GB2329036A - Optical system for redistributing optical extent and illumination source - Google Patents

Abstract

An optical system is provided for redistributing optical extent, for instance to provide an illumination source capable of providing a relatively thin light source in one dimension. The system comprises a plurality of optical paths, for instance formed by a split Fresnel lens 10 or a multiwedge array (10, Fig.4), or an arrangement of mirrors and lenses (Figs 5,6), or a split ellipsoidal reflector (16, Fig 9). The device 10 samples input light, for instance from a light source 15 and a reflector 16, and images the light at an image plane with a different spatial distribution, such as a linear distribution. An optical combiner, for instance in the form of a split mirror 11, aligns the light propagation directions from the optical paths.

Description

OPTICAL SYSTEM FOR REDISTRIBUTING OPTICAL EXTENT AND ILLUMINATION SOURCE.

The present invention relates to an optical system for redistributing optical extent and to an illumination source using such an optical system. Such illumination systems may be used in projection displays.

The term "optical extent" as used herein is defined to mean the product of the size and optical divergence of an object or image.

Certain optical systems are inherently one dimensional in that they effectively integrate the optical intensity in the dimension orthogonal to the one dimension and are therefore tolerant to optical extent in the orthogonal dimension. Examples of such systems include projection systems, for instance projection television systems, which modulate light by deflecting it in one dimension and which require light sources which are physically small in that dimension. Known systems of this type use small light sources which have lifetime restrictions making them unsuitable or undesirable for commercial products. Metal halide lamps with relatively large electrode gaps are efficient in terms of conversion of electricity into light output and have relatively long lifetimes. Such lamps are therefore suitable for use in home consumer projection systems.

However, it is desirable to provide smaller light sources with longer lifetimes so as to improve the performance of projection displays.

H. Roder, H.j. Ehrke, R. Gerhard-Multhaupt, E. Ipp and Imenzel, "Full Color Diffraction-based Optical System for Light Valve Projection Displays" Jn. Display vol. 16 No.1 1995 pp 27-33 and David Armitage "Design Issues in Liquid Crystal Projection Displays", pp.41-51 SPIE Proceedings Vol. 2650 Projection Displays II Editor(s): Ming H. Wu, Hamamatsu Corp., Bridgewater, NJ, USA. ISBN: 0-8194-2024-7, 308 pages, published 1996 disclose a known type of system generally referred to as a Schlieren optical system. Such a system is suitable for use with projection displays, for instance as disclosed in "The Grating Light Valve: revolutionising Display Technology" D.M. Bloom, Photonics West/Electronic Imaging '97 SPIE. European Patent Application No.

97303990.2. However, such systems suffer from the disadvantages described hereinbefore of requiring a small light source of high efficiency.

Figure 1 illustrates a known technique for sacrificing the optical extent in one dimension for that in the dimension orthogonal thereto so as to provide a more nearly one dimensional output. This arrangement makes use of a fibre bundle 1 of multimode optical fibres whose relative positions are changed along the length of the bundle. This allows the effective image of a source to be altered in aspect ratio without altering its divergence. Thus, a slit source 2 can be formed from a circular source 3 with the optical divergence substantially unaffected.

The type of arrangement shown in Figure 1 is commonly used in optical spectrometers which are based on one dimensional diffraction to analyse optical chromaticity. The source size in one dimension must be small in order to provide high resolution but the size in the other dimension is unimportant because the signal is integrated along the long axis of the source. Optical divergence must match the capture angle of the instrument source size so that alteration must be performed without affecting divergence.

Although this type of arrangement could in principle be used to alter illumination source size for a television projection system, a disadvantage is that the fibres cannot be closely packed together. Thus, the "fill factor" is relatively low and leads to loss of light from the light emitter. This leads to reduced brightness performance or increased illumination requirements.

Examples of other devices which require illumination sources which are small in one dimension are disclosed in "Digital Light Processing for Projection Displays: A Progress Report" Larry J. Horn beck, Proceedings Society of Information Display 16th International Display Research Conference 1009 pp 67-71 and H. Hamada et al "A New Bright Single Panel LC-Projector System without a Mosaic Color Filter" IDRC '94 Proceedings, 422 (1994) and C. Joubert, B. Loiseaux, A. Delboulbe and Huignard j-P "Dispersive Holographic Microlens Matrix for LCD Projection" SPIE Proceedings Vol. 2650 Projection Displays II Editor(s): Ming H. Wu, Hamamatsu Corp., Bridgewater, NJ, USA. ISBN: 08194 2024-7, 308 pages. Published 1996.

