A reflective optical system

Abstract: A wide angle large reflective unobscured system (10) includes a primary convex reflective element (12) for receiving a broad range of energy, a secondary concave reflective element (14) for reflecting the energy from the primary reflective element (12) to reimage a virtual entrance pupil at a real aperture stop. A beamsplitter element (16) is provided for reflecting a first portion of the energy, such as visible energy, to a first tertiary concave reflective mirror (18), while transmitting a second portion of the energy, such as IR light, to a second tertiary concave reflector (20). Each tertiary reflector is capable of focusing the received energies to dual focal planes wherein a compact detector array (22, 24) assembly can convert the images to electronic signals. The beamsplitter element (16) has aspherical surfaces (16a,16b) for controlling aberrations. The system (10) is used in head mounted displays.

Description: The present invention relates to a reflective optical system and, more particularly, to a compact all-reflective optical system to accommodate a dual color / or wavelength image system with a wide field of view and very fast optical speed that can be used for helmet-mounted applications. All-reflective optical systems have been utilized in various imaging applications where chromatic aberrations, thermal behavior, size, weight, or cost restricts the use of conventional refractive lenses. Retroreflective systems with folded optical paths are frequently utilized in combination with lenses to provide telescopic optical systems. Wide angle large reflective unobscured systems (WALRUS) capable of forming a high quality image with an extremely wide angle object field on a flat image surface are known. Such a system is illustrated in an article entitled "Easily Fabricated Wide Angle Telescope" by R. Calvin Owen, 430 SPIE, Vol. 1354, International Lens Design Conference (1990). A relevant wide angle large reflective system having a fast optical speed can be found in US-A- 5,331,470. Additional background references representing the work of the present inventor can also be found in US-A-4 265 510, US-A-4 733 955, US-A-4 834 517, and US-A-5 170 284. Examples of helmet- and head-mounted display systems for commercial and military applications are known in US-A-4 361 384, US-A-4 878 046, US-A-5 003 300, and U.S. statutory registration No. H7779. From US-A-5 181 145 a reflective optical system according to the preamble of claim 1 is known which comprises: a primary reflector adapted to receive energy; a secondary reflector for receiving energy reflected from the primary reflector, the primary and the secondary reflectors cooperating to reimage a virtual entrance pupil to a real aperture stop; and a beamsplitter element. The beamsplitter element receives radiation reflected from the secondary reflector and defines two output beams which are directed to a detector, such as a wide field of view CCD detector on the one hand, and to a second detector, which may be a narrow field of view CCD detector. Further reference is made to US-A-5 331 470 which discloses a wide angle large reflective unobscured system having a primary, secondary, tertiary and fold mirror. The primary and secondary mirrors act as a non-reimaging afocal telescope of the Galilean type and relay a virtual entrance pupil to the fold mirror which is positioned at the system aperture stop. The fold mirror directs the energy to the tertiary mirror which acts to direct and focus the energy at a viewing plane. In view of this, it is an object of the present invention to provide an improved optical system, particularly for application and compact optical-electronic systems, such as helmet-mounted applications and small turret pilotage/navigational applications, which provides very fast optical speeds and very wide field angles that can further accommodate a dual colour or wavelength application. This object is achieved by a reflective optical system according to claim 1. The present invention provides a wide angle large reflective unobscured system utilizing a primary mirror to initially receive incident radiation, such as visible and infrared radiation. The primary mirror has a negative power and directs the incident energy to a secondary mirror of a positive power. The secondary mirror focuses the energy at an aperture stop so that a virtual entrance pupil is re-imaged at the aperture stop and a beamsplitter element, such as a Ge substrate beamsplitter can be operatively positioned at the aperture stop for reflecting a first portion of the energy, such as visible light, while transmitting a second portion of the energy, such as inftared radiation. The faces of the beamsplitter can be advantageously aspherical because of their position at the aperture stop to provide a correction of spherical aberration. The reflected first portion of energy can be focused by a first tertiary mirror onto a first viewing plane, while the transmitted second portion of energy can also be reflected from a second tertiary mirror to a second viewing plane. Each of the tertiary mirrors have a positive power and the arrangements of the mirrors are such to be on-axis in aperture, but off-axis in field. The optical system is extremely compact, since the tertiary mirrors can be arranged to provide their viewing planes parallel to each other and thereby accommodate a detector array with the respective detector surfaces on either side and in alignment with the viewing plane of the respective tertiary mirrors. A thermal/electric cooler can be sandwiched by the detector surfaces to reduce any dark current generated within each array. As one example, a field of view of such an optical system can be 30 degrees elevation by 40 degrees azimuth with an optical speed of F/4.5 in elevation and F/1.5 in