WO1998054611A2

WO1998054611A2 - Projection system and light source for use in a projection system

Info

Publication number: WO1998054611A2
Application number: PCT/GB1998/001157
Authority: WO
Inventors: Graham Harry Moss; Paul Michael Everett; Ray Fielding; Martin Kavanagh
Original assignee: Digital Projection Limited
Priority date: 1997-05-27
Filing date: 1998-04-21
Publication date: 1998-12-03
Also published as: WO1998054611A3; GB9710909D0

Abstract

A projection system includes a sealed beam arc lamp. An optical means is provided for directing light from the sealed beam arc lamp onto a spatial light modulator in the form of a digital micromirror device. The sealed beam arc lamp is mounted such that the output beam from the sealed beam arc lamp is directed substantially vertically downwards.

Description

PROJECTION SYSTEM AND LIGHT SOURCE FOR USE IN A

PROJECTION SYSTEM

This invention relates to projection systems and light sources for use in such a projection system. In particular the invention relates to projection systems of the type for use with a spatial light modulator for producing a spatially modulated beam of light which may be projected onto a display surface.

The spatial light modulator may take the form, for example of a digital micromirror device (DMD) or a liquid crystal device (LCD) .

A digital micromirror device comprises an array of deflectable mirror devices, each mirror device being mounted on a torsion element over a control electrode. Applying an electric field between each mirror device and the associated control electrode causes the mirror device to pivot. Thus the direction of light reflected from each mirror device may be changed by application of suitable electrical address signals to the digital micromirror device, the electrical address signals usually being derived from an input video signal.

Each mirror device of the digital micromirror device may be caused to reflect light either in an "on" direction towards the projector lens for projection onto a display screen, or in an "off" direction towards a beam dump. It is thus possible to spatially modulate a beam of light which illuminates the array of mirror devices, the pixels of the image displayed on the display surface being derived from one or more of the mirror devices within the digital micromirror device.

In order to provide a sufficiently intense light beam to address such a digital micromirror device, a sealed beam arc lamp has been used. One such lamp is described in the Applicants ' copending European application EP-A-

0646284, corresponding to US Patent Application Serial

No. 08/356303, the contents of which are incorporated herein by reference. An alternative sealed beam arc lamp is described in the Applicants' pending European patent application EP-A-07635253 corresponding to US Patent application Serial No. 08/7502212, the contents of which are also incorporated by reference.

A further example of a sealed beam arc lamp which may be used is described in US Patent 4,633,128.

An example of the use of a sealed beam arc lamp 101 in a projector system incorporating a digital micromirror device 117 is illustrated schematically in Figure 1. As can be seen in Figure 1 the sealed beam arc lamp 101 is arranged to direct an output beam through a condenser lens 103 effective to focus an inverted image of the light source onto the central region of the input face of an integrator rod, 105. The integrator rod 105 by means of multiple internal reflections within the integrator rod evens out spatial aberrations in the cross section of the light beam output from the arc lamp 101, and produces a light beam having a more uniform light distribution across the cross section of the beam. The integrator rod 105 is also effective to create an output beam of a cross sectional aspect ratio matched to that of the digital micromirror device 117.

Light from the output surface of the integrator rod 105 is focused by a relay lens system 107, 109 and directed via a reflective surface 111 onto a prism system 113 including an air gap 115, one surface of which is effective to direct light onto the digital micromirror device 117. The digital micromirror device 117 is arranged such that light reflected from the array of mirror devices along the "on" path will pass through both surfaces defining the air gap 115 into a projection lens 119 for projection onto a display surface (not shown).

Whilst, for simplicity, in Figure 1 only one digital micromirror device 117 has been shown, the prism assembly 113 will in practice be a complex prism assembly array including two dichroic surfaces (not shown in Figure 1) effective to direct light of different colors onto two further digital micromirror devices, such that light within the separate red, green and blue wave length bands is spatially modulated by a respective digital micromirror devices. The spatially modulated red, blue and green light beams are then recombined within the prism system 113, and directed through the projector lens 119. Details of suitable prism systems, which may include more than one air gap, are shown in the Applicants' copending Applications' EP-A-0746948A (corresponding to US Application Serial No. 08/696860) and International application W096/36184. Details of the use of an integrator rod in a projector system are disclosed in the Applicants' pending International application PCT/GB98/00523.

