WO2010060143A1

WO2010060143A1 - A compact critical imaging optical system for an lcd or lcos projector

Info

Publication number: WO2010060143A1
Application number: PCT/AU2009/001543
Authority: WO
Inventors: Danny Jung
Original assignee: Digislide Holdings Limited
Priority date: 2008-11-26
Filing date: 2009-11-26
Publication date: 2010-06-03

Abstract

An optical engine apparatus for the production of an image using an LCoS. A light source emits light that passes through a lenslet array and then a pre-polariser that polarises the light so that it can pass through a beamsplitter, the light then passing though a quarter-wave retarder and to a concave mirror that reflects the light back through the retarder adapted to change the direction of polarisation of the light so that it is reflected from the beamsplitter and onto the LCoS where it is reflected so that the reflected light from the LCoS passes through the beamsplitter and then projection lenses to be displayed as an image on a surface.

Description

A compact critical imaging optical system for an LCD or LCOS projector

FIELD OF THE INVENTION

The present invention relates to an optical engine for a projection system and in particular a LCD or LCoS miniature projector with an optical system that uses critical imaging to provide illumination to an LCD or LCoS projector system. A mirror or combination of lenses and mirrors is used together with a polarising beamsplitter to illuminate the LCoS/LCD in a compact configuration.

BACKGROUND OF THE INVENTION

Optical engines for image projection utilising reflective LCoS panels are well known. The engines comprise illumination systems to illuminate an LCoS panel and projection systems to project the image from the panel onto a screen.

The illumination system typically has one of three basic configurations.

1 A white LED emitting light through a concentrator and/or lenses and a polarising beamsplitter, the p polarised component of which is then transmitted onto a colour filter LCoS panel or the s polarised component reflected onto the colour filter LCoS panel depending on whether transmission or reflection mode is chosen.

2 Three colour LED's emitting light through a concentrator and/or lenses and then utilizing dichroic surfaces to combine the three colours which light is then reflected or transmitted onto a field sequential LCoS panel by a polarizing beam splitter.

3 A colour cluster LED emitting light through a concentrator and/or lenses and a polarising beamsplitter, which light is then reflected or transmitted onto a field sequential LCoS panel. Where a field sequential LCoS panel is used the colour LED's are switched in sequence with the image generating electronics system to ensure colour synchronisation.

The LCoS panel contains liquid crystals that rotate the polarisation of light, with the amount of rotation controlled by an electric field. The liquid crystals are placed on top of a pixelated highly reflective mirror surface. The mirror is the top layer of a silicon chip which produces the electric field for each pixel. The polarised light is focused on the panel, with the brightness of each pixel controlled by varying the electric field at its location.

The light reflected from the LCoS panel is directed through a projection system comprising a projector lens with a focus mechanism, and viewed on an external screen or surface.

Both LCD and LCoS projectors require polarised telecentric illumination to function. Telecentricity is a special property of certain multi-element lens designs in which the chief rays for all points across the object or image are collimated. For example, telecentricity occurs when the chief rays are parallel to the optical axis, in object and/or image space. Another way of describing telecentricity is to state that the entrance pupil and/or exit pupil of the system is located at infinity.

There are generally two methods which one can use, Koehler illumination or critical imaging. Illumination by critical imaging can provide more efficient and uniform illumination but the optical path is usually longer than that using Koehler illumination. In addition a telecentric design requires larger lenses and is more complex than a non- telecentric design.

It is an object of the present invention to overcome the above-mentioned problems or to provide the public with a useful alternative. The present invention successfully addresses these problems by providing a mirror or combination of lenses and mirrors together with a polarising beamsplitter making it possible to use critical imaging to illuminate the LCoS/LCD in a compact configuration. SUMMARY OF THE INVENTION

In a first form of the invention there is proposed an optical engine apparatus for the production of an image using an LCoS said apparatus including: a light source to emit light, the light passing through a lenslet array and then a pre- polariser adapted to transmit polarised light that can pass through a beamsplitter, a quarter-wave retarder and to a concave mirror that reflects the light back through the retarder adapted to change the direction of polarisation of the light so that it is reflected from the beamsplitter and onto the LCoS where it is reflected so that the reflected light from the LCoS passes through the beamsplitter and then projection lenses to be displayed as an image on a surface.

