Optical: systems and elements – Projection screen
Reexamination Certificate
1997-11-26
2002-12-03
Le, N. (Department: 2858)
Optical: systems and elements
Projection screen
C359S443000
Reexamination Certificate
active
06490086
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image display apparatus, and more particularly to an image display apparatus for guiding the light beam containing the image information from image generating means to the eyes of the observer through the optical system thereby displaying an enlarged virtual image of the image displayed on the image generating means, and adapted for use as so-called head mounted display (hereinafter defined as HMD) to be mounted on the head of the observer.
2. Related Background Art
The head mounted display (HMD) is an apparatus for guiding the light beam, containing the image information and generated from an image displayed on the image generating means such as a liquid crystal display apparatus (LCD) or a cathode ray tube (CRT), to the pupils of the observer through the optical system thereby presenting an enlarged virtual image of the above-mentioned image, wherein the observer achieves observation by matching the exit pupil of the HMD optical system with the pupil position of the observer.
Also the HMD is preferably compact and light in weight since it is to be mounted in use on the head of the observer and, for this reason, the image generation means is often constituted by a small-sized LCD. In the HMD utilizing LCD, the light beam diverging from each of the LCD pixels shows the highest contrast in a direction which is very slightly displaced from in the normal direction to the display surface of the LCD. For this reason, the optical system of the HMD is preferably so constructed as to have the entrance pupil of the LCD side at an approximately infinite position, thereby realizing so-called telecentric system at the entrance side, and most of the proposed HMD's have the optical system of such entrance side telecentric configuration. Also in the HMD's utilizing small-sized CRT, the entrance side of the HMD is usually constructed as a telecentric system.
While the exit pupil of the optical system of the HMD is fixed, the pupils of the observer are movable by the movement of the visual axis. For this reason, the exit pupil of the optical system of the HMD is preferably made large, so as to be able to cover the movement of the pupils of the observer.
FIG. 2
is a schematic view showing the range of movement of the pupil of the observer, resulting from the movement of the visual axis, wherein a schematically represented eyeball
100
of the observer rotates about a center
101
of rotation when the visual axis moves. I
C
indicates the pupil of the observer, with a pupil diameter p, when the visual axis of the observer is directed straight ahead. The pupil I
C
is positioned at a distance r from the center of rotation of the eyeball. For an entire angular width w of the displayed image, the pupil of the observer moves to a position I
R
when the visual axis is shifted to the right-hand end, or a position I
L
when the visual axis is shifted to the left-hand end.
Consequently, with the movement of the visual axis within the field of view w, the pupil of the observer moves within a range represented by a line segment AB, of which length s is given by the following equation (1):
s=2·{r·sin (w/2)+(p/2)·cos (w/2)} (1)
The diameter of the exit pupil of the HMD optical system is preferably at least equal to the length s given by the equation (1). Consequently, the length s is called the desirable exit pupil diameter. In the equations of the present text, a mark·indicates multiplication.
In general, the field of view W
H
of the HMD in the horizontal direction is selected larger than that W
V
in the vertical direction. On the other hand, the light illuminating the LCD is generally symmetrical about the axis perpendicular to the light-emitting face of the light source and, if certain means is provided, for example for increasing the numerical aperture of the illuminating light beam, in order to obtain an exit pupil diameter sufficiently large for the image display angle in the horizontal direction, the exit pupil diameter may become excessively large for the vertical direction to deteriorate the efficiency of utilization of the LCD illuminating light.
On the other hand, because of the characteristics inherent to the LCD, within the light beam outgoing from each of the liquid crystal pixels, the light beam showing a high contrast and suitable for image display is limited to a certain diverging angle (&agr;/2) with respect to the normal line to the display surface of the LCD. The entire width a will be called an effective viewfield angle of the LCD.
FIG. 7
is a view showing the relationship, in the optical system of HMD, among the effective viewfield angle &agr;, the size h of the LCD display area, the field of view w of the HMD and the exit pupil diameter q of the optical system, wherein shown are a schematically illustrated optical system
2
of the HMD, an LCD
3
and an exit pupil
3
of the HMD optical system. The light beam entering from the LCD
3
into the HMD optical system
2
is assumed to be telecentric (a ray outgoing from the display surface of the LCD
3
perpendicularly thereto being the principal ray). In this state there stands the following relation (2) wherein f is the focal length of the HMD optical system
2
and h is the width of LCD:
f=h/{2·tan (w/2)} (2)
As the virtual image displayed by the HMD optical system
2
is generally distanced from the pupil by as large as 1 to 2 meters, the light beams from the different points on the display surface of the LCD reach the exit pupil of the optical system in substantially parallel light beams. As shown in
FIG. 7
, a pair of peripheral rays R
1
, R
2
outgoing from a point C on the LCD
3
within the diverging angle a in the plane of drawing become parallel light beams after passing the optical system
2
and pass the exit pupil plane in this state. The distance between a pair of peripheral rays R
1
, R
2
at the position of the exit pupil represents the exit pupil diameter q in the direction of the plane of drawing paper.
Consequently the following relation (3) stands among the effective viewfield angle &agr; of the LCD, the entire focal length f of the HMD optical system
2
and the exit pupil diameter of the optical system:
q=2·f·tan (&agr;/2) (3)
The relations (2) and (3) lead to the following relation among the exit pupil q, the field of view w of the HMD and the size h of the LCD display surface:
q=h·tan (&agr;/2)/tan (w/2) (4)
The value q represented by the equation (4) indicates the maximum exit pupil diameter formed by the light beam of high image contrast within the effective viewfield angle &agr; of the LCD and will be called the exit pupil diameter of the optical system.
By substituting, for the distance r from the center of the eye to the pupil and the pupil diameter p, r=13 mm and p=2 mm as the average values for human being in the equation (1) and also substituting &agr;=30° as the effective viewfield angle of ordinary small-sized LCD's in the equation (4), there are obtained:
desirable exit pupil diameter s=2·{13·sin (w/2)+cos (w/2)} (1)
optical system exit pupil diameter q=h·tan (15°)/tan (w/2) (4)
and the comparison of the equations (1)′ and (4)′ indicates that the desirable exit pupil diameter of the HMD optical system increases with the increase of the entire angular display width w while the exit pupil diameter q of the optical system, formed by the light beam within the effective viewfield angle &agr; of the LCD decreases with the increase of the field of view w.
Consequently, for a given size h of the LCD display surface, the field of view w becomes limited if the display is made with the light beam of high image contrast within the effective viewfield angle &agr; of the LCD.
Also according to the equation (4)′, the exit pupil diameter q of the optical system is proportional to the size h of the LCD display surface. In case
Le N.
LeRoux Etienne P
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