WO2010068974A1

WO2010068974A1 - An optical engine for miniature projection utilising a retro-reflective polarisation recycling system and a reflective field sequential or colour filter lcos panel

Info

Publication number: WO2010068974A1
Application number: PCT/AU2009/001622
Authority: WO
Inventors: Danny Jung; Peter Rubinshtein; David Kessler
Original assignee: Digislide Holdings Limited
Priority date: 2008-12-15
Filing date: 2009-12-15
Publication date: 2010-06-24

Abstract

The present invention relates to an optical engine for a projection system and in particular for a miniaturised device in which a projected image is provided by the use of white or coloured light sources, a light collection system, a light shaping system, a polarisation recycling system, a polarising plate beam splitter, an LCoS (Liquid Crystal On Silicon) panel and an image projection system. The projection optics may also incorporate a cylindrical surface to compensate for aberrations caused by the plate beam splitter. The recycling system ensures the efficiency of the engine wherein light that does not pass through the pre-polariser is reflected back along the optical path to the light source where it is absorbed and re-emitted. This is achieved by having a symmetrical lenslet array that is closely coupled to the prepolariser.

Description

An optical engine for miniature projection utilising a retro-reflective polarisation recycling system and a reflective field sequential or colour filter LCoS panel

FIELD OF THE INVENTION

The present invention relates to an optical engine for a projection system and in particular for a miniaturised device in which a projected image is provided by the use of white or coloured light sources, a light collection system, a light shaping system, a polarisation recycling system, a polarising plate beam splitter, an LCoS (Liquid Crystal On Silicon) panel and an image projection system. The projection optics may also incorporate a cylindrical surface to compensate for aberrations caused by the plate beam splitter.

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 beam splitter where one component of the polarised light is reflected or transmitted onto a colour filter LCoS panel (depending on whether the system is imaging in reflection or imaging in transmission).

2 Three colour LED's emitting light through concentrators and/or lenses and then utilising dichroic surfaces to combine the colours which are then reflected or transmitted onto a field sequential LCoS panel by a polarising beam splitter.

3 A colour cluster LED emitting light through a concentrator and/or lenses and a polarising beam splitter where one component of the polarised 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.

Each of the individual light beams hits a designated LCoS micro-device. Each micro-device has a liquid crystal layer that is sandwiched between a clear thin- film transistor (TFT) and a silicon semiconductor. The coloured beam passes through polarising filters to strike the micro-device. The liquid crystal acts as a control device that manages the amount of light which hits the pixelated reflective surface of the silicon semiconductor. The more voltage a particular pixel receives, the more light it allows to pass through.

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

Some of the difficulties of current systems utilising reflective LCoS panels are that the light does not illuminate the LCoS uniformly and one component of the polarised light from the illumination system is lost, so reducing the efficiency of the optical engine immediately by 50%. Also the polarising plate beam splitter can introduce aberrations in the projected image.

It is therefore an object of the present invention to provide an optical engine for miniaturised projection applications that overcomes the aforementioned problems or provides a useful alternative.

It is a further object of the present invention to provide an optical engine that illuminates the LCoS panel uniformly and recovers some of the lost component of the polarised light from the illumination system by the use of a retroreflective polarisation recycling system.

It is a further object of the present invention to provide an optical engine that compensates for the aberration introduced by the plate beam splitter.

SUMMARY OF THE INVENTION

The present invention is for an optical engine that uses a reflective LCoS colour panel to produce an image and a four or five element projection lens to project this image onto a screen. This LCoS panel is illuminated by a high brightness LED via a concentrator or two condenser lenses, a lenslet array with a prepolariser and combiner lenses by the critical imaging method. A polarising plate beamsplitter acts to reflect this light onto the LCOS. The LCOS changes the state of the polarised light and it is then transmitted through the beamsplitter and the projection optics onto the screen.

Therefore in one 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 concentrator and into a symmetrical lenslet array and then an adjacent pre-polariser, some of the light reflected back along the same path from the prepolariser to the light source to be emitted a second time and some of the light being transmitted through the pre-polariser.

In preference the light source is a white LED including a diffuse phosphor coating that absorbs and re-emits the reflected light.

In preference the light passing through the pre-polariser then passes through combiner lenses, a beamsplitter, and onto the LCoS, a field lens and a plurality of projector lenses and an aperture to project an image onto a surface.

In preference the optical engine further includes a window to protect the projector lenses.

In a further form of the invention there is proposed an optical engine including a LCoS panel to produce an image, said panel adapted to be illuminated by a high brightness LED, there extending between the LED and the panel a symmetrical lenslet array closely coupled to a pre-polariser wherein at least some of the light is reflected from the pre-polariser back through the lenslet array and into the LED to be absorbed and re- emitted a second time. In a still further form of the invention there is proposed an optical engine for image projection, including: a light source adapted to emit a light, the light passing through a concentrator or condenser lenses and onto a lenslet array and then through a pre-polarizer; some of the light being reflected back through the lenslet array and to the light source to be absorbed and re-emitted, the rest of the light now pre-polarised light passing through combiner lenses adapted to provide critical imaging so that the light is then reflected to an LCoS panel by a polarising plate beam-splitter wherein the reflected light from the LCoS panel changes the state of polarisation, the changed light then passing back through the beam splitter and through projection optics onto a screen.

