Light concentration devices are used to capture light over a wide angular range and emit the light as an output beam over a narrower range of output angles. A light concentration device may be used to collect light from a large field created by a plurality of spaced apart light sources or an extended light source subtending a large angle at the device, and convert the light into a beam that is emitted over a smaller angle. The devices may also be used to collect light from different colored light sources and combine the light to form a white output beam.

Conventional light concentration devices typically utilize technologies such as integrating rods, fiber optic light pipes, or optical elements having axi-symmetric or asymmetric forms based on reflection or refraction. One drawback to these conventional devices is that they are subject to constraints imposed by the Lagrange invariant (or etendue), which effectively imposes a limitation on the angular range over which incident light can be collected. The Lagrange invariant can be expressed as follows: nl2AI (sinul) 2 = n22A2 (sinu2) 2 where: Ai is the area of the input aperture

A2 is the area of the output aperture n, is the refractive index of the input medium n2 is the refractive index of the output medium ul is the angle of the input beam; u2 is the angle of the output beam; and (ul and u2 are defined by the axis of symmetry of the optical system (i. e., the optical axis and the beam directions)).

The angle of the output beam u2 is determined by the maximum light incidence angle at which the light detection system can generate a useful output signal of the device (e. g., light detection system) to which the concentration device supplies light. This places a restriction on the size and position of the light source used at the input end of the concentration device. Furthermore, in the case where the concentration device is used to collect light from different colored light sources, the limitations imposed on the input beam may result in poor color mixing.

There is, therefore, a need for a light concentration system in which the angular range over which the incident light can be collected is not limited by the constraints imposed by the Lagrange invariant.

SUMMARY OF THE INVENTION A light concentration system of the present invention generally comprises a light concentrator having an input end and an output end, and configured to receive light at the input end, concentrate the light, and emit the concentrated light at the

output end over a range of angles of emergence. The system further includes a holographic device operable to receive light from a range of angles of incidence and diffract the light onto the input end of the light concentrator. The holographic device comprises a plurality holographic optical elements each configured to diffract light received from different angles of incidence. The range of angles of emergence is narrower than the range of angles of incidence.

The holographic optical elements may be switchable between an active state wherein light incident on the element is diffracted and a passive state wherein light incident on the element is transmitted without substantial alteration. The system may also include a controller operable to sequentially switch the elements between their active and passive states. The holographic optical elements comprise a hologram interposed between two electrode layers operable to apply an electrical field to the hologram. The hologram may be formed from a polymer and liquid crystal material, for example. The holographic optical elements may be configured to diffract light having different wavelength bands.

In another aspect of the invention a light concentration system generally comprises a light concentrator having an input end and output end, and configured to receive light at the input end within a predetermined angular range, concentrate the light, and emit the concentrated light at the output end over a range of angles of emergence. The system further includes holographic device operable to receive light from a range of angles of incidence and diffract the light onto the input end of the light concentrator within the predetermined angular range. The range of angles of emergence is narrower than the range of angles of incidence.

The above is a brief description of some deficiencies in the prior art and advantages of the present invention. Other features, advantages, and embodiments of the invention will be apparent to those skilled in the art from the following description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is side view of a first embodiment of a light concentration system of the present invention shown collecting light from three discrete light sources.

Fig. 2 is a side view of the light concentration system of Fig. 1 shown collecting light originating from a single extended light source.

Fig. 3 is a perspective of a holographic optical element and light source for use with the optical system of Fig. 2.

Fig. 4 is a partial front view of the holographic optical element of Fig. 3 illustrating an electrode and electric circuit of the holographic optical element.

Fig. 5 is a schematic of a holographic device having three holographic optical elements each optimized to diffract red, green, or blue light.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION The following description is presented to enable one of ordinary skill in the art to make and use the invention. Descriptions of specific embodiments and applications are provided only as examples and various modifications will be readily

apparent to those skilled in the art. The general principles described herein may be applied to other embodiments and applications without departing from the scope of the invention. Thus, the present invention is not to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail.

