Abstract:
An apparatus includes first and second portions. The first portion has optics which cause first radiation within a selected waveband to travel along a path of travel and to have a selected field of view. The second portion introduces second radiation within the selected waveband into the field of view, without any significant degradation of a transmission efficiency of the first radiation along the path of travel. The second radiation then travels with the first radiation along the path of travel.

Description:
This application claims the priority under 35 U.S.C. §119 of provisional application No. 60/552,263 filed Mar. 10, 2004. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to techniques for combining optical information and, more particularly, to techniques for combining optical information within an optical sight. 
     BACKGROUND OF THE INVENTION 
     Optical sights are used for various purposes, one example of which is mounting a sight on a weapon in order to help a user accurately aim the weapon. The optical sight takes image information from a distant scene, and presents this image information within a field of view which is visible to the eye of a user. 
     In some situations, it is desirable to be able to provide some supplemental information within this same field of view, such as alphanumeric information generated by circuitry within the sight. For example, the sight might include a laser rangefinder which can determine a distance to a target, and then generate alphanumeric indicia representing this distance. One possible approach for showing supplemental information of this type to a user would be to present it on a display which is physically and spatially separate from the sight&#39;s optical field of view. However, this would force the user to take his or her eye off the target or scene in order to observe the supplemental information. 
     A different approach would be to use an image detector to digitize the optical field of view containing information from the scene, then use a microprocessor to digitally combine this information with the supplemental information, and then display the combined information on a digital display such as a color liquid crystal display (LCD). However, existing full-color LCDs are sometimes difficult to see in direct sunlight. Further, in the event of a battery power loss, the entire sight becomes non-functional. 
     Still another possibility would be to use a wideband beam splitter to inject the supplemental information into the sight&#39;s optical field of view. However, this would cause a significant portion of the brightness of the sight&#39;s optical image to be lost. As a result, there would be a significant degradation in the transmission efficiency of this radiation, which in turn would significantly reduce the utility of the sight in low light conditions, such as at dawn and dusk. 
     SUMMARY OF THE INVENTION 
     One form of the invention involves causing first radiation within a selected waveband to travel along a path of travel and to have a selected field of view, while introducing second radiation within the selected waveband into the field of view without any significant degradation of a transmission efficiency of the first radiation along the path of travel, the second radiation thereafter traveling with the first radiation along the path of travel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of the present invention will be realized from the detailed description which follows, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagrammatic view of an apparatus which is an optical sight for a weapon, and which embodies aspects of the present invention; 
         FIG. 2  is a diagrammatic depiction of a circular field of view for image information embodied in radiation which reaches an eyepiece of the sight of  FIG. 1 ; 
         FIG. 3  is a diagrammatic fragmentary sectional side view of a portion of the sight of  FIG. 1 , in a significantly enlarged scale; 
         FIG. 4  is a graph depicting a curve representing a filter characteristic for a filter which is a component of the sight of  FIG. 1 ; and 
         FIG. 5  is a graph for an alternative embodiment of the sight of  FIG. 1 , and includes a curve similar to the curve shown in  FIG. 4 , as well as two additional curves representing additional filter characteristics. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a diagrammatic view of an apparatus which is an optical sight  10  for a weapon, and which embodies aspects of the present invention. For example, the sight  10  could be mounted on a rifle, in order to assist a user in aiming the rifle at a target within a distant scene.  FIG. 1  does not depict all of the structure of the sight  10 , but only selected components which facilitate an understanding of the present invention. 
     The sight  10  has a housing, which is represented diagrammatically in  FIG. 1  by a broken line  11 . A broken line  12  represents a path of travel of visible radiation which embodies an image of a remote scene  13 , the radiation traveling from the scene  13  through the sight  10  to an eye  14  of a user. The scene  13  could be any of a wide variety of different things, and is therefore depicted diagrammatically in  FIG. 1  by a broken line. 
