WO1996027142A1

WO1996027142A1 - Projection lenses for light valve projection systems

Info

Publication number: WO1996027142A1
Application number: PCT/IB1996/000063
Authority: WO
Inventors: John Alfred Clarke
Original assignee: Philips Electronics N.V.; Philips Norden Ab
Priority date: 1995-02-25
Filing date: 1996-01-26
Publication date: 1996-09-06
Also published as: GB9503859D0

Abstract

A projection lens (20) for projecting an enlargement of an image in a light valve (30) onto a screen (12) in a light valve projection system has, in succession from the light valve side, first, second and third lens groups (G1, G2, G3) whose relative powers lie within prescribed ranges which are such that a space is determined between the second and third groups (G2, G3) in which a mirror (40) is disposed that folds the optical path through the lens. In a rear projection system, a compact cabinet (10) can then be achieved. For a full colour projection system using the projection lens together with three light valves, each illuminated with light of a different colour, optical combining means (24) for combining the three light valve outputs is accommodated between the light valves and the second lens group. In a single light valve system, a further mirror may precede the second lens group.

Description

DESCRIPTION

PROJECTION LENSES FOR LIGHT VALVE PROJECTION SYSTEMS

This invention relates to projection lenses for projecting an enlargement of an image in a light valve onto a screen which comprise first, second and third lens groups, the first and third lens groups being respectively at the light valve side and the screen side and the second lens group being situated between the first and third lens groups, and in which the optical path through the projection lens is folded. The invention relates also to light valve projection systems incorporating such a lens.

It should be understood that the term lens group as used herein is intended to include a single lens element as well as a plurality of lens elements in combination.

Examples of projection lenses of the above kind and a rear projection system using the lens are described in US-A-5042929. It is common in light valve rear projection systems for the image in the light valve, for example comprising a matrix liquid crystal display panel, to be projected by the projection lens onto a viewing screen via at least one mirror by means of which the optical path between the lens and the screen is folded so as to allow the size of the cabinet required to be reduced. To some extent this is similar to known rear projection systems using CRTs, although the projection optical system in other respects, and particularly the projection lens, differ and are not interchangeable because very different optical considerations apply. A major difference is that in a LC projection system, the illumination system defines the range of angles in the beam of light passing through the LC panel. In many cases the beam is approximately perpendicular to the panel over the whole area of the panel. In a CRT projection system light is emitted from the phosphor on the CRT faceplate over a complete hemisphere, with the projection lens selecting a limited range of these angles. Although near the optical axis this range of angles may be close to the perpendicular to the faceplate of the CRT, for points well away from the optical axis this does not hold. At the corners of the picture the range of angles often approaches grazing incidence. Projection lenses used in light valve rear projection systems can typically be around 20 to 30 cms in length and, therefore, play an important part in determining the size of the cabinet. The projection lens described in the aforementioned specification has an advantage in this respect in that the optical path therethrough is folded and this provides greater flexibility when arranging the components of the optical projection system. The folding of the optical path in the projection lens conveniently permits the illumination system for the light valve to be disposed out of line with the main optical axis of the light output from the lens. The described projection lens consists of a front lens arrangement of the retroreflecting type, a spaced rear lens group, comprising a single lens element, which is situated closely adjacent the liquid crystal light valve, and an inclined planar mirror which is disposed in the space between the front lens arrangement and the rear lens. The lens's optical axis is diverted through ninety degrees at the mirror. The front lens arrangement consists of six lens elements arranged in two closely spaced lens groups, the group closest to the mirror having four elements and the group remote from the mirror having two elements.

In the described projection system using this lens, the light input to the projection lens is provided by a single LC light valve. In an embodiment of projection system described in this specification for providing a full colour display, three separate projection lenses are arranged side by side and each lens is associated with a respective LC light valve. The three LC light valves are illuminated with red, green and blue light respectively which colours are obtained by splitting white light from a white light source using dichroic mirrors. The requirement for three separate projection lenses adds significantly to the cost of such a projection system.

Whilst it is known in other LC light valve projection systems to use three LC light valves, each operating with a respective light colour, in combination with a single projection lens by providing an optical combining unit, for example a dichroic mirror or prism arrangement, between the light valves and the projection lens to combine their outputs, such an arrangement is not feasible with the folded projection lens of US-A-5042 929 because the location of the folding mirror prevents known types of combining optics being used at that region.

