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Researchers Submit Patent Application, "Apparatus for Condensing Light from Multiple Sources Using Bragg Gratings", for Approval

July 24, 2014



By a News Reporter-Staff News Editor at Politics & Government Week -- From Washington, D.C., VerticalNews journalists report that a patent application by the inventors POPOVICH, Milan Momcilo (Leicester, GB); WALDERN, Jonathan David (Los Altos Hills, CA), filed on December 19, 2013, was made available online on July 10, 2014.

No assignee for this patent application has been made.

News editors obtained the following quote from the background information supplied by the inventors: "This invention relates to an apparatus for illuminating a display, and more particularly to an illuminator device based on Bragg gratings.

"Recent developments in microdisplays and Light Emitting Diode (LED) technology are driving the development of a range of consumer applications such as compact projectors and thin form factor rear projection televisions. Current microdisplays employ a variety of technologies including liquid crystals, micro-mechanical mirrors (MEMs) micro-mechanical diffraction gratings and others. Liquid Crystal Displays (LCDs) are the most well-known examples. The most efficient method of illuminating microdisplays is to present red, green and blue illumination sequentially with the display image data being updated in the same sequence. Such procedures require that the display update rate is fast enough for the sequential single-color images to appear to the observer as a full color image.

"Prior art illumination system have employed color wheels which suffer from the problems of noise and mechanical complexity. FIG. 1 shows an example of a prior art illumination system. The illumination system comprises an incoherent light source 1001, condenser mirror 1002, focusing lens 1003, color wheel 1004, collimating lens 1005 and filter 1006. The ray directions are generally indicated by the arrowed lines 2000. A projection display would further comprise a microdisplay 1007 and a projection lens 1008 forming an image on a screen 1009. Illumination systems based on incoherent sources such as UHP lamps, for example, suffer from the problems of bulk, warm up time lag, high heat dissipation and power consumption, short lamp lifetime, noise (resulting from the color wheel) and poor color saturation.

"Many of the above problems can be solved by using LED illumination. One commonly used illuminator architecture uses dichroic beam splitters known as X-cubes. The prior art illuminator shown in FIG. 2 comprises red, green and blue LED sources 1010a,1010b,1010c each comprising LED die and collimators, an X-cube 1011, focusing lens 1012, light integrator 1013, a further relay lens 1014 which directs light from the integrator onto the surface of a microdisplay 1015. The ray directions are generally indicated by the arrowed lines 2010. However, illuminators based on LEDs suffer from several problems. Although LEDs provide high lumen output they have large emittance angles, making the task of collecting and relaying light through the narrower acceptance cones of a microdisplay a very challenging optical design problem. LEDs require fairly large collimators, making it difficult to achieve compact form factors. LED triplet configurations using a shared collimation element suffer from thermal problems if the die are configured too closely. In the case of X-cube architectures such as the one shown in FIG. 2, the resulting image is barely bright enough, with the X-cube itself losing around one third of the light from the LEDs. X-cubes also present alignment, bulk and cost problems. Thus there exists a need for a compact, efficient LED illuminator for microdisplays

"Diffractive optical elements (DOEs) offer a route to solving the problems of conventional optical designs by providing unique compact form factors and high optical efficiency. DOEs may be fabricated from a range of recording materials including dichromated gelatine and photo-polymers.

