Abstract:
The present invention pretains to a device for protecting the human eye by means of a narrow bandwith interference filter which filters out one or more specific wavelengths of light emitted in the form of a coherent, concentrated beam that is laser radiation and in coherent light, an image intensifier for amplifying an image passing through the narrow bandwidth filter to an observable light level, and a neutral density filter which reduces image reflected illuminance to avoid detection.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to eye protection. In particular, this invention relates to an optical filtering apparatus for protecting the eye against radiation from a laser. 
     2. Description of the Prior Art 
     The increasing use of leaser light beams in military and other applications has underscored the need for protective systems and, particularly eye wear for protecting individuals from retinal damage. One method for dealing with this is the individual. However, due to the fact that such goggles or safety glasses filter out a wide range of wavelengths, the outside world tends to undergo drastic coloration when viewed through such a shield. Further, if protection from a number of wavelengths is desired, the use of a multilayer conventional color filter reduces the quality of the system form marginal to unacceptable. 
     Additionally, while such laser safety eyeglasses or goggles do protect the eye of the user from radiation which essentially comes head on and enters through the protective filters, these safety eyeglasses due leave regions open between the upper edge of the mounting frame and the eyebrows of the user and between the lateral edges of the mounting frame and the temple. When working with a laser beam apparatus, it is quite possible that when the user of such safety eyeglasses drops or rotates the head, laser radiation can gain access to the eye of the user through these unprotected regions causing damage thereto. 
     An alternative to such conventional color filters is the holographic notch filter. Such notch filters, generally, have the desirable characteristic that upon being exposed with light of a given wavelength, in the proper holographic configuration, a very narrow bandwidth effectively reflective surface (which is optically a diffractive surface) will be formed holographically. This reflective surface or “notch filter” comprises recorded interference patterns in a photosensitive material such as dichromated gelatin. Such a holographically exposed gelatin layer will exhibit reflective properties along a very narrow range of wavelengths substantially identical to the recording wavelengths. 
     However, a serious limitation of such systems is the fact that dichromated gelatin is not sensitive to the various laser hazard wavelengths against which one desires to protect. 
     With the-disadvantages inherent in the design of prior art laser protective goggles or safety glasses the present invention was conceived and one of its objects is to provide a laser radiation protection device for the human eye which will not interfere with the forward or peripheral vision of the user. 
     It is yet another object of the present invention to provide a laser eye protection device which is responsive to a broad range of laser frequencies. 
     It is a further object of the present invention to provide a laser eye protection device which is relatively simple in design, convenient to wear and which will not interfere with the normal activities of the wearer. 
     These and other objects of the present invention will become apparent from the following detailed description of the preferred embodiment. 
     SUMMARY OF THE INVENTION 
     The present invention is a device for protecting the human eye by means of a narrow bandwidth interference filter which filters out one or more specific wavelengths of light emitted in the form of a coherent, concentrated beam that is laser radiation, an image intensifier for amplifying an image passing through the narrow bandwidth filter to an observable light level, and a neutral density filter which reduces image reflected illuminance to avoid detection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a perspective view of an observer utilizing the laser protection system constituting the present invention; 
     FIG. 2 is a schematic representation of the laser protection system constituting the present invention; and 
     FIG. 3 is a graph illustrating the operating frequency of various lasers with respect to the laser protection system of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, there is shown a helmet mounted night vision goggle system  11  comprising an optical support structure  13  adapted to be mounted on a helmet  15 , which is worn by an observer  17 . Optical support structure  13  includes a rotatable support bar  19  which has attached thereto a pair of image intensifier tubes  21  and  23  in optical alignment with the eyes  25  of observer  17 . Each image intensifier tube has mounted therein the optical components of a laser protection system  27 , FIG. 2, constituting the present invention. 
     At this time it should be noted that the night vision goggle system  11  selected for use with laser protection system  27  is a Litton, Model M-927, Aviator&#39;s Night Vision System which has a pair of second generation 18 mm microchannel plate image intensifiers for image enhancement. 
     In addition, it should be noted that the second generation image intensifier may be replaced with a third generation image intensifier to upgrade system  11  to a model M-929 Aviator&#39;s Night Vision System. It is also pointed out, however, that any like quality image intensifier can be equally well employed in the goggle system  11 , provided the image intensifier is optically modified as described herein. In fact, any third or successive generation of image intensifier likewise may be employed. 
