OPTICAL FILTER SYSTEM
RELATED APPLICATIONS
The present application claims the benefit of U.S. Provisional Application Serial No. 60/165,223, filed November 12, 1999.
BACKGROUND OF THE INVENTION
The present invention relates generally to light generating systems, and more particularly, to a holographic optical filter for use in a light generating system.
Image display systems typically include a display screen configured to display monochrome images. When a multi-color display is required a sequence of images is displayed and illuminated sequentially with red, green, and blue light. The switching from one image to the next is performed rapidly (e.g., at a rate faster than the response time of the human eye) so that a color image is created in the viewer's eye due to the illumination of red, green, and blue monochrome images. This allows the viewer to see a full color image generated from a display system having a display screen operable to produce only monochrome images. The display system typically includes a white light source and a rotating color wheel having red, green, and blue filters to provide color sequential illumination of a display device. However, these rotating filters are often susceptible to mechanical failures and tend to be large and noisy.
The problem of mechanical failures, noise, and size may be reduced by using solid state techniques such as liquid crystal polarization switches. However, these switches work only with polarized light, thus, half of the light produced by a light source is never projected from the display screen, resulting in low illumination. Furthermore, the liquid crystal polarization switches and mechanical rotating wheels often do not provide sufficiently fast switching speeds between the different colors. If the switching speed is too slow, the image projected from the display screen will not appear as a multi-color image.
In other words, the red, green, and blue image components will not be properly fused into a full color image.
There is, therefore, a need for reliable, efficient, and fast switching optical system operable to generate red, green, and blue light from a white light source.
SUMMARY OF THE INVENTION
An optical filter system is disclosed. The system includes a first optical device positioned to receive light from a light source and configured to reflect light in a first wavelength band in an output direction and a second optical device positioned to receive light from the light source and configured to deflect the light in a second wavelength band. The system further includes a third optical device positioned to receive light deflected by the second optical device and direct the light into the output direction.
In another aspect of the invention a display system comprises a display device operable to display an image, a light source, and an optical filter system. The optical filter system comprises a first optical device positioned to receive light from the light source and configured to reflect light in a first wavelength band in an output direction and a second optical device positioned to recieve light from the light source and configured to deflect light in a second wavelength band. The optical filter further comprises a third optical device positioned to receive light deflected by the second device and operable to direct the light into the output direction.
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 a side view of a light generating system having a first embodiment of an optical filter of the present invention.
Fig. 2 is a perspective of a holographic optical element and light source for use with the display system of Fig. 1. Fig. 3 is a partial front view of the holographic optical element of Fig. 2 illustrating an electrode and electric circuit of the holographic optical element.
Fig. 4A is a side view of the light generating system of Fig. 1 with the optical filter in a first operating mode.
Fig. 4B is a side view of the light generating system of Fig. 1 with the optical filter in a second operating mode.
Fig. 4C is a side view of the light generating system of Fig. 1 with the optical filter in a third operating mode.
Fig. 5 is a plan view of the optical filter of Fig. 1.
Fig. 6 is a side view of a light generating system having a second embodiment of an optical filter of the present invention.
Fig. 7 A is a side view of the light generating system of Fig. 6 with the optical filter in a first operating mode.
Fig. 7B is a side view of the light generating system of Fig. 6 with the optical filter in a second operating mode.
Fig. 7C is a side view of the light generating system of Fig. 6 with the optical filter in a third operating mode.
Fig. 8 A is a side view of a third embodiment of the optical filter operating in a first mode. Fig. 8B is a side view of the third embodiment of the optical filter operating in a second mode.
Fig. 8C is a side view of the third embodiment of the optical filter operating in a third mode.
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 generating system having a first embodiment of an optical filter system of the present invention is shown and generally indicated at 10. The system of the present invention may be used in a color sequential display system, in which a display device 12 is used to display monochrome images in sequence. The monochrome images correspond to the red, green, and blue components of the final image and the light generating system illuminates the display in sequence with red, green, and blue light according to the color component image being displayed at any particular time. The present invention may be used, for example, in an application where the source has a spectral or color bias, such as with metal halide lamps. Such lamps have strong emission characteristics in the green region of the spectrum, but tend to be less efficient in the red and blue regions. Many high intensity discharge lamps (also referred to as arc lamps) exhibit prominent peaks in the green region of the spectrum.