Referring now to the drawings, and first to Fig. 1, a light concentration system, generally indicated at 10, is shown. The light concentration system 10 may be used as a solar collector cell, a wide angle detector for use over a large field of view for robotic tracking of light sources, a light integrator for use in a color sequential microdisplay illumination system, a surveillance system for detecting the presence of light sources within a specific field of view, or a device for monitoring average sky brightness, for example. The system 10 collects light over a large angle and concentrates the light to a narrow field to focus the light onto a light detector such as a photovoltaic cell, fiber optic light guide, or photoelectronic detector array such as a charge-coupled device (CCD), for example. The system includes a light concentrator 12 operable to receive light at an input end 14 over a predetermined angular range and emit the light at an output end 16 over a range of angles of emergence, and a holographic diffraction device generally indicated at 18, operable to diffract light incident thereon onto the input end of the light concentrator. The holographic diffraction device 18 is disposed adjacent to the input end of the light concentrator and preferably comprises a plurality of holographic diffraction elements 20,22,24 each of which is operable to diffract light incident thereon from a

respective direction or area over a range of angles of incidence onto the light concentrator 12. The holographic optical elements 20,22,24 are preferably switchable between an active (diffracting) state and a passive (non-diffracting). It is to be understood that in the passive state the incoming light may still be slightly diffracted, however, the light is substantially unaltered. Switching of the holographic elements 20,22,24 is controlled by a controller 50 which operates to switch each of the elements between their active and passive states.

As shown in Fig. 1, the light concentrator 12 is positioned to gather light from three separate light sources 30,32,34. The sources 30,32,34 are spaced from one another and emit light beams B1, B2, B3 respectively, from different directions. Each of the holographic diffraction elements 20,22,24 is operable to receive light from one of the sources 30,32,34 and diffract the light onto the light concentrator 12. The light beams B1, B2, B3 are diffracted into light beams D1, D2, D3, respectively, at angles such that each beam is incident on the input end of the light concentrator 12. This allows light from the light sources 30,32,34 to be emitted from the concentrator 12 within a common angular range (as indicated by beams C1, C2, C3, respectively), even though the light sources emit light from a wide range of directions.

Light beams Bl, B2, B3 emitted from the light sources 30,32,34, respectively, may be gathered at one time by the holographic diffraction device 18, in which case the holographic optical elements 20,22,24 are switched by the controller 50 at approximately the same time into their active states. Alternatively, the light beams can be gathered by each of the holographic optical elements 20,22,24 at different times, in which case the controller 50 switches the elements sequentially into and out of their active states.

The light sources 30,32,34 may be polychromatic and provide incoherent white light or may each provide light within a single color wavelength band (e. g., red, green, or blue). As further described below, the holographic optical elements 20,22, 24 may each be designed to diffract light of a different wavelength band. The elements 20,22,24 may be switched to their active state at the same time to combine the light from the different color light sources into a white output beam.

Alternatively, the elements 20,22,24 may be activated in turn by the controller 50, so that the system emits an output beam which is sequentially colored red, green, and blue. The output beam may be used to illuminate display panels for color sequential displays, such as disclosed in U. S. Patent Application Serial No. 09/565,678, filed May 4,2000, by M. Popovich et al., for example.

Figure 2 illustrates the light concentration device of Fig. 1 receiving light beams B1 l, B21, B31 from different areas in the overall field of collection of the light concentration device 12. The light may originate from a single extended source or from different regions of an overall field of view.

The holographic optical elements 20,22,24 each include a hologram interposed between two electrodes 40 (Figs. 3 and 4). The hologram may be a Bragg (thick or volume) hologram or Raman-Nath (thin) hologram. Raman-Nath holograms are thinner and require less voltage to switch light between various modes of the hologram, however, Raman-Nath holograms are not as efficient as Bragg holograms.

The Bragg holograms provide high diffraction efficiencies for incident beams with wavelengths close to the theoretical wavelength satisfying the Bragg diffraction condition and within a few degrees of the theoretical angle which also satisfies the Bragg diffraction condition.

