GB2425167A

GB2425167A - Illumination System and Projector.

Info

Publication number: GB2425167A
Application number: GB0507333A
Authority: GB
Inventors: Grant Bourhill; Marina Khazova; Allan Evans; Gregory Gay; Nigel Joseph Copner
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 2005-04-12
Filing date: 2005-04-12
Publication date: 2006-10-18
Also published as: GB0507333D0

Abstract

An illumination system is provided for a projector. The system comprises a light source (30) which directs light along a main optical path (32) through the system and through the projector. A deflector (34) deflects part of the spectrum from the light source (30), such as ultraviolet radiation, out of the main path while passing the remainder of the light along the main path. A wavelength converter (35, 36) is disposed outside the optical path. The converter (35, 36) gathers the incident radiation and converts part of this to a different part of the spectrum, for example for augmenting spectral deficiencies in the light from the light source (30). The converted visible radiation is then supplied to a combiner (37) in the main path for combining the light from the converter (35, 36) with light from the light source (30), for example for illuminating one or more spatial light modulating panels in a projector.

Description

Illumination System and Projector The present invention relates to an

illumination system and to a projector including such an illumination system.

Image projection systems have been used for many years to project motion and still pictures onto screens for viewing. Presentations using multimedia projection systems are widely used to deliver information in diverse fields, such as sales, demonstrations, business meetings and education.

Many types of projection systems use non-emitting spatial light modulators in combination with an illumination source to generate an image. Colour image projection displays operate on the principle that colour images are produced from three primary colours, generally Red (R), Green (G) and Blue (B) projected onto a screen, either at the same time or sequentially in time. The light of wavelength regions corresponding to these primary colours is generally separated from the broad band illumination emitted from a light source by using optical filters. The separated light is then modulated by one or more spatial light modulators, such as liquid crystal displays (LCD) or Digital Light Processors (DLP).

Viewers evaluate display systems based on many criteria including image size, resolution, contrast ratio, colour purity and brightness. Image brightness is a particularly important metric in many display markets since the available brightness can limit the image size of a projected image and controls how well the image can be seen in venues having a high level of ambient light. For a given projection engine architecture, the light source can be identified as one of the main factors that determine the brightness and colour reproduction of the projected image.

Image projection systems typically employ a high-intensity ultra-high pressure mercury lamp (UHP lamp), that provides a high luminous efficiency in the visible region.

Figure 1 of the accompanying drawings shows the emission spectrum of a typical UHP lamp. Sufficient light intensity is provided in the blue and green spectral regions.

However, the light intensity is insufficient in the red wavelength region above 600nm.

For this reason, in projectors employing such a UHP lamp, the light intensity in the blue and green wavelength regions is reduced in order to adjust the balance with the light intensity in the red wavelength region to ensure acceptable colour reproduction.

If the light intensity in the blue and green wavelength regions is reduced to provide light intensity balance with the light intensity in the red wavelength region, for example, by dimming the 0 and B channels or increasing the angular size of a red segment of a colour wheel, part of the illumination from the light source is wasted.

In order to try to solve the problem of colour imbalance, the light source may comprise a xenon lamp having an emission spectrum with better intensity uniformity than that of a UHP lamp. However, the luminous efficiency of such a xenon lamp is lower than that of a UHP lamp. Thus, the power consumption of a Xenon lamp is markedly higher than that of a Ul-IP lamp of equivalent brightness.

Another known technique for reducing the red deficiency of projection systems is to combine a UHP lamp with an auxiliary light source, which is chosen such that its emission energy content minimises the colour imbalance introduced by the UHP lamp.

Practical implementations of this approach are described hereinafter.

US 6,561,654 suggests combining the light from a UHP lamp with that from a semiconductor laser emitting light in a spectral range that is different from the emission spectrum of the main (UHP) source. An illumination device disclosed by this patent is shown in Figure 2 of the accompanying drawings. The light from the UHP lamp 1 is substantially homogenised and converted into a single linear polarised parallel beam by the polarisation conversion unit 2 and a relay lens 3. An auxiliary light source comprising a semiconductor laser 4 with a wavelength of approximately 650nm is arranged so that the optical axis of the laser beam is nearly orthogonal to the optical axis of the main source. The beam shape of the laser beam is corrected via an optical system 5, which also corrects the intensity distribution and the divergence angle of the laser light. In the auxiliary light source, the tilting angle of the semiconductor laser 4 is chosen so that the polarisation plane of the auxiliary light corresponds to the polarisation plane of the main light after the relay lens 3. A light combiner 6 is placed at the intersection of the optical paths of the main and auxiliary sources and is a spectrally selective element, such as volume reflection hologram or dichroic film. In the wavelength region where the intensity of the main source is less than that of the auxiliary source, the main source is partially replaced by the auxiliary source: the spectrally selective element 6 efficiently transmits light from the main source except for the wavelength range of the auxiliary source 4. Light in the spectral range emitted from the main source is reflected back to the main source and is therefore lost.