The light generating system 10 includes a light source 14 and the optical filter. The light source 14 may be polychromatic and preferably provides incoherent white light. The light source 14 may also comprise a plurality of light emitting elements (e.g., LEDs or lasers) which emit light of red, green, and blue wavelengths. The light source 14 includes red, green, and blue bandwidth light components (i.e., light emissions having different wavelength ranges). The filter receives light from the light source in a predetermined direction of incidence, as indicated by arrow I. The filter is configured to emit light in a predetermined output direction, as indicated by arrows OR, OQ, OB-
The filter system includes a reflection hologram (first optical device) 22 operable to act upon a wavelength band of light in the green region of the visible spectrum while allowing all other wavelengths to pass therethrough substantially undeflected, as indicated by arrow I'. The characteristics of the refection hologram shown in Fig. 1 are selected such that light in the green wavelength band is reflected by the device 22 in the output direction indicated by arrow O -
A second optical device 24 is disposed optically behind the first optical device, with respect to the direction of incidence of light from the light source. The second optical device 24 includes two transmission holograms 26, 28, one configured to act upon a wavelength band in the red region of the visible spectrum and the other configured to act upon a wavelength band in the blue region. All other wavelength bands pass through the respective components substantially unaltered. The transmission holograms 26, 28 are operable to deflect red and blue light, respectively, away from the predetermined incidence direction, as indicated by arrows AR and Aβ.
A third optical device 30 is disposed optically behind the second device and receives light deflected by the second device. The third device 30 includes a plane and two transmission holograms 32, 34 and a plane reflector 36. The first transmission hologram 32 is configured to act upon the same red wavelength component as hologram 26, and deflects the red light onto the reflector, as indicated by arrow BR. The reflector 36 then reflects this deflected light in the predetermined output direction, as indicated by arrow OR. Similarly, hologram 34 is configured to act upon the same blue wavelength as hologram indicated by arrow BB. The reflector 36 then reflects the light in the predetermined output direction, as indicated by arrow OB- The output direction is preferably generally the same for each wavelength band (OG, OR, OB).
The holograms 22, 26, 28, 32, 34 are each located on a holographic optical element that preferably switchable between an active (diffracting) state and a passive (non-diffracting) state. It is to be understood that in the passive state (non-diffracting state), the incoming light may still be slightly diffracted, however, the light is not substantially altered. Switching of the holographic elements 22, 26, 28,32, 34 is controlled by controller 40, which operates to switch each of the elements between their active and passive states. The holographic optical elements 22, 26, 28, 32, 34 each include a hologram interposed between two electrodes 40 (Figs. 2 and 3). 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 wavelength selectivity of the Bragg holograms serves to avoid significant cross talk between the red, green, and blue wavelength bands. In order to eliminate cross talk, the optical devices are configured as switchable.
The hologram is used to control transmitted light beams based on the principles of diffraction. Fig. 2 illustrates operation of the transmission holograms 26, 28, 32, 34. The hologram selectively directs an incoming light beam from light source 14 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. Light passing through the hologram in the same direction as the light is received from the light source 14 is referred to as the zeroth (0th) order mode 44 (Fig. 3). When no voltage is applied to the electrodes 40, liquid crystal droplets within the holographic optical element 34 are oriented such that the hologram is present in the element and light is diffracted from the zeroth order mode to a first (1st) order mode 46 of the hologram. When a voltage is applied to the holographic optical element 34, 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.
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 in its entirety. 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 22, 26, 28, 32, 34 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. 3). 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. The transmission may be increased to above 98% by adding suitable antireflection coatings to the electrodes. Further 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. 3). Initially, with no voltage applied to the electrodes 40, the hologram is in the diffractive (active) state and the holographic optical element 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, 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.
It is to be understood that the holographic diffraction elements may be different than described herein without departing from the scope of the invention. The display system 20 may also include additional holographic elements that perform optical functions other than light directing.
The holograms are sequentially enabled with a refresh rate (e.g., less than 150 microseconds) which is faster than the response time of a human eye so that a color image will be created in the viewer's eye due to the integration of the red, green, and blue monochrome images created from each of the red, green, and blue holograms. Consequently, the display device 12 will be illuminated sequentially by red, green, and blue lights so that the final viewable image will appear to be displayed as a composite color. The red, green, and blue holographic optical elements may be cycled on and off in any order. Additional optical components (not shown) may also be provided to generate desired optical characteristics in the red, green, and blue beams.
Referring now to Figs. 4A, 4B, and 4C, three modes of operation of the optical filter of Fig. 1 are shown. The first mode of operation is shown in Fig. 4A. The first device 22 is activated and the second and third devices are deactivated 24, 30. In this mode, light in the green wavelength band is reflected by the first device and the red and blue wavelengths are not acted upon by the second and third devices 24, 30. Thus, red and blue light is lost from the system, as indicated by arrow C.