According to a first aspect of the invention, there is provided an optical system for redistributing optical extent, comprising: a plurality of optical subsystems, each of which has an input aperture and is arranged to image light from a source, the subsystems forming a spatial distribution of source images whose relative positions are different from the relative positions of the input apertures; and an optical combiner for aligning the light propagation directions from the subsystems.

The input apertures may be arranged as a two dimensional array and the spatial distribution of source images may comprise a one dimensional array.

Each subsystem may comprise an optical imaging element. At least one of the imaging elements may comprise a converging lens. At least one of the imaging elements may comprise a concave mirror.

The subsystems may comprise respective first light deflecting elements and a common imaging element. Each of the first light deflecting elements may comprise a wedge. The common imaging element may comprise a further converging lens.

The optical combiner may comprise a respective second light deflecting element for each subsystem. Each second light deflecting element may be disposed at an image forming region of the respective subsystem. Each second light deflecting element may comprise a plane mirror.

According to a second aspect of the invention, there is provided an illumination source comprising a light emitting source and a system according to the first aspect of the invention for redistributing the optical extent of the light emitting source.

The illumination source may comprise a collimator disposed between light emitting source and the system. The collimator may comprise at least one cylindrical lens.

The light emitting source may comprise a light emitter and a concave reflector. The concave reflector may have a plurality of portions forming the subsystems.

It is thus possible to provide an optical system which can redistribute the optical extent. For instance, the optical extent in one dimension may be reduced at the expense of increasing the optical extent in an orthogonal dimension so that it is possible to provide an illumination source having a relatively small size in one dimension. Thus, an efficient one dimensional illumination source having good lifetime may be provided relatively inexpensively. Such illumination sources are advantageous in projection display systems such as projection television systems for domestic use.

The relatively high efficiency of conversion of electricity to light gives such display systems of improved brightness for a given power consumption.

The increased lifetime makes such display systems more convenient, particularly for domestic use.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagram illustrating a known type of optical system; Figure 2 illustrates diagrammatically an optical system constituting a first embodiment of the invention; Figure 3 illustrates diagrammatically an illumination source constituting a second embodiment of the invention; Figure 4 illustrates diagrammatically an illumination source constituting a third embodiment of the invention; Figure 5 illustrates diagrammatically an illumination source constituting a fourth embodiment of the invention; Figure 6 is a diagrammatic plan view of the illumination source of Figure 5; Figure 7 illustrates diagrammatically a projection display including an illumination source of the type shown in Figure 3; Figure 8 illustrates another projection display using an illumination source of the type shown in Figure 3; and Figure 9 illustrates diagrammatically an illumination source constituting a fifth embodiment of the invention.

Like reference numerals refer to like parts throughout the drawings.

The optical system shown in Figure 2 comprises two optical paths defined by a split Fresnel lens 10 and an optical combiner 11 in the form of a split mirror disposed in the common image plane of the lens 10. The two halves of the lens 10 are vertically displaced with respect to each other so as to form images of an object 12 at respective parts of the split mirror 11.

The object 12 is shown as having a horizontal size x and a vertical size y with a horizontal divergence + and vertical divergence 0. Each half of the lens 10 forms an image of the object 12 with reduced divergence ' in the horizontal direction and with reduced horizontal size x'. The split mirror 11 corrects for the different light propagation directions from the halves of the lens 10 and a compound source image is formed having increased vertical size y' and increased divergence 0'. it is therefore possible to reduce the extent in the horizontal direction, for instance by a factor of 2, by sacrificing the extent in the vertical direction which is increased in proportion to the reduction in the horizontal direction. The region illuminated by light from the object via the optical system is shown at 13.

The optical system of Figure 2 is based on producing multiple images using free space optical multi-elements. The separate optical elements (the lens halves shown in Figure 2) add to fill the full system aperture so that individually their apertures are small. Multiple sub-source images are formed and all have reduced extents in both dimensions normal to the direction of propagation. By optically positioning these images in a linear array and correcting for any optical propagation direction mismatch by a further optical multi-element (the split mirror 11 in Figure 2) in the image plane, the compound image of the source has a different aspect ratio from the source itself.