"An important category of DOE known as an Electrically Switchable Holographic Bragg Gratings (ESBGs) is formed by recording a volume phase grating, or hologram, in a polymer dispersed liquid crystal (PDLC) mixture. Typically, ESBG devices are fabricated by first placing a thin film of a mixture of photopolymerizable monomers and liquid crystal material between parallel glass plates. Techniques for making and filling glass cells are well known in the liquid crystal display industry. One or both glass plates support electrodes, typically transparent indium tin oxide films, for applying an electric field across the PDLC layer. A volume phase grating is then recorded by illuminating the liquid material with two mutually coherent laser beams, which interfere to form the desired grating structure. During the recording process, the monomers polymerize and the HPDLC mixture undergoes a phase separation, creating regions densely populated by liquid crystal micro-droplets, interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating. The resulting volume phase grating can exhibit very high diffraction efficiency, which may be controlled by the magnitude of the electric field applied across the PDLC layer. When an electric field is applied to the hologram via transparent electrodes, the natural orientation of the LC droplets is changed causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to very low levels. Note that the diffraction efficiency of the device can be adjusted, by means of the applied voltage, over a continuous range from near 100% efficiency with no voltage applied to essentially zero efficiency with a sufficiently high voltage applied. U.S. Pat. No. 5,942,157 and U.S. Pat. No. 5,751,452 describe monomer and liquid crystal material combinations suitable for fabricating ESBG devices. A publication by Butler et al. ('Diffractive properties of highly birefringent volume gratings: investigation', Journal of the Optical Society of America B, Volume 19 No. 2, February 2002) describes analytical methods useful to design ESBG devices and provides numerous references to prior publications describing the fabrication and application of ESBG devices. DOEs based on HPDLC may also be used as non-switchable devices. Such DOEs benefit from high refractive index modulations.

"Typically, to achieve a satisfactory display white point it is necessary to provide significantly more green than red or blue. For example, to achieve a white point characterised by a colour temperature of 8000K we require the ratio of red:green:blue light to be approximately 39:100:6. It is found in practice that providing adequate lumen throughput and white point simultaneously requires more than one green source. Although DOEs may be designed for any wavelength, providing a separate DOE for each source may be expensive and may lead to unacceptable attenuation and scatter when the elements are stacked. Methods for recording more than one grating into a hologram are well known. For example, one grating may be used to diffract light from two or more different sources. However such devices suffer from reduced diffraction efficiency and throughput limitations imposed by the etendue of a grating.

"Another approach to combining light from more than one LED of a particular colour is to exploit the angle/wavelength selectivity of Bragg gratings. High efficiency can be provided in different incidence angle ranges for different wavelengths according to the well-known Bragg diffraction equation. However, if we consider the wavelength ranges of typical sources the resulting incidence angle range will not be sufficiently large to separate the LED die. For example, if green sources with peak wavelengths at the extremities of the green band of the visible spectrum were provided the resulting incidence angles would differ by just a few degrees. This would make it at best extremely difficult to integrate the LED die and condenser optics into a compact package.

"There is a requirement for a compact, efficient LED illuminator based on Bragg gratings.

"There is a further requirement for a compact and efficient illuminator capable of combining two light sources having similar peak wavelengths using a single grating.

"There is a yet further requirement for a complete colour sequential illumination device in which light of at least one primary colour is provided by means of a single grating that combines light from more than one source."

As a supplement to the background information on this patent application, VerticalNews correspondents also obtained the inventors' summary information for this patent application: "It is an object of the present invention to provide compact, efficient LED illuminator based on Bragg gratings.

"It is a further object of the present invention to provide a compact and efficient illuminator capable of combining two light sources having similar peak wavelengths using a single grating.

"It is a yet further object if the present invention to provide a complete colour sequential illumination device in which light of at least one primary colour is provided by means of a single grating that combines light from more than one source.

"The objects of the invention are achieved in a first embodiment comprising a LED module, a condenser lens and an Electrically Switchable Bragg Grating (ESBG) device configured as a stack of separately switchable ESBG layers. Said optical elements are aligned along an optical axis normal to the surface of each element Each ESBG layer is recorded in HPDLC sandwiched between transparent substrates to which transparent conductive coatings have been applied. Each ESBG has a diffracting state and a non-diffracting state. Each ESBG diffracts light in a direction substantially parallel to the optical axis when in said active state. However, each ESBG is substantially transparent to said light when in said inactive state. Each ESBG is operative to diffract at least one wavelength of red, green or blue light.

