WO2000045608A1

WO2000045608A1 - Optical sensor

Info

Publication number: WO2000045608A1
Application number: PCT/US2000/001191
Authority: WO
Inventors: Milan M. Popovich; Jonathan D. Waldern; Michael R. Adams
Original assignee: Digilens Inc.
Priority date: 1999-01-29
Filing date: 2000-01-18
Publication date: 2000-08-03
Also published as: AU2730000A

Abstract

A system for detecting an image comprising at least two optical devices (90, 92) switchable between an active state wherein light is diffracted by the holographic device and a passive state wherein light is not diffracted by the holographic device. The system further includes a detector (22) operable to detect light impinging upon a surface thereof. A first of the two optical devices (90) is operable in its active state to diffract light passing therethrough to form on the detector (22) an image of a first field of view (A1) and a second of the two optical devices (92) is operable in its active state to diffract light passing therethrough to form on the detector (22) an image of a second field of view (A2). The second field of view (A2) is different from the first field of view (A1).

Description

OPTICAL SENSOR

RELATED APPLICATION

The present application claims the benefit of U.S. Provisional Application Serial No. 60/117,825, filed January 29, 1999.

BACKGROUND OF THE INVENTION

The present invention relates generally to optical sensors, and more particularly, to optical sensors operable to image different fields of views.

Many applications such as surveillance systems and video conferencing utilize panoramic cameras. The problem of acquiring panoramic images has been treated extensively over the years and multiple commercial products have been developed. A number of prior art approaches employ complex catadioptric systems with multiple aspherical reflective and refractive optical elements. These imaging systems are typically very expensive to produce due to the large number of custom elements. Panoramic images have also been created by synthesizing frames taken by multiple cameras or by a single camera at successive adjacent intervals. However, a single camera that scans introduces undesirable artifacts for moving images and the use of multiple cameras quickly becomes cost prohibitive.

There is, therefore, a need for an optical sensor operable to capture both narrow and wide angle field of views, which utilizes few optical elements while providing a high resolution image.

SUMMARY OF THE INVENTION

An optical system for detecting an image is disclosed. The system comprises at least two optical devices switchable between an active state wherein light is diffracted by the holographic device and a passive state wherein light is not diffracted by the holographic device, and a detector operable to detect light impinging upon a surface thereof. A first of the two optical devices is operable in its active state to diffract light passing therethrough to form on the detector an image of a first field of view and a second of the two optical devices is operable in its active state to diffract light passing therethrough to form on the detector an image of a second field of view. The second field of view is different from the first field of view.

The optical devices may include holographic optical elements having a hologram recorded therein optimized to diffract red, green, or blue light. The holographic optical element may also be configured to diffract infrared or ultraviolet light. 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 schematic of an optical sensor of a first embodiment of the present invention configured for forming an image of a relatively wide field of view.

Fig. 2 is a schematic of the optical sensor of Fig. 1 configured for forming an image of an off-center field of view.

Fig. 3 is a schematic of the optical sensor of Fig. 1 configured for forming an image of a relatively narrow field of view.

Fig. 4 is schematic of a holographic device of the optical sensor shown in Figs. 1-3.

Fig. 5 is a perspective view of a holographic optical element of the holographic device of Fig. 4.

Fig. 6 is a partial front view of the holographic optical element of Fig. 5 showing an electrode and electric circuit of the holographic optical element.

Fig. 7 is a schematic of the holographic device of Fig. 4 with three holographic optical elements each optimized to diffract red, green, or blue light and a controller operable to switch each of the holographic optical elements between an active state and a passive state.

Fig. 8 is a schematic of a second embodiment of the optical sensor of Fig. 1 configured to form images with different fields of view on different areas of a light detector.