The hologram is used to control transmitted light beams based on the principles of diffraction. The hologram selectively directs an incoming light beam from light source 30 either towards or away from a viewer and selectively diffracts light at certain wavelengths into different modes in response to a voltage applied to the electrodes 40 (Figs. 3 and 4). Light passing through the hologram in the same direction as the light is received from the light source 30 is referred to as the zeroth (Oth) order mode 44 (Fig. 3). When no voltage is applied to the electrodes 40, liquid crystal droplets within the holographic optical element 20,22,24 are oriented such that the hologram is present in the element and light is diffracted from the zeroth order mode to a first (lst) order mode 46 of the hologram. When a voltage is applied to the holographic optical element 20,22,24 the liquid crystal droplets become realigned effectively erasing the hologram, and the incoming light passes through the holographic optical element in the zeroth order mode 44.

It is to be understood that the holographic optical elements 20,22,24 may also be reflective rather than transmissive as shown in Fig. 3 and described above. In the case of a reflective holographic optical element, the arrangement of the holographic devices and light concentrator 12 would be modified to utilize reflective properties of the hologram rather than the transmissive properties described herein.

The light that passes through the hologram is diffracted by interference fringes recorded in the hologram. Depending on the recording, the hologram is able to perform various optical functions which are associated with traditional optical elements, such as lenses and prisms, as well as more sophisticated optical operations.

The hologram may be configured to perform operations such as deflection, focusing, or color filtering of the light, for example.

The holograms are preferably recorded in a photopolymer/liquid crystal composite material (emulsion) such as a holographic photopolymeric film which has been combined with liquid crystal, for example. The presence of the liquid crystal allows the hologram to exhibit optical characteristics which are dependent on an applied electrical field. The photopolymeric film may be composed of a polymerizable monomer having dipentaerythritol hydroxypentacrylate, as described in PCT Publication, Application Serial No. PCT/US97/12577, by Sutherland et al, which is incorporated herein by reference. The liquid crystal may be suffused into the pores of the photopolymeric film and may include a surfactant.

The diffractive properties of the holographic optical elements 20,22,24 depend primarily on the recorded holographic fringes in the photopolymeric film.

The interference fringes may be created by applying beams of light to the photopolymeric film. Alternatively, the interference fringes may be artificially created by using highly accurate laser writing devices or other replication techniques, as is well known by those skilled in the art. The holographic fringes may be recorded in the photopolymeric film either prior to or after the photopolymeric film is combined with the liquid crystal. In the preferred embodiment, the photopolymeric material is combined with the liquid crystal prior to the recording. In this preferred embodiment, the liquid crystal and the polymer material are pre-mixed and the phase separation takes place during the recording of the hologram, such that the holographic fringes become populated with a high concentration of liquid crystal droplets. This process can be regarded as a"dry"process, which is advantageous in terms of mass production of the switchable holographic optical elements.

The electrodes (electrode layers) 40 are positioned on opposite sides of the emulsion and are preferably transparent so that they do not interfere with light passing through the hologram (Fig. 4). The electrodes 40 may be formed from a vapor deposition of Indium Tin Oxide (ITO) which typically has a transmission efficiency of greater than 80%, or any other suitable substantially transparent conducting material. An anti-reflection coating (not shown) may be applied to selected surfaces of the switchable holographic optical element, including surfaces of the ITO and the electrically nonconductive layers, to improve the overall transmissive efficiency of the optical element and to reduce stray light. The electrodes 40 are connected to an electric circuit 48 operable to apply a voltage to the electrodes, to generate an electric field (Fig. 4). Initially, with no voltage applied to the electrodes 40, the hologram is in the diffractive (active) state and the holographic optical element 20,22,24 diffracts propagating light in a predefined manner. When an electrical field is generated in the hologram by applying a voltage to the electrodes 40 of the holographic optical element 20,22,24, the operating state of the hologram switches from the diffractive state to the passive state and the holographic optical element does not optically alter the propagating light. It is to be understood that the electrodes may be different than described herein without departing from the scope of the invention. For example, a plurality of smaller electrodes may be used rather than two large electrodes which substantially cover surfaces of the holograms.