It is an object of the present invention to provide an improved folded-path projection lens for light valve projection systems which offers flexibility in use.

It is another object of the present invention to provide a folded-path projection lens suitable for use in a light valve rear projection system which is reasonably simple and inexpensive to manufacture and which allows a compact cabinet to be used. It is another object of the present invention to provide a folded-path projection lens which overcomes the aforementioned limitations.

It is a further object of the present invention reasonably to provide a full colour projection system which is inexpensive to produce.

According to a first aspect of the present invention, there is provided a projection lens of the kind described in the opening paragraph which is characterised in that the powers of the first, second and third lens groups, K-, K₂ and K₃ respectively, are selected so that O.SK^OJK, 0.25K<K₂<0.5K and -0.6K<K₃<0, where K is the total power of the projection lens, and in that a mirror is disposed between the second and third groups which folds the optical path through the projection lens. The prescribed ranges for the relative powers of the lens groups are such that a space is determined between the second and third groups in which a folding mirror can be accommodated. With such a lens it then becomes possible, for example, to accommodate the kind of optical combiner units needed to provide a multi-colour display when using separate light valves for different colours while at the same time still retaining the space saving benefits of a folded optical path in the projection lens.

The invention stems from an appreciation that by suitably designing the lens groups to have appropriate relative powers then the desired folding of the optical path can be accomplished between the second and third groups as a spacing between the second and third groups sufficient to take a reflection element can be achieved. By situating the reflective element between the second and third groups, then limitations of the kind found in the projection lens of US-A-5042929 caused by the presence of a path-folding mirror between the first and second lens groups, which practically restrict the projection lens to use with a single light valve, are removed. Importantly, the projection lens of the present invention can be used in conjunction with more than one light valve.each operating with light of a different colour, which together with the colour combining optics can now be accommodated optically preceding the second lens group, which arrangement is prohibited with the lens of US-A- 5042929 because of the presence in that lens of a mirror preceding the second lens group. Thus, a colour projection system using a single projection lens in conjunction with three (red, green, and blue) light valves becomes viable.

According to a second aspect of the present invention, therefore, there is provided a light valve projection display system comprising at least one light valve for generating a display image, illuminating means for illuminating the light valve, a viewing screen, and a projection lens for projecting an enlargement of the image in the light valve onto the viewing screen, which is characterised in that the projection lens comprises a projection lens according to the first aspect of the present invention. Whilst the projection lens can be used in a front projection system, the lens is particularly beneficial when used in a rear projection system because it offers the capability of lay-outs being used which result in compact cabinets.

In an embodiment of projection system offering full colour display, the system includes three light valves each arranged to be illuminated with light of a respective and different colour, and optical combining means disposed between the three light valves and the second lens group of the projection lens. Unlike the full colour projection display system described in US-A-5042929, which requires a separate projection lens for each light valve, this system is considerably less expensive to produce since all three colours are projected via the same lens and offers the further advantage that any problems due to misregistration of the three separately projected colours at the viewing screen, which may occur in the known system, are avoided.

The projection lens of the present invention could, however, be utilised in a projection system using a single light valve, which can be of a monochrome type or a multi-colour, for example red, green and blue, type employing a colour microfilter array or other colouring means through which different groups of elements of a matrix array of elements in the light valve serve to modulate light of respective different colours, thereby giving benefits similar to those offered by the lens of US-A-5042929 in terms of reduced cabinet size requirements by virtue of the folded optical path in the projection lens. The projection lens of the present invention has the further advantage, however, that a second mirror can be disposed between the light valve and the second lens group so that the optical path through the lens is folded twice. The optical path folding obtained by this second mirror may be a plane orthogonal to the plane in which the optical path is folded by the first-mentioned mirror, that is, the optical axis of the light from the light valve can be orthogonal to the plane containing the optical axis of the light through the second lens group and the optical axis of light through the third lens group after folding by the first mentioned mirror. It will be appreciated that this offers further flexibility in the configuration of the components of the projection optics.