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 first embodiment of an optical sensor, generally indicated at 20, is shown. The optical sensor 20 may be used, for example, in video surveillance, robotics, video conferencing, or panoramic imaging. As shown in Fig. 1, the optical sensor 20 includes a light sensitive detector 22, an optical system, generally indicated at 24, comprising dynamic optical devices, and a controller 32 for controlling the optical devices. As shown in Fig. 1, the optical system 24 is preferably positioned on a common optical axis with the detector 22. The optical system 24 described below includes switchable holographic devices, 26, 28, 30 however, it is to be understood that other dynamic optical devices may be used, without departing from the scope of the invention. A dynamic optical device includes devices which are capable of being switched from a state in which it performs a specified optical function (e.g., imaging) to a state in which it has minimal effect on incident light. Further, it is to be understood that the term light as used herein includes infrared and ultra-violet light, as well as light in the visible spectrum.

The holographic devices 26, 28, 30 are each configured to form an image of a different field of view on the detector 22. The controller 32 is operable to switch each of the holographic devices 26, 28, 30 between a passive state in which the device has substantially no impact on incoming light and an active state in which the device diffracts light to form an image on the detector 22. Depending on which of the three holographic devices 26, 28, 30 are active, the optical system 24 will form an image having a relatively wide

central field of view (angular FOVcci) (Fig. 1), an off-center field of view (angular FOVα₂) (Fig. 2), or a relatively narrow field of view (angular

FOV ₃) (Fig. 3). Fig. 1 shows device 28 in its active state and devices 26 and

30 in their passive states, Fig. 2 shows device 26 in its active state and devices 28 and 30 in their passive states, and Fig. 3 shows device 30 in its active state and devices 26 and 28 in their passive states. Images corresponding to different fields of view are selectively projected onto the detector 22 by switching the holographic devices 26, 28, 30 between their passive and active states. The field of view is determined by the angular bandwidth of the holographic device 26, 28, 30 which is defined by the recording of holographic optical elements within the device. The holographic device 30 may also be configured to provide magnification along with a focusing function so that switching between devices 28 and 30 creates images corresponding to a wide field of view and a selected area of interest contained within that wide field of view, for example.

Fig. 4 schematically illustrates one of the holographic devices 26 of the optical system 24 shown in Figs. 1-3. Structurally, the holographic devices 26, 28, 30 are essentially identical, except for the interference fringes recorded in the holographic optical elements of the devices. Therefore, only device 26 will be described in detail. The holographic device 26 includes three holographic optical elements 36, 38, 40 which are selectively activated and deactivated to transmit a resultant image which is formed by sequentially manipulating different colors (Fig. 4). When the holographic optical element 36, 38, 40 is passive all light incident on the element is transmitted substantially unaltered through the element. When the holographic optical element 36, 38, 40 is active, a portion of the light incident on the holographic element within the angular wavelength and bandwidth of the holographic

optical element is diffracted such that the diffracted light is incident on the detector 22.

The holographic optical elements 36, 38, 40 each include a hologram interposed between two electrodes 52 (Figs. 5 and 6). The hologram may be a Bragg (thick or volume) hologram or Raman-Nath (thin) hologram. Raman- Nath holograms require less voltage to switch light between various modes of the hologram than Bragg holograms, however, Raman-Nath holograms are not as efficient as Bragg holograms. The hologram is used to control transmitted light beams based on the principles of diffraction. The hologram selectively directs an incoming light beam 46 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 52. Light passing through the hologram in the same direction as the light is received is referred to as the zeroth (0th) order mode 48 (Fig. 5). When no voltage is applied to the electrodes 52, liquid crystal droplets within the holographic optical element 36, 38, 40 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 50 of the hologram. When a voltage is applied to the holographic optical element 36, 38, 40, 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 48.

It is to be understood that the holographic optical elements 36, 38, 40 may also be reflective rather than transmissive as shown in Figure 5 and described above. In the case of a reflective holographic optical element, the arrangement of the holographic device and optical components 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 to form an image. 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, magnification, or color filtering of the light, for example.

The holograms are preferably recorded on a photopolymer/liquid crystal composite material (emulsion) 60 such as a holographic photopolymeric film which has been combined with liquid crystal, for example (Fig. 6). 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. The liquid crystal may be suffused into the pores of the photopolymeric film and may include a surfactant.