Such an arrangement differs in performance from systems based on simple cylindrical lenses. In particular, although cylindrical optics can vary the image size and divergence independently in the horizontal and vertical directions, the extents in these directions are maintained in accordance with the Constant Brightness Theorum, which is well known in optical physics and is disclosed, for instance, in "Handbook of Optics", Vol. 1, pp 45, McGraw-Hill (1996). Cylindrical optical systems cannot therefore alter the optical extent as in the case of the optical system shown in Figure 2.

Figure 3 illustrates a light source which makes use of the optical system illustrated in Figure 2. The illumination source comprises a light source 15 and an ellipsoidal reflector 16. Light from the source and reflector is collimated by cylindrical collimating lenses 1 7a and 1 7b is supplied to the optical system of Figure 2. The focal length 8f" of the lens 1 7b is equal to twice the focal length 8f' of the lens 17a. Each of the lenses 1 7a and 1 7b alters the divergence in one dimension with a corresponding alteration in that dimension of the image size. This equalises the divergence of the light source and allows a uniform illumination cone to be produced. This has the effect of producing an oval image of the circular source when imaged by the split Fresnel lens 10. Offsetting the images produced by the lens halves results in two images having their extents reduced by approximately a half in the horizontal direction. The split mirror 11 at the image plane adjusts the direction of optical propagation from these two images and produces a near uniform illumination region 13 suitable for illuminating a projection optical system as described hereinafter.

The optical system shown in Figures 2 and 3 provides two optical paths by dividing the Fresnel lens 10 into two halves. However, the Fresnel lens 10 could be split into more than two sub-elements with an individual plane mirror element of the slit mirror 11 for each sub-element of the lens 10.

This allows further reduction in the horizontal extent at the expense of increasing the vertical extent.

Figure 4 illustrates an illumination source having six parallel free space optical paths. The light source 15 is provided with a parabolic reflector 16 which results in a non-uniform intensity profile. In this embodiment, the optical paths are effectively defined by a multi-element array 10 which comprises six wedges for spatially sampling the input light and spatially redistributing the images. Light from the array 10 is imaged by a lens 20 as a combined image 21 in the form of a linear one dimensional vertical array of images. An optical combiner, such as a split mirror having six plane mirrors (not shown), is disposed at the image plane for aligning the divergences or directions of propagation so as to form a light source having greatly reduced horizontal extent at the expense of increased vertical extent.

Figures 5 and 6 illustrate an arrangement in which the light source 15 is associated with a plurality of optical elements having moderate numerical apertures for relaying a plurality of images into a linear array. The optical elements surround the light source 15 and comprise two spherical reflectors 1 0a and two lenses 10b. For instance, the reflectors 1 0a may have an f number of 0.5 and the lenses lOb may have an f number of one.

The elements 10a and 10b form four images in a linear array which images are corrected for uniform illumination and for propagation direction by a split mirror 11 comprising four plane mirrors. The reflectors 10a are offset slightly in angle so that the reflected light does not pass back through the source 15. However, for sources which are not adversely affected by the superposition of reflected images, overlap of the reflected image and source may be achieved so that only two images are relayed.

Such an arrangement reduces the optical extent in the dimension in which the images are replicated.

The arrangements shown in Figures 4 to 6 ae particularly applicable to systems which are grossly optically mismatched. In such systems, not all of the light from the light source can be efficiently collected which, in the absence of the present invention, would lead to an illumination source of poor efficiency. In the systems shown in Figures 4 to 6, the light which is collected is more uniform and in the highest intensity regions of the light source output. This leads an illumination source whose efficiency is substantially improved compared with known arrangements and, therefore, represents a substantial advantage.

Figure 7 illustrates the use of the illumination system illustrated in Figure 3 in a projection display system of the type disclosed in European Patent Application No. 97303990.2. Light from the illumination source is directed via a field lens 30 onto a diffractive device 31 in the form of a diffractive liquid crystal phase-only grating. The device 31 is pixellated and each pixel is controlled to be in a reflective state for producing a bright pixel or a diffractive state for producing a dark pixel. When in the reflective state, the pixel reflects light from the illumination source to a projection lens 32 which images the pixel on a screen 33. When in the diffractive state, the pixel diffracts light out of the projection system. In order to improve the brightness and contrast ratio of such a projection display, a one dimensional illumination source of small extent in the orthogonal direction is required. The illumination source illustrated in Figures 3 and 7 supplies this requirement and allows a projection display of high brightness and contrast ratio to be provided while using a light source 15 of relatively large size and good working lifetime.