Projection lenses for light valve projection systems, and projection systems incorporating such, will now be described, by way of example, with reference to the accompanying drawings, in which:-

Figure 1 illustrates schematically a typical layout of a light valve rear projection video display system in which a projection lens according to the invention is used; Figures 2, 3 and 4 show three embodiments of projection lenses according to the present invention and example ray paths therethrough;

Figures 5, 6 and 7 show the modulation transfer functions, (MTF), and defocus functions for the lenses of Figures 2, 3 and 4 respectively; Figures 8, 9 and 10 illustrate schematically three possible arrangements of a main optical unit for an embodiment of a full colour projection system which unit includes three LC light valves; and

Figure 11 shows schematically in section an example construction of a main part of one of the embodiments of projection lens. It should be understood that the Figures are merely schematic and are not drawn to scale. In particular, certain dimensions may have been exaggerated whilst other dimensions may have been reduced. Also, the same reference numerals have been used throughout the Figures to denote the same or similar parts.

Referring to Figure 1 , the LC rear projection video system consists of a cabinet 10 having a transmissive, rear projection, planar display screen 12 onto which an enlarged display image is projected via a mirror 14 from a projection lens 20. The optical path in the rear projection system is folded to achieve a compact cabinet. As in known rear projection systems a substantial part of this folding is done in the space between the projection lens 20 and the screen 12 by means of the mirror 14. Increased compactness is achieved by using a projection lens within which the optical path is folded. This permits greater flexibility in the arrangement of the components of the optical system within the cabinet, and by suitable configuration, results in a reduction in the overall size of the cabinet.

This embodiment of projection system comprises a full colour projection system in which three separate light valves, each comprising an active matrix, twisted nematic liquid crystal panel with polarising layers at the input and output sides, are used which are each illuminated by light of a respective different colour (red, green and blue). The modulated light outputs of the panels are combined and projected by a single projection lens onto the screen 12. Each panel is driven, in conventional manner, according to the appropriate colour component of a video, e.g. T.V., signal so that upon combining their respective modulated outputs a full colour TV picture is obtained. Illumination of the panels is accomplished using a white light source, such as a metal halide lamp together with a reflector and a condenser lens and a heat filter in a known configuration, here represented by the block 22, which produces a substantially parallel white light beam that is directed into a beam splitter/combiner unit 24 containing the three LC panels. Optical splitter and combiner arrangements are well known and example units will be described shortly. Briefly, however, the white light is split in this unit into red, green and blue components using dichroic beam splitters with each component then being directed through an associated LC panel where it is modulated according to the applied video signal. The individual panel outputs are then combined using dichroic beam- combining filters.

The projection lens 20 comprises three mutually spaced groups, or units, the first and third groups being positioned respectively on the LC panel side and the screen side and the second group being disposed between the first and third groups. The optical path through the lens is folded by means of a planar reflecting mirror element situated between the second and third groups which diverts the optical path so that the optical axis at the output side is at ninety degrees to the optical axis through the second group.

The powers of the three lens groups are selected not only to provide good performance but also to allow sufficient spacing between the second and third groups to accommodate the mirror element. To this end the lens group powers satisfy the following relationship:- 0.3K<K,<0.7K, 0.25K<K₂<0.5K, and -0.60<K₃<0 where K is the total power of the projection lens (equal to the reciprocal of its focal length), and K K₂ and K₃ are respectively the powers of the first, second and third lens groups (equal to the reciprocal of their focal lengths).

The second lens group of the projection lens also has adequate back clearance to accommodate various possible beam combining dichroic element configurations, as will be described hereinafter. Three preferred embodiments of the projection lens will now be described with reference to Figures 2, 3 and 4 which illustrate, schematically, the main components of the lens systems and show example ray paths. These three embodiments are designed primarily for use with a telecentric pupil as this gives uniform contrast over the whole picture. For simplicity, the projection lenses are shown in these Figures in conjunction with a single LC panel, here referenced at 30 in each case, and the optical elements of the illumination system are omitted. Referring to all of Figures 2, 3 and 4, the three groups of each embodiment of projection lens are identified by the references G1 , G2 and G3 and the optical path folding mirror is shown at 40. The first group G1 , nearest the LC panel 30, has in each case only one lens element, acting as a field lens. The second group, G2, which immediately precedes the space for the folding mirror element 40, consists of four elements (Figure 2) or three elements (Figures 3 and 4) and in each case has at least one single positive lens element and a cemented doublet. The third group G3, following the folding mirror 40, for the embodiments of Figures 2 to 4 respectively consists of two, four, or three elements, and in each case the final lens element is a negative lens of the meniscus type and one other lens in the third group G3 is a positive lens.