     The sight  10  has an objective lens doublet  16 , and two removable lenses  17  and  18  that determine the magnification of the sight  10 . The sight  10  also has a prism assembly which includes three prisms  21 - 23 . The prisms  21 - 23  have surfaces  31 - 35 , and each of these surfaces has at least a portion covered by a reflective coating of a type which is well known in the art. For clarity, the coatings are not separately shown in  FIG. 1 . Radiation from the scene  13  which is propagating along the path of travel  12  passes successively through the lens doublet  16  and the lenses  17 - 18 , and then successively through the prisms  21 - 23 , while being successively reflected at each of the surfaces  31 - 35 . 
     The sight  10  also has a lens assembly  41 , and a lens  42 . After exiting the prism  23 , radiation propagating along the path of travel  12  passes successively through the lens assembly  41  and lens  42 , and then travels to the eye  14  of the user. 
     A broken line  51  diagrammatically represents a fusion section  51  of the sight  10 . The fusion section  51  generates some radiation which embodies image information, and this radiation then travels along the path of travel  12  from the prism  23  through the lens assembly  41  and lens  42  to the eye  14  of the user. The purpose and function of the fusion section  51  will be explained in more detail below, with reference to  FIG. 2 . 
     More specifically,  FIG. 2  is a diagrammatic depiction of a circular field of view (FOV) for the image information embodied in the radiation that reaches the eye  14  along the path of travel  12 . This image information includes an image of the scene  13 . In addition, this image information includes two lines of alphanumeric indicia  61  and  62 , which are superimposed on the image of the scene  13  by the fusion section  51  of  FIG. 1 , in a manner which is described in more detail later. In addition, the image information shown in  FIG. 2  includes a selected reticle  64 . The reticle  64  is superimposed on the image of scene  13  by the optics of the sight  10 , in a manner which is known in the art and therefore not described here in detail. 
       FIG. 3  is a diagrammatic fragmentary sectional side view of a portion of the sight  10  of  FIG. 1 , in a significantly enlarged scale. In particular,  FIG. 3  shows the fusion section  51 , and an adjacent portion of the prism  23 . As mentioned earlier, the surface  35  of the prism  23  is at least partially covered by a reflective coating of a known type, and this coating is shown at  71  in  FIG. 3 . In the disclosed embodiment, the reflective coating  71  is a thin layer of aluminum or silver. However, it could alternatively be implemented in any other suitable manner.  FIG. 3  shows an opening  73  through the coating  71 . In the disclosed embodiment, the opening  73  is formed by selectively etching the coating  71 , but the opening  73  could alternatively be formed in any other suitable manner. 
     The fusion section  51  includes a glass plate  76 , which is supported adjacent the coating  71  so as to cover the entire opening  73 . The fusion section  51  also includes a multi-layer thin-film filter  78 , which is formed on the side of the glass plate  76  that faces the coating  71 . 
     In more detail, in the disclosed embodiment, the filter  78  includes a plurality of thin layers of different materials, which are selected and ordered so that the filter  78  has certain specific properties with respect to radiation which impinges on the filter  78  in a specified direction. In particular, the filter  78  is transmissive to radiation having wavelengths within a narrow passband of approximately 4 nm, with a center wavelength of approximately 630 nm. The filter  78  is highly reflective to other visible radiation with wavelengths outside this passband. The passband is within the spectrum of visible light and, as seen by the human eye, is effectively one shade of red from the section of the visible spectrum that contains a variety of different shades of red. However, the invention is not limited to radiation corresponding to the color red, and the passband could alternatively be selected from some other portion of the electromagnetic spectrum. Further, although the disclosed embodiment uses a multi-layer filter of a known type, it would alternatively be possible to use any other suitable type of filter structure. 
     Although the disclosed embodiment has a passband with a width of 4 nm, the passband could alternatively have some other width. For example, in some applications the advantages of a relatively narrow passband such as 4 nm may justify the added manufacturing cost, whereas for other applications a wider passband such as 8 nm may be adequate, especially where it can be manufactured at a lower cost. 
     The following is one exemplary prescription for the specific filter  78  which is used in the disclosed embodiment, using a notation form which is well-known to those skilled in the art:
         AIR 0.91227D 0.78233Q 0.18881D (0.429D 0.901Q 0.429D)11 (0.338D 0.75Q 0.358D)11 0.57796D 0.70852Q 0.3663D 1.04079D (1.09273Q 1.04079D)4 1.04079D (1.09273Q 1.04079D)9 5.20393D (1.09273Q 1.04079D)9 9.36708D (1.09273Q 1.04079D)9 5.20393D (1.09273Q 1.04079D)9 1.04079D (1.09273Q 1.04079D)4 0.31224D 1.42055Q GLASS
 
