. As will be explained below, this frame is included to make possible a reasonable amount of head motion without losing view of the display. If the same amount of motion is also required along the vertical axis, the size of the vertical field can be determined as follows for an aspect ratio of 1:1.

(11) FOV_H = 136.9 mrad = 7.85 deg

Hence, the vertical size of the lens will be:

(12) D_v = 2 ER tan (F0V_v/2)

Following the numerical example above:

(12a) D_v = 2 (830) tan (7.85/2) = 113.9 mm

It should be pointed out that an aspect ratio of 1 :1 may not be necessary. It is entirely possible to display sufficient amount of information in strips having shorter vertical extent, e.g., as would be the case with a more conventional aspect ratio of 3/4. In this regard, it should be mentioned that the optimal organization and display of the various bits of information in the strips will be subject to the precise application and under the control of the designers implementing the present invention. For example, it may not prove practical to have each symbol in a specific location in the FOV, since usually it is not necessary that all symbols be simultaneously activated. It may be highly advantageous to organize the display on a time multiplex basis, i.e., using the same location on the strip to show different data at different times. According to this approach a microprocessor or other electronic or quasi-mechanical means can be used to implement Multi-Function Displays (MFD) at different locations on the strips. In this manner considerably more information can be displayed given the limited size of the two vertical strips. For example, the right and the lef turn verification signals which may not be activated simultaneously, can be displayed as two different symbols but on the same MFD. Furthermore, based on the time multiplexed principle, different symbols which may be activated simultaneously for relatively long periods of time can be alternately displayed by a single MFD. For example, if both the engine overheat and the low brake fluid conditions are active, display space could be conserved by periodically exchanging the two symbols rather than displaying both signals simultaneously at different locations. The form and size of the active field-of-view are determined by two main requirements. First, the single symbol display must be appear large enough to make it discernable and easily recognizable to the viewer. Second, allowance must be made for reasonable head movement without losing the displayed symbols and without allowing any point in the field to be exposed to both eyes simultaneously, in accordance with the underlying principle of maintaining strict separation between the ocular fields of the two eyes.

These are two conflicting demands. The greater the size of the symbol used, the less freedom the viewer will have to move his head while still maintaining the symbol in view. Conversely, the more freedom of head movement the viewer desires, the smaller will have to be the size of the symbols. The optimum combination of symbol size and freedom of head movement will be determined for each application by trading off the benefits and disadvantages.

To see how such a tradeoff might be effected for a particular application one must first note that each point on the projected symbol has a corresponding horizontal fan of rays which fans out to a size nearly equal to the interpupillary distance, at the eye plane. This can be illustrated by using the "0" field point at the center of the image plane of Figure 2. The horizontal fan of virtual rays which reach it have an angular width of 2k, equal to the angle size subtended by the lens from the image plane. At the eye plane this fan spreads to a width of IP. Actually the size is IP - d₀e„y₀e only to prevent a possible exposure of the beam to both eyes simultaneously. d_eye is the pupil diameter of the eye under conditions of dim light

(13) d_eye = 5.0 mm

It should be noted that for a different field angle, &_H, the width of the beam decreases further to (IP - d_eye) cos($_H), but since only relatively small field angles are involved, one can ignore the cosine term. A certain symbol is made visible to one eye, say the right eye. Its angular width is designated by ψ , and its three most relevant fans are shown in Figure 3. Shown are the central fan, related to the symbol's center point, the fan furthest to the right, related to the fan's left edge, and the left fan, related to the symbol's right edge. The position of this symbol will be determined about the Θ_H field angle so that the principal ray of the central fan falls at the center of the eye's pupil, IP/2, to__ the right of the central meridional plane. Therefore©"_H is obtained by dividing this distance by the eye relief length, ER:

Using the numerical example, one gets:

(15) &_χ = 0.5 (65/830) = +/-39.2 mrad

At this stage it remains to determine the amount of horizontal head freedom (HHF) allowed, before the right edge of the symbol disappears.. When the right eye is situated in its nominal position, IP/2 to the right of the center plane, this situation first arises when the head moves left, up to the left edge of the right fan. With 50% vignetting to eliminate the edge of the picture, the amount of head freedom allowed can be computed by the method depicted in Figure 3:

(16) HHF = IP/2 - {< ER + eeyyee'²⁾

Given ER, IP and d_eye, the choice of too large a symbol,

I, will restrict head motion and unnecessarily hide the view outside. Similarly, too much freedom of head movement entails reducing the size of the symbol to such an extent as to make it difficult to identify. Figure 4 presents a plot of equation (13) in which <j> is displayed in mrad over the entire possible range of φ equalling from 0 to 36 mrad.

