Patent Publication Number: US-11032485-B2

Title: Optical assembly for superimposing images from two or more sources

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 14/961,377, filed Dec. 17, 2015, which application claims priority under 35 USC § 119(e) to U.S. Provisional Patent Application 62/089,326 filed Dec. 9, 2014, the content of which is incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     This document concerns an invention relating generally to superimposing images from two or more sources, and more specifically to systems and methods for importation of video imagery into optoelectronic devices (such as night vision goggles) for superimposition of imported imagery with imagery of the optoelectronic devices, and exportation of superimposed images to other devices. 
     BACKGROUND OF THE INVENTION 
     Night vision goggles enhance the ability of users to see in low-light conditions. It is often useful to inject additional imagery into images of a night vision goggle to provide the user with more information. This allows the user to simultaneously view the night vision images along with the imported imagery. The imported video imagery can be imagery from thermal detectors, commonly called thermal cameras, or other imagery such as a map, compass, training video, etc. Such a system that contains night vision imagery along with injected imagery is often called a “fused” system. 
     Referring initially to  FIG. 1 , a prior art fused system  10  includes a night vision goggle (with an image intensifier (“I2”) channel)  12  and a clip-on thermal imager (with a thermal channel)  14 . The image intensifier (“I2”) channel  12  consists of an objective lens  16  for collecting light from the night vision scene  18 , an image intensifier tube  20  for the detection, amplification, and redisplay of the night vision scene  18  for an observer  20 , and an eyepiece lens  22  for optical collimation of the redisplayed image so the eye can properly focus. The thermal channel  14  consists of a thermal lens  24 , thermal detector  26 , electronics  28 , and a means of injecting or importing the thermal imagery into the objective path of the image intensifier channel (discussed below). This approach is discussed in U.S. Pat. Nos. 7,842,921, 7,755,047, and 7,541,581. For simplicity the lenses are shown as a simple lens element but it should be understood that the lenses are composed of several elements. The I2 tube  20  includes a tube light emitting surface  21 , and is encapsulated in a plastic or aluminum housing  40 . The whole assembly is commonly called a night vision goggle, or goggle for short. The human observer  20  views the image intensifier through the eyepiece. 
     The thermal channel  14  is an independent assembly which is mechanically mounted (using clip-ons or clamps) externally to the I2 goggle  12  and feeds an image into the objective of the I2 goggle  12 . The thermal channel  14  has its own optics and power sources. It consists of the thermal objective lens  24  for collecting the thermal imagery from a thermal scene  19 , the thermal detector  26  for detection and conversion to an electronic image, and electronics  28  for converting the electronic image into either digital or analog video. A hard wire link  42  sends the video to a display  44  via a flex circuit and other electrical hardware, where the electronic image is converted to a light image that can be observed. The thermal image is imported or injected into the I2 channel  12  via cable  46 . Thus the thermal image and the night vision image are simultaneously imaged onto the I2 tube  20 . The I2 tube  20  then amplifies both the injected image and the night vision image together and displays them on the output of the tube  20  through the eyepiece  22 . 
     This approach suffers from multiple shortcomings. First, the thermal channel imagery has the same green color as the I2 imagery. Ideally, a viewer can distinguish targets in the I2 and thermal channels by having a color contrast, or differences in brightness and resolution between the two images. Because the color in the two channels is identical, a person has to rely on brightness and resolution to distinguish targets in the two channels. But if the brightness of the two channels is nearly equal, then the thermal channel may not be distinguished and targets missed. Second, the thermal brightness is determined by the tube gain. As the night vision scene becomes brighter, automatic brightness control circuitry of the image intensifier starts reducing the tube gain and thus thermal brightness contrast is again reduced. To overcome this weakness manual brightness adjustment can be added to the thermal display. But manual brightness adjustment would not be practical, as many mission scenarios have rapid scene brightness changes and the soldier using the goggle does not need the distraction of constantly adjusting the thermal channel brightness. Third, this method cannot provide export of the image intensified image. That is only possible by gathering the imagery from the output of the tube. Export of the I2 imagery is sometimes desirable to allow (for example) recording of video from a mission for training purposes. What is needed is an approach that overcomes these and other shortcomings of traditional approaches. 