The respective powers of individual groups in each embodiment are selected so as to achieve, inter alia, a spacing between the groups G2 and G3 of adequate size to accommodate the mirror element 40 and for this reason lie within certain prescribed limits. The space between groups G2 and G3 for the folding mirror element 40 can also be expressed as a ratio to the focal length of the projection lens, or to the distance from the LC panel 30, defining the object image plane, to the second group G2. The first of these ratios varies considerably in the three embodiments but the second of these ratios is always close to 0.5. The approximate power of each lens group relative to the total power of the lens, and the approximate size of the space between G2 and G3 relative to both the focal length and the distance from the LC panel 30 to group G2 for each embodiment is given in the following table, Table 1, in which the lens embodiments of Figure 2, 3 and 4 are referred to as lenses 1 , 2 and 3 respectively.

Table 1

Lens Focal Relative Power Relative Spacing No. length Group 2 to Group 3 cm. Group 1 Group 2 Group 3 Focal LC to length Group 2

1 10.8 .36 .38 -.18 .97 .47

2 4.16 .38 .40 -.51 1.90 .46

3 4.21 .60 .32 -.37 2.43 .54

The following tables, Tables 2, 3 and 4, give the detail design numerical data for the three lens embodiments of Figures 2, 3 and 4 respectively. In these tables, the surface number denotes the surface of the lens elements in succession starting with the surface (No. 1) of the (single) lens of group G1 facing the LC panel 30 and the separation is measured along the lens axis. The object is the LC panel 30. Table 2

Lens No. 1 (Figure 2)

Numerical Aperture = .1375 Focal Length = 108 mm.

SURFACE RADIUS CURVATURE SEPARATION REFRACTIVE

(cm) (cm ¹ ) (cm) INDEX

OBJECT PLANE

1 PLANE 11.630 1.00000

2 -18.93358 -.0528162 1.720 1.62038

3 9.79910 .1020502 9.100 1.00000

4 PLANE 1.540 1.62038

5 5.72340 .1747213 .100 1.00000

6 PLANE 1.700 1.65856

7 4.34300 .2302556 .464 1.71665

8 -9.38150 -.1065928 1.520 1.00000

9 -14.40899 -.0694011 .756 1.71665

10 MIRROR 5.0

11 13.73600 .0728014 5.469 1.00000

12 -180.37193 -.0055441 1.300 1.64769

13 -8.48510 -.1178536 7.385 1.00000

14 -37.67202 -.0265449 .685 1.65238

I MAG IE PLANE 123.000 1.00000

Table 3

Lens No. 2 (Figure 3)

Numerical Aperture = .167 Focal Length = 41.6 mm

SURFACE RADIUS CURVATURE SEPARATION REFRACTIVE (cm) (cm ¹ ) (cm) INDEX

OBJECT PLANE

1 PLANE 2.030 1.000

2 -6.76177- .1478903 1.071 1.62293 Aspheric coefficients .1387E-02 -.3623E-04 .3774E-0E > .6733E-07

3 13.02358 .0767838 14.000 1.00000

4 -17.69774 -.0565044 1.115 1.62293

5 12.96417 .0771357 .100 1.00000

6 -5.59288 -.1787989 1.503 1.66161

7 24.14876 .0414100 .400 1.72327

8 MIRROR 3.901

9 -5.98065 -.1672058 4.0 1.0000010-

10 -10.20236 -.0980165 .500 1.66161

11 9.85483 .1014731 1.212 1.00000

12 -10.79285 -.0926539 1.692 1.65235

13 -7.12226 -.1404048 1.883 1.00000

14 15.11348 .0661661 .500 1.66161

15 -4.50028 -.2222083 2.383 1.00000

16 -15.13306 -.0660805 .700 1.66161

IMAGE PLANE 82.000 1.00000

The folding mirror is located between surfaces 7 and 8. Table 4

Lens No. 3 (Figure 4)

Numerical Aperture = .167 Focal Length = 42.1 mm

SURFACE RADIUS CURVATURE SEPARATION REFRACTIVE (cm) (cm^"1 ) (cm) INDEX

OBJECT PLANE

1 PLANE .830 1.00000

2 -4.38545 -.2280266 1.251 1.62293 Aspheric coefficients .1821 E-02 .8592E-05 .2053E-04 .1242E-06