A similar approach is disclosed in US 6,398,389 as shown in Figure 3 of the accompanying drawings. A projection display includes a main light source 7 and a solid state auxiliary light source 8. Light from the main source 7 and the auxiliary colour compensation source 8 are combined by a beam combiner 9. The beam combiner 9 transmits the emission spectrum from the main light source 7, except for the spectral region of the light emitted by the auxiliary source 8, and reflects the radiation of the spectral region of the auxiliary source 8, from both the arc lamp and the solid state source. As a result, this spectral component of the main source is lost and substituted by the emission of the auxiliary source.

US 6,623,122 discloses an illumination system for a projector comprising two or more lamps with mutually different spectral distributions and a condensing optical system which combines the light from the two or more sources. The patent describes embodiments using two lamps and, in particular, a combination of a halogen lamp and a high pressure mercury lamp. The illumination system is bulky, which substantially increases the size of the image projector, and is inefficient.

WO 02/10 1459 suggests adding an auxiliary light source for the compensation of colour imbalance to the illumination system of an image projector, as schematically shown in Figure 4a of the accompanying drawings. The auxiliary light source 144 is disposed near the entrance of an optical integrating device, such as an integrator rod 110 or light tunnel, so that the compensating light coincides with the primary light path at a location upstream of the place where the first paraxial reflection occurs and co-propagates with the polychromatic light along the primary light path. The auxiliary light source 144 is a laser diode, LED or arc lamp with a spectral emission which minimises the colour imbalance of the main source, specifically for correcting red deficiency. The auxiliary source 144 is coupled to the integrator rod 110 by an optical fibre or by a prism 158, as illustrated in Figure 4b of the accompanying drawings. In order to improve the colour balance of such an image projector, at least 30 high power LEDs are required which makes this design bulky and impractical.

Further embodiments include direct coupling of auxiliary light into the arc of the main source, as shown in Figure 5 of the accompanying drawings. In this embodiment light from an auxiliary source 10 is transmitted through an area 11 of a lamp reflector 12 and focused into the arc area 13. The area 11 of the lamp reflector also transmits the light emitted by the arc of the lamp and does not reflect it towards the integrator so that this light is lost. WO 03/034145 additionally suggests adjusting the brightness of the auxiliary source to achieve a desired colour temperature.

An alternative approach to increase the brightness of an image projector is disclosed in D.Dewald, S.Penn and M.Davis "Sequential Colour Recapture and Dynamic Filtering: a Method of Scrolling Colour", 51D2000 Digest, 40. 2 and US 6,591,022. A display device comprises a light source, a recycling integrator rod, a spiral colour wheel acting as a sequential colour filter, and a DMD (digital micromirror device) chip. The principle of operation is explained schematically in Figure 6a of the accompanying drawings and details of the optical engine and spiral colour wheel are shown in Figure 6band6c.

The integrator rod has a mirror coating on its input side, leaving a circular transparent area of approximately one third of the integrator cross-section, as illustrated in Figure 6b.

The colour wheel of Figure 6c has sets of three or four colour filters, whose boundaries form a "spiral of Archimedes". Each set includes one of each of the primary colours, and a clear, or white, segment that allows all visible light to pass through it. Each primary colour segment transmits one of the primary colours and reflects the other two primary colours. The spirals are designed and aligned to the spatial light modulator such that the tangent of the boundaries between adjacent segments is approximately parallel to the rows of the spatial light modulator. The number of RGB stripes determines the speed of wheel rotation.

Light from a small-arc lamp is focused onto the input aperture of the integrator and homogenised by multiple reflections off the integrator rod walls. When the white light reaches the colour wheel, light of a given colour (Red, for example) is transmitted through the corresponding section of the wheel, which reflects the Green and Blue towards the input end of integrator rod. At this point two thirds of the light is reflected by the mirror surface on the input end of the integrator rod and one third passes through the transparent aperture to the lamp.

The reflected G and B light is homogenised again and may be transmitted by a corresponding G or B segment of the colour wheel, as described above. This process is repeated several times until all the light that entered the input aperture from the lamp is either transmitted to the modulator, absorbed by the lamp or scattered, as illustrated in Figure 6a.

Although such colour re-capture increases light throughput, it does not reduce the red deficiency of the illuminator and it does not improve the colour balance of the projected image.