azimuth. The geometrical 80% blur diameters of the design can average about 0.0008 inches over the field with the best blur being 0.00035 inch and the worst blur being 0.0016 inch. The reflective optics system of the present invention can be combined with a display system for portable application to a user, e.g., mounted on a helmet or other head supported structure to provide not only an infrared image to a soldier but also enhanced night vision by capturing photon energy in the near-visible spectrum that would not be seen by the naked eye. Figure 1 is a schematic elevational ray trace view of one example of an optical system applied with a detector array; Figure 2 is an azimuth ray trace section with a detector unit in phantom lines; and Figure 3 is a schematic view of the reflective optics applied to a helmet-mounted application. The objects and features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages, may best be understood by reference to the following description, taken in connection with the accompanying drawings. The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventor of carrying out his invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the general principles of the present invention have been defined herein specifically to provide a dual wavelength wide angle large reflective unobscured system. Referring to Figure 1, a schematic of one application of a compact all-reflective optical system with a wide field of view and very fast optical speed, including a beamsplitter and detector array, is illustrated in a schematical manner for ease of understanding and designated with the reference number 10. The optical system 10 includes a primary mirror of a negative power 12 that will reflect a range of optical energy desired for utilization by a detector array. For exampie. helmet-mounted applications and small turret pilotage/navigation applications frequently require the capability of both visible and IR imaging. Detector arrays, such as a staring array, and particularly ambient temperature arrays in the IR range, such as dual color/wavelength sensors have become smaller, simpler, and less expensive. Our optical system 10 discloses a compact arrangement of both a visual and IR detector system and the primary mirror 12 is designed to have a reflective surface that is applicable over this desired wavelength range. Optical system 10 could be utilized, for example, in a helmet-mounted sensor for foot soldier use, as shown schematically in Figure 3, a small turret pilotage/navigation sensor for aircraft and a weapon-mounted sight for alignment on a target, including that of a rifle. Each of these applications can provide a visible imagery using a CCD array 24 and night sight imagery or LWIR imagery using an ambient temperature silicon microbolometer array 22. The primary mirror surface can be a higher order aspheric surface and will direct the desired energy to a positive powered secondary mirror 14 which can also utilize a higher order aspheric surface configuration. By using an all-reflective optical system, the present design is capable of the desired and necessary wavelength coverage, fields of view, and optical speeds. A dichroic beamsplitter, such as a Ge substrate beamsplitter 16, is located at the aperture stop where the entrance pupil of the primary mirror is realized through the optics. In this embodiment of the invention, the dichroic beamsplitter element 16 reflects visible wavelength energy to a first positive power tertiary mirror 18, while transmitting the infrared energy through the beamsplitter 16 to a second positive power tertiary mirror 20. By positioning a relative thick dichroic beamsplitter 16 at the aperture stop in a nearly collimated region midway between the secondary and the respective tertiary mirrors, a very compact arrangement can be provided. The respective tertiary mirrors 18 and 20 can focus the respective visible light and infrared light onto a pair of parallel image planes. The positions of the image planes can be varied during the optical design process by varying the thickness of the beamsplitter substrate 16. As with the primary mirror 12 and secondary mirror 14, the respective tertiary mirror surfaces 18 and 20 can have an aspherical configuration to address aberrations, such as spherical, coma, and astigmatic aberrations. The front and back surfaces of the beamsplitter substrate 16 can be also be used for the correction of spherical aberration by providing an aspherical configuration. For example, the beamsplitter surface can be subject to a diamond turned lathe process to provide the appropriate aspherics. Since the beamsplitter surfaces have a unique location adjacent the aperture stop of the optical system in a nearly collimated region of the optical rays, the provision of an aspherical surface at both the entrance and exit surfaces of the beamsplitter 16 are particularly advantageous. As can be particularly appreciated, the exit surface of the beamsplitter can be particularly utilized to the address the longer wavelength radiation of the infrared spectrum. The exit surface 16b of the beamsplitter 16, in correlation with the aspherical design of the reflective surface of the second tertiary mirror 20, can ensure an adequate control of aberrations, to thereby improve the image at a focal plane common with the second detector surface, for example, of an ambient temperature silicon microbolometer array 22. Likewise, the reflective configuration of the entrance surface 16a of the beamsplitter 16 for reflecting the light to the first tertiary mirror 18 can also be advantageously designed with an aspheric surface complementary to the aspheric surface of the first tertiary mirror 18 to, again, provide an improved imagery at a focal plane common to the detector surface of the detector array 24, such as a