Details of the electrical address systems for driving a digital micromirror device are disclosed for example, in the Applicants' European Patent EP-A-0707303 and corresponding US application Serial No. 08/050290, European application EP-A-0557362 and corresponding US application Serial No. 08/050293, EP-A-0664917 and corresponding US application Serial No. 08/416801, EP-A- 0755556 and corresponding US application Serial No. 08/716173. The contents of each of the above mentioned applications is incorporated herein by reference.

A problem which occurs with the arrangement shown in Figure 1 is that in order to provide a sufficiently high intensity light beam, typically up to 2 kw, the closed beam arc lamp 101 is filled with a gas, for example xenon, at a relatively high pressure, typically 200 psi with the lamp off, this pressure increasing by typically 50% at normal operating temperatures. The inventors for the present application have observed that the use of such high pressures produces turbulence within the gas within the lamp, this being shown schematically in Figure 2 which illustrates the interior of the arc lamp as seen through the output window of the lamp.

As shown in Figure 2, areas of turbulence are observed within the gas within the lamp, this creating corresponding variations in intensity in the cross section the light beam produced by the lamp which are not totally removed by the integrator rod 105. Furthermore if the gas pressure within the lamp is increased to an even higher pressure, regions of gas at the lower edge of the lamp are formed which have a liquid non turbulent appearance in contrast to the increased turbulence in the upper part, this increasing the asymmetry of the intensity and angular distribution of the light beam produced by the lamp .

It is an object of the present invention to provide a projection system and a light source for use in such a projection system wherein this problem of aberrations in the light beam caused by turbulence within the lamp is reduced .

It is a further object of the present invention to provide a light source which has a lower running temperature, and thus a longer lifetime.

According to a first aspect of the present invention there is a provided a projection system including a sealed beam arc lamp arranged to illuminate a spatial light modulator, wherein the sealed beam arc lamp is arranged to emit an output beam in a substantially vertical direction.

According to a second aspect of the present invention there is provided a light source comprising a sealed beam arc lamp, and means for mounting the sealed beam arc lamp to produce a output beam in a substantially vertical direction.

In order to strike an arc in the sealed beam arc lamp, it is necessary to apply a high voltage, typically of 20 to 30 kv between the anode and the cathode. As the anode and cathode have electrical connectors on the exterior of the lamp housing, the spacing between which is necessarily limited, this may lead to electrical shorts between these electrical connectors. It is a further object of the present invention to provide a light source in which this problem is at least alleviated.

According to a third aspect of the present invention, there is provided a sealed beam arc lamp comprising: an insulating means forming part of the enclosure; first and second metallic sleeves at least partially carried on the insulating means exterior to the enclosure electrically connected to the anode and cathode; wherein said insulating means has an arcuate portion separating said first and second metallic sleeves, at least part of said arcuate portion extending beyond the external surfaces of the first and second metallic sleeves.

Fabrication of the lamp normally involves brazing of the lamp window to the surround, and further brazing to produce metallic to ceramic airtight seals for the enclosure. Such brazes may be difficult to perform, and may produce stresses. It is a further object of the present invention to provide a lamp structure wherein these problems may be at least alleviated. According to a fourth aspect of the present invention, there is provided a sealed beam arc lamp comprising: an insulating means forming part of the enclosure for the arc lamp; and at least one metallic sleeve at least partially carried by the insulating means exterior to the enclosure having a tapered portion at a region of the metallic sleeve which is brazed to the insulating means.

The window of a sealed beam arc lamp is typically mounted in a metallic bracket, which is brazed to the window. Such brazing can lead to cracking of the window due to the high temperatures involved.

According to a fifth aspect of the present invention, there is provided a sealed beam arc lamp wherein the window for the lamp is mounted in a bracket supported by a metal flange, the flange having a recess encircling the bracket .

