Abstract:
A self-contained, military usage compatible, stabilized illumination calibration system and method of using same to enable in-the-field accurate determination of the low-level light falling on the surface of a night vision apparatus resolution chart and attending adjustment of a night vision device. Use of such charts, such as the chart of U.S. Pat. No. 4,607,923, preferably includes known levels of illumination at several different expected night vision goggle compatible lower-levels in order to both evaluate and adjust the performance of a night vision device prior to its use or during extended use periods. The disclosed system is digital computer controlled and provides an accurate, easy to use, and low cost alternative to previous laboratory-like methods of determining night vision evaluation illumination. The disclosed system provides go or no go indications of achieved chart illumination.

Description:
RIGHTS OF THE GOVERNMENT 
   The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
   COMPACT DISC APPENDIX 
   The present document includes a single compact disc appendix, submitted in two identical disc copies identified as “Copy 1” and “Copy 2”, each containing only the 16 kilobyte file identified as “GNG-PO.txt”. This file provides a program listing for software used in operating the apparatus of the present invention and is referred-to in the application specification. This GNG-PO file is listed in ASKII code as is accomplished with the “plain text” option of a “Windows 2003 XP” operating system. The disclosed computer code is in the format of the Parallax Inc. Stamp BASIC Interpreter programming language. This file may also contain a reference to the present invention using one or more of the names GNG tester, Go No-Go tester and Discrete-Level NVIS-Weighted Irradiance Tester. A transmittal letter provides supplemental information regarding the “Copy 1” and “Copy 2” discs. The content of the GNG-PO compact disc file is hereby incorporated by reference herein. The GNG-PO file was created on May 15, 2003 and bears a May 19, 2005 present disc creation date. 
   BACKGROUND OF THE INVENTION 
   Currently, United States military personnel use the U.S. Air Force Resolution Chart of U.S. Pat. No. 4,607,923 issued to Task et al. in 1986 when evaluating or adjusting night vision goggle devices. This resolution chart is now a standard for use by U.S. and allied military units performing night vision goggle missions. This chart has a series of increasingly finer pairs of black and white bars, bars that, when viewed through night vision goggles, allow maintenance personnel to evaluate and adjust the image quality and allow the war fighter to personally perform night vision adjustments such as objective lens focusing, interpupillary distance selection, tilt angle adjustment, eyepiece selection and battery checking prior to a night vision mission. For language convenience purposes the several functions accomplished with aid of the present invention may be referred-to as “tuning” of a night vision device. 
   Proper use of this resolution chart however requires it to be precisely irradiated at several different levels, levels corresponding to no moon presence, quarter moon presence, half moon presence and full moon presence during usage. This chart usage irradiance is also preferably accomplished with the aid of an irradiance-measuring instrument such as a photometer or a radiometer or less preferably with human estimation of irradiance level. Photometer and radiometer instruments range in value from $5,000 to $28,000 or greater as may be observed in the catalog or on the web site of one supplier of such instruments, Hoffman Engineering Corporation of Stamford, Conn., http://www.hoffmanengineering.com. Such instruments are also generally unsuited for use under military field conditions as is dictated by their cost and their substantially fragile nature. 
   The present invention is believed to provide an answer for these difficulties. 
   The U.S. Pat. No. 4,607,923 and each other patent document and reference document identified herein are also hereby incorporated by reference herein. 
   SUMMARY OF THE INVENTION 
   The present invention provides in the field achievement of “standardized” irradiance for evaluation and adjustment of night vision systems. 
   It is an object of the invention to measure night vision related irradiance levels accurately and economically. 
   It is an object of the invention to provide a test irradiance measuring system for the irradiance levels used with night vision apparatus under military field conditions. 
   It is an object of the invention to provide a test irradiance measuring system for the plurality of irradiance levels used with military night vision apparatus. 
   It is an object of the invention to provide a portable night vision irradiance measuring system adapted to successful usage by unskilled personnel. 