Polychromatic light from a UHP lamp also contains the invisible radiation of the ultra violet (UV) and infrared (IR) spectral regions, as shown in Figure 1. This light is not used to form a projected image and may be removed by UVIIR cut-off filters placed immediately after the lamp to reduce degradation effects to the downstream optical elements of the image projector. However, spectral conversion of invisible UV and IR radiation to visible light is known and can be found in number of publications, for

example:

F.Auzel et al, "Rare earth doped vitroceramic: new efficient, blue and green materials for infrared Up-Conversion", J.Electrochem.Soc.: Solid State Science And Technology, v122,nl,pplOI-107, 1975; W.Miniscaico, "Optical and Electronic Properties of rare earth ions in glasses" - in "Rare Earth doped fiber lasers and amplifiers", Ed.M.Digonnet, NY, 1993; US 5,585,640; and US 6,207,229.

EPO1 994009 discloses luminescent aluminoborate and/or aluminosilicate glass which is activated by rare earth metals for used as luminescent screens in discharge lamps or cathode-ray tubes. These luminescent glasses contain terbium or cerium in the form Tb+3 or Ce+3 as an activator and have high quantum efficiency upon UV excitation.

JP 2001-264880 discloses the use of wavelength converting elements, which change UV emission of the arc lamp to blue radiation and JR lamp emission to red radiation, to improve colour balance of a three-panel LCD image projector. Two suggested arrangements are shown in Figure 7a and Figure 7b of the accompanying drawings.

Wavelength converting elements 14, 17 and 18 are of a filter-type and arranged within colour separation optics. In the arrangement shown in Figure 7a, the wavelength converting element 14 is arranged between a lamp 15 and a condensing lens 16 and is described as a rare-earth doped glass element which changes UV emission to blue and JR to red.

In the arrangement illustrated in Figure 7b, the wavelength converting filters 17 and 18 are placed in front of transmissive LCDs 19, 20. The wavelength converting filter 17 changes the IR emission of the lamp to red and the yellow peak of the lamp to red but no details of implementation of such a filter are given. The wavelength converting filter 18 changes the UV lamp emission to blue radiation.

As the fluorescent material of the wavelength converting filters emits light over the full solid angle, the collection efficiency of the converted spectral component is very low since the projection system is designed for a limited angular cone of illumination light.

This LCD projector also does not use polarisation conversion and homogenisation optics and suffers from low efficiency of light utilisation and brightness non-uniformity.

Furthermore, any polarisation conversion optics, if placed before the wavelength converting elements, would be damaged or destroyed by harmful UV and IR radiation.

Japanese Patent Application JP2002-90883 discloses the use of an integrator rod 22 as a wavelength converting element to reduce the red deficiency of an arc lamp 21 in an image projector by changing the UV emission to red or green radiation as shown in Figure 8 of the accompanying drawings. The beam from the arc lamp 21 is focussed onto the input side of the glass rod 22 doped with europium, terbium or erbium in the form Eu+2, Eu+3, Tb+3 or Er+3 where it is homogenised by multiple reflections off the walls. The integrator rod 22 is surrounded by a reflective mirror 23. Part of the incident UV/IR is converted into light of the visible wavelength region inside the rod and the proportion of UV/IR in light exiting the surface 24 is reduced as the red or green spectral component is increased.

However, the overall improvement of the colour balance in a projection system with such a wavelength converting element is very small because fluorescent material of the integrator rod emits light over the full solid angle but only 2-3% of emitted light can be utilised by downstream optics in a projector, as schematically illustrated in Figure 9 of the accompanying drawings. If such an illumination system is used in an LCD image projector, light of one polarisation is lost as no polarisation conversion elements are provided to increase light throughput. Furthermore, due to the low broadband absorption cross-section of rareearth ions, a substantial part of UV and IR propagates through the integrator rod 22 unconverted and is lost from the system. This system does not use any re-circulation elements and UV light passes through the rod only once. In order to achieve high UV to visible light conversion in a single pass glass rod doped with a rare earth compound, it is necessary to increase either the length of the rod or the concentration of the rare earth material. A longer rod has the disadvantage of increasing the size of a projector.

To improve the efficiency of utilisation of emitted light in a projection system with a wavelength converting integrator rod, W02004/046809 discloses shaping the light receiving end of the integrator rod as a parabolic or elliptical reflector R, as illustrated in Figure 10 of the accompanying drawings. Fluorescent material is placed in a focal plane F2 of such a reflector. This focal plane coincides with the focal plane of the lamp reflector: all the light from the lamp is focused onto this point, but not at a colour wheel or the entrance of a rod, where beam size is much bigger. To enable light from the lamp to couple into the integrator rod, a light entering, non-reflective, aperture must be greater than the beam diameter. Thus, the size of the integrator rod is substantially increased. Furthermore, the integrator rod has a columnar shape that does not provide a rectangular beam cross-section of the output beam.