     Referring now to FIG. 2 there is shown a laser  29  which emits an intense collimated beam of radiant energy  31  of a particular energy level and wavelength along an optical or light path  33 . It should be noted that laser  29  may be an argon laser, a helium neon laser, a ruby laser or any like apparatus for generating a very narrow, intense beam of coherent light. Spatially disposed downstream from laser  29  along optical path  33  is the laser protection system  27  constituting the present invention. Laser protection system  27  comprises a neutral density filter  35 , a narrow-band interference filter  37  spatially disposed downstream from neutral density filter  35  along optical path  33 , and an image intensifier  39  spatially disposed downstream from narrow-band interference filter  37  along optical path  33 . Positioned downstream from filter  37  along optical path  33  is a first lens  41  while there is positioned in front of eye  25  along optical path  33  a second lens  43 , with lenses  41  and  43  being components of image intensifier  39 . 
     Referring now to FIGS. 2 and 3 neutral density filter  35 , which may be fabricated from inconel-coated glass or fused silica, absorbs part of the incident radiation from laser  29  as well as part of the incident visual/visible image forming light  45  from an image, not shown, viewed by observer  17 . This partial absorption of incident laser radiation and visual light by neutral density filter  35  reduces reflected illuminance to a point where reflected light from laser protection system  27  is not visible to the human eye which, in turn, allows observer  17  to avoid detection. 
     Narrow-band interference filter  37  will reflect all out-of-band laser radiation, generally indicated by arrow  31 , which passes through neutral density filter  35 . Visible imaging forming light, generally indicated by arrows  45 , which passes through neutral density filter  35  and which is within the passband wavelength of narrow-band interference filter  37  will pass through filter  37  to lens  41 . 
     The preferred narrow-band interference filter  37  used in laser protection system  27  is a dielectric interference filter manufactured by Melles Griot having a passband  32  angstroms wide which is centered at 750 nanometers. As is best illustrated in FIG. 3, a filter having the reflection characteristics of filter  37  will reflect laser radiation from an argon laser operating at approximately 520 nanometers, a helium neon laser operating at approximately 620 nanometers, and a ruby laser operating at approximately 695 nanometers. Filter  37  will also reflect laser radiation from a neodymium yttrium aluminum garnet (YAG) laser operating at approximately 1064 nanometers, a double neodymium YAG laser operating at approximately 532 nanometers, and all other like radiation having wavelengths outside of the passband of filter  37 . 
     Lens  41  inverts and focuses the image forming light  45  passing through filter  37  on to an input fiber optic faceplate  47 . Faceplate  47  then directs the image forming light  45  to a photocathode  49  which converts the photons from the light  45  to electrons in proportion to the amount of light falling thereon. It should be noted that photocathode  45  may be a tri-alkali photocathode which is typically used in a second generation image intensifier or a gallium arsenide photocathode which is typically used in a third generation image intensifier. The third generation image intensifier, in turn, typically provides an enhanced spectral response over the second generation image intensifier. 
     The electrons emitted by photocathode  49  are directed to a microchannel plate  51  which generally consist of microscopic hollow glass conduction capillaries or channels fused into a thin disc-shaped array less than one millimeter thick, and having approximately two million channels. The glass conduction channels of microchannel plate  51  are connected in parallel to a direct current voltage potential of approximately 3000 volts and each channel well emits secondary electrons when electrons emitted by photocathode  49  collide with the channel walls. Repeated collision of the secondary electrons with the channel walls of microchannel plate  51  initiates a cascade of secondary electrons that continuously multiplies as the electrons progress through the channels. The electron gain of microchannel plate  51  produced by this process is controlled by varying voltage across the plate and is achieved in a small volume without image deterioration. 
     The secondary electrons emitted by microchannel plate  51  are directed to a phosphor screen  53  which converts the electrons to an enhanced optical image which is then re-inverted by and passes through an output fiber optic bundle  55  to lens  43  which functions as an eyepiece so as to allow the enhanced visual image, provided by image intensifier  39  and designated generally by arrows  57  to be viewed by the eye  25  of observer  17 . 
     As is best illustrated in FIG. 3 a second-generation tri-alkali photocathode is responsive from approximately 400 nanometers to 870 nanometers, while a third generation gallium arsenide photocathode is responsive from approximately 570 nanometers to 920 nanometers. Thus, any filter which is centered between approximately 570 nanometers and 870 nanometers and has a bandpass of approximately 30 angstroms or less may be used with laser protection system  27  as long as the filter selected is not centered at the operating wavelength of a helium neon, a ruby or any other like laser not illustrated in FIG.  3 . 
     It should be noted that use of a third generation image intensifier with laser protection system  27  as opposed to a second generation image intensifier will provide an enhanced visual image to the eyes  25  of observer  17  as is best illustrated in FIG.  3 . 
     Tables 1 and 2 below show the results of tests on laser. protection system  27 . These tables give the center wavelength of narrow-band interference filer  37 , the filter line width of filter  37 , the illuminance from a target/visual image, and the optical density (OD) for a minimum detectable scene. Table 1 illustrates data where the center wavelength is varied and the line width is held constant, while table 2 illustrates data where the line width is varied and the center wavelength is held constant. Table I. Variable wavelengths, constant line widths. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Variable wavelengths, constant line widths. 
               