One must find the minimum reasonably small size required for quick and easy identification of the symbol. The human eye has an angular resolution of 1.0 min. of arc. This angle applies to normal visual acuity (V.A. = 1.0). Thus, the human eye is capable of discerning and identifying a five-element letter, each element of which subtends an angle of 1 min. of arc. To facilitate identification of the symbol, one can choose to operate with elements which are three times larger than the minimum size which can be resolved by the human eye, that is, to require that each of the five elements have an angle width subtending 3.0 min. of arc. Consequently a one digit element will subtend an angle of

(17) <^■_> = 5 (3) { /180/60} = 4.4 mrad

To account for the fact that the background against which the image will be projected will often be rather cluttered and confused, providing for a visual acuity of less than 1.0, < ought to be increased somewhat, say, to about 6 mrad.

According to equation (15) and Figure 4, for a single digit symbol of 6 mrad, the HHF is +/- 25.0 mm, and with a two digit symbol HHF reduces to +/-20 mm, etc. It is seen that out of the gross field-of-view, which is square in shape with a border of 136.9 mrad according to equations (10) and (11), only a limited area of two vertical strips, as shown in Figure 5, is allocated to the active display field. The display strips, ψ = 12 mrad width each, are located 2#_H = 78.4 mrad apart center-to-center, symmetrically about the center plane. The strips are also encircled by a dark frame having a width of:

(18) w = F0V_H/2 - Θ-_H - j>/2 = 23.3 mrad

in order to allow for head movements up and down and from side to side with reasonable freedom.

Unlike the horizontal size of the active field, the vertical direction is flexible and freely expandable. Thus, with an aspect ratio (AR) of V/H = 1/1, it can be widened beyond the standard proportion (V/H = 3/4), if so desired. In this connection, another essential difference should be pointed out between the horizontal and the vertical imaging, relating to the stop position. Whereas in the horizontal position the stop is determined by the lens aperture or close to it, the stop for the vertical imaging is located at the ocular plane.

The two strips of the active field-of-view make possible a head motion box (HMB) whose size is:

(19) HMB = 2 HHF = 40.0 mm

along the vertical and horizontal directions, (40.0 x 40.0 mm), without losing any detail of the instantaneous field of view. It is to be noted that a considerably extended HMB, roughly of size:

(20) 2 [(IP/2) 3] = 195 mm along the horizontal direction would make it possible to maintain observation of at least one half of the field, that is, at least one strip or the other, to avoid any possibility of "completely losing" the displayed image. In the vertical direction the span would remain 40.0 mm.

Various lens and lens geometries, including various low grade lenses, would adequately serve in the context of the present invention. The precise specifications of the lens can be determined with regard to the precise application envisioned. As explained above, since the lens need only function over rather narrow field of view (+/-2.7 deg) , it is relatively free of the influences of chromatic aberrations and it is relatively immune from distortions, such as those caused by display accuracy and parallax errors. Determination of the lens' effective focal length f, is subject to several considerations. First, it should be selected to be sufficiently long so that every element in the symbol is larger than the smallest possible lighting element, e.g., an LCD board would have a minimal element of size 0.2 - 0.3 mm. Furthermore, a longer f will relax production tolerances and contribute toward making the lens largely immune from misalignments. On the other hand, the choice of the lens focal length is subject to limitations of the volume envelope for the physical size of the entire system. With regard to this point, determination of the lens' effective focal length is not really hampered, since the light path can be compactly folded by using one or more auxiliary mirrors.

The HUD system can be mounted in the vehicle in any convenient fashion. For example, the system can be mounted with the projection of the image from the source oriented upward in a vertical direction, as in the conventional approach and as shown in Figure 1. Alternatively, the image can be projected downward from the image source, as shown in Figure 1a. Although the conventional, upward projection, approach makes it possible to easily use a combiner which is attached as a semi-reflecting coating directly to the windshield, it may be preferable to use an independently mounted combiner with a downward projection of the image. Such a system offers certain advantages in that the independent combiner can be freely adjusted to accommodate drivers of varying heights, in the manner in which today's rear view mirrors are adjusted. Also, a downward image projection serves to significantly limit or eliminate disturbances due to solar or other extraneous lighting effects. In addition, such a configuration would obviate the need for moving or redesigning the traditional dashboard.