In preference the beamsplitter is a cube beamsplitter. In preference the image is produced using a LCD instead of an LCoS. In preference located before the projection lenses is a post polariser. In preference the pre-polariser transmits p polarised light. In a further form of the invention there is proposed a method of producing an image using a LCoS, said method including passing light through a lenslet array and then through a pre-polariser adapted to transmit polarised light that can pass through a beamsplitter and then through a quarter-wave retarder and to a concave mirror that reflects the light back through the retarder adapted to change the direction of polarisation of the light so that it is then reflected from the beamsplitter and onto the LCoS where it is reflected and the direction of polarisation changed so that the reflected light from the LCoS passes through the beamsplitter and then through projection lenses.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several implementations of the invention and, together with the description, serve to explain the advantages and principles of the invention. In the drawings:

Figure 1 is a schematic illustration of the optical system embodying the present invention having a LCoS;

Figure 2 is an alternate embodiment of an optical system of the present invention having a LCoS and where the beamsplitter is a cube beamsplitter; Figure 3 is an alternate embodiment of an optical system of the present invention having a LCoS and where the mirror is a coated surface; and Figure 4 is an alternate embodiment of an optical system of the present invention where there is a transmissive LCD panel instead of an LCoS.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description of the invention refers to the accompanying drawings. Although the description includes exemplary embodiments, other embodiments are possible, and changes may be made to the embodiments described without departing from the spirit and scope of the invention. The reader should note that different reference numbers between the Figures may be used throughout the drawings and the following description to refer to the same and like parts.

Referring to Figure 1 there is provided a light source 10, such as a LED, that with appropriate optics illuminates a lenslet array 12. At the exit of this array 12 is a prepolariser 14 orientated to transmit polarised light. This polarised light presents itself as p polarised light to the polarising beamsplitter 16 and therefore passes through the beamsplitter 16, a quarter-wave retarder 18 and onto a concave mirror 20. The mirror 20 reflects this light through the retarder 18 a second time to change p polarised light to s polarised light, thereby allowing it to reflect of the beamsplitter 16 and onto the LCoS 22. The LCoS changes s light to p light by the LCoS pixels in their on state and passes through the beamsplitter 16 and projection optics 24 onto the screen (not shown).

A post polariser 26 is necessary to block any residual s light directly reflected from the light source by the beamsplitter 16 into the projector lens 24. The advantage of this arrangement is that the entire optical path necessary for the critical imaging method from the lenslet array 12 to the LCoS 22 is contained within the space required for the beamsplitter. In another embodiment and as illustrated in Figure 2 the polarising beamsplitter may be a cube beamsplitter 26 which could include lens surfaces on its entry/exit faces as part of its optical design. The concave mirror and quarter- wave retarder 18 could be glued onto the cube beamsplitter. The polarisers 26 may also be glued onto the cube beamsplitter. In a further embodiment and as illustrated in Figure 3 the mirror 20 could be a coated surface on the retarder. An additional lens 28 would be needed which may be part of the beamsplitter.

The above arrangements are also of benefit in transmissive LCD projectors to minimise space while utilising the advantages of critical imaging. The use of a LCD is illustrated in Figure 4. A post polariser may not be necessary in such an arrangement

The reader should now appreciate the advantages of the present invention whereby the use of a mirror and a retarded in this design provides telecentric light to the LcoS resulting in a compact arrangement.

For the purposes of the specification the word "comprising" means "including but not limited to", and the word "comprises" has a corresponding meaning. Also a reference within the specification to document or to prior use is not to be taken as an admission that the disclosure therein constitutes common general knowledge in Australia.

Further advantages and improvements may very well be made to the present invention without deviating from its scope. Although the invention has been shown and described in what is conceived to be the most practical and preferred embodiment, it is recognized that departures may be made therefrom within the scope and spirit of the invention, which is not to be limited to the details disclosed herein but is to be accorded the full scope of the claims so as to embrace any and all equivalent devices and apparatus.

Claims

1. An optical engine apparatus for the production of an image using an LCoS said apparatus including: a light source to emit light, the light passing through a lenslet array and then a pre- polariser adapted to transmit polarised light that can pass through a beamsplitter, a quarter-wave retarder and to a concave mirror that reflects the light back through the retarder adapted to change the direction of polarisation of the light so that it is reflected from the beamsplitter and onto the LCoS where it is reflected so that the reflected light from the LCoS passes through the beamsplitter and then projection lenses to be displayed as an image on a surface.

2. An optical engine apparatus as in claim 1 wherein the beamsplitter is a cube beamsplitter.

3. An optical engine apparatus as in claim 1 wherein the image is produced using a LCD instead of an LCoS.

4. An optical engine apparatus as in any one of the above claims wherein located before the projection lenses is a post polariser.

5. An optical engine apparatus as in claim 1 wherein the pre-polariser transmits p polarised light.

6. A method of producing an image using a LCoS, said method including passing light through a lenslet array and then through a pre-polariser adapted to transmit polarised light that can pass through a beamsplitter and then through a quarter-wave retarder and to a concave mirror that reflects the light back through the retarder adapted to change the direction of polarisation of the light so that it is then reflected from the beamsplitter and onto the LCoS where it is reflected and the direction of polarisation changed so that the reflected light from the LCoS passes through the beamsplitter and then through projection lenses.