The foregoing prescription assumes that the prism  23  is made from a glass material having a refractive index of 1.52, and the prism is assumed to be sufficiently thick so that the opposite side of the prism can be effectively ignored. The prescription is configured for random polarization, with incidence on the filter in glass at 22.5°, and with incidence on the filter in air at 35.6° (and then exiting into the glass prism).
       

     In the prescription, each “D” and “Q” represents a respective layer with an optical thickness of one quarterwave at normal incidence for the design wavelength of 630 nm. The number preceding each “D” or “Q” is a coefficient representing a thickness adjustment. The number after each set of parentheses represents the number of times that the layer or sequence of layers within the parentheses is repeated. When the repeated layers within all of the parentheses are expanded, the disclosed design has a total 142 layers. The “D” layers have a refractive index of 2.1 and can, for example be implemented with tantalum pentoxide. The “Q” layers have a refractive index of 1.444 and can, for example, be implemented with silicon dioxide. The exact values may vary slightly in dependence on fabrication considerations such as the method of deposition, residual gases and rates of deposition. Alternatively, other high-index coating materials could be used with similar scalable results, including niobium pentoxide, zirconium oxide, and/or titanium dioxide. 
     Breaking the layers of the foregoing prescription into four optically sequential groups, the layers in the following group serve to reflect blue and green light:
         0.91227D 0.78233Q 0.18881D (0.429D 0.901Q 0.429D)11 (0.338D 0.75Q 0.358D)11 0.57796D 0.70852Q 0.3663D.
 
Next, the following layer serves as an impedance matching layer:
   1.04079D.
 
Then, the layers in the following group serve to define the bandpass filter that passes red light at the selected 630 nm wavelength and that reflects green yellow and deep red:
       

                                                         (1.09273Q    1.04079D) 4    1.04079D   (1.09273Q            1.04079D) 9    5.20393D   (1.09273Q    1.04079D) 9            9.36708D   (1.09273Q    1.04079D) 9    5.20393D           (1.09273Q    1.04079D) 9    1.04079D   (1.09273Q            1.04079D) 4.                            
Finally, the layers in the following group serve as impedance matching layers:
         0.31224D 1.42055Q.
 
If the glass material of the prism has a refractive index other than 1.52, the two groups of impedance matching layers would need slight changes. It is emphasized that the foregoing prescription for the filter  78  is merely one possible way of implementing the filter  78 . The invention encompasses this approach, as well as any other suitable approach.
       