In preference the light is a white LED or colour cluster. In preference the beam splitter is a cube beam splitter.

In preference the optical engine is adapted to be embedded in a mobile device such as but not limited to the group including mobile phones, PDA's, laptops, calculators, GPS devices.

In a still further form of the invention there is proposed a method of producing an image using a LCoS, said method including passing light emitted from a LED through a lenslet array and then through a pre-polariser wherein some of the light is adapted to be reflected back to the LED wherein it is absorbed and re-emitted, the light passing through the pre-polariser being reflected from a 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 to be projected onto a surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which is incorporated in and constitute a part of this specification, illustrates the invention and, together with the description, serves to explain the advantages and principles of it, where;

Figure 1 illustrates schematically the optical engine of the present invention;

Figure 2 illustrates schematically the optical engine working in a reflection mode; Figure 3 illustrates schematically the optical engine with the polarising cube beamsplitter instead of a polarising plate beamsplitter;

Figure 4 illustrates schematically the optical engine with condenser lenses rather than a concentrator;

Figure 5 illustrates the light cone from the lenslet array; and

Figure 6 illustrates the lenslet array at the entrance pupil of the combiner lenses and the LCoS at the focal plane.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following detailed description of the invention refers to the accompanying drawings. Although the description includes an exemplary embodiment, other embodiments are possible, and changes may be made to the embodiment described without departing from the spirit and scope of the invention.

Referring to Figure 1 , a high brightness white LED 1 and concentrator 2 are used to illuminate a lenslet array 3. A prepolariser/recycler 4 located at the exit of the lenslet array 3 transmits s polarised light and works with the lenslet array 3 to retro-reflect p polarised light. This p polarised light is returned to the LED 1 via the concentrator 2 where some of it is depolarised and reflected to be recycled and added to the light emitted by the LED 1. It is the white diffuse phosphor coating on the LED die that depolarisers and reflects the light to forward a second time.

The lenslet array 3 has a lenslet aspect ratio of 4:3 to match the aspect ratio of the

LCoS panel 6. Each lenslet of the array projects a rectangular cone of light. The two combiner lenses 5 superimpose all these cones of s polarised light onto the LCOS panel 6 via reflection by beamsplitter 8. This illumination method of critical imaging is more efficient than Koehler illumination because the light is shaped into the correct aspect ratio to suit the LCoS panel 6. The LCoS panel 6 is also illuminated more uniformly. The field lens 7 is part of the overall optical design to assist in minimizing the size of the optical engine whilst ensuring the LCoS panel 6 receives telecentric light. The s polarised light is changed to p polarised light by the LCoS pixels in their on-state. This p light then passes through the beamsplitter, projection optics 9, aperture 10 and window 11 onto the screen (not shown). The window has a cylindrical surface to compensate for the field independent astigmatism induced by the plate beamsplitter.

In a second alternate embodiment illustrated in Figure 2 the LCoS panel 6 is located in the reflection mode. The cylindrical surface on the window 11 would then not be necessary.

In a third alternate embodiment and as illustrated in Figure 3 the plate beamsplitter 8 is replaced with a cube beamsplitter. The cylindrical surface on the window 11 would then not be necessary. The cube beamsplitter entry/exit faces could incorporate some of the lens surfaces thereby reducing complexity.

In a fourth alternate embodiment as illustrated in Figure 4, condenser lenses 2 are used to illuminate the lenslet array 3.

The reader should now appreciate the advantages and the operation of the present invention.

The optical engine in the present invention uses a Himax SVGA LCoS colour panel which is illuminated with polarized telecentric light from a high brightness white LED via condenser lenses, a lenslet array, combiner lenses and a polarizing beamsplitter. The image displayed on the LCoS is projected onto a wall or screen with a projection lens which allows for focussing of the projected image from a distance of 500mm to infinity. To facilitate an understanding of the optical engine, it can be divided into five groups of components and the following explanation details the function of each group. The design of each group must be in such a manner that they work correctly with the other groups to maximize efficiency.

The five groups of components are • LED and condenser lenses or modified concentrator • Lenslet array and prepolarizer/recycler

• Combiner lenses

• Polarizing beamsplitter/analyser

• Projection lens The high brightness white LED 1 is the light source for the projector. The role of the condenser lenses or concentrators 2 is to collect light emitted from the LED and illuminate the entrance side of the lenslet array 3. The design goal of the condenser lenses is to collect as much light as possible from the LED and illuminate the lenslet array within the array's acceptance angle and aperture. This acceptance angle is determined by the design of the lenslet array 3. Any light incident on the array outside this angle is lost. The aperture is determined by the design of the combiner lenses 5. The condenser lens design must also take into account the package size and emitting surface area of the selected LED.