Fluorescent material is integrated into the glass rod. Inorganic Eu3+ compounds suggested for VV/Red wavelength conversion have very low crosssection and, therefore, the efficiency of UV light utilisation is low. Fluorescent material is concentrated in a plane F2 so that efficiency of photoluminescence suffers due to concentration quenching.

According to a first aspect of the invention, there is provided an illumination system for a projector, comprising a light source for directing light along a main optical path, a deflector for deflecting light in a portion of the frequency spectrum emitted by the light source out of the main path into a subsidiary optical path and for passing light in the frequency spectrum outside the portion along the main path, a converter disposed outside the main path in the subsidiary path for converting at least part of the light deflected by the deflector to a different frequency band, and a combiner in the main path for combining light from the converter passing along the subsidiary path with light from the light source passing through the deflector along the main path.

The deflector may be arranged to deflect invisible light. The invisible light may comprise ultraviolet light. The invisible light may comprise infrared light.

The deflector may be arranged to deflect part of the visible light produced by the light source.

At least one of the deflector and the combiner may comprise a frequencydependent reflector. The frequency-dependent reflector may comprise one of a prismatic element, a dichroic mirror and a cholesteric mirror.

The deflector may be arranged to deflect light substantially perpendicularly to the main path.

The combiner may comprise a lightpipe. The converter may be arranged to direct light through a shadow region into the lightpipe. The system may comprise coupling optics at or adjacent the shadow region. The coupling optics may comprise a first grating.

The converter may comprise a reflector.

The system may comprise a further reflector for returning at least part of the light not converted by the converter to the light source for recirculation.

According to a second aspect of the invention, there is provided an illumination system for a projector, comprising a light source for directing light in a light beam, a first deflector for deflecting the direction of the light beam, a converter for converting part of the light deflected by the first deflector to a different frequency band, and a reflector for re-imaging the light beam from the first deflector and the light from the converter.

The system may comprise a second deflector for deflecting light re-imaged by the reflector. The second deflector may be arranged to deflect the light into an output beam which is substantially coaxial with the light beam produced by the light source.

The all or each deflector may comprise a mirror.

The first deflector may be arranged to deflect light substantially perpendicularly to the light beam produced by the light source.

The reflector may have a concave conic surface. The reflector may be parabolic or ellipsoidal.

The different frequency band may be in the visible light spectrum.

The different frequency band may be in a spectral region in which the light from the light source is deficient.

The different frequency band may be in the red visible spectrum.

The converter may comprise a phosphor material. The phosphor material may comprise at least one of zinc sulphide, cadmium selenide and cadmium telluride.

The converter may comprise a fluorescent material. The material may be a compound including a rare earth element. The rare earth element may comprise at least one of europium, samarium, terbium and yterbium. The rare earth element may be in a ligand bound form. The converter may comprise a film of the material adjacent a metal film.

The converter may comprise a second grating.

The light source may comprise a reflector. The reflector may have a concave conic surface. The reflector may be parabolic or ellipsoidal.

The light source may comprise an ultrahigh pressure mercury arc lamp.

The system may comprise an homogeniser.

The system may comprise a polarisation converter.

According to a third aspect of the invention, there is provided a projector comprising a system according to the first or second aspect of the invention.

The projector may comprise at least one spatial light modulator arranged to modulate light from the system. The or each modulator may comprise a liquid crystal device.

It is thus possible to provide an illumination system which gives improved colour balance of projected images without substantially compromising image brightness.

Light throughput in a projector may be improved and it is possible to increase the intensity of a spectral component or range which is deficient in the emission of the light source. Efficiency of collection and use of light from the converter may be increased and a wide range of materials may be used in the converter. No substantial change in projection architecture or lamp design is necessary and the illumination system may be used with all types of spatial light modulators, such as digital light processors (DLPs), liquid crystal devices (LCDs) and liquid crystal on silicon (LCoS) as used in projection arrangements. For example, such an illumination source may be used in a DLP sequential colour recapture system in order to overcome or reduce the effects of colour