3 28.37000 .0352485 17.000 1.00000

4 -15.07584 -.0663313 1.244 1.62293

5 22.17713 .0450915 .100 1.00000

6 -5.93373 -.1685282 1.935 1.66161

7 109.32666 .0091469 .400 1.72327

8 MIRROR 5.0

9 9.98613 .1001389 5.242 1.00000

10 -20.55042 -.0486608 2.651 1.65235

11 -8.37149 -.1194531 2.547 1.00000

12 6.33810 .1577759 .500 1.66161

13 -5.57693 -.1793102 2.839 1.00000

14 -23.68501 -.0422208 .700 1.66161

IMAGE PLANE 83.000 1.00000

The performances of the three lens embodiments of Figures 2, 3 and 4, expressed as the modulation transfer function (MTF) for on axis and five off axis points at intervals out to the corners of the picture are shown in Figures 5, 6 and 7 respectively. The curves shown are for one colour of light. The left hand curves give the MTF at spatial frequencies from 0 to 10c/mm (cycles/mm) for the mean focal position for the tangential (tan) and sagittal (sag) directions, while the right hand curves show how the MTF at 5c/mm in the case of Figure 5 and at 3c/mm in the case of Figures 6 and 7 varies as the focal position (FP) in mm of the light valve is changed. The lowermost curve is for on-axis. The object, "o" and image "i" distances from the axis involved are given alongside (in mms). Also shown for the four off- axis picture points is the relative pupil area, "p", which is a measure of the vignetting. As shown by these Figures, all three lens embodiments demonstrate good performance.

The projection lenses of Figures 2, 3 and 4 can be used in conjunction with a single LC panel to provide a monochrome display in which white light, or light of one colour, is modulated by the matrix array of display elements in the panel, or a colour display in which the matrix array of display elements is associated with a colour filter array so that different groups of display elements provide different colour light outputs. In this case, the arrangement of the lens in relation to the LC panel will be as illustrated in Figures 2 to 4 with the panel illumination means (not shown) comprising, for example, a white light source condenser lens and a heat filter which in combination direct a collimated beam of generally parallel light onto the LC panel.

With such a single light valve projection system, the space available between either the light valve and the first lens group G1 in the case of lens No. 1 (Figure 2) or between the first lens group G1 and the second lens group G2 in the case of all lenses Nos. 1 , 2 and 3, (Figures 2, 3 and 4), can be utilised, if desired, to accommodate a second mirror so that the optical path through the projection lens is folded twice. The use of a second folding mirror offers still further flexibility in the arrangement of the optical projection components. With one mirror, the optical path of light through the projection lens can be diverted in two dimensions. With two mirrors, the optical path of light through the lens can be diverted in three dimensions. Thus for example, the plane in which the optical path is diverted by the second mirror may be orthogonal to the plane in which the optical path is directed by the mirror 40. The direction (optical axis) of light from the illuminating means 22 through the light valve is then perpendicular to the plane in which the directions (optical axis) of light from the second lens group G2 to the mirror 40 and light from the mirror 40 to the third lens group G3 both lie. The light beam from the illuminating means 22 does not, therefore, need to be coaxial 5 with the second lens group G2.

For the projection system embodiment of Figure 1 , which uses three LC panels for modulating different light colours, it is necessary to combine the modulated outputs of each LC panel using the beam splitter/combiner unit 24. Two examples of such a unit for use with the projection lens o embodiments of Figures 3 and 4 are shown schematically in Figures 8 and 9. In these Figures, W, R, G and B denote white, red, green and blue light, and the three LC panels are referenced 30R, 30B and 30G accordingly. The components referenced 41 are plain mirrors while the components referenced 45 and 46 are wavelength selective light splitting dichroic mirror s filters and light combining dichroic mirror filters respectively. For these arrangements, the projection lens has three separate first lens groups, GIR, GIB and GIG, each comprising a field lens element. Each lens group is associated with, and positioned closely adjacent to, a respective one of the panels 30. Both arrangements ensure that each LC panel 30R, 30G and o 30B, which defines an object plane, and its associated lens unit G1 R, G1 B and G1G, which is positioned a predetermined distance from its output side, are substantially optically equidistant from the second unit G2 of the projection lens. The second and third units G2 and G3 of the projection lens are represented in Figures 8 and 9 by the box 48. 5 An example of the construction of the unit 48, comprising lens groups G2 and G3 and the folding mirror 40, for the projection lens embodiment of Figure 2 is illustrated in Figure 11.