The function of the lenslet array is to convert the light incident on the entrance side into light with the same aspect ratio as the LCoS panel 6 and together with the combiner lenses provide an area of light with uniform intensity and the correct size at the LCoS. Figures 3 and 4 show an individual lenslet in one plane only. One explanation to describe the manner in which a lenslet works is to consider that the lens surface on the exit side acts to project an image of a virtual illuminated plane located at the entrance side out to infinity. The lens surface on the entrance side acts as a field lens to refract a cone of light towards the exit lens surface. If each lens surface is rectangular with an aspect ratio of 4:3 then the output angle has the same ratio, i.e. 10 x 13 degrees.

A reflective prepolarizer 4 is located 0.5 mm on the exit side of the array. This component transmits light polarized in one plane and reflects the other. As the lenslet array is symmetrical, the reflected light retraces the same path and returns to the LED. The principle of polarization recycling is utilized by this arrangement. The returned light is depolarized and reflected by the white diffuse phosphor surface of the LED back to the prepolarizer to repeat the process.

The purpose of the combiner lenses 5 is to collect the light from the lenslet array and superimpose each lenslet's output at the LCoS. Figure 5 illustrates the light cone from two individual lenslets. The lenslet 20 passes the light through prepolarizer 22, through combiner lenses 24 field lens 26 and to the LCoS 28. The polarizing beamsplitter has been omitted for clarity. The second function of the combiner lenses is to provide this light as telecentric illumination at the LCoS. Figure 6 illustrates the lenslet array at the entrance pupil of the combiner lenses and the LCoS at the focal plane. Thus the light from the lenslet apertures 30 is telecentric at LCoS 32. A field lens 7 is used to reduce the airspace between the lenslet array and the first combiner lens and thus reduce overall size of the optical engine. The LCoS is a reflective component and therefore the back focal length of the illumination system and the projector lens must overlap. This is achieved by the use of the polarizing beamsplitter 8. The beamsplitter reflects s-polarized light from the lenslet array and prepolarizer via the combiner lenses onto the LCoS panel. The pixels which are in their "on" state rotate the plane of polarization from s to p and are reflected back towards the beamsplitter. Light reflected by pixels in the "off state maintain their s polarized condition. The beamsplitter now functions as an analyser, transmitting p light from "on" pixels and reflecting s light from "off pixels.

The function of the projection lenses 9 is to project the image on the LCoS onto a wall or screen. The focal length is chosen to give a particular size image at a certain distance and the focal ratio must match the focal ratio of the combiner lenses. This lens must also move to allow for focussing at a range of distances. The design of the projector lens must be telecentric at the LCoS. A cylindrical component is included in the projector lens design to compensate for the astigmatism introduced by the beamsplitter.

The reader should now appreciate that the present invention can either use a concentrator or two condenser lenses. Testing has however revealed that the efficiency of the system is better using tow condenser lenses. In relation to the projector lens design it is not intended to limit the invention to either a four or a five element design. However a four element design requires an aspheric surface on one of the lenses. Thus a five element design can be used to improve image quality. 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 concentrator and into a symmetrical lenslet array and then an adjacent pre-polariser, some of the light reflected back along the same path from the prepolariser to the light source to be emitted a second time and some of the light being transmitted through the prepolariser.

2. An optical engine as in claim 1 wherein the light source is a white LED including a diffuse phosphor coating that absorbs and re-emits the reflected light.

3. An optical engine as in any one of the above claims 1 wherein the light passing through the pre-polariser then passes through combiner lenses, a beamsplitter, and onto the LCoS, a field lens and a plurality of projector lenses and an aperture to project an image onto a surface.

4. An optical engine as in claim 3 further including a window to protect the projector lenses.

5. An optical engine including a LCoS panel to produce an image, said panel adapted to be illuminated by a high brightness LED, there extending between the LED and the panel a symmetrical lenslet array closely coupled to a pre-polariser wherein at least some of the light is reflected from the pre-polariser back through the lenslet array and into the LED to be absorbed and re-emitted a second time.

6. An optical engine for image projection, including: a light source adapted to emit a light, the light passing through a concentrator or condenser lenses and onto a lenslet array and then through a pre-polarizer; some of the light being reflected back through the lenslet array and to the light source to be absorbed and re-emitted, the rest of the light now pre-polarised light passing through combiner lenses adapted to provide critical imaging so that the light is then reflected to an LCoS panel by a polarising plate beam-splitter wherein the reflected light from the LCoS panel changes the state of polarisation, the changed light then passing back through the beam splitter and through projection optics onto a screen.

7. An optical engine as in claim 6 wherein the light is a white LED or colour cluster.

8. An optical engine as in claim 6 wherein the beam splitter is a cube beam splitter.

9. An optical engine as in any one of the above claims adapted to be embedded in a mobile device such as but not limited to the group including mobile phones, PDA's, laptops, calculators, GPS devices.

10. A method of producing an image using a LCoS, said method including passing light emitted from a LED through a lenslet array and then through a pre-polariser wherein some of the light is adapted to be reflected back to the LED wherein it is absorbed and re-emitted, the light passing through the pre-polariser being reflected from a 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 to be projected onto a surface.