flickering in DLP field sequential projectors.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a graph illustrating the spectrum or distribution of light from a typical UHP lamp; Figure 2 is a cross-sectional diagram of a first known type of projector illumination system; Figure 3 is a cross-sectional diagram of a second known type of projector illumination system; Figure 4a and Figure 4b are prospective and cross-sectional diagrammatic views, respectively, of a third known projector illumination system; Figure 5 is a cross- sectional diagram of a fourth known type of projector illumination system; Figures 6a, 6b and 6c are different diagrammatic views of parts of a fifth known projector illumination system; Figures 7a and 7b are cross- sectional diagrams of a sixth known type of projector illumination system; Figure 8 is a cross-sectional diagram of a seventh known type of projector illumination system; Figure 9 is a diagram of part of the illumination system of Figure 8 illustrating a disadvantage thereof; Figure 10 is a cross-sectional diagram of an eighth known type of projector illumination system; Figures 11 a and 11 b are cross-sectional diagrams illustrating a projector illumination system constituting an embodiment of the invention; Figure 12 is a cross-sectional diagram illustrating a projector illumination system; constituting another embodiment of the invention; Figure 13 is a cross-sectional diagram illustrating a projector illumination system constituting a further embodiment of the invention; Figure 14 is a cross-sectional diagram illustrating a projector illumination system constituting another embodiment of the invention; Figure 15 is a cross-sectional diagram of a projector illumination system constituting another embodiment of the invention; Figure 16 is a cross-sectional diagram of a projector illumination source constituting a further embodiment of the invention; Figures 17 and 18 are diagrammatic views illustrating single panel and multiple panel projectors, respectively, constituting embodiments of the invention; and Figure 19 is a cross-sectional diagram illustrating a light source of the type shown in Figure 11 a together with an homogeniser and polarisation conversion optical system.

The illumination system illustrated in Figures 1 la and lib comprises a light source 30 in the form of a small arc ultrahigh pressure (UHP) mercury lamp. The lamp 30 is deposed at or near the focal point of a concave reflector 31, for example in the form of a parabolic (or ellipsoidal) mirror, which directs light along a main optical path, for example as indicated by the arrows 32, 32', 32" through the illumination system. Light emitted by the lamp 30 has a spectral distribution of the form illustrated in Figure 1 and comprising a visible region together with ultraviolet and infrared radiation components.

Light from the lamp 30 is deficient at wavelengths above about 600nm (the red visible region) and would result in colour imbalance in a projected image if used on its own.

Light from the lamp 30 and the reflector 31 passes along the main optical path 32 to a crossed arrangement 33, which acts as a combined deflector and combiner. In particular, the arrangement 33 includes an ultraviolet reflector 34, which reflects ultraviolet radiation perpendicularly out of the light path 32 into a subsidiary light path but substantially transmits visible light from the lamp 30 along the main path.

Ultraviolet (UV) radiation is reflected by the reflector 34 towards a converter, which comprises an element 35 comprising a fluorescent material and a non-imaging light concentrator 36 for reflecting UV and visible light. For example, the concentrator 36 may comprise a parabolic (or ellipsoidal) concave mirror. The concentrator 36 is arranged to concentrate the UV radiation reflected by the reflector 34 into an optimally small volume and the fluorescent material 35 is disposed in the UV beam waist so that the emitting area is minimised and emitted light is returned with a similar etendue. The material 35 converts the UV radiation by fluorescence into red light, which is returned substantially in the opposite direction along the subsidiary path towards the prism 33.

The prism 33 also comprises a narrow-band red reflective filter 37, which is arranged to reflect red light in the relatively narrow band produced by the fluorescent material 35 and to transmit light outside this narrow band substantially without deviation from the lamp 30 and the reflector 31. Light from the material 35 thus copropagates with light from the lamp 30 to the illumination system output and augments or at least partially compensates for the red deficiency in the light produced by the lamp 30. Thus, light from the illumination system may be used to form a projected image with reduced or no colour imbalance.

Rare earth ions may be used as the fluorescent material 35, for example such as Eu+3, Eu+2, Sm+2, Sm+3, Tb+3, Yd+3 and others. The rare earth compounds may be in a ligand bound form so as to increase the UV absorption cross-section. Ligand bound compositions also provide substantial elimination of concentration quenching and thus enable thin film elements to be used as the emitters of light concentrators.

As an alternative to the fluorescent material 35, the converter may be made of inorganic phosphors, such as zinc selenide, cadmium sulphide or cadmium telluride. Such inorganic phosphors may be in the form of quantum dots, for instance as available from Evident Technologies inc.

For example, where the fluorescent material 35 includes Eu+3, light emission following UV irradiation is in a relatively narrow band so that the red reflecting filter or mirror 37 can have a relatively narrow bandwidth. For example, the bandwidth may be in the range of 4 to 20 nm so that only a relatively small amount of light from the lamp 30 is reflected out of the system by the reflector 37. This results in a substantial increase in brightness for a correctly colour-balanced illumination source and the results of specific examples are illustrated in the following table.