     Still referring to  FIG. 3 , the fusion section  51  includes a liquid crystal display (LCD)  81 , which is supported adjacent a side of the glass plate  76  opposite from the filter  78 . In addition, the fusion section  51  includes a backlight  82 , which is supported adjacent the side of the LCD  81  opposite from the glass plate  76 . In the disclosed embodiment, the LCD  81  is a device of a known type, and has a two-dimensional array of pixels with a resolution of ¼ VGA (or in other words one-fourth as many pixels in each direction as a display conforming to the Video Graphics Array industry standard). 
     The LCD  81  generates an image which is the alphanumeric indicia shown at  61  and  62  in  FIG. 2 . In this regard, at any given point in time, the portions of the LCD  81  which correspond to displayed alphanumeric characters are transmissive to visible radiation, while other portions of the LCD  81  are opaque. The backlight  82  emits radiation with a wavelength of approximately 630 nm, and this radiation passes through the LCD  81  so that the alphanumeric image being generated by the LCD  81  becomes embedded in the radiation. This radiation then passes through the glass plate  76  and the filter  78 , and into the prism  23 , as indicated diagrammatically by two arrows  86  and  87  in  FIG. 3 . 
     In  FIG. 3 , arrow  91  represents a portion of the radiation from the scene  13 , which impinges on the reflective coating  71  in a direction that forms a small angle with respect to a not-illustrated line perpendicular to the plane of the coating  71 . The coating  71  is highly reflective to all radiation in the visible spectrum which impinges on it in this direction, and thus reflects substantially all of the incident energy of all wavelengths of visible light within the radiation  91 , as indicated diagrammatically at  92 . 
     Arrow  93  represents other radiation from the scene  13 , which is traveling parallel to the radiation  91 , and which impinges on the filter  78  rather than on the coating  71 . The filter  78  is highly reflective to virtually all radiation in the visible spectrum which impinges on it in this direction, except for radiation within the filter&#39;s narrow 4 nm passband, which is centered at a wavelength of 630 nm. Consequently, as indicated diagrammatically by the arrow  94 , the filter  78  reflects substantially all of the incident energy of all wavelengths of visible light within the radiation  93 , with the exception of radiation having wavelengths within the  4  nm passband of approximately 628 nm to 632 nm. 
       FIG. 4  is a graph that helps to explain the operation of the filter  78 . The horizontal axis represents wavelength, increasing from left to right. The broken line  101  represents the spectrum which is visible to the human eye, and which runs from approximately 400 nm to approximately 700 nm. The curve  106  represents the transmission characteristic of the filter  78 . 
     In particular, it will be noted that the filter  78  is generally reflective rather than transmissive throughout the entire visible spectrum  101 , except for a spike  111  that represents a high transmissivity for wavelengths within a narrow passband  112  of approximately 4 nm, which in this embodiment is centered at a wavelength of 630 nm. The filter  78  also happens to be transmissive to infrared (IR) radiation, which is permissible and does not affect the operation of the filter  78  within the visible spectrum that is of interest. In fact, partial or full IR transmissivity of the filter  78  could be advantageous in a situation where an IR laser rangefinder and/or a near IR laser pointer is incorporated into the sight  10 . 
     Still referring to  FIG. 4 , it will be understood that radiation from the scene  13  which impinges on the filter  78  will virtually all be reflected, except for a very small portion thereof which falls within the passband  112 . Therefore, a very small portion of the red light part of the visible spectrum will effectively be extracted out of the radiation from the scene  13  which reaches the filter  78 . Of course, this is an extremely small portion of the total energy represented by the area under the curve  101  in  FIG. 4 . In fact, as a practical matter, the human eye  14  ( FIG. 1 ) will be effectively unable to detect that any radiation is missing from the image of the scene  13 . Stated differently, the integral of the energy under the photopic/scotopic response curve  101  less the portion of this energy which is within the passband  112  is not detectably different to a user (or at least is not distractingly different to the user) than the integral of the energy under the photopic/scotopic response curve  101 . In other words, the ratio of these two values is approximately 1. 
     On the other hand, and as discussed above, the backlight  82  emits radiation which falls within the passband  112 . Consequently, this radiation passes through the LCD  81  and the filter  78 , and then in effect is optically combined with or superimposed on the visible radiation which is outside the passband  112 , and which arrives at  93  and is reflected at  87 . 
     Persons skilled in the art will recognize that the curve  106  of  FIG. 4  is somewhat idealized. The curve  106  is presented in idealized form in order to easily convey an accurate understanding of the principle of the invention. Persons skilled in the art will recognize that specific implementations of the filter  78 , such as the exemplary prescription discussed above, will typically have transmittance curves that approximate the idealized curve  106 , but are not completely identical to it. For example, a given implementation of the filter  78  may have a transmittance for infrared radiation which is not uniform throughout the infrared spectrum, but instead varies somewhat. 
     In an alternative embodiment of the sight  10  of  FIG. 1 , which is not separately illustrated, there are three fusion sections of the type shown at  51  in  FIGS. 1 and 3 . In particular, radiation traveling from the scene  13  to the eye  14  is reflected successively off three surfaces, each of which is associated with a fusion section similar to the fusion section  51 . These three fusion sections would differ in that each would be configured for use with a different passband. 
     More specifically,  FIG. 5  is a graph which includes all of the same information shown in  FIG. 4 , and which also has two additional curves  201  and  202 . The curves  201  and  202  depict respective filter characteristics that are highly reflective to visible radiation, except within respective different passbands  206  and  207 , where they are highly transmissive to visible radiation at selected wavelengths. In particular, the passband  206  has a center wavelength of approximately 440 nm, and a width of approximately 4 nm. This passband is within the spectrum of visible light and, as seen by the human eye, is effectively one shade of blue from the section of the visible spectrum that contains a variety of different shades of blue. Similarly, the passband  207  has a center wavelength of approximately 510 nm, and a width of approximately 4 nm. This passband is within the spectrum of visible light and, as seen by the human eye, is effectively one shade of green from the section of the visible spectrum that contains a variety of different shades of green. Thus, each of the three fusion sections in the alternate embodiment could superimpose information in a respective different color onto the radiation from the scene,  13 . 
     In still another alternative embodiment, which is not separately depicted in detail, the sight  10  would include a single fusion section similar to that shown at  51  in  FIG. 3 . However, the filter  78  would be replaced with a filter which is configured to define two or three separate passbands at respective different wavelengths that correspond to different colors. Further, the LCD  81  would be replaced with a multi-color LCD that can produce two or three colors of radiation which each fall within a respective one of the passbands of the filter. As still another alternative, the LCD  81  and backlight  82  of  FIG. 1  could both be replaced with a light emitting diode (LED) display, which could be a single color display in the case of a filter with a single passband, or which would be a multi-color LED display in the case of a filter with two or more distinct passbands. 
     The disclosed embodiments permit radiation at one or more specific wavelengths to be efficiently combined with or superimposed on other radiation, such as that from a scene, without substantially degrading the overall transmission efficiency of the latter. This allows the optical sight to be readily used when the display is off or when a battery for operating the display is discharged, or in low light situations such as at dawn or dusk. By effectively wavelength division multiplexing supplemental information into the field of view, the efficiency of obtaining sufficient contrast of the supplemental information relative to the sight&#39;s field of view is maximized, thereby lowering the necessary emission brightness of the light source which generates the supplemental information, and thus the power requirements of that light source, which in turn maximizes battery life in portable configurations. 
     Although selected embodiments have been illustrated and described in detail, it will be understood that various substitutions and alterations are possible without departing from the spirit and scope of the invention, as defined by the following claims.