     SUMMARY OF THE INVENTION 
     The invention, which is defined by the claims set forth at the end of this document, is directed to optical assemblies for superimposing images from two or more sources which at least partially alleviate the aforementioned problems. A basic understanding of some of the features of preferred versions of the invention can be attained from a review of the following brief summary of the invention, with more details being provided elsewhere in this document. To assist in the reader&#39;s understanding, the following review makes reference to the accompanying drawings (which are briefly reviewed in the “Brief Description of the Drawings” section following this Summary section of this document). 
     This invention relates to a new approach for importing video imagery into a night vision device (or other optoelectronic device) and/or exporting imagery from the night vision device. The night vision device may be a binocular system such as the ANVIS goggle worn by aviators for pilotage at night, or a monocular such as the AN/PVS-14 goggle worn by infantry men for terrain or building navigation and short range enemy engagement, or rifle sights for longer range enemy engagement. Imagery is inserted into the optical train of the goggle such that both the night vision scene from the goggle and the injected imagery can be simultaneously observed. The combined imagery of the night vision scene can be transmitted to external systems for observation purposes, such as active mission feedback. 
     Referring to  FIGS. 2 and 3 , an exemplary optical assembly  100  and method for superimposing/combining images from at least two image sources includes a non-contact receiver  102  for wirelessly receiving a first image (generated externally), and a display  104  for displaying the first image. The assembly  100  can be positioned between an eyepiece  108  and an image tube  110  of an optoelectronic device  112  (such as a night-vision goggle). The display  104  (and its resultant image) is at 90 degrees with respect to the tube image plane. A 45-degree semi-transparent (i.e., partially reflective, partially transmissive) mirror  106  is positioned to intercept light from the display  104 , allowing some of the light to pass therethrough and some of the light to reflect there off. Some light from the display  104  (which displays the first image) hits a mirror first side  116  and is reflected into the optical path of the eyepiece  108 . Some light from a second image (generated internally, passing through objective lens  109  and image tube  110 ) approaches the mirror  106  from a mirror second side  120  and is transmitted through the mirror  106  to reach the eyepiece  108 . The first image is superimposed on the second image for viewing at the eyepiece  108 . Optionally, a camera  122  can be included to capture the combination of transmitted light from the first image (hitting the mirror first side  116  and passing therethrough) and reflected light from the second image (hitting the mirror second side  120  and reflecting into the optical path of the camera  122 ). The combined image captured by the camera  122  can be wirelessly sent via a camera transmitter  127  to a camera receiver  128 , which can send to a transmitter  129  for transmission of the combined image as captured by the camera  122  to another device. 
     The display  104  and fold mirror  106  are positioned such that a display-to-eyepiece portion  114  of the first image hits the mirror first side  116  and is reflected to the eyepiece  108 , and a tube-to-eyepiece portion  118  of the second image hits the (opposing) mirror second side  120  and passes through the mirror  106  to reach the eyepiece  108 . The camera  122  is positioned in the optical assembly  100  such that a display-to-camera portion  115  of the first image hits the mirror first side  116  and passes through the mirror  106  to reach the camera  122 , and a tube-to-camera portion  126  of the second image hits the mirror second side  120  and is reflected to the camera  122 . The camera  122 , which includes a camera lens  125  and a camera sensor  131 , is thus positioned to receive a superimposition of light from the display  104  that passes through the mirror  106 , and light from the image tube  110  that reflects off the mirror  106 . The transmitter  129  allows wireless transmission of images out of the goggle. A brightness detector  130  is preferably included to allow the adjustment of brightness level of the display  104  depending on the brightness of the image from the tube  110  in case one image would overwhelm the other. The brightness detector  130  allows for the automatic adjustment of brightness level via electronic circuitry or software. 