"In a further embodiment of the invention the illuminator further comprises a diffractive optical element (DOE) for beam intensity shaping. The DOE is operative to alter the wavefronts of incident red green and blue light to control the spatial distribution of illumination. Diffusion characteristics may be built into the ESBG devices. The diffusing properties of the ESBGs and the CGH may be combined to produce a desired illumination correction.

"In a further embodiment of the invention the ESBG device comprises a, green diffracting ESBG layer a red diffracting ESBG layer and a blue diffracting ESBG layer. The red and green LEDs are disposed with their emission axes in a common plane. The blue LED is disposed with its emission axis disposed in an orthogonal plane.

"In alternative embodiments of the invention the red, green and blue LEDs may be configured to lie in a common plane.

"In a further embodiment of the invention the ESBG device comprises a first ESBG into which two superimposed red and green Bragg gratings have been recorded and a second ESBG into which a blue Bragg grating has been recorded.

"In a further embodiment of the invention the ESBG device comprises red and green diffracting layers only. The blue LED is disposed with its emission axis parallel to the optical axis. The light from the blue LED is collimated by the lens system but is not deflected by the ESBG instead continuing to propagate without substantial deflection parallel to the optical axis.

"A further embodiment of the invention comprises a LED module, a condenser lens, a group of ESBGs configured as a stack of separately switchable ESBG layers, a DOE and a relay lens. Each ESBG layer is recorded in HPDLC sandwiched between transparent substrates to which transparent conductive coatings have been applied. Each ESBG has a diffracting state and a non-diffracting state. Each ESBG diffracts light in a direction substantially parallel to the optical axis when in said active state. However, each ESBG is substantially transparent to said light when in said inactive state. Each ESBG is operative to diffract at least one wavelength of red, green or blue light. The LED module comprises two green emitters, a blue emitter and a red emitter. The ESBG group comprises green diffracting ESBG layers and a red diffracting ESBG. The DOE is operative to alter the wave fronts of incident red green and blue light to control to spatial distribution of illumination at the display panel. The output from the DOE comprises diffused light. Advantageously, the DOE is a Computer Generated Hologram (CGH) operative to diffract and diffuse red green and blue light. The ESBGs may also have diffusing properties that operate on light at the diffraction wavelength. The diffusing properties of the ESBGs and the CGH may be combined to produce a desired illumination correction

"In a further embodiment of the invention light from at least one LED is directed towards the ESBG device by means of a dichroic beam splitter.

"In further embodiments of the invention elements of the illuminator may be configured in folding configurations to provide a compact form factor when the apparatus is not in use

"In further embodiment of the invention the LED die are disposed on a curved substrate.

"In further embodiment of the invention refracting elements are disposed in front of each LED die to modify the LED emission angular distribution.

"In a further embodiment of the invention a polarization insensitive illuminator is provided in which the ESBG groups in any of the above embodiments further comprise a half wave plate and further ESBG layers.

"In alternative embodiments of the invention the ESBGs may be replaced by non-switchable Bragg gratings. In such alternative embodiments colour sequential illumination is provided by switching red, green and blue LEDs in sequence.

"In preferred operational embodiments of the invention more efficient use of LED emission may be achieved by running two identical pulse sequentially driven LEDs.

"In a further embodiment of the invention the illuminator further comprises a polarization rotating filter operative to rotate the polarization of at least one primary colour through ninety degrees.

"In a further embodiment of the invention the illuminator may incorporate at least one light guide for one or more of the red green and blue lights. The light guide is disposed in the optical path between the LEDs and the ESBG device.

"In further embodiments of the invention diffusing characteristics are encoded within one or more of the Bragg gratings.

"In alternative embodiment of the invention the LED module comprises a multiplicity of emitters arranged in a circular pattern on a substrate. The ESBGs are disposed on a rotating substrate containing at least one ESBG.