CCD array. The CCD array with its broad wavelength sensitivity can act to enhance night vision. To improve the performances of the respective CCD array 24 and the silicon microbolometer array 22, a common thermal electric cooler unit 26 can be sandwiched between the respective detector arrays to reduce any dark current generated within each array. As a result of this design, an extremely fast and compact optical system is provided wherein the field of view of the optical systems is 40 degrees elevation by 53 degrees azimuth. The optical speed is F/4.5 in elevation, and F/1.5 in azimuth. The area equivalent F number will be F/2.3. The geometric 80% blur diameters of the design, shown in Figures 1 and 2, can average about 0.0008 inch over the field of view, with the best blur being 0.00035 inch, and the worst blur being 0.0016 inch. The detector arrays 22 and 24 can also include entrance windows (not shown) to transmit the energy to the detector plane and to protect the detector array from contaminants. Advantageously, such windows can be manufactured from materials appropriate to the spectral bandpass of each individual detector array. The power of the positive mirrors 14, 20, and 18 are designed to balance the negative power of the primary mirror 12 to basically provide a zero curvature or a flat field condition. The primary 12 and secondary mirrors 14 form a non-reimaging afocal telescope of the Gallilean type with an afocal magnification of about 2X of the pupil. These two mirrors relay the virtual entrance pupil (not shown), which would be located behind the primary mirror on the right side of Figure 1, to a real aperture stop located at the beamsplitter 16. As can be appreciated, the reflective surface design of tertiary mirror 20 can take into account the transmission of the infrared spectrum through the dichroic beamsplitter 16. Optical communication through this reflective and compact optical system permits an increase in the optical speed while providing to the user a large non-circular aperture for viewing the wide field of view. One example of the optical system of the present invention can be found in the following: No. Element RD CC AD AE AF AG THK Matl 12 Primary mirror 1.723 -0.14348 -0.18866 x 10 -1 -0.64627 x 10 -2 -0.47399 x 10 -3 -0.16639 x 10 -3 -1.315 REFL 14 Secondary mirror 4.044 -0.77170 -0.37004 x 10 -2 -0.34845 x 10 -3 0.34859 x 10 -4 -0.91481 x 10 -5 1.460 REFL 16a Beamsplitter aperture stop ∞ 0 0.44574 x 10 -3 0.31835 x 10 -2 -0.25449 x 10 -1 0.54572 x 10 -1 -1.531 REFL 18 First tertiary mirror 2.814 0.47014 -0.26124 x 10 -2 0.20758 x 10 -3 -0.15947 x 10 -2 0.44579 x 10 -3 1.334 REFL 24 First focal plane ∞ --- --- --- --- --- --- --- 16a Beamsplitter aperture stop ∞ 0 0.44574 x 10 -3 0.31835 x 10 -2 -0.25449 x 10 -1 0.54572 x 10 -1 0.20 GE 16b Beamsplitter ∞ 0 0.82506 x 10 -4 0.89348 x 10 -2 -0.53581 x 10 -1 0.10602 1.5235 AIR 20 Second tertiary mirror -2.806 0.41520 0.22562 x 10 -2 -0.16526 x 10 -3 0.13431 x 10 -2 -0.31664 x 10 -3 -1.329 REFL 22 Second focal plane ∞ --- --- --- --- --- --- --- Field of view: 40° elevation x 53° azimuth Aperture stop size: 0.36 elevation x 1.08 azimuth, racetrack Focal length: 0.589 paraxial Field offset: 52° to FOV center Nominal spectral bands: Visible and 8-12 microns One example of an application of the reflective optics system of the present invention is shown schematically in Figure 3. A helmet 28 can mount the optical system 10 within an appropriate housing, including a source of power (not shown). Alternatively, goggles or other head-mounted support members can be used instead of a helmet. The detector arrays 24 and 22 can drive a display generator 30 with a display, such as an LCD, which can be projected and focused by projection optics 32 onto a beamsplitter 36 to be further reflected and realized as a virtual image on a partially reflective combining mirror surface 34 to allow the user to view both reflected images and the transmitted outside real world scene. As can be appreciated, variations can be made in the present invention within the skill of a person in the optical field. Anti-reflective coatings can be utilized and tailored to the particular effective range of radiation impacting on a surface. As noted, all wavelengths of the light will share the common primary mirror 12 and secondary mirror 14. The visible light will be reflected from the first surface of the beamsplitter 16 and while a Ge substrate beamsplitter is contemplated, it can be readily appreciated that other forms of beamsplitters are known and can be used in this field. A simple reference to any computerized optical data base under the definition of beamsplitter in the optical field will provide to a person of ordinary skill in this field, numerous examples of different beamsplitters along with their optical characteristics. The visible light reflected from the beamsplitter 16 is focused by the tertiary mirror 18, while the LWIR energy will pass through the beamsplitter will be focused by the tertiary mirror 20. Again, different forms of visible and LWIR focal plane detector arrays can be utilized and can also be ascertained by a person of ordinary skill in the field by reference to an appropriate search data base under semiconductor detector arrays. While the present invention has been designed specifically to address helmet-mounted small turret pilotage/navigation aircraft sensors and weapon sight sensors, it can be appreciated that other applications of the present invention can be readily determined, wherever there is a desire to have a relatively inexpensive compact wide angle fast optical system. Those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiment can be configured without departing from the scope of the invention as defined in the appended claims. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.