     It is noted that the light rays  114 ,  115 ,  118 ,  126  are intended to be illustrative representations of the paths of light that are reflected and transmitted from the display  104  and the tube  110 . The line style indicates the source of the light, such that matching line styles are from the same source; here, the lines with the finer dots ( 114 ,  115 ) are from the display  104 , and the lines with the longer dots ( 118 ,  126 ) are from the tube  110 . Analogously, the arrowheads represent which images are superimposed, such that matching arrowheads indicate the lines that are superimposed. Here, the chevron arrowheads ( 114 ,  118 ) are associated with the images that are superimposed at the eyepiece  108 , and the solidly triangular arrowheads ( 115 ,  126 ) are associated with the images superimposed at the camera  122 . 
     The optical assembly  100  can be a separate module (“insert”) that is installable in an existing goggle (such as a night vision goggle or other optoelectronic device) when inserted between its image tube  110  and eyepiece  108  to retrofit the goggle. In such a retrofitted goggle, the first image would be received via the non-contact receiver  102 , and the second image would be generated by the goggle itself. Preferably, retrofit versions of the insert  100  (i.e., versions configured to be installed in an existing goggle) include a power interface that allows the optical assembly  100  to receive power from the goggle in which the insert  100  is retrofitted. This can eliminate the need to include a separate power source for primary power, helping save space, weight, and costs. Retrofitting the insert  100  in the goggle generally involves removal of the eyepiece  108  from the goggle, insertion of the insert  100  between the image tube  110  and the eyepiece  108 , and replacement of the eyepiece  108  on the goggle (see also  FIG. 14 , further discussed below). Rather than being a separate module for retrofitting, the optical assembly  100  can also be an integral part of an optoelectronic device which captures images using an (internal) image source (housed by the optoelectronic device itself). The integral optoelectronic device is able to receive images from a separate (external) image source (such as another optoelectronic device) and combine images from the two image sources for viewing at the eyepiece. 
     Exemplary versions of the present invention can: (i) provide a truly retrofittable solution for previously-fielded optoelectronic devices so that fused imagery may be applied to systems already in use with no or minimal modifications either optically, mechanically, or electrically; (ii) provide color imagery if desired; and (iii) provide for the export of imagery. The optical assembly can be disposed against the already existing tube in the existing space between the tube and eyepiece of the optoelectronic device. For convenience, the optical assembly will be referred to as “insert assembly” or simply “insert”; the optoelectronic device housing the optical assembly will be referred to as the “internal image source,” “primary image source,” “night vision channel,” “night vision device,” “night vision goggle,” or simply “goggle,” and the imagery generated/captured by the goggle will be referred to as the “primary image,” “internal image,” “goggle image,” or “night vision scene.” The external source of imagery (i.e., the optoelectronic or other device capturing or generating images to be imported by the insert, injected into the optical path of the goggle, and superimposed with the google image) will for the sake of convenience be referred to as the “secondary image source,” “external image source,” “thermal detector,” “thermal channel,” “thermal imager,” or “thermal camera.” These terms are used only for convenience and are not intended to limit the scope, functionality, or applicability of exemplary versions of the devices discussed herein. For example, the thermal detector is only one example of an external image source that can be used to augment the imagery of goggles. The optical wavelength for the second source is generic and could be, for example, a second intensifier channel, such as a weapon sight or a short-wave infrared camera. 
     The exemplary versions of the insert  100  depicted in  FIGS. 2 and 3  include a fold mirror with a notch filter, a display, a camera, a projection lens, a non-contact wireless receiver—which serves as a non-invasive means of communication in and out of the goggle (or other optoelectronic device)—circuit cards, mechanical members for retaining the assembly in the goggle, a photo-sensor for measuring the tube brightness, and a flex cable for obtaining electrical power from the existing goggle system. Retrofitability to existing goggles is particularly enhanced by the non-invasive means of communication, the method of obtaining power from the goggle, and the mechanical ability to fit in existing goggles. The mechanical means of insertion will also provide the basic mechanics to rough-align the display to the tube, but final alignment can be left under the control of the user. When assembled in an existing goggle, the insert provides the means of communication with a thermal channel that can be clipped on to allow (for example) the thermal detector to generate imagery of the same area that is being imaged by the night vision channel. A communications adaptor can be clipped on for picking up the camera signal from the thermal detector, or entirely wireless communication with an external camera processing unit can be implemented. Color imagery and export imagery can be accomplished by the fold mirror into the eyepiece section of the I2 optical channel. 