A number of embodiments of the invention will now be disclosed by way of example only with reference to the accompanying figures in which:

Figure 1 illustrates schematically a prior art projection system including a sealed beam arc lamp;

Figure 2 illustrates the view seen through the output window of the sealed beam arc lamp incorporated in the system of Figure 1;

Figure 3 illustrates a projection system in accordance with an embodiment of the present invention;

Figure 4 illustrates a sealed beam arc lamp incorporated in the projection system of Figure 3;

Figure 5 illustrates the view seen through the output window of the arc lamp of Figure 4;

Figures 6a and 6b illustrate on a smaller scale to that of Figure 4, an under plan view and a partially sectioned side view of the sealed beam arc lamp of Figure

4 positioned on a mounting plate and surrounded by heat sinks;

Figures 7a and 7b illustrate respectively an under plan view and a side view of the heat sink shown in Figures 6a and 6b;

Figure 8 illustrates schematically the under plan view of the mounting plate of Figure 6;

Figure 9 illustrates the positioning of the arc lamp of Figure 4 on the mounting plate of Figure 8 using a jig;

Figure 10 illustrates the arc lamp of Figure 4 on the rest of the optical system included in the projection system of Figure 3;

Figure 11a and lib schematically illustrate, respectively, a plan view and a side view of a clamping arrangement for clamping the lamp of Figure 4 on the optical system;

Figures 12 is a schematic plan view showing the arc lamp clamped in position together with part of the cooling system used to cool the arc lamp during operation of the projector system;

Figure 13 illustrates an alternative projection system in accordance with an embodiment of the invention; and

Figure 14 illustrates an alternative sealed beam arc lamp for use in a projection system in accordance with an embodiment of the invention.

Referring now to Figure 3, the projection system in accordance with an embodiment of the invention to be described is basically an adaptation of the projection system shown in Figure 1. The output beam from a sealed beam arc lamp 301 is arranged to illuminate a spatial light modulator 317 in the form of a digital micromirror array, to produce a spatially modulated beam for projection through a projection lens 319. The output beam from the lamp 301 is directed through a condensing lens 303, integrating rod 305, a relay lens system 307,309 and reflected by a reflector 311, through a prism system 313 including an air gap 315 onto the digital micromirror device 317.

As so far described, the optical arrangement is as in the prior art arrangement described above. However in the projection system in accordance with an embodiment of the invention instead of being mounted horizontally as in the prior art arrangement shown in Figure 1, the sealed beam arc lamp 301 is mounted such that the output beam from the lamp is directed vertically downwards onto a cold mirror 321. The cold mirror 321 comprises a refractory glass slide coated with a dichroic coating 323 effective to transmit light within the infrared and ultraviolet wave length regions, and to reflect all other wave lengths . Thus unwanted radiation within the light beam emitted from the sealed beam arc lamp 301 passes through the coating 323 and into a heat sink 325. The rest of the light beam from the lamp 301, with the infrared and ultraviolet components removed, is directed through the condenser lens 303 to pass through the rest of the optical system onto the digital micromirror device 317.

Turning now to Figure 4, this figure shows on an enlarged scale details of the sealed beam arc lamp 301 of Figure 3. The lamp comprises an anode 401 and a cathode 403 within an enclosure defined by a ceramic body 405 formed, for example, of alumina, a copper block 407, a nickel alloy flange 409 and a sapphire output window 411. The enclosure is filled with high pressure Xenon gas, at for example 200 psi . Alternatively other gases such as Argon or Neon may be used. The internal surface of the ceramic body 405 is formed in the shape of a parabola, a reflective coating 413 being formed on the parabola surface so as to constitute a reflector, the arc gap defined between the anode 401 and the cathode 403 being positioned at the focal point of the parabola reflector such that a substantially parallel output beam will be produced .

The sapphire window 411 is mounted in a bracket carried by the nickel alloy flange 409, the flange being formed with a recess encircling the window 411. The reason for the recess is to reduce stress on the window 411 during the high temperatures used to braze the sapphire window 411 to the nickel alloy flange 409. The lamp is designed to have particularly efficient heat transfer characteristics. In particular, the anode 401 is mounted on the copper block 407 such that the block 407 constitutes a heat sink for the anode 401. As best seen in Figure 5 which shows the view into the lamp as seen through the sapphire window 411, the cathode 403 is suspended via three molybdenum strips 415 in a spider configuration from the metal flange 409. The strips 415 are designed for maximum heat conduction so as to remove heat from the cathode 403 during operation of the lamp, but to provide minimal obscurance of the light beam emitted from the lamp 301. The cathode 403 is arranged to have an enlarged surface area and in particular to have a series of grooves 417 in order to increase the surface area of the cathode, thereby increasing heat dissipation from the cathode 403 into the arc lamp enclosure .

Furthermore, as heat from the lamp 401 will naturally travel upwards, the copper block 407 is more effective to conduct heat away from the lamp, as will be described in more detail hereafter, leading to more efficient running of the lamp and a longer lamp life time.