Lamp Lamp + l6nm l2nm 8nm 4nm only light concentrator +2Onm ____________ _______ reflector Fluorescence usage = 0.3 Fluorescence usage = (UVabsorption) x (Quantum Efficiency) x (Fluorescence collection) _______ ____________ __________ __________ __________ _____________ Size of R 0.55 0.41 0.40 0.39 0.38 0.36 G 0.25 0.33 0.34 0.35 0.35 0.36 B 0.20 0.26 0.26 0.26 0.27 0.28 segments in colour wheel ______ ____________ _________ Brightness, 1025 1286 1310 1336 1362 1388 lumens ______ (+25.5%) (+27.8%) (+30.3%) (+32.9%) (+35.5%) Fluorescence usage 0.5 Size of R 0. 55 0.32 0.31 0.305 0.30 0.29 G 0.25 0.39 0.39 0.395 0.40 0.40 B segments 0.20 0.29 0.30 0.30 0.30 0.31 in colour wheel ______ ____________ _________ Brightness, 1025 1481 1496 1513 1530 1548 (+51.0) lumens ______ (+44.5%) (+46%) (+47.6%) (+49.3%) ____________ The table represents a range of results for an illumination system used with a colour wheel of variable size RGB segments to achieve a D65 White Point (x = 0.313; y = 0.329). Brightness comparisons are made against a corresponding system in which the prism 33, the fluorescent material 35 and the concentrator 36 are omitted. Results are given for reflectors 37 of different bandwidths with percentage improvements in brightness being given in parenthises.

Although the illumination system illustrated in Figure 1 la makes use of fluorescent material 35 which converts ultraviolet light from the lamp 30 into visible red light in order to make up at least partially for the deficiency in the red part of the spectrum produced by the lamp 30, other conversion schemes are possible. For example, as an alternative or in addition to converting the ultraviolet light, the infrared light may be converted to a more useful part of the visible spectrum. Thus, such an arrangement may be used with a lamp which emits excess infrared light. Alternatively or additionally, it is possible to convert part of the light in part of the visible spectrum to light in another part of the visible spectrum. For example, where the lamp produces excess blue and/or green light, part of this may be directed along the subsidiary path for conversion by the material 35 to the red part of the spectrum so as to improve the colour balance of the output light.

Figure 12 illustrates another illumination system which differs from that shown in Figure 1 la in that the prism 33 is replaced by separate frequency-dependent reflectors 34 and 37 forming the deflector and combiner, respectively. In addition to reflecting the ultraviolet component of the light from the lamp 30, the reflector 34 also reflects red visible light along the subsidiary path towards the material 35 and the reflector 36.

Thus, most or all of the red light from the lamp 30 is deflected into the subsidiary path and is not lost from the illumination system by reflection from the reflector 37.

The ultraviolet light is converted as described hereinbefore to red visible light and returned along the subsidiary path to the reflector 37 together with the red light reflected by the reflector 34. Ultraviolet light which has not been converted by the material 35 is reflected by an ultraviolet reflector 38 back along the subsidiary path to the reflector 34, where it is reflected back to the light source comprising the lamp 30 and the reflector 31 for re-circulation. Thus, by making full use of the red light supplied by the lamp 30 and by re-circulating the ultraviolet light for further conversion, the efficiency of the illumination system is improved.

Figure 13 illustrates an illumination system which is similar to that illustrated in Figure 12 but in which the ultraviolet reflector 38 is omitted and the reflectors 34 and 37 are mirrors which reflect all of the radiation incident thereon and comprising mainly the visible (RGB) light and the ultraviolet light. Thus, all of the radiation passes along the same optical path and the material 35 converts, for example, the ultraviolet light to red visible light so as to augment the output spectrum of the lamp 30. The reflector 36 re- images the light from the deflector 34 together with the light emitted by the material 35 and such an arrangement provides a compact and efficient illumination system for a projector. The ultraviolet reflector 38 as shown in Figure 12 may also be provided in the illumination system of Figure 13 to re-circulate ultraviolet light and thus improve the efficiency of the system. Also, the material 35 may perform the additional or alternative frequency changing schemes described hereinbefore.