     Further advantages and features of the invention will be apparent from the remainder of this document in conjunction with the associated drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a prior art clip-on thermal device which uses a cable to feed images to the objective lens of a night vision goggle. 
         FIG. 2  represents an optoelectronic device retrofitted with an exemplary optical assembly (“insert”). 
         FIG. 3  depicts the use of wireless communication in general in an exemplary optical assembly. 
         FIG. 4  is an end view of an exemplary insert in night vision goggles with eyepiece removed, showing a wireless transceiver  405 , mirror mounts  410 , display  415 , wireless receiver  420 , wireless transmitter  425 , tube screen  430 , tube envelope  435 , and camera  440 . 
         FIG. 5  represents the use of a fiber optic extender for the display for maintaining focal position in an exemplary optoelectronic device. 
         FIG. 6  represents the use of radio frequency wireless communication in an exemplary optoelectronic device. 
         FIG. 7  represents the use of capacitively coupled wireless communication in an exemplary optoelectronic device. 
         FIG. 8  depicts the use of two fold mirrors with a display that is secured to an electronics card in an exemplary optoelectronic device. 
         FIG. 9  depicts an exemplary snap fit retention mechanism for mounting the exemplary optical assembly in an optoelectronic device. 
         FIG. 10  depicts an exemplary diffractive optical element in an optoelectronic device. 
         FIG. 11  is a close-up view of the exemplary diffractive optical element of  FIG. 10 . 
         FIG. 12  depicts design details of exemplary diffractive elements that match RGB of the display. 
         FIG. 13  shows exemplary flat and curved diffractive elements that could be used in the optoelectronic device of  FIG. 10 . 
         FIG. 14  is an exemplary flowchart with typical steps involved in retrofitting a night vision goggle with the exemplary optical assembly. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED VERSIONS OF THE INVENTION 
     The exemplary version of  FIG. 2  is a (retrofittable) optical assembly  100  that is inserted into an I2 goggle located between the image intensifier tube  110  and the eyepiece  108 , and is mounted on a tube surface  132  mechanically held by the side walls of the goggle body  134 . For use as a retrofit kit, reference is made to  FIG. 14 . To effect the retrofit ( 1400 ) in common night vision goggles, the eyepiece is removed from the goggle ( 1410 ), as are the tube retention ring ( 1420 ) and low battery indicator and image intensifier tube ( 1430 ). If the goggle has a light pipe, it is removed as well before the image tube is removed. To install the kit, the power flex circuit is inserted into the goggle so as to contact the power source of the goggle ( 1440 ) by pressing the flex against the power contacts. The intensifier tube ( 1450 ), electronics card ( 1460 ), and fold mirror (if separate from the electronics card) ( 1470 ) are also inserted. The optical assembly is thus inserted into the goggle and placed on the output of the tube. If the mechanical mounting features are independent of the electronics card these are now reinstalled. Optical alignment between the I2 and thermal channel may be made at this point. The fold mirror is aligned with the display of the thermal detector ( 1480 ) and the eyepiece is replaced ( 1490 ) on the goggle. 
     The optical assembly  100 , shown close up in  FIG. 3 , includes a plastic frame  136  that holds display  104 , thin fold mirror  106 , and CMOS or CCD camera sensor  131 . The frame  136  may also contain an electronics card  138 ; the plastic frame  136  can be mounted on the electronics to make the electronics card  138  a mechanical member for the optical assembly  100 . The electronics card  138  preferably has one or more I/O section(s) for communicating through the goggle body using one or more of the non-contact communication mechanisms (further discussed below). For importing imagery into the goggle from (for example) a thermal channel  205 , such as a clip-on on thermal imager, the thermal channel  205  has a wireless transmit unit  210  and an electronics card  215 . The wireless transmit unit  210  may be placed physically opposite the receiver  102  for best signal transfer to help implement (for example) an electro optical method or capacitively coupled method of communication (further discussed below); a small pigtail (“to thermal camera”) may be needed to route the signal to the proper location on the goggle body. If an RF method is used for the wireless transmission (see  FIG. 6 ) and the power is low enough, the wireless transmission can be made through the goggle body to the receiver, making the pigtail unnecessary. 