An annular opening 419 is formed between the ceramic block 405 and the copper block 407 to enable movement of the xenon gas within the enclosure into a gap 421 behind the ceramic block 405. A copper bracket 423 is inserted behind the ceramic block 405 in order to produce a heat conductive path between the ceramic block 405 and a metallic sleeve 425 is arranged to encircle the bracket 423 and gap 421 and part of the ceramic body 405 so as to produce a gas tight seal. A further metallic sleeve 426 encircles part of the metal flange 409 and a further part of the ceramic body 405 so as to encapsulate the connections to the spider configuration support 415 for the cathode 403.

The ceramic block 405 includes a ridge 427 having between the metallic sleeve 425 and the metallic sleeve 426 edges extending beyond the plane of the sleeves 425, 426 for reasons which will be explained later. Each of the sleeves 425, 426 includes a tapered portion adjacent the edges of the ridge 427. These tapered portions are provided to facilitate the brazing of the metallic sleeves to the ceramic block 405 to produce the required gas tight seals.

In use of the arc lamp, a high voltage, of typically 20 to 30 kv, is applied between the metallic sleeve 425 and the metallic sleeve 426 so as to ignite a discharge between the anode 401 and the cathode 403. The form of the ridge 427 reduces the possibility of an arc occurring during application of this high voltage between the two metallic sleeves 425, 426 whilst increasing the surface area of the ceramic block, thus further increasing the cooling of the lamp when cooling air is directed over the lamp, and without increasing the weight of the lamp. After ignition the voltage applied is reduced to a level of approximately 20 v to sustain the discharge.

Turning now also to Figure 5, by virtue of the lamp being mounted with its optical axis vertical (that is at right angles to the arrangement shown in Figures 1 and 2), the turbulent regions of the gas seen in the prior art arrangement of Figure 2 are removed, with less obtrusive turbulent areas being symmetrically distributed around the lamp enclosure behind the spider configuration cathode mounting 415. This turbulence is less obtrusive than the turbulence illustrated in Figure 2 in that the intensity and angular distribution of the light is more symmetrical, and an increase in light intensity of the beam produced by the lamp of several percent is exhibited. The inventors have found that by use of the downwards pointing lamp, the lamp can be run at pressures of around 700 psi.

Turning now also to Figures 6 and 7, around the lamp there are clamped forward and rear heat sinks 601,603, the forward heat sink 601 being clamped around the window

411 and metal sleeve regions of the lamp, the rearward heat sink being clamped around the metal block 407 region of the lamp. Within the rearward heat sink 603 is carried a copper insert 605, this being bolted to the copper block 407.

The heat sinks 601,603 include a series of radially directed copper vanes 607, only a small number of vanes 607 being shown in Figures 6 and 7 for the sake of clarity. These vanes 607 will enhance turbulent air flow through the heat sinks 601,603, when, as will be described later, a fan is used to direct cooling air through the heat sinks 601,603. As seen in Figure 6, a cover 615 is positioned around the heat sinks 601,603, this having a pair of diametrically opposed lugs 617 which is used to mount the lamp 301 in position as will be described in detail hereinafter. A thermocouple for a heat sensor 618 is attached to the heat sink 601.

Referring now also to Figures 8 and 9, in order to mount the lamp shown in Figure 4 on the optical system for the projector system of Figure 3 such that the beam from the lamp is substantially vertical in order to maximise the advantages of the lamp, a plate 619 is attached to the forward heat sink 601 clamped round the window and forward regions of the lamp 301. The alignment of the plate 619 with the lamp 301 enables the lamp to be mounted with its optical axis vertical as will now be described.

The plate 619 includes a circular groove 601 designed to locate the lower edge of the heat sink housing 615. In the plate 619 is mounted a sliding safety shutter 803 effective to shield the sapphire window 411 of the lamp 301 when not mounted in the projection system. The plate 619 includes, at its perimeter, three alignment means 805,807,809 for ensuring that the plate 619 is mounted horizontally so as to ensure that the optical axis of the lamp 301 (which will be perpendicular to the plate 619) is in the vertical direction.

