Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
  • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
  • G02F1/015—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on semiconductor elements having potential barriers, e.g. having a PN or PIN junction
  • G02F1/025—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on semiconductor elements having potential barriers, e.g. having a PN or PIN junction in an optical waveguide structure
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/0001—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems
  • G02B6/0011—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form
  • G02B6/0033—Means for improving the coupling-out of light from the light guide
  • G02B6/0035—Means for improving the coupling-out of light from the light guide provided on the surface of the light guide or in the bulk of it
  • G02B6/0045—Means for improving the coupling-out of light from the light guide provided on the surface of the light guide or in the bulk of it by shaping at least a portion of the light guide
  • G02B6/0046—Tapered light guide, e.g. wedge-shaped light guide
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N9/00—Details of colour television systems
  • H04N9/12—Picture reproducers
  • H04N9/31—Projection devices for colour picture display, e.g. using electronic spatial light modulators [ESLM]
  • H04N9/3141—Constructional details thereof
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S385/00—Optical waveguides
  • Y10S385/901—Illuminating or display apparatus

Definitions

• This invention is concerned with a way of eliminating distortion in a tapered-waveguide display of the kind in which light enters the blunt end of a tapered slab waveguide .
• Such a tapered-waveguide display comprises a video projector, a flat slab waveguide, known as an "input” or “expansion” waveguide, for spreading light from the projector in the width direction, and a tapered sheet of transparent material acting as a leaking waveguide, light exiting from its face at a point determined by the angle at which it entered.
• the taper starts at the same thickness as the expansion waveguide and decreases usually to zero. Light entering the taper at some angle up to the critical angle bounces ever more steeply until it exits at a distance along the taper that depends on the initial angle of injection.
• the system is described in the applicant's earlier application WO 01/72037.
• a tapered-waveguide display apparatus including an input waveguide in the form of a slab, into one end of which light can be injected so that it bounces along the waveguide and exits at the other end, and a tapered output waveguide adapted to receive light from the input waveguide at the thick end of the taper to allow it to propagate a certain distance by reflection within the taper and then to emit the light over one of the faces of the tapered waveguide when the angle within the taper exceeds the critical angle; in which the tapered waveguide's thickness profile is such that light bounces the same number of times in total, passing through the two waveguides, before leaving the tapered waveguide .
• the wedge shape of the tapered waveguide had planar faces, which meant that there could be gaps where light bouncing to and fro as it was internally reflected along the input slab and then the tapered slab stayed within the taper for one more bounce before reaching the critical angle. It has been found that a very slight deviation from the flat in the faces of the tapered waveguide can compensate for this effect, by making sure that the number of bounces is always the same. This is possible because there is already an inbuilt tendency balancing out the number of bounces: rays entering at a shallow angle will bounce less often in the input slab but will stay longer in the output slab, and vice versa.
• the shape of the tapered slab can be calculated by ray tracing, and this calculation is particularly simple if the input or expansion slab, which is used to spread the image in one dimension, in the plane of the slab, is a (flat) cuboid.
• the input slab can be integral with or separate from the taper; and it can be in the same plane or "folded” by optical means.
• the input and output waveguides have about the same shape in plan, so folding halves the footprint of the device.
• the device is described more in terms of a display it could equally be run backwards as a camera, as described in the applicant's earlier PCT/GB 01/5266. If the light source is a simple point source then the -apparatus becomes simply a light source, useful for LCD or hologram illumination.
• Figure 1 contains an example algorithm for calculating an appropriate thickness profile for the tapered waveguide
• Figure 2 plots the profile of the tapered waveguide calculated by the algorithm of Figure 1;
• Figure 3 traces rays through the tapered waveguide, with Figure 3a showing the situation after each ray has undergone 574 bounces, and Figure 3b showing the situation after each ray has undergone 575 bounces;
• Figure 4 gives the thickness (t) versus length (z) co-ordinates of an embodiment of the shaped taper
• Figure 5 shows how the slab and taper should be folded with a bulk mirror collimator at the short edge of the slab;
• Figure 6 shows how a set of louvers and fine scattering sites can be used to reduce blurring and improve contrast ratio;
• Figure 7 shows the display apparatus schematically.
• Figure 7 shows the general layout contemplated by the invention.
• a projector 6 (or camera if the apparatus is operated backwards, or just a point light source for producing area illumination) emits (or receives) light forming an image made of an array of pixels into an edge of a rectangular slab waveguide 2 where it propagates by reflection alternately off the main faces.
• This slab will usually have flat parallel faces, though this is not- necessary.
• the shaped taper is made by using an algorithm to. calculate for the taper a series of thicknesses and distances between these thicknesses, then passing these data to a numerically controlled machine which either shapes the tapered waveguide directly, or shapes a mould for making it. Other methods of manufacture can also be contemplated.
• the algorithm starts by considering a ray whose angle is equal to the critical angle for total internal reflection off the faces of the slab and taper at the slab/taper boundary. This ray will have bounced the maximum possible number of times as it travelled along the input slab and will leave the tapered waveguide at the first bounce.
• the algorithm determines what change in ray angle is needed for a ray to undergo one double bounce less in the input slab, then calculates at what taper thickness this ray will emerge from the taper. The algorithm then calculates what distance is needed from the taper entrance (i.e. the thick end) to the point at which the ray emerges in order for the ray to undergo one extra double bounce in the tapered waveguide, thus keeping the total number of bounces constant, and allocates this extension of the taper to the new value for the taper's thickness. This procedure is repeated for successive ray angles until the thickness of the taper has decreased to zero.
• the change in ray angle needed for a ray to undergo one double bounce less in the input slab is found by calculating the progress of trial rays through the slab - a procedure known as ray tracing.
• the input or expansion slab has parallel faces, and the ray angle ⁇ ⁇ n is given by trigonometry as :
• b is the number of bounces less than those for a ray at the critical angle
• t in is the thickness of the expansion slab and of the input to the taper
• 2L is the length of the expansion slab
• ⁇ c is the critical angle. It is here assumed that the slabs are made of glass, but any suitable material could be used.
• the taper thickness, t cr at which a ray will emerge from the taper is calculated from the angle of the ray, ⁇ n , relative to the normal to the glass/air interface as the ray passes into the taper, as follows:
• the profile of the taper for this embodiment is shown in Figure 2, and was calculated using the algorithm of Figure 1 with the single change that in the third line of executable code, crit is set equal to the inverse sine of 1/1'491756 (i.e. the critical angle of light in acrylic) .
• Figure 3 Two traces of four sample rays through a 1270 mm long, initially 2-5 ram thick slab, then a taper with the profile shown in Figure 2 , are shown in Figure 3, which shows progress along the taper on the vertical axis against propagation perpendicular to the plane, on the horizontal axis.
• Figure 3a the rays have been traced through 574 bounces and none have emerged from the taper.
• the code is slightly crude, so the shape of the taper should then be adjusted by trial and error using a ray- tracing program such a Zemax to get a profile where all rays leave the taper at approximately the same emergent angle after the same number of bounces. This is done by tracing a ray such as to emerge near to the thick end of the wedge, then tracing rays at gradually decreasing angles of injection. If a ray emerges at too shallow an angle or with too many bounces, the section of taper between the position of emergence of the current ray and that of the previously traced ray should be reduced. If a ray emerges at too large an angle or with too few bounces, the section of taper between the position of emergence of the current ray and that of the previously traced ray should be increased.
• a ray- tracing program such as Zemax to get a profile where all rays leave the taper at approximately the same emergent angle after the same number of bounces. This is done by tracing a
• the precise shape of the taper is not important - it is the thickness along its length that matters. Once face could be flat, or both faces could undulate, or whatever.
• Figure 4 shows the thickness and length co-ordinates of a taper embodying this invention which in this case is 15 mm thick at the thick end, is approximately 1130 mm long, and is butted to a slab which is 1270 mm long and 15 mm thick.
• Figure 5 shows, in elevation and plan, the taper 1 and expansion slab 2 together, with a fold 3 in the slab 2 and, in order to merge the transition from the slab 2 to the shaped taper 1, a 44 mm section either side of the slab/taper interface which is smoothed to a curve 4 with a radius of curvature of 8500 mm.
• both slab and taper should be scaled linearly. If a longer taper is required, the length of both slab and taper should be scaled linearly.
• FIG. 5 Not shown in Figure 5 is a fold within the slab which should be introduced if it is desired to place the video projector .6 at the centre of one of the longer sides of the display, and a translucent -screen on top of the tapered waveguide to make the image visible in various directions .
• a set of opaque louvers 8 is placed over the tapered waveguide 7 as shown in
• a scattering element 9 such as a phosphor or refractive surface should be placed at the end of each louver, and provided the scattering elements are kept small then most ambient light 12 will be absorbed by the louvers.
• the elements 9 can be emissive, such as a phosphor activated by short-wavelength, possibly UV, input light. The result is a thin screen which has no blurring and a good contrast ratio but needs no anti- reflection coating.