A modified form of the arrangement of Figure 8 for use with the projection lens of Figure 2 is illustrated in Figure 10. In this modified o arrangement the lens elements G1R, G1 B and G1G are replaced by just two lens elements, one of which, GIRG, is positioned in the optical path between 15

the two combining filters 46 so as to operate with the LC panels 30R and 30G and the other of which, GIB, is positioned in the optical path between the mirror 41 following the LC panel 30B and the final combining filter 46. The arrangement of Figure 9 could be modified similarly in this way for use with the lens of Figure 2.

If will be appreciated that other arrangements, for example involving the use of dichroic prism blocks, could be used in the unit 24. However, prism blocks would add significantly to the weight and cost of the unit and for these reasons are less preferred. It should also be appreciated that the lay out of the components of the rear projection system shown in Figure 1 may be changed. For example, the components 20, 22 and 24 may be arranged with their main axis extending horizontally and parallel to the screen near the base of the cabinet 10 and with the projection lens 20 directing its output onto the screen 12 via an inclined mirror, corresponding to the mirror 14 in Figure 1.

Light from the projection lens may be directed onto the screen via two mirrors rather than just one as shown in Figure 1, as in the system described in US-A-5042929.

Although the mirror 40 is shown in the Figures as diverting the optical path in the projection lens though substantially 90 degrees, it will be appreciated that other diversion angles, for example in the range of 70 to 110 degrees, could be employed instead.

In the above described embodiments of projection systems transmissive LC panels are used as the light valves. Other types of light valves, for example reflective LC panels, mirror type light valves or light valves using PLZT or electro-optic crystal materials, may be employed.

In, for example, a projection system using a single reflective type of light valve, light from the illuminating means would be directed onto the light valve via a prism block or semi-reflecting mirror disposed between the light valve and either the first lens group or the second lens group which allows light reflected from the light valve to pass therethrough undiverted to the first or second lens group as the case may be.

From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the field of light valve projection systems and component parts thereof and which may be used instead of or in addition to features already described herein.

Claims

1. A projection lens for projecting an enlargement of an image in a light valve onto a screen which comprises first, second and third lens 5 groups, the first and third lens groups being respectively at the light valve side and the screen side and the second lens group being between the first and third lens group, and in which the optical path through the projection lens is folded, characterised in that the powers of the first, second and third lens groups, K_{1 t} I , and K₃ respectively, are selected so that ιo O.SK-cK^OJOK, 0.25K<K₂<0.50K and -0.6K<K₃<0, where K is the total power of the projection lens, and in that a mirror is disposed between the second and third lens groups which folds the optical path through the projection lens.

i5 2. A projection lens according to Claim 1 , characterised in that the relative powers of the first, second and third lens groups are approximately 0.36, 0.38 and -0.18 respectively.

3. A projection lens according to Claim 1 , characterised in that the 0 relative powers of the first, second and third lens groups are approximately

0.38, 0.40 and -0.51 respectively.

4. A projection lens according to Claim 1 , characterised in that the relative powers of the first, second and third lens groups are approximately 5 0.60, 0.32 and -0.37 respectively.

5. A light valve projection system comprising at least one light valve for generating a display image, illuminating means for illuminating the light valve, a viewing screen, and a projection lens for projecting an enlargement 0 of the image in the light valve onto the viewing screen, characterised in that the projection lens comprises a lens according to any one of Claims 1 to 4.

6. A light valve projection system according to Claim 5, characterised in that the projection system comprises a rear projection system in which the projection lens projects the image onto the rear of the viewing screen.

7. A light valve projection system according to Claim 5 or Claim 6, characterised in that a second mirror is disposed between the light valve and the second lens group which folds the optical path between the light valve and the second lens group.

8. A light valve projection system according to Claim 7, characterised in that the plane in which the optical path is folded by the second mirror is orthogonal to the plane in which the optical path is folded by the first-mentioned mirror.

9. A light valve projection system according to Claim 5, or Claim 6, characterised in that the system includes a plurality of light valves each arranged to be illuminated with light of a respective and different colour, and optical combining means for combining the outputs of the plurality of light valves arranged between the light valves and the second lens group of the projection lens.

10. A light valve projection system according to Claim 9, characterised in that the system includes three light valves and in that the illuminating means comprises a source of white light and beam splitting means for splitting the white light into red, green and blue components, each component being used to illuminate a respective one of the light valves.