Tn the illumination system illustrated in Figure 14, the prism 33 shown in Figure 1 la is omitted and replaced by a deflector 34 in the form of a UV reflector which reflects UV radiation and transmits visible light, and a combiner in the form of a lightpipe 40 comprising a glass integrating rod. The system comprises a reflector 36 for concentrating the reflected UV radiation from the reflector 34 onto the fluorescent (or other) material 35, which is located at an aperture on the axis of the reflector 36. The material 35 is also located at an on-axis aperture in a further similar reflector 41, which directs red light from the material 35 via mirrors 42 and 43 into the tightpipe. For example, red light reflected by the mirror 43 enters the lightpipe 40 at a shadow region and is recombined with light propagating along the optical path 32.

The illumination system shown in Figure 15 combines the arrangement of the material and the reflector 36 shown in Figure 1 Ia with the reflector 34 and the light pipe 40 shown in Figure 14. However, in this embodiment, the axis of the reflector 36 is offset relative to the perpendicular to the light path 32 50 that red light from the material 35 and reflected by the reflector 36 is sent directly to the lightpipe 40 for combination and copropagation with light from the lamp 30 passing along the optical path 32. The lightpipe 40 is provided with coupling optics 45 at the shadow area so as to improve the coupling of the red light from the material 35 into the lightpipe 40. For example, the coupling optics 43 may be in the form of a diffractive element.

In the illumination system shown in Figure 16, UV radiation from the lamp 30 is reflected by a dichroic filter 34 to the element 35. The element 35 comprises a thin film of fluorescent material adjacent a thin metal film, for example a silver film of 30 to 6Onm thickness or an aluminium film of 10 to 3Onm thickness. In this arrangement, the directionality of fluorescence is controlled by surface plasmon coupled emission. In particular, the incident UV radiation excites the fluorescent material, whose atoms are coupled to surface plasmons at the metal interface. As a result, the fluorescence is coupled into a highly directional emission so that no further means are required for directing the resulting red light into the light pipe 40. This light is coupled via the grating 45, which directs the red light into the acceptance angle of the lightpipe 40 through the shadow region of the lightpipe. The lightpipe 40 combines and homogenises light from the lamp 30 and red light from the element 35.

Plasmon coupled emission is disclosed, for example, in W.Barnes, A.Dereus, T.Ebbesen, "Surface Plasmon Subwavelengths Optics", Nature, vol 424, pp 824-830, 2003.

The aluminium or silver film may be corrugated so as to form a grating to provide further control or manipulation of the direction of emission from the element 35. It is thus possible to achieve a very compact design with the highly directional red emitted light being efficiently coupled into the light pipe 40.

Figure 17 illustrates a projector of the single panel type including an illumination system illustrated in simplified form at 50. The illumination system 50 is used in combination with colour wheel 51, including a colour recapture wheel, of known type.

The colour wheel 51 is used as a colour splitting element and is disposed immediately after the illumination system 50. Light of each primary colour is modulated by a light modulator 52, for example in the form of a liquid crystal device, and is projected by suitable projection optics onto a projection screen 53.

Figure 18 illustrates a three panel projector including the illumination system 50.

Broadband white light from the illumination system 50 is spatially separated into three primary colour components by colour splitting elements 55, 56 and 57, for example in the form of dichroic mirrors. Each of the primary colour beams is then modulated by a respective spatial light modulator 60R, 60G and 60B, for example in the form of a digital light processor, a liquid crystal device or liquid crystal on silicon device.

In the case of spatial light modulators which require polarised input light, such as liquid crystal devices or liquid crystal on silicon devices, efficiency of utilisation of light may be improved by passing light from the illumination system through a polarisation conversion optical system whose output polarisation matches the input polarisation requirements of the spatial light modulators. Figure 19 illustrates the use of such a polarisation conversion optical system (PCOS) 70 downstream of an illumination system of the type shown in Figure ha. The PCOS comprises microlens arrays 71 and 72, an array of polarisation splitting microprisms 73, and an array of halfwave retarders such as 74. Such a system 70 is well known and passes light of a first linear polarisation without modification while converting light of the orthogonal polarisation to the first polarisation.

It is thus possible to provide illumination systems for projectors with high brightness and improved colour balance. Excess light, such as UV radiation, from the light source may be extracted from the main optical path through the illumination system and converted to visible light. The directionality of the converted light may be manipulated by optical means or by controlling the direction of the emission. The converted visible light may then be inserted back into the main optical path. This avoids the need for manipulation of directionality within the main light path, which could affect all of the light passing through the system. Thus, light passing directly along the main optical path from the light source is not substantially affected. Fluorescent material is not required to be incorporated in the glass matrix of an integrator rod and this enables a wider choice of fluorophores with an increased absorption cross-section in the case of fluorescent wavelength converters. A relatively compact system may be provided.