     For the CMOS/CCD camera output, the order is reversed, i.e. the transmitter  127  is inside the goggle whereas the receiver  128  is exterior to the goggle body. This output can be routed to a transmitter  129  on the helmet mount. A photo-detector  130  can be mounted proximal to the display  104  or tube output screen  150  to measure the tube output. The output of the photo-detector  138  is routed to a display brightness control loop so that the display brightness approximates or matches the tube brightness. Finally, a thin flex circuit is mounted to the electronics card  138  to receive power from the goggle. 
     Optically, the frame holds the display  104  and camera  122  in the same focal plane as the output of the image intensifier tube  110 . As an insert kit, the optical assembly  100  can be free to rotate about the optical axis for angular alignment of the display  104  and camera  122  to the I2 imagery. (It is also free to be inserted and removed without major disassembly of the goggle.) Rotational alignment can be accomplished by one of at least two means: (1) the mirror/display/camera assembly can be rotated relative to the electronics card as power and signals can be provided by a thin flex circuit; or, preferably, (2) the whole optics and electronics card can be rotated in the goggle body. This is in contrast to prior art goggles in which components are part of the goggle, in which case alignment takes place in the manufacturing process; this adds costs and increases production and yield risks. When components are part of an insert, those risks do not come into play. The flexibility of rotation allows both the display and camera to be located optically in the proper place, while at same time permitting the display or the camera to be located so there is no physical interference with the eyepiece. If, for particular goggles, this flexibility does not permit installation without interference, the display or camera may optionally have a fused fiber-optic bundle attached thereto to permit physical relocation of the component while maintaining the proper optical location (as depicted in  FIG. 5 ). The optical principle that the display and tube output must be the same distance from the mirror is also shown in  FIG. 5 , but this principle does not apply to the camera and lens. 
     In illustrated versions, the display is shown to be located 90 degrees relative to the electronics card. While this may be desirable from an optics perspective, it might not be desirable from a display electronics card manufacturing perspective for certain designs, depending on goggle constraints. It thus may be desirable to mount the display flat on the electronics card. To properly present the display image to the eyepiece, an additional fold mirror can be placed over the display to bend the image 90 degrees.  FIG. 8  illustrates the use of a second fold mirror  805  along with the main fold mirror  104  in relation to the eyepiece  108 . As an additional alternative, a 90-degree fiber optic can be bonded to the display (not pictured). In either case, the display is optically rotated 90 degrees to present the image properly to the fold mirror and eyepiece. 
     The non-contact communications of the optical assembly can involve any electromagnetic wireless technology involving, for example, radio signals, light, magnetic or electric fields, the use of sound, etc. This includes, but is not limited to: 1) optically by, for example, taking advantage of the transmissive properties of plastic; 2) electrically via capacitive or inductive pick-up; and/or 3) radio (RF) communication. Because the display receives an input from the exterior thermal channel and the CMOS/CCD camera exports to the exterior, the communications links are effectively functionally identical: each has a send and receive through the goggle body. For these communication methods, wavelengths/frequencies should be chosen such that each transmit/receive channel will not interfere with other channels. 
