Contrast improvement in liquid crystal light valve projectors

Details Contrast improvement in liquid crystal light valve projectors Contrast improvement in liquid crystal light valve projectors

1999-11-12

2001-05-01

Dowling, William (Department: 2851)

Optics: image projectors

Polarizer or interference filter

C349S005000, C353S069000, C353S070000

Reexamination Certificate

active

06224215

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to contrast improvement in liquid crystal light valve projection systems, more particularly to a light valve image projector with a disposition for improving the contrast of the by that projector projected optical image.
BACKGROUND OF THE INVENTION
For an enlarged projection of images on a screen by means of a light valve projector, an optical image is put on a light valve and bundled light is sent through said light valve. Said light then passes through an appropriate projection lens system, which focuses an image on the screen. An appropriate light valve for such a projection system is a liquid crystal display, commonly referred to as an “LCD”.
Two kinds of liquid crystal light valve projectors exist: OFF-axis projectors and ON-axis projectors.
FIG. 1
shows the optical path in an OFF-axis projection system. Said projection system comprises an OFF-axis projector
1
and a screen
2
. The OFF-axis projector
1
comprises an illumination system
3
, a field lens
4
, a polariser
5
, an LCD
6
, an analyser
7
and a projection lens
8
. The illumination system
3
comprises a lamp, a reflector, and an integrating system which converts the circular and non-uniform light distribution coming from the reflector into a rectangular and uniform illumination of the LCD
6
. The field lens
4
images the illumination system into the entrance pupil of the projection lens
8
. In this way, it ensures that the light with which the LCD
6
is illuminated is directed into this entrance pupil of the projection lens
8
. The projection lens
8
images the illuminated LCD
6
on the screen
2
. The line through the optical centres of the illumination system
3
, the field lens
4
and the projection lens
8
is further called the optical axis
9
of the projector
1
.
In case the OFF-axis projector
1
is a multi-light valve projector (for example a projector with three light valves), the optical path is more complex, because by means of a set of dichroic mirrors the light is splitted into 3 colours (red, green and blue), and after modulation of the light, these colours are recombined again. The OFF-axis projector then comprises an illumination system, three field lenses, three sets consisting of a polariser, an LCD and an analyser, and a projection lens. A line through the optical centres of the illumination system, one of the field lenses and the projection lens is an optical axis of the projector. The schematic description of the illumination and imaging system shown in
FIG. 1
is not altered by these colour-splitting mirrors. In what follows the state of the art is described as if there were only one illumination system, one field lens, one set consisting of a polariser, an LCD and an analyser, and one projection lens.
The direction of the optical axis
9
of the projector
1
is different from the direction of the optical axis
10
of the projection lens
8
. Both optical axes
9
and
10
include an OFF-axis angle &agr; as shown in FIG.
1
. This angle &agr; can have any positive value, but in the most common OFF-axis configuration, the projector
1
is set up in such a way that the optical axis
10
of the projection lens
8
intersects with the bottom of the image of the screen
2
.
If the OFF-axis angle &agr; is zero, or in other words, if the direction of the optical axis
9
of the projector
1
is equal to the direction of optical axis
10
of the projection lens
8
, the projector is called an ON-axis projector.
FIG. 10
a
shows the minimum configuration of an ON-axis liquid crystal light valve projection system. It comprises an ON-axis projector
11
and a screen
2
. The ON-axis projector
11
comprises the same parts as the OFF-axis projector
1
described hereinabove. Said same parts have been denoted by means of the same reference numerals.
FIG. 2
shows an image source of as well OFF-axis and ON-axis liquid crystal light valve projectors
1
and
11
. Said image source comprises a polariser
5
, an LCD
6
and an analyser
7
. Although represented in
FIG. 2
as three separate parts, two of the parts or the three parts can form one unit. The polariser
5
and the analyser
7
are both light polarising elements and have in their planes each an absorbing direction and a non-absorbing direction, the absorbing direction being perpendicular to the non-absorbing direction.
The LCD
6
comprises, with reference to the light direction, a front glass plate
15
, a back glass plate
16
and sealed in between both glass plates
15
,
16
a twisted nematic liquid crystal layer. The sides of the glass plates
15
and
16
not touching the twisted nematic liquid crystal layer, each have a layer with transparent image forming pixel electrodes. The twisted nematic LCD (TN-LCD) may be addressed with an active matrix. In such an active matrix TN-LCD, a switching device, such as a thin film transistor or a number of thin film diodes, is integrated on each pixel.
In
FIG. 2
, the non-absorbing direction
17
of the polariser
5
is perpendicular to the rubbing direction
18
of the front glass plate
15
of the LCD
6
. Said rubbing direction
18
defines the preferred direction of the liquid crystal molecules at the interface with the front glass plate
15
. The non-absorbing direction
19
of the analyser
7
is perpendicular to the rubbing direction
20
of the back glass plate
16
. In a twisted nematic liquid crystal display, the rubbing directions at the opposite glass plates are perpendicular, and therefore the molecules form a 90 degrees twisted helix in the bulk as shown in
FIG. 3
a
and
FIG. 3
b
.
FIG. 3
a
shows the distribution of the molecules
21
in a twisted nematic LCD between the front glass plate
15
and the rear glass plate
16
in the bright state.
FIG. 3
b
shows the corresponding distribution in the dark state.
To obtain an optimum black level in an image projected by a projection system containing a polariser, an LCD and an analyser, the non-absorbing directions of these three elements must be correctly matched. Therefor, the analyser or the polariser may be installed rotatable around their normal.
The contrast of the image obtained with an LCD as light valve depends strongly on the angle of incidence of the light entering the LCD. It is by consequence LCD area and, by consequence, screen area dependent, what is explained by the following.
FIG. 4
shows the typical viewing angle characteristics of a twisted nematic LCD. The graphs are lines of equal contrast between the bright and dark state at the two corresponding driving voltages, with relative values between 3 and 300. The vertical axis shows the vertical angle of incidence between −40° and 40°. It presents the component of the light ray within the plane perpendicular to both the plane of the LCD and the horizontal image scanning direction of the LCD. The horizontal axis gives angles of incidence between −40° and 40° and is the component of the light ray within the plane perpendicular to the plane of the LCD and the vertical image scanning direction of the LCD.
FIG. 4
thus demonstrates the dependence of the contrast in function of the incident angle of the light which enters the LCD. The contrast changes much more in the vertical direction than in the horizontal direction of the screen. Moreover, the maximum contrast is not obtained for light rays with normal incidence but for light rays having a small incident angle in the vertical direction. This angle corresponds with the angle &agr;
v
shown on
FIG. 3
b
. The angle &agr;
v
corresponds very well with the remaining tilt at dark state of the molecules in the twisted nematic liquid crystal layer.
FIG.
1
and
FIG. 10
a
show that light rays enter the LCD under different angles. As a consequence of the phenomena shown in
FIG. 4
, without further measures being taken, an image projected by a projector using as light valve a twisted nematic LCD, will be non-uniform, which is differently contrasted in vertical direction.
FIG. 5
shows in another way how the LCD light transmission (vertical axi

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