             
          
           
               
                   
                   
                   
                   
                 TOTAL 
               
               
                   
                   
                 ILLUMI- 
                   
                 OPTICAL 
               
               
                   
                   
                 NANCE 
                 TOTAL 
                 ATTENUATION 
               
               
                 CENTER 
                   
                 FROM 
                 OPTICAL 
                 FOR MIN 
               
               
                 WAVE- 
                 LINE 
                 TARGET 
                 ATTENUATION 
                 DET SCENE 
               
               
                 LENGTH 
                 WIDTH 
                 KILO FOOT 
                 FOR MIN DET 
                 NORMALIZED 
               
               
                 (nm) 
                 (nm) 
                 CANDLE 
                 SCENE 
                 TO 1-kfc 
               
               
                   
               
               
                 900 
                 10 
                 1.5 kfc 
                 5.9 OD 
                 5.7 OD 
               
               
                 800 
                 10 
                 1.5 kfc 
                 6.5 OD 
                 6.3 OD 
               
               
                 750 
                 10 
                 1.5 kfc 
                 6.8 OD 
                 6.6 OD 
               
               
                 700 
                 10 
                 1.1 kfc 
                 6.6 OD 
                 6.5 OD 
               
               
                 660 
                 10 
                 1.5 kfc 
                 7.0 OD 
                 6.8 OD 
               
               
                 620 
                 10 
                 1.2 kfc 
                 6.8 OD 
                 6.7 OD 
               
               
                 600 
                 10 
                 1.2 kfc 
                 6.8 OD 
                 6.7 OD 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 Variable line widths, constant wavelengths. 
               
             
          
           
               
                   
                   
                   
                   
                 TOTAL 
               
               
                   
                   
                 ILLUMI- 
                   
                 OPTICAL 
               
               
                   
                   
                 NANCE 
                 TOTAL 
                 ATTENUATION 
               
               
                 CENTER 
                   
                 FROM 
                 OPTICAL 
                 FOR MIN 
               
               
                 WAVE- 
                 LINE 
                 TARGET 
                 ATTENUATION 
                 DET SCENE 
               
               
                 LENGTH 
                 WIDTH 
                 KILO FOOT 
                 FOR MIN DET 
                 NORMALIZED 
               
               
                 (nm) 
                 (nm) 
                 CANDLE 
                 SCENE 
                 TO 1-kfc 
               
               
                   
               
             
          
           
               
                 755 
                 1 
                 1.5 kfc 
                 5.0 OD 
                 4.9 OD 
               
               
                 755 
                 6.2 
                 1.1 kfc 
                 6.1 OD 
                 6.1 OD 
               
               
                 755 
                 10 
                 1.1 kfc 
                 6.1 OD 
                 6.1 OD 
               
               
                 755 
                 20 
                 1.1 kfc 
                 6.7 OD 
                 6.7 OD 
               
               
                 755 
                 40 
                 1.1 kfc 
                 7.0 OD 
                 7.0 OD 
               
               
                   
               
             
          
         
       
     
     Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.