In a second mode of operation shown in Fig. 4B, the first device 22 is deactivated, the red components 26, 32 of the second and third devices 24, 30 are activated and the blue components 28, 34 of the second and third devices are deactivated. This results in the red wavelength being deflected by holograms 26 and 32 and reflected by the reflector 36 into the predetermined output direction, as indicated by arrow OR. The green and blue wavelengths are lost from the system, as indicated by arrow D.
In the third mode of operation, shown in Fig. 4C, the first device 22 is deactivated, the red components 26, 32 of the second and third devices 24, 30 are deactivated, and the blue components 28, 34 of the second and third devices are activated. The blue wavelength is thus deflected by holograms 28 and 34 and reflected by the reflector 36 into the predetermined output direction, as indicated by arrow Oβ. The red and green wavelengths are lost from the system, as indicated by arrow E.
The switching of the filter between these three modes is preferably performed in very rapid succession so that light emitted in the output direction is viewed as a blend of the red, green, and blue wavelengths (i.e., white light). If the optical devices uses a Bragg reflection hologram, the hologram will have a relatively narrow bandpass (e.g., few tens of nanometers). Accordingly, the wavelength of the green light reflected by the device is fairly narrow. In contrast, holograms 26, 28, 32, and 34 are all of the transmission ty e, which tend to have a rather broad bandpass (e.g., 100 to 150 nanometers).
Consequently, the wavelength bandwidths of the red and blue light acted upon by the devices 24 and 30 are relatively broad. The filter can thus be used to increase the amount of red and blue light emitted in the output direction as compared with the green wavelengths, thereby compensating for the color bias of a lamp (e.g., metal halide lamp) having stronger emission characteristics in one region. This provides a broad color range and a good white point.
For certain applications, it may be desirable to increase the bandpass of the filter. This can be achieved by constructing the filter from a stack of holographic optical elements which have different bandpass characteristics. Each of these elements is equivalent to a filter that operates over a narrow bandwidth, and the overall effect of the stack is to produce reflection characteristics which are a combination of those of the individual hologram optical elements. In the case where the first device 22 is switchable, these holographic elements may be switched simultaneously. Since the switching electrodes will introduce transmission losses, it is preferred that all of the elements are sandwiched between a common pair of electrodes, rather than a separate pair of electrodes for each element. Since holograms 26, 28 32, and 34 are transmission holograms, they will be sensitive to the polarization state of the incident light. In particular, the diffraction efficiency of the transmissive elements is considerably higher for p- polarized light than for s-polarized light. For example, the response to s- polarized light may be around 1% of that for p-polarized light. In order to make use of the full output of the light source, the display system may include optical filters which make use of both the p-polarized light and s-polarized light, such as disclosed in U.S. Patent Application Serial Number 09/478,150, filed January 5, 2000, (Attorney Reference No. 5454-00700/RDP029) which is incorporated herein by reference in its entirety. For example, pairs of holographic diffraction elements may be used with one element in the pair acting on the p-polarized component and the other acting on the s-polarized components. This may be achieved either by interposing a polarization rotator between the elements in the pair or by arranging for the interference fringes in the elements of each pair to be mutually crossed. In a system as shown in Figs. 4A-4C, the polarization rotator would be inserted between elements 24 and 30. If reflection holograms are used, these additional provisions are not required since reflection holograms only start to become polarization sensitive at large angles of incidence, typically much greater than 45 degrees. In the case where the holographic diffraction elements comprise Bragg holograms, the angular and wavelength selectivity of such holograms in many practical cases avoids cross talk between the various wavelength bands. Under these circumstances, suitable illumination of the display device 12 may be achieved by using a broadband incoherent light source and using holographic diffraction elements that are not switchable. Alternatively, since there is little or no polarization sensitivity for light incident at angles of between 0° to approximately 45°-50°, the geometry of the system may be arranged such that the light is always incident at such angles. The above described problem does not arise with the hologram of the first optical device 22 because reflection holograms are not polarization sensitive.
It is to be understood that the arrangement of components within the system 10 may be different than shown in Fig. 1 without departing from the scope of the invention. For example, the first device may operate on red wavelengths and the second and third device act upon green and blue wavelengths. This arrangement may be used, for example, in cases where the light source has a strong infra-red component. The filter would thus substantially eliminate the infra-red from the red light emitted from the filter to avoid unwanted heating effects.