Claims: A reflective optical system (10), comprising: a primary reflector (12) adapted to receive energy; a secondary reflector (14) for receiving energy reflected from said primary reflector (12), said primary and secondary reflectors (12, 14) cooperating to reimage a virtual entrance pupil to a real aperture stop; and a beamsplitter element (16), the system further comprising a pair of tertiary reflectors (18, 20), wherein said beamsplitter element (16) is arranged for reflecting a first portion of the energy incident from the secondary reflector (14) to one (18) of said tertiary reflectors (18, 20) and for transmitting a second portion of the energy incident from the secondary reflector (14) to the other (20) of said tertiary reflectors (18, 20), and wherein the tertiary reflectors (18,20) focus the energy onto a pair of image planes. The system of claim 1, characterized in that said beamsplitter element (16) is positioned at the aperture stop of the system (10). The system of claim 1 or 2, characterized by having a field of view of about 40 degrees of elevation by 53 degrees azimuth. The system of any of claims 1 - 3, characterized in that the area equivalent optical speed is about F/2.3. The system of any of claims 1 - 4, characterized in that the system comprises a detector array assembly (22, 24) at said pair of image planes for receiving respective first and second portions of energy from the pair of tertiary reflectors (18, 20). The system of claim 5, characterized in that said detector array assembly (22, 24) includes a first detector (22) having a first detector surface for receiving the first portion of energy, and includes a second detector (24) having a second detector surface for receiving the second portion of energy, the respective detector surfaces being in planes parallel to each other. The system of claim 6, characterized in that the first and second detector surfaces face away from each other. The system of any of claims 5 - 7, characterized in that the primary mirror (12) is positioned to receive and reflect energy from a scene to be viewed; said primary and secondary mirrors (12, 14) form a real image of said virtual entrance pupil at said aperture stop; said pair of tertiary mirrors (18, 20) are positioned to receive energy from said secondary mirror (14) and reflecting and focusing energy to a viewing position; and in that said detector array assembly (22, 24) is positioned at the viewing position for receiving the energy from each tertiary mirror (18, 20). The system of any of claims 5 - 8, characterized in that the system comprises a cooler unit (26) connected to the detector array assembly (22, 24). The system of any of claims 1 - 9, characterized in that the beamsplitter element (16) is adapted to reflect a visible range of light energy and transmit infrared light energy. The system of any of claims 1 - 10, characterized in that the beamsplitter element (16) has at least one aspherical surface (16a, 16b). The system of any of claims 1 - 11, characterized in that said primary mirror (12) is a negative power mirror. The system of any of claims 1 - 12, characterized in that said secondary mirror (14) is a positive power mirror. The system of any of claims 1 - 13, characterized in that said tertiary mirrors (18, 20) are positive power mirrors. The system of claim 1, characterized in that said beamsplitter element (16) is of aspherical configuration; and in that the system comprises a detector (22, 24) being in optical communication with at least one tertiary reflector (18). The system of any of claims 15, characterized in that said detector (22, 24) is adapted to receive infrared energy. The system of any of claims 1 - 16, characterized in that said reflectors (12, 14, 18, 20) and beamsplitter element (16) are positioned on-axis in aperture and off axis in field. The system of claim 15, 16 or 17, characterized in that the system comprises a cooler unit (16) which is sandwiched by a pair of detector surfaces on the detector (22, 24), in that one of the tertiary reflectors (18) provides optical communications to one detector surface while the other tertiary reflector (20) provides optical communications to the other detector surface. A head-mounted viewing system (10) comprising the reflective optical system of any of claims 1 - 18, characterized in that the head-mounted viewing system further comprises a support member (28) adapted to be mounted on the head of a user; a reflective optics system supported on the support member (28), including, said primary and said secondary reflectors (12, 14), said pair of tertiary reflectors (18, 20) and said beamsplitter element (16); said tertiary reflector elements (18, 20) being supported on the support member (28); and a display unit supported on the support member (28) and connected to the first and second detector surfaces for displaying the detected portions of energy to the user.

Inventors: COOK LACY G, COOK, LACY G.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1998
Main classification: G02B17
IPC codes: G02B27, G02B17, G02B13