"In an alternative embodiment of the invention the ESBGs are disposed on a rotating substrate. The ESBG configuration comprises two displaced ESBGs disposed such that while one ESBG overlaps the beam path of a first LED, the second ESBG is ready to overlap the beam path of an adjacent LED.

"In an alternative embodiment of the invention the illuminator comprises an LED module comprising a substrate and an array of LED die, a printed circuit board containing apertures, an array of lens elements disposed on a substrate and a stack of ESBGs.

"In a further embodiment of the invention directed at providing a compact and efficient illuminator capable of combining two light sources having similar peak wavelengths using a single grating there is provided an illuminator comprising: a first LED characterised by a first wavelength; a second LED characterised by said first wavelength; a collimating lens; and a first Bragg grating. The grating is recorded by means of a first recording beam incident normal to the grating and a second recording beam incident at an angle to the grating. The lens collimates and directs light from the first and second LEDs towards said grating at first and second angles respectively. The second angle is substantially equal to the incidence angle of the second recording beam. The grating has a maximum acceptance angle for light beams whose average direction corresponds to that of said first recording beam, said acceptance angle being defined by the angle at which the diffraction efficiency of said grating falls to a predetermined value. The first angle is greater than said maximum acceptance angle. The normal to the surface of the grating defines an illumination direction. The first grating diffracts light from said second LED into said illumination direction.

"In a further embodiment of the invention directed at providing a compact and efficient illuminator capable of combining two light sources having similar peak wavelengths using a single ESBG there is provided an illuminator comprising: a first LED characterised by a first wavelength; a second LED characterised by said first wavelength; a collimating lens; and a first ESBG. The ESBG is recorded by means of a first recording beam incident normal to the ESBG and a second recording beam incident at an angle to the ESBG. The lens collimates and directs light from the first and second LEDs towards said ESBG at first and second angles respectively. The second angle is substantially equal to the incidence angle of the second recording beam. The ESBG has a maximum acceptance angle for light beams whose average direction corresponds to that of said first recording beam, said acceptance angle being defined by the angle at which the diffraction efficiency of said ESBG falls to a predetermined value. The first angle is greater than said maximum acceptance angle. The normal to the surface of the ESBG defines an illumination direction. The first ESBG diffracts light from said second LED into said illumination direction.

"In one particular embodiment of the invention directed at providing a complete colour sequential illumination device in which light of at least one primary colour is provided by means of a single grating that combines light from more than one source there is provide an illuminator comprising: a holographic optical element into which superimposed third and fourth Bragg gratings have been recorded; a third LED emitting light of a second wavelength; and a fourth LED emitting light of a third wavelength. The lens diffracts said second and third wavelength light at a third and fourth angles respectively with respect to said holographic optical element. The second and third wavelength light is diffracted into a direction normal to said holographic optical element.

"In a further embodiment of the invention based on said particular embodiment the Bragg grating is a first ESBG and the holographic optical element is a second ESBG.

"In a further embodiment of the invention based on said particular embodiment the Bragg grating is a first ESBG and the holographic optical element is a second ESBG. The apparatus further comprises in series a half wave plate; a third ESBG and a fourth ESBG. The third ESBG is identical to said first ESBG and the fourth ESBG is identical to said second ESBG.

"A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings wherein like index numerals indicate like parts. For purposes of clarity details relating to technical material that is known in the technical fields related to the invention have not been described in detail.

BRIEF DESCRIPTION OF THE DRAWINGS

"FIG. 1 is a schematic side view of a first prior art illuminator.

"FIG. 2 is a schematic side elevation view of a second prior art illuminator.

"FIG. 3 is a schematic side elevation view of a further embodiment of the invention.

"FIG. 4 is a schematic side elevation view of a further embodiment of the invention.

"FIG. 5 is a schematic side elevation view of a further embodiment of the invention.