Referring now particularly to Figure 8 which illustrates the under plan view of the plate 619, the first alignment means 805 comprises a conical indentation within the plate 619, the indentation being effective at its base to define a fixed point. The second alignment means 807 comprises an inclined plane effective to define a line directed towards the fixed point defined by the conical indentation 809. The third alignment means comprises a flat plane 809 parallel to the plane of the plate 619, the three alignment means 805,807,809 thereby uniquely defining the required horizontal plane.

Turning now particularly to Figure 9, this figure illustrates the alignment of the plate 619 on a jig having a horizontal reference surface 901, the alignment being achieved optically. The surface 901 carries three conical projections 903,905,907 arranged to mate with the three alignment means 805,807,809 in the plate 619. The three point coupling between the top of the jig and the plate ensures that the plate 619 is parallel to the horizontal reference surface 901 of the jig. The arc lamp is loosely clamped on the jig by the clamping arrangement 908.

The rest of the jig includes an optical system comprising a cold mirror 915, a condensing lens 917, an integrating rod 919 and a first relay lens 921 identical to the components 307,309,311 in the projector system of Figure 3, with the optical axis of the jig between the cold mirror 915 and the first relay lens 921 being arranged to lie in a horizontal plane parallel to the reference surface 901. Beyond the first relay lens 921, attached to the optical system via a boot 923 there is provided a closed sphere 925 having an internal titanium white coating. The optical system is carried in a plastics housing 927 which is mounted on the base 928 of the projector system housing. A detector 929 is arranged to detect the amount of light passing through the optical system and entering the hemisphere 925.

If the optical axis of the particular sealed beam arc lamp 301 being aligned is not vertically downwards, the light reflected from the cold mirror 915 will not be focused onto the central portion of the input face of the integrator rod 919 by the lens 917. The amount of light reaching the output face of the integrator rod 919 will thus decrease, and a decreased amount of light will enter the hemisphere 925 to be detected by the detector 929. The orientation of the arc lamp 301 relative to the plate 619 is adjusted by means of three screw adjustments 931,933,935 so as to maximise the light detected within the hemisphere 925 by the detector 929. When the maximising adjustment has been completed, the position of the lamp 301 relative to the plate 619 is fixed, and the arc lamp surrounded by the heat sink attached to the plate is removed from the jig and placed on the optical system of the projector system as shown in Figure 10 which is arranged in a projector housing.

Referring now also to Figure 11 the arc lamp 301 which is now in a fixed orientation relative to plate 619, is attached to the projector housing carrying the optical system using a support arrangement comprising two vertical upper pillars 953,955 supporting an upper circular bracket 957 and carried by a lower circular bracket 959. Four lower vertical pillars 961 , 963 , 965 , 967 support the lower circular bracket 959. The lower pillars 961,963,965,967 are attached to the base of the projector system housing via screws, in a position aligned with the position of the cold mirror 321.

The support arrangement includes three conical projections 969,971,973 arranged to locate in the respective alignment means 805,807,809 of the plate 619 in which the projections of the jig were located during the alignment process. This ensures that the plate 611 is again aligned in the horizontal direction, to thereby ensure that the optical axis of the lamp is in the vertical direction. The uppermost circular bracket 957 is arranged to clamp over the lugs 617 formed on the heat sink cover 615 thereby clamping the arc lamp 301 in position, while still allowing rotational movement of the arc lamp (if this is required) with the optical axis remaining in the vertical direction. The plate 619 is provided with two diametrically opposed cut outs effective to allow a limited amount of rotation relative to the uprights 953,955.

Referring now particularly to Figure 12, when the arc lamp 301 is clamped in position in the projection system housing, the heat sink arrangement 601, 603 is positioned in the through path of cooling air from a fan (not shown) positioned in a venting arrangement in the projection system housing so that cooling air passes up and through the heat sinks 601,603. The heat sensor 618 is used to monitor the temperature of the heat sinks 601,603, to enable corrective adjustment of the speed of the fan to maintain a required temperature .

It will be appreciated that a light source in accordance with an embodiment of the invention is suitable for use in projection systems having different configurations to the projection system shown in Figure 3. In particular referring now to Figure 13, (in which corresponding features to those of Figure 3 are labelled by the same reference numerals as in the previous embodiment) it can be seen that the condenser lens, integrator rod and relay lens system used to shape the beam in the embodiment of Figure 3 can be replaced by a beam shaping arrangement comprising two lenticular lens plates of the form described in the Applicant's International Application WO96/08743, the contents of which are incorporated herein by reference. In this arrangement a first lenticular lens plate 961 includes an array of convex lenslets, each lenslet acting as a condensing lens to focus an image of the beam produced by the arc lamp 301 onto a corresponding lenslet in the second integrating plate 963. Lenslets in the second integrating plate 963 then each act as a field lens to focus an image of the lenslets in the first integrating lens plate 961 on the active surface of the digital micromirror device 317. By such an arrangement a beam of the required aspect ratio is produced.