Each holographic optical element 20,22,24 may be holographically configured so that only a particular monochromatic light is diffracted by the hologram. For example, holographic optical element 20 may have a hologram which is optimized to diffract red light, the optical element 24 may have a hologram which is

optimized to diffract green light, and the optical element 26 may have a hologram which is optimized to diffract blue light. The holographic device controller 50 drives switching circuitry 64 associated with the electrodes 40 on each of the optical elements 20,22,24 to apply a voltage to the electrodes (Figs. 4 and 5). The electrodes 40 are individually coupled to the device controller 50 through a voltage controller 68 which selectively provides an excitation signal to the electrodes 40 of a selected holographic optical element 20,22,24 switching the hologram to the passive state. The voltage controller 68 also determines the specific voltage level to be applied to each electrode 40.

The electrodes 40 may be switched to their active states at the same time or electrodes associated with only one of the three holographic optical elements 20,22, 24 may be energized at one time. For example, the voltage controller 68 may switch the green and blue holograms 22,24 to the passive state by applying voltages to their respective electrodes 40. The supplied voltages to the electrodes 40 of the green and blue holograms 22,24 create a potential difference between the electrodes, thereby generating an electrical field within the green and blue holograms. The presence of the generated electrical field switches the optical characteristic of the holograms 22, 24 to the passive state. With the green and blue holograms 22,24 in the passive state and the red hologram 20 in the diffractive state, only the red hologram optically diffracts the emitted light. Thus, only the portion of the visible light spectrum corresponding to the red light is diffracted. The green hologram 22 is next changed to the diffractive state by deenergizing the corresponding electrodes 40 and the electrodes of the red hologram 20 are energized to change the red hologram to the passive state so that only green light is diffracted. The blue hologram 24 is then

changed to the diffractive state by deenergizing its electrodes 40 and the electrodes of the green hologram 22 are energized to change the green hologram to the passive state so that only blue light is diffracted. The holograms are sequentially enabled with a refresh rate preferably less than 150 microseconds. The holographic elements 20,22, 24 may be cycled on and off in any order, or more than one element may be switched to the active state at one time.

It is to be understood that the holographic diffraction elements may be different than described herein without departing from the scope of the invention. For example, holographic diffraction device 18 may include additional holographic elements that perform optical functions other than color filtering. Also, the device 18 may include more than one holographic optical element configured to diffract each color wavelength band. Further, the holographic diffraction elements 20,22,24 do not need to be switchable. Since the angular separation between the red, green, and blue beams B 1, B2, B3 is relatively large (i. e., larger than the angular bandwidth of the Bragg holograms), the Bragg angular and wavelength selectivity will be sufficient to ensure that there is no appreciable cross-talk between the red, green, and blue wavelengths. Minimizing cross talk is important in color sequential illumination applications and less of a problem where the concentrator is used to mix different colored sources into a white output beam. It is also possible to record the red, green, and blue interference patterns separately in a single holographic optical element. The holographic diffraction device 18 would then comprise a single holographic element which embodies fringes combining the functions of the three individual elements.

The purpose of the concentrator 12 is to gather as much light as possible by the sight angle of acceptance of its input opening and then guide the light to an output

opening as a concentrated beam of uniform light. The light concentrator 12 may be a solid glass or polymer device or a hollow device with a reflective coating, for example, where the light is preferably confined by internal reflection. The light concentrator 12 may also comprise two or more separate components. For example, the light concentrator 12 may comprise a plurality of prisms, each performing a double internal reflection wherein about half of the incoming rays are reflected back and recycled unless they can be output within the narrower output angular range. The light concentrator 12 may also include other optical components such as condenser lenses and relay lenses.

It will be observed from the foregoing that the light concentration system of the present invention has numerous advantages. The placement of the holographic optical elements 20,22,24 at the input end 14 of the light concentrator 12 permits light received from different regions to be emitted by the concentrator within a common angular range. This is not possible with conventional light concentration devices due to the constraints imposed upon the geometry of the system by the Lagrange invariants.

Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations made to the embodiments without departing from the scope of the present invention. For example, although the invention has been described with reference to the visible band, the same principles also apply to infrared radiation. It is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.