"FIG. 6 is a schematic plan view of a further embodiment of the invention.

"FIG. 7 is a schematic side elevation view of a further embodiment of the invention.

"FIG. 8 is a schematic plan view of a further embodiment of the invention.

"FIG. 9 is a schematic side elevation view of a further embodiment of the invention.

"FIG. 10 is a schematic plan view of a further embodiment of the invention.

"FIG. 11 is a schematic side elevation view of a further embodiment of the invention.

"FIG. 12 is a schematic plan view of a further embodiment of the invention.

"FIG. 13 is a schematic side elevation view of a further embodiment of the invention.

"FIG. 14 is a front elevation view of an LED device.

"FIG. 15 is a schematic front elevation view of a first LED configuration.

"FIG. 16 is a schematic front elevation view of a second LED configuration.

"FIG. 17 is a schematic front elevation view of a third LED configuration.

"FIG. 18 is a schematic plan view of a LED configuration incorporating a dichroic beam splitter.

"FIG. 19 is a schematic side view of a LED configuration incorporating a dichroic beam splitter.

"FIG. 20A is a diagram defining LED emission angles used in the charts in FIG. 13B and FIG. 14

"FIG. 20B is a chart showing normalized LED intensity as a function of angle.

"FIG. 21 is a chart showing normalized LED luminous flux as a function of angle.

"FIG. 22 is a schematic side elevation view of another embodiment of the invention.

"FIG. 23 is a schematic side elevation view of another embodiment of the invention.

"FIG. 24 is a schematic side elevation view of a further embodiment of the invention.

"FIG. 25A is a schematic front elevation view of a further embodiment of the invention.

"FIG. 25B is a schematic front elevation view of a further embodiment of the invention.

"FIG. 25C is a schematic side elevation view of a further embodiment of the invention.

"FIG. 26 is a schematic front elevation view of a further embodiment of the invention.

"FIG. 27 is a schematic three-dimensional view of a further embodiment of the invention.

"FIG. 28 is a schematic three-dimensional view of a further embodiment of the invention.

"FIG. 29 is a schematic side elevation view of a further embodiment of the invention.

"FIG. 30 is a schematic plan view of the further embodiment of the invention in FIG. 27.

"FIG. 31 is a schematic side elevation view of a further embodiment of the invention.

"FIG. 32 is a chart illustrating a LED drive scheme for use with the invention.

"FIG. 33 is a schematic side elevation view of a further embodiment of the invention.

"FIG. 34 is a schematic plan view of the further embodiment of the invention in FIG. 31.

"FIG. 35 is a schematic side elevation view of a further embodiment of the invention.

"FIG. 36 is a schematic side elevation view of a further embodiment of the invention.

"FIG. 37 is a schematic side elevation view of a further embodiment of the invention.

"FIG. 38 is a schematic side elevation view of a further embodiment of the invention.

"FIG. 39 is a schematic side elevation view of a further embodiment of the invention.

"FIG. 40 is a schematic side elevation view of a further embodiment of the invention.

"FIG. 41 is a schematic side elevation view of a yet further embodiment of the invention.

"FIG. 42 is a schematic side elevation view illustrating elements of said yet further embodiment of the invention.

"FIG. 43 is a schematic side elevation view illustrating elements of said yet further embodiment of the invention.

"FIG. 44 is a chart showing characteristics of one particular embodiment of said yet further embodiment of the invention.

"FIG. 45 is a schematic side elevation view of a particular embodiment of the invention."

For additional information on this patent application, see: POPOVICH, Milan Momcilo; WALDERN, Jonathan David. Apparatus for Condensing Light from Multiple Sources Using Bragg Gratings. Filed December 19, 2013 and posted July 10, 2014. Patent URL: http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=4636&p=93&f=G&l=50&d=PG01&S1=20140703.PD.&OS=PD/20140703&RS=PD/20140703

Keywords for this news article include: Patents.

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