In another embodiment of the present invention, the semi- transparent combiner is replaced with a mirror. The back surface of the mirror is opaque to light and will not allow sunlight or light from other extraneous sources to enter the system and cause glare thereby degrading the system performance. Of course, such a mirror would also be opaque to light coming from the outside view and would also block this desirable light from reaching the viewer's eyes. However, because of the small size of the mirror used, and for the same reasons which make the present invention operable, the size of the mirror is small enough in relation to the viewer's interpupillary distance that beyond the projection distance T it will never completely block any object from the view of the driver. This is because the mirror, being smaller than the interpupillary distance, will at most hide an object from one but never from both eyes of the viewer. An incidental benefit of using an opaque mirror is that the back side of the mirror can be used to conveniently mount and adjust the mirror. It is to be noted that using an opaque mirror in place of a combiner makes it possible to use the conventional upward projection configuration rather than the downward projection system described above and allows for the use of a mirror rather than a semi-transparent device, thereby simplifying and reducing the costs of the system.

In yet another embodiment of the system according to th present invention, the display would be shifted sideways fro the field center. This would leave the display easily visibl to the viewer yet it would keep the central field-of-vie region free of displays and available exclusively for viewin road activity. The angle of the projection can be any convenient angle. It is envisioned that the angle will be in the range of from slightly more than 0 degrees from the perpendicular to approximately 45 degrees from the perpendicular. Figures 6 and 7 show schematic plan views of the projections of the images from combiners or mirrors 60 to the eyes of the viewer 62 in a perpendicular and in a sideways projection, respectively. Such a configuration would also obviate the need for moving or redesigning the traditional dashboard.

It is seen that the present invention and the embodiments disclosed herein are well adapted to obtain the ends set forth at the outset. Certain changes can be made in the method without departing from the spirit and the scope of this invention. It is realized that changes are possible and it is further intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner. It is intended to cover the invention broadly in whatever form its principles may be utilized. The present invention is, therefore, well adapted to attain the ends and advantages mentioned, as well as others inherent therein. Those skilled in the art may find many variations and adaptations thereof, and all such variations and adaptations, falling within the true scope and spirit of applicant's invention, are intended to be covered thereby.

Claims

WHAT IS CLAIMED IS:

1. A head-up display system, for communicating information to a viewer's eyes, comprising:

(a) a display source capable of transmitting light images containing the information;

(b) a lens capable of forming said light images;

(c) directing means for directing said light images to the eyes of the viewer; and

(d) an aperture stop for creating images to be viewed separately by each eye of the viewer.

2. A head-up display system as in claim 1 wherein said aperture stop is no larger than the interpupillary distance of the viewer.

3. A head-up display system as in claim 1 wherein said aperture stop is determined by the clear width of said lens.

4. A head-up display system as in claim 1 wherein said aperture stop is determined by the clear width of said directing means.

5. A head-up display system as in claim 1 wherein said aperture stop is determined by the clear width of an independently mounted diaphragm.

6. A head-up display system as in claim 5 wherein said diaphragm is mounted between said display source and said lens.

7. A head-up display system as in claim 5 wherein said diaphragm is mounted between said lens and said directing means.

SUBSTITUTE SHEET

8. A head-up display system as in claim 1 wherein said means for directing said light images to the eyes of the viewer is a combiner.

9. A head-up display system as in claim 1 wherein said means for directing said light images to the eyes of the viewer is a mirror.

10. A head-up display system as in claim 1 wherein said light images are projected substantially upward from below said lens.

11. A head-up display system as in claim 1 wherein said light images are projected substantially downward from above said lens.

12. A head-up display system as in claim 1 wherein said light images reach the viewer from an angle of from 0 to 45 degrees from the normal forward-looking direction of the eyes of the viewer.

13. A head-up display system as in claim 1 wherein said light images reach the viewer from an angle substantially along the normal forward-looking direction of the eyes of the viewer.

14. A head-up display system as in claim 1 wherein said system is installed in a vehicle.

15. A head-up display system as in claim 14 wherein said vehicle is an automobile.

16. A head-up display system as in claim 15 wherein said means for directing said light images to the eyes of the viewer is mounted onto a windshield.

SUBSTITUTE SHEET

17. A head-up display system as in claim 15 wherein said means for directing said light images to the eyes of the viewer is mounted separately from the windshield.

SUBSTITUTE SHEET