The refractive properties of the holographic optical elements 36, 38, 40 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 36, 38, 40. Recording of the hologram may be accomplished by a traditional optical process in which interference fringes are created by applying beams of light. Alternatively, the interference fringes may be artificially created by using highly accurate laser writing devices or other optical replication techniques. As further described below, the optical properties of the holographic optical elements 36, 38, 40 primarily depend on the recorded holographic fringes in the photopolymeric film.

The electrodes (electrode layers) 52 are positioned on opposite sides of the emulsion 60 and are preferably transparent so that they do not interfere with light passing through the hologram. The electrodes 52 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 electrodes 52 are connected to an electric circuit 58 operable to apply a voltage to the electrodes, to generate an electric field (Fig. 6). Initially, with no voltage applied to the electrodes 52, the hologram is in the diffractive (active) state and the holographic optical element 36, 38, 40 diffracts propagating light in a predefined manner. When an electrical field is generated in the hologram by applying a voltage to the electrodes 52 of the holographic optical element 36, 38, 40, 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 52 may be different than described herein. 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 36, 38, 40 is holographically configured such that only a particular monochromatic light is diffracted by the hologram (Figs. 4 and 7). The red optical element 36 has a hologram which is optimized to diffract red light, the green optical element 38 has a hologram which is optimized to diffract green light, and the blue optical element 40 has a hologram which is optimized to diffract blue light. The device controller 32 drives switching circuitry 64 associated with the electrodes 52 on each of the optical elements 36, 38, 40 to apply a voltage to the electrodes (Figs. 6 and 7). The electrodes 52 are individually coupled to the device controller 32 through a voltage controller 68 which selectively provides an excitation signal to the electrodes of a selected holographic optical element 36, 38, 40, switching the hologram to the passive state. The voltage controller 68 also determines the specific voltage level to be applied to each electrode 52.

Preferably, only one pair of the electrodes 52 associated with one of the three holographic optical elements 36, 38, 40 is energized at one time. In order to display a color image, the controller 32 operates to sequentially display three monochromatic images of the color input image. The electrodes 52 attached to each of the holograms 36, 38, 40 are sequentially enabled such that a selected amount of red, green, and blue light is directed towards the viewer. For example, when a red monochromatic image is projected, the controller 32 switches the green and blue holograms 38, 40 to the passive state by applying voltages to their respective electrodes 52. The supplied voltages to the electrodes 52 of the green and blue holograms 38, 40 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 38, 40 to the passive state. With the green and blue holograms 38, 40 in the passive state and the red hologram 36 in the diffractive state, only the red hologram optically diffracts the projected red image. Thus, only the portion of the visible light spectrum corresponding to the red light is diffracted to the viewer. The green hologram 38 is next changed to the diffractive state by deenergizing the corresponding electrodes 52 and the electrodes of the red hologram 36 are energized to change the red hologram to the passive state so that only green light is diffracted. The blue hologram 40 is then changed to the diffractive state by deenergizing its electrodes 52 and the electrodes of the green hologram 38 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 (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. The red, green, and blue holographic elements 36, 38, 40 may be cycled on and off in any order. The controller 32 is operable to switch the elements between their active and passive states in a cyclic mode such that red, green, and blue images are projected in rapid succession onto the detector 22. The holographic devices 26, 28, 30 may also be configured to diffract infrared or ultra-violet light. Thus, the number of holographic optical elements within each holographic device 26, 28, 30 may be different than shown and described herein. For example, the optical sensor may be configured for sensing only ultra-violet light, in which case each holographic device would include only one holographic optical element. The holographic devices 26, 28, 30 may also be configured to perform more complex optical operations. For example, a variable magnification (zoom) function may be encoded in one or more of the devices 26, 28, 30 by including an additional switchable holographic optical element.

It is to be understood that the components within the optical sensor 24 or the arrangement of components may be different than shown herein without departing from the scope of the invention. For example, the optical system 24 may also include conventional optical components to correct aberrations produced by the holograms or for relaying images to the detector 22.