Claims

CLAIMS: I. An illumination system for a projector, comprising a light

source for directing light along a main optical path, a deflector for deflecting light in a portion of the frequency spectrum emitted by the light source out of the main path into a subsidiary optical path and for passing light in the frequency spectrum outside the portion along the main path, a converter disposed outside the main path in the subsidiary path for converting at least part of the light deflected by the deflector to a different frequency band, and a combiner in the main path for combining light from the converter passing along the subsidiary path with light from the light source passing through the deflector along the main path.
2. A system as claimed in claim 1, in which the deflector is arranged to deflect invisible light.
3. A system as claimed in claim 2, in which the invisible light comprises ultraviolet light.
4. A system as claimed in claim 2 or 3, in which the invisible light comprises infrared light.
5. A system as claimed in any one of the preceding claims, in which the deflector is arranged to deflect part of the visible light produced by the light source.
6. A system as claimed in any one of the preceding claims, in which at least one of the deflector and the combiner comprises a frequencydependent reflector.
7. A system as claimed in claim 6, in which the frequency-dependent reflector comprises one of a prismatic element, a dichroic mirror and a cholesteric mirror.
8. A system as claimed in any one of the preceding claims, in which the deflector is arranged to deflect light substantially perpendicularly to the main path.
9. A system as claimed in any one of the preceding claims, in which the combiner comprises a lightpipe.
10. A system as claimed in claim 9, in which the converter is arranged to direct light through a shadow region into the lightpipe.
11. A system as claimed in claim 10, comprising coupling optics at or adjacent the shadow region.
12. A system as claimed in claim 11, in which the coupling optics comprise a first grating.
13. A system as claimed in any one of the preceding claims, in which the converter comprises a reflector.
14. A system as claimed in any one of the preceding claims, comprising a further reflector for returning at least part of the light not converted by the converter to the light source for recirculation.
15. An illumination system for a projector, comprising a light source for directing light in a light beam, a first deflector for deflecting the direction of the light beam, a converter for converting part of the light deflected by the first deflector to a different frequency band and a reflector for reimaging the light beam from the first deflector and the light from the converter.
16. A system as claimed in claim 15, comprising a second deflector for deflecting light reimaged by the reflector.
17. a system as claimed in claim 16, in which the second deflector is arranged to deflect the light into an output beam which is substantially coaxial with the light beam produced by the light source.
18. A system as claimed in any one of claim 15 to 17, in which the or each deflector comprises a mirror.
19. A system as claimed in any one of claims 15 to 18, in which the first deflector is arranged to deflect light substantially perpendicularly to the light beam produced by the light source.
20. A system as claimed in any one of claims 13 to 19, in which the reflector has a concave conic surface.
21. A system as claimed in claim 20, in which the reflector is parabolic or ellipsoidal.
22. A system as claimed in any one of the preceding claims, in which the different frequency band is in the visible light spectrum.
23. A system as claimed in any one of the preceding claims, in which the different frequency band is in a spectral region in which the light from the light source is deficient.
24. A system as claimed in claim 22 or 23, in which the different frequency band is in the red visible spectrum.
25. A system as claimed in any one of the preceding claims, in which the converter comprises a phosphor material.
26. A system as claimed in claim 25, in which the phosphor material comprises at least one of zinc suiphide, cadmium selenide and cadmium telluride.
27. A system as claimed in any one of claims I to 24, in which the converter comprises a fluorescent material.
28. A system as claimed in claim 27, in which the material is a compound including a rare earth element.
29. A system as claimed in claim 28, in which the rare earth element comprises at least one of europium, samarium, terbium and ytterbium.
30. A system as claimed in claim 28 or 29, in which the rare earth element is in a ligand bound form.
31. A system as claimed in any one of claims 27 to 30, in which the converter comprises a film of the material adjacent a metal film.
32. A system as claimed in claim 31, in which the converter comprises a second grating.
33. A system as claimed in any one of the preceding claims, in which the light source comprises a reflector.
34. A system as claimed in claim 33, in which the reflector has a concave conic surface.
35. A system as claimed in claim 34, in which the reflector is parabolic or ellipsoidal.
36. A system as claimed in any one of the preceding claims, in which the light source comprises an ultrahigh pressure mercury arc lamp.
37. A system as claimed in any one of the preceding claims, comprising an homogen iser.
38. A system as claimed in any one of the preceding claims, comprising a polarisation converter.
39. A projector comprising a system as claimed in any one of the preceding claims.
40. A projector as claimed in claim 39, comprising at least one spatial light modulator arranged to modulate light from the system.
41. A projector as claimed in claim 40, in which the or each modulator comprises a liquid crystal device.