In the embodiment described above, it is important that the filtered light is separated from light which emerges from the filter by other means, such as by zero order diffraction or by specular reflection from the surfaces of the optical devices. Fig. 5 illustrates how this can be achieved. Fig. 5 is a view of the light generating system of Fig. 1 taken at a right angle relative to Fig. 1. The optical devices 22, 24, 30 are all oriented generally parallel to one another and inclined at an angle Ui relative to the direction of incidence of light emitted from the light source, indicated by arrow I. Any light that is specularly reflected by the surfaces of the holograms 22, 26, 28, 32, 34 will propagate at an angle U2 with respect to that direction (e.g., arrows Ri to R ). Based on the laws of geometric optics, the angle U will be twice Uj.
As for diffraction, zero order diffracted light will essentially pass straight through the components 26, 28, 32, 34 (as indicated by arrow Z) and will be reflected by the plane reflector 36 at the angle U2, as indicated by arrowt. First order diffracted light will be deflected (e.g., arrow X) and will be reflected by the plane reflector 36 at an angle U to emerge in the output direction, as indicated by arrow O. By optimizing the angle Ui, it is possible to separate the diffracted light O from the specularly reflected and zero order diffracted light Rl s R2, R3, R4. This arrangement may also be applied to the second and third embodiments described below.
Figs. 6, 7A, 7B, and 7C illustrate a second embodiment 60 of the light generating system shown in Fig. 1. The first and second optical devices 22, 24 of the second embodiment 60 are generally the same as those of the first embodiment 10. The third optical device comprises a reflective holographic device 62 which receives light deflected by the second device 24, as indicated by arrows AR and Aβ. The third optical device 62 then diffracts the received light into the output direction, as indicated by arrows OR and Oβ. The device 62 may be used simultaneously to correct chromatic aberrations introduced by the components 26, 28 of the device 62. The device 62 may also be configured such that, in conjunction with the components 26 and 28, it provides color correction over the entire visible spectrum or a substantial portion thereof. The third device 62 is switchable by controller 64 between its active and passive states simultaneously with the second device. The third device 62 may also be a nonswitchable hologram or grating. The third device 62 may also be a stack of switchable holograms having spectral responses matched to elements 26 and 28.
The embodiments of Figs. 6 and 7 may also incorporate the additional layers 32 and 34 together with a polarization rotator to ensure that s and p polarized light can be used.
Figs. 7A, 7B, and 7C illustrate three modes of operation of the image generating system 60 of Fig. 6. In the first mode of operation (Fig. 7 A), the first optical device 22 is activated and the second and third devices 24, 62 are deactivated. Green wavelengths from the light source are reflected by the reflection hologram, as indicated by arrow OG- Red and blue wavelengths pass through the first and second devices 22, 24 substantially unaltered and are lost from the system, as indicated by arrow C (when the third device is switchable) or arrow C" (when the third device is not switchable).
As shown in Fig. 7B, in the second mode of operation, the first device 22 is deactivated, the red component 26 of the second device 24 is activated, and the blue component of the second device is deactivated. The red, green, and blue wavelength light passes through the first device 22 substantially unaffected. The red wavelength light is deflected by the activated component 26 and subsequently reflected by the third device 62 into the predetermined output direction, as indicated by arrow OR. The green and blue wavelengths pass through the second device substantially unaltered and pass out of the system, as indicated by arrows D' or D".
Fig. 7C illustrates the third mode of operation. The first device 22 and the red component 26 of the second device 24 are both deactivated, and the blue component 28 of the second device is activated. The red, green, and blue wavelengths all pass through the first device 22 substantially unaffected to reach the second device 24. The blue wavelengths are then deflected by the activated component 28 and are subsequently deflected by the third device 62 into the output direction, as indicated by arrow Oβ. The red and green wavelengths pass through the second device 24 substantially unaffected and pass out of the system, as indicated by arrows E' and E". As with the first embodiment, cycling between the three modes of operation is performed very rapidly so that the sequence of light emitted is perceived as comprising white light.
Figs. 8A, 8B, and 8C illustrate a third embodiment of the present invention, generally indicated at 80. The system 80 is similar to the first embodiment 10 except that the first and third optical devices 82, 86 are configured to have optical diffusing properties so that the light emitted in the output direction is slightly scattered. Fig. 8 A illustrates scattering of the green wavelengths by the first device 82. Fig. 8B illustrates scattering of the red wavelengths by the third device 86. Fig. 8C illustrates scattering of the blue wavelength by the third device 86.
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. Accordingly, 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.