     Regarding the first method (optically taking advantage of the transmission properties of the plastic materials of the goggle body), reference is made to  FIG. 3 . Here, the sending unit/camera transmitter  127  is a photodiode (LED) that emits light and a photo sensor, such as a phototransistor, is the wireless receiver  128 . The photodiode  127  can be mounted to the electronics card  152 , and is electrically connected to the display circuitry. The wavelength is chosen to be in a transmission region of the plastic part of the goggle body. There is much flexibility in the sources and receivers that can be used, as long as the fundamental principle of choosing a wavelength to which the goggle body is transmissive is followed. Likewise, the receiver is  128  chosen to sense the emission wavelength of the source. Most plastics have transmissive windows in the near infrared, so a 1.55 micron LED, such as Edmund stock number 5-73, might be suitable in many cases. This diode could be incorporated into the thermal camera, and would transmit into the goggle body where the matching receiver is on the electronics card. The photodiode  127  would also be attached to the electronics card to send the CMOS/CCD video through the goggle body. The match receiver would then be selected and respectively mounted on the electronics card and goggle to receive the signal from the LED. If necessary, a minor modification to the goggle body could be made if the goggle body is not transmissive, such as for a goggle body made of aluminum or high carbon Ultem plastic. The modification would involve forming an aperture (by, for example, drilling a hole) and securing a transmissive window (using, for example, glue or other suitable securing means). The transmissive window would be shaped to the form of the goggle body and be large enough to permit angular rotation of the insert for rough alignment. 
     To use this method, the exterior unit should be optically aligned with the interior source. The thermal camera receiver can be mounted on a “pig tail” that runs to the rest of the thermal camera assembly, which contains the thermal lens, detector, and image processing subsystem. The pigtail can be mounted to the monocular via (for example) a Velcro strip or something with similar functionality. To align with the photo-sensing section of the optical assembly, both the I2 channel and thermal channel are turned on. The pigtail is then moved by the operator until video signal is displayed. Video signal will be displayed when the photo source picks up the signal from the pigtail. It is noted that the thermal channel “sends” and the display assembly “receives,” while with the video camera, the opposite will occur. 
     The capacitive pick up/inductive pick-up and RF communication approaches all function in a similar manner and are represented in  FIGS. 6 and 7 , respectively. A send and receive function occurs on each side of the goggle plastic body, with the source and receiver able to be aligned manually by movement of the exterior component. In  FIG. 6 , the RF approach can involve incorporation of low power RF send  610  and receive  615  circuits into the electronics card. For example, a Bluetooth sending unit can be incorporated in the clip-on thermal channel and a Bluetooth receive unit can be incorporated in the electronics card. Referring to  FIG. 7 , the capacitive pick up method, as a special case of external communication, takes advantage of the dielectric properties of the goggle body  700 . A common type of capacitor is composed of a dielectric sandwiched between two metal plates. A conductive plate or button such as copper film  705 ,  710  can be attached to the interior of the goggle body. The goggle body would function as the dielectric. A similar conductive film  715 ,  720  is placed on the exterior of the goggle body opposite the interior conductor. Thus the goggle behaves as a capacitor. When a video signal is applied to one plate the signal appears on the other plate. Size and materials can be chosen as found suitable for the particular application. 
     Mechanical assembly can be accomplished via a friction fit of the plastic optical frame with the goggle body. The primary intent would be to use the threaded ring that is already used to retain the tube in the goggle. However, if insufficient thread length exists, other means can be employed, such as a reverse dagger snap fit  905 , as depicted in  FIG. 9 , or simply a press fit. In  FIG. 9 , a retaining ring and four threaded fingers  910  snap into the existing retaining ring threads  915 . The optical assembly is an annular cartridge containing a frame for holding the fold mirror, display, and/or camera. Optionally, the frame may hold the circuit card; the circuit card could also be sandwiched between the back of the tube and the optical frame. If the frame holds the circuit card, the frame has a series of mounting ridges. Upon insertion and rotational alignment, the eyepiece is replaced onto the goggle body. For operation with a thermal channel, the thermal channel can be secured to the goggle and the sending unit aligned with the receiving unit by simply adjusting to maximize the signal. For video output the same sort of alignment takes place. The goggle may then be put in service. 