However it is found that where the arc lamp is mounted horizontally in an equivalent manner as shown in Figure 1, as the lenslets plates 961,963 are not as effective to mix the light within the light beam from the arc lamp as an integrating rod, even greater abberations due to turbulence in the lamp are produced. Thus, by use of a projection system in accordance with an embodiment in the invention as shown in Figure 13 the effect of turbulence is even more dramatic, an increase in light beam intensity of approximately 10 per cent being achievable over a prior art arrangement in which the light source is mounted horizontally.

Referring now to Figure 14, this figure illustrates an alternative sealed beam arc lamp to the arc lamp shown in Figure 4. This alternative arc lamp is, however, a modification of the arc lamp shown in Figure 4 and thus corresponding components are labelled accordingly.

The arc lamp shown in Figure 14 differs from that shown in Figure 4 in that the reflector in the form of a reflective coating 413 on the ceramic body 405 of the lamp shown in Figure 4 is replaced by a self supporting electroformed metallic reflector 1401. This is designed to increase heat flow from the lamp, and enable more accurate positioning of the reflector.

The reflector 1401 is mounted via a ring 1403 formed integrally with the reflector 1401 in the electroforming process onto a ridge 1402 on the copper block 407. A gap 1404 is left around the anode mount and the ring 1403 is formed with four spaced apertures enabling gas to flow into an enlarged chamber behind the reflector 1401. The cooling of the hot gas in this chamber is aided by gas cooling fins 1405 formed from the copper block 407.

The rim of the reflector 1401 is loosely located against a shoulder 1406 in the ceramic body 405, via a flange 1413 formed integrally with the reflector 1401 in the electroforming process. The flange 1413 is formed with cutouts enabling gas flow past the flange 1413.

The electroformed reflector may be formed of any suitable material. Particularly suitable materials are silver or copper or copper/silver backed with nickel for support. Such an arrangement will produce a highly reflective surface for the reflector combined with high thermal conductivity.

It will be appreciated that mounting the arc lamp of this particular embodiment with its window 411 facing downwards particularly assists hot gasses to flow behind the reflector 1401, cooling being achieved by contact with the cooling fins 1405 which are made possible by the reduction of the size of the ceramic body 405.

It will be appreciated that many other sealed beam arc lamps and optical systems may be used in a projection system in accordance with the invention. For example, the reflector for the lamp may be elliptical, rather than parabolic. This will produce a focused beam, thus enabling the condenser lens 303 shown in Figure 3 to be omitted.

It will be appreciated that whilst the use of an air gap defining an internally reflecting surface is a particularly advantageous arrangement to separate the input and output beams to and from the digital micromirror array, thus enabling a close spacing of the projection lens and the input path to the digital micromirror array, it is possible for a projection system in accordance with the invention not to include such an air gap. Likewise instead of including dichroic surfaces in order to split the incoming light into red, green and blue wavelengths bands in order to optically address three separate digital micromirror arrays, it is possible to use a single digital micromirror array with, for example a color wheel effective to direct red, green and blue light sequentially through the system. Whilst the invention has been described in relation to a digital micromirror array, it will be apparent that the invention is also applicable to projection systems including other forms of spatial light modulators for example ferroelectric liquid crystal devices .

Claims

1. A projection system including: a sealed beam arc lamp; optical means for directing light from the sealed beam arc lamp onto a spatial light modulator; wherein the projection system includes mounting means for mounting the sealed beam arc lamp such that the output beam from the sealed beam arc lamp is directed substantially vertically downwards.

2. A projection system according to claim 1, including a cold mirror arrangement effective to transmit light within the infra-red and ultraviolet wavebands in said output beam on to a heat sink and to reflect the rest of the light towards the spatial light modulator.