The light-sensitive detector 22 includes a photodetector array comprising a plurality of detector elements (e.g., charge-coupled detectors (CCDs), photo capacitors, photo resistors, photo diodes, or any other suitable light-sensitive device). The detector elements may be arranged in rows and columns, or other suitable arrangements. The light detector 22 may communicate with a computer capable of storing information representative of the image incident on the detector, or the image may be captured on film, for example. The detector 22 may be configured to operate in full color (e.g., visible red, green, and blue light described above) or in monochrome. For example, the detector 22 may be an infrared image intensifier coupled with a CCD array responsive to infrared radiation.

A second embodiment of the optical sensor is shown in Fig. 8 and generally indicated at 80. The second embodiment 80 is similar to the first embodiment 20 except that the holographic devices 26, 28, 30 of the first embodiment 20 formed their images on a common area of the detector 22 (Figs. 1-3), whereas the optical sensor 80 of the second embodiment forms images on different areas of the detector 22 (Fig. 8). This allows the images to be tiled on the detector 22 in areas laterally spaced from one another on the detector. The images may be formed substantially simultaneously with one another by having controller 84 switch rapidly between holographic devices 90, 92, 94. For example, as shown in Fig. 8, holographic device 90 is

configured to form an image with an off-center field of view (angular FOV α₄)

on an area Al of the detector 22, and device 92 is configured to form an image

with a central field of view (angular FOV α₅) on an adjacent area A2 of the

detector.

The principles illustrated in the first and second embodiments 20, 80 may also be combined so that tiled images are formed on different areas of the detector 22 with one or more of the images being switchable between different fields of view. It is to be understood that the type and number of holographic devices 26, 28, 30, 90, 92, 94 may be different than shown and described herein without departing from the scope of the invention. For example, the system 20, 80 may include only two holographic devices. Although the invention has been described in terms of transmission holograms, reflective holograms or other forms of dynamic optical devices 26, 28, 30, 90, 92, 94 may also be used. The holographic devices may also be arranged to act upon ultra-violet or infrared radiation other than visible light. In this case, the detector 22 would be responsive to those particular wavelengths. For example, the detector 22 may be an infrared image intensifier coupled with a CCD array for an optical sensor responsive to infrared radiation.

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.

Claims

CLAIMSWHAT IS CLAIMED IS:

1. A system for detecting an image comprising:

at least two optical devices switchable between an active state wherein light is diffracted by the holographic device and a passive state wherein light is not diffracted by the holographic device; and

a detector operable to detect light impinging upon a surface thereof;

wherein a first of the two optical devices is operable in its active state to diffract light passing therethrough to form on the detector an image of a first field of view and a second of the two optical devices is operable in its active state to diffract light passing therethrough to form on the detector an image of a second field of view, said second field of view being different from said first field of view.

2. The system of claim 1 wherein each of the optical devices is operable to form an image at a different location on the detector.

3. The system of claim 1 wherein said first field of view is a relatively wide field of view and said second field of view is a relative narrow field of view.

4. The system of claim 1 wherein said first field of view is a central field of view and wherein said second field of view is an off-center field of view.

5. The system of claim 1 further comprising a controller operable to switch one of the optical devices to its active state while switching the other of the optical devices to its passive state.

6. The system of claim 1 wherein the first and second optical devices each comprise a holographic device having a hologram interposed between two electrode layers operable to apply an electrical field to the hologram.

7. The system of claim 6 wherein the hologram is formed from a polymer and liquid crystal material.

8. The system of claim 1 wherein the first and second optical devices each comprise three holographic optical elements.

9. The system of claim 8 wherein the three holographic optical elements each have a hologram recorded therein which is optimized to diffract red, green, or blue light.

10. The system of claim 9 wherein each hologram is interposed between two electrode layers operable to apply an electrical field to the hologram to diffract the red, green, or blue light.

11. The system of claim 10 further comprising a controller operable to sequentially supply voltage to and remove voltage from the electrode layers of each holographic optical element to create a sequence of monochrome images which are combined to form a color image.

12. The system of claim 1 wherein the optical devices are operable to diffract ultra-violet light.

13. The system of claim 1 wherein the optical devices are operable to diffract infrared radiation.

14. The system of claim 1 wherein at least one of the optical devices is operable to magnify the image.