     As suggested above, a photo detector can be placed in the output path from the image tube to control the output brightness of the tube and thermal channel. The photo detector could directly measure the tube brightness and provide a signal to the display electronics that are on the electronics card. The detector may be mounted by the tube or placed after the mirror, as deemed suitable. This provides at least two advantages. First, the tube power supply does not need to be modified, and second, the thermal camera can be completely cut out of the control loop. The video receiving section then transmits camera imagery either by wire or wirelessly to a recorder or display. Both the output section of the thermal channel, or the video receiving unit, transmit and receive to the optical assembly without modification of the goggle, as already suggested. The optical assembly can also be mounted internally to the goggle body, as discussed below. 
     Providing the optical assembly with electrical power is very advantageous for a versatile retrofit into a goggle. Power can be obtained from the existing power contacts. In presently-fielded goggles, the contacts are commonly leaf spring contacts or “pogo” pin contacts. Both methods make electrical contact to the tube power supply by pushing down on the matching contact on the tube. Electrical contact could be made by sandwiching a thin flex circuit between the tube and the spring contacts. The flex circuit would have exposed contacts on both sides so that the spring pins would push against the flex providing power to the circuit card and push the flex against the tube so that power is supplied by the tube. 
     In exemplary versions, the mirror surface can be provided with a neutral density coating to achieve a uniform spectral reflectance of a white output from the display. A notch filter is used where the transmission on the tube optical path is around 90% in the peak wavelength of the tube and zero elsewhere. In the display channel, the reflectance is zero at the wavelength of the tube and around 90% in all other wavelengths. As a consequence the visual imagery of the goggle is not reduced. Thus performance metrics such as resolution, modulation transfer function (MTF), range detection, and reliability are not significantly degraded. The small amount of light removed from the display channel does not significantly impact the white output. Application of such a filter to a night vision goggle in the tube output train is one of the inventive aspects of exemplary versions of the invention. 
       FIGS. 10-13  depict alternative exemplary methods of transmitting the display image into the eyepiece optical path so that an observer can simultaneously view another image source such as an image intensifier tube. This method optically projects image from a display  1005  so that it appears to be approximately coincident with the image intensifier tube image plane. A light pipe  1010  translates the image by internal reflection. In the case of a diffractive element as shown, the display is mounted on flat on its circuit card which is then mounted flat on the tube. A diffractive optical element  1015 ,  1015 A,  1015 B bends the entrance light in such a manner that the image is translated along the light pipe unit it is in the position for exitance. Another diffractive optical element  1020 ,  1020 A,  1020 B is located to rebend the light for exitance. One alternative to the light pipe  1010  is to use a dove prism instead of a diffractive as deemed suitable in particular applications. 
     Referring in particular to  FIGS. 10 and 11 , an input diffractive element  1015  is included on the entrance face of a flat piece of optical glass  1010 . That is, the diffractive surface is placed as an input on the glass. The input diffractive element  1015  bends the light from the display  1005  at such an angle that image is transmitted to the output by total internal reflection (TIR). Preferably, to help maximize efficiency, the diffractive surface is matched to each of the color centers in the display, as illustrated in  FIG. 12 . The image is then transmitted to the center of the tube by total internal reflection. The surfaces of the optical element thus should be optically parallel. Finally, an output diffractive surface  1020  is placed on the output of the element. This diffractive surface breaks up total internal reflection and thus allows the light to exit the element to be observed through the eyepiece  1050 . The output diffractive may be placed on the surface closest to the tube image plane, in which case it functions in reflection mode, or on the surface farthest away, where it acts in transmission mode. Either may be suitable depending on the application, and the selection can be based on efficiency and how close the tube and display image planes must be with respect to each other. 
     The optical glass can have different configurations. For example, the glass can be stepped so the display image is very close to the tube output, as shown on the left side of  FIG. 13 , or it can be stepped with a curvature to match the tube output, as shown on the right side of  FIG. 13 . The output glass can also be made circular so there is no line of demarcation between the intensified and displayed image. 
     Preferred versions of the invention have been described above in order to illustrate how to make and use the invention. The invention is not intended to be limited to these versions, but rather is intended to be limited only by the claims set out below. Thus, the invention encompasses all different versions that fall literally or equivalently within the scope of these claims.