Landscapes

• Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• General Physics & Mathematics (AREA)
• Nonlinear Science (AREA)
• Signal Processing (AREA)
• Multimedia (AREA)
• Engineering & Computer Science (AREA)
• Projection Apparatus (AREA)
• Devices For Indicating Variable Information By Combining Individual Elements (AREA)
• Light Guides In General And Applications Therefor (AREA)
• Transforming Electric Information Into Light Information (AREA)
• Illuminated Signs And Luminous Advertising (AREA)
• Optical Integrated Circuits (AREA)

Priority Applications (6)

Applications Claiming Priority (2)

Publications (2)

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Cited By (10)

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• 2001
  • 2001-08-02 GB GBGB0118866.3A patent/GB0118866D0/en not_active Ceased
• 2002
  • 2002-07-29 TW TW091116924A patent/TWI222536B/zh not_active IP Right Cessation
  • 2002-08-01 CN CNB028152271A patent/CN1330189C/zh not_active Expired - Fee Related
  • 2002-08-01 WO PCT/GB2002/003578 patent/WO2003013151A2/en not_active Ceased
  • 2002-08-01 DE DE60208049T patent/DE60208049T2/de not_active Expired - Lifetime
  • 2002-08-01 US US10/485,888 patent/US7410286B2/en not_active Expired - Fee Related
  • 2002-08-01 EP EP02745694A patent/EP1417843B1/en not_active Expired - Lifetime
  • 2002-08-01 JP JP2003518193A patent/JP4291141B2/ja not_active Expired - Lifetime
  • 2002-08-01 AU AU2002317406A patent/AU2002317406A1/en not_active Abandoned
  • 2002-08-01 KR KR1020047001271A patent/KR100871521B1/ko not_active Expired - Fee Related

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