The illumination system shown in Figures 3 and 7 may also be used to achieve similar advantages with displays of the type disclosed in "Digital Light Processing for Projection Displays: A Progress Report" Larry J.

Hombeck, Proceedings Society of Information Display 16th International Display Research Conference 1009 pp 67-71.

The illumination system illustrated in Figure 3 may also be used in colour separation systems in which the primary colours (RGB) are separated spatially using a micro-optic array. Such an arrangement is illustrated in Figure 8. Light from the light source is collimated by a lens 35 and supplied to RGB dichroic mirrors 36. A spatial light modulator 37 comprises a micro-optic array which images onto a display panel so that the RGB subpixels of the panel, which are arranged side by side in one dimension, receive red, green and blue light respectively. The output light is then supplied to a projection lens 32.

Figure 9 illustrates another illumination source in which the light source 15 is associated with a split ellipsoidal reflector 16. The optical axes of the two halves of the reflector 16 are displaced vertically or at an angle in the vertical plane so as to define the two optical paths. The split mirror 11 realigns the divergence or direction of propagation and is disposed at the image plane of the reflector 16.

The optical system and illumination system may be used in various other applications, some of which are as follows.

The fibre optic system illustrated in Figure 1 may be replaced by the optical system disclosed herein, for instance at the input to a spectrometer or other similar optical instruments such as optical transfer function measurement systems.

The optical system may be used for replicating images with uniform extent and rearranging them to coincide with the fibre entrance positions of a fibre optic coupling system in which light is coupled from a source into many fibres but the fibres are in a nontircular array.

White light time-of-flight velocimetres ideally require a plane of white light imaged from a single bright source. By reducing the extent in one dimension, a line of small sources may be formed having low divergence in the dimension of the small source image. The optical systems may be used to imitate a sheet of light by superimposing light from such sources.

In an alternative embodiment, the multi-element device such as the split Fresnel lens 10 may be replaced by active elements such as spatial light modulators to allow active alteration of the extent at the output of the optical system.

Claims (16)

1. An optical system for redistributing optical extent, comprising: a plurality of optical subsystems, each of which has an input aperture and is arranged to image light from a source, the subsystems forming a spatial distribution of source images whose relative positions are different from the relative positions of the input apertures; and an optical combiner for aligning the light propagation directions from the subsystems.

2. A system as claimed in Claim 1, in which the input apertures are arranged as a two dimensional array and the spatial distribution of source images comprises a one dimensional array.

3. A system as claimed in Claim 1 or 2, in which each subsystem comprises an optical imaging element.

4. A system as claimed in Claim 3, in which at least one of the imaging elements comprises a converging lens.

5. A system as claimed in Claim 3 or 4, in which at least one of the imaging elements comprises a concave mirror.

6. A system as claimed in Claim 1 or 2, in which the subsystems comprise respective first light deflecting elements and a common imaging element.

7. A system as claimed in Claim 6, in which each of the first light deflecting elements comprises a wedge.

8. A system as claimed in Claim 6 or 7, in which the common imaging element comprises a further converging lens.

9. A system as claimed in any one of the preceding claims, in which the optical combiner comprises a respective second light deflecting element for each subsystem.

10. A system as claimed in Claim 9, in which each second light deflecting element is disposed at an image forming region of the respective subsystem.

11. A system as claimed in Claim 9 or 10, in which each second light deflecting element comprises a plane mirror.

12. An illumination source comprising a light emitting source and a system as claimed in any one of the preceding claims for redistributing the optical extent of the light emitting source.

13. An illumination source as claimed in Claim 12, comprising a collimator disposed between the light emitting source and the system.

14. An illumination source as claimed in Claim 13, in which the collimator comprises at least one cylindrical lens.

15. An illumination source as claimed in any one of Claims 12 to 14, in which the light emitting source comprises a light emitter and a concave reflector.

16. An illumination source as claimed in Claim 15, in which the concave reflector has a plurality of portions forming the subsystems.