3. A projection system according to either of the preceding claims, in which the sealed beam arc lamp includes xenon.

4. A projection system according to claim 3, wherein the pressure of the xenon is at least 200 psi.

5. A projection system according to any one of the preceding claims, in which the arc lamp is surrounded by heat sinks .

6. A projection system according to claim 5, in which the mounting means is mounted on said heat sinks.

7. A projection system according to any one of the preceding claims, wherein the arc gap of the lamp is set within a reflector, the reflector being an electroformed reflector mounted so as to enable gas flow out of the enclosure partially defined by the reflector.

8. A projection system according to claim 7, wherein the reflector is at least partially surrounded by cooling fins in the region of the lamp exterior to said enclosure.

9. A projection system according to any one of the preceding claims, wherein said arc lamp comprises: an anode and a cathode defining between them an arc gap in a gas filled enclosure; an insulating means forming part of the enclosure; first and second metallic sleeves at least partially carried on the insulating means exterior to the enclosure; and first and second electrical connector means arranged to form respective electrical connections between said first metallic sleeve and said anode and said second metallic sleeve and said cathode; wherein said insulating means is formed with an arcuate portion separating said first and second electrical connectors, at least part of said arcuate portion extending beyond the external surfaces of the first and second metallic sleeves .

10. A projection system according to any one of the preceding claims, wherein said arc lamp comprises: an anode and a cathode defining between them an arc gap in a gas filled enclosure; an insulating means forming part of the enclosure; first and second metallic sleeves at least partially carried on the insulating means exterior to the enclosure; and first and second electrical connector means arranged to form respective electrical connectors between said first metallic sleeve and said anode and said second metallic sleeve and said cathode; wherein each of said metallic sleeves has a tapered portion at a region of the metallic sleeve which is brazed to the insulating means .

11. A projection system according to any one of the preceding claims, wherein said arc lamp includes: an anode and cathode defining an arc gap in an enclosure; wherein the window for the lamp is mounted in a metallic flange, the flange having a recess at least partially surrounding the window.

12. A projection system according to any one of the preceding claims, including a display surface.

13. A projection system according to any one of the preceding claims, including a spatial light modulator.

14. A projection system according to claim 13, in which the spatial light modulator is a digital micromirror device .

15. A sealed beam arc lamp comprising: an anode and a cathode defining between them an arc gap in a gas filled enclosure; an insulating means forming part of the enclosure; first and second metallic sleeves at least partially carried on the insulating means exterior to the enclosure; and first and second electrical connector means arranged to form respective electrical connections between said first metallic sleeve and said anode and said second metallic sleeve and said cathode; wherein said insulating means is formed with an arcuate portion separating said first and second metallic sleeves, at least part of said arcuate portion extending beyond the external surfaces of the first and second metallic sleeves.

16. A sealed beam arc lamp comprising: an anode and a cathode defining between them an arc gap in a gas filled enclosure; an insulating means forming part of the enclosure; first and second metallic sleeves at least partially carried on the insulating means exterior to the enclosure; and first and second electrical connector means arranged to form respective electrical connectors between said first metallic sleeve and said anode and said second metallic sleeve and said cathode; wherein each of said metallic sleeves has a tapered portion at a region of the metallic sleeve which is brazed to the insulating means.

17. A sealed beam arc lamp including; an anode and cathode defining an arc gap in an enclosure; wherein the window for the lamp is supported by a metallic flange, the flange being formed with a recess at least partially surrounding the window.

18. A sealed beam arc lamp including an anode and cathode defining an arc gap at the focal point of a reflector, wherein the reflector is an electroformed reflector mounted so as to enable gas flow out of the enclosure partially defined by the reflector.

19. A method of aligning a sealed beam arc lamp in a projection system including optical means for directing light from the sealed beam arc lamp onto a spatial light modulator and wherein the output beam from the sealed beam arc lamp is substantially vertically downwards, comprising the steps of: mounting said sealed beam arc lamp on a jig, the jig including means equivalent to at least part of said optical means; adjusting the position of the arc lamp relative to a mounting means so as to maximise light passing through said equivalent optical means; and using said mounting means to align the arc lamp in the projection system such that the output beam is substantially vertically downwards.

20. A method according to claim 19, wherein said jig includes a first surface and said mounting means defines a second surface, one of said first and second surfaces including three alignment means effective to define, respectively, a reference point, a reference line and a reference plane, and the other of said first and second surfaces including three pivot points, each engaging one of said alignment means such that the first and second surfaces are parallel.