   It is an object of the invention to provide a stable, battery-operated, computerized measuring system for night vision compatible low irradiance levels. 
   It is an object of the invention to provide an irradiance measuring system compatible with an existing night vision apparatus resolution chart. 
   It is an object of the invention to provide an irradiance measuring system based on the stable characteristics of a photodiode transducer device. 
   It is an object of the invention to provide an irradiance measuring system that is disposable in a plurality of physical mounting arrangements. 
   It is an object of the invention to provide an irradiance measuring system producing a simple output indication in response to a plurality of sought after irradiance conditions. 
   It is an object of the invention to provide an irradiance measuring system providing an easily comprehended digital-like irradiance level output indication. 
   It is an object of the invention to provide an irradiance measuring system producing an irradiance level output indication that is remotely comprehensible. 
   It is an object of the invention to provide an irradiance measuring system providing an output indication that is free of night vision device interference generation. 
   It is an object of the invention to provide an irradiance measuring system capable of both convenient field use and replacement of more costly laboratory equipment. 
   These and other objects of the invention will become apparent as the description of the representative embodiments proceeds. 
   These and other objects of the invention are achieved by portable military field use night vision device evaluation apparatus comprising the combination of: 
   a standardized military night vision device performance assessment chart including a plurality of night vision device resolution patterns of selected line density and line spacing character; 
   a source of military night vision device compatible selectable irradiance levels disposable in illuminating proximity with said military night vision device performance assessment chart; 
   a portable irradiance intensity measuring apparatus disposable within an area irradiated by said source of military night vision device compatible selectable irradiance levels including low level irradiance in physical proximity with said military standardized night vision device performance assessment chart; 
   said portable irradiance intensity measuring apparatus including a received irradiance to first electrical signal transducer element, a plurality of selectably accessed standardized ambient irradiance-related reference second electrical signals and an electrical signal comparison apparatus connecting with each said first and second electrical signals; 
   said electrical signal comparison apparatus including analog to digital converter and signal processing elements coupled to an evaluation apparatus contained remotely visible graphic multiple luminous element military night vision device-indiscernible irradiance level indicating display. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The accompanying drawings incorporated in and forming a part of the specification, illustrate several aspects of the present invention and together with the description serve to explain the principles of the invention. In the drawings: 
       FIG. 1  shows use of a dimly illuminated resolution chart to adjust and evaluate a set of night vision goggles. 
       FIG. 2  includes the views of  FIG. 2   a ,  FIG. 2   b  and  FIG. 2   c  in which: 
       FIG. 2   a  shows the spectral energy distribution of a typical nighttime ambient irradiation. 
       FIG. 2   b  shows the spectral energy band pass of a typical night vision device. 
       FIG. 2   c  shows the spectral energy distribution of a typical variable incandescent light source. 
       FIG. 3  shows an overall external view of an apparatus made in accordance with the present invention. 
       FIG. 4  shows an electrical schematic diagram of a preamplifier circuit used in the  FIG. 3  apparatus. 
       FIG. 5  includes the portions  FIG. 5   a ,  FIG. 5   b ,  FIG. 5   c ,  FIG. 5   d  and  FIG. 5   e  and shows an electrical schematic diagram of a computer inclusive embodiment of the present invention. 
       FIG. 6  shows a flow chart relevant to computer software used with the invention, software disclosed in the above identified compact disc appendix of the present document. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  in the drawings illustrates the general method by which U.S. military personnel use the resolution chart of U.S. Pat. No. 4,607,923 to adjust and evaluate a set of night vision goggles or other night vision apparatus. In the  FIG. 1  drawing an observer  100  is viewing the resolution chart  104  along the line of sight  103  by way of the night vision goggles  102 . The chart  104  is irradiated with a controlled source of irradiance  110  using the aid of a measuring device  112  made in accordance with the present invention to determine the level of night vision goggle sensitive irradiance received on the chart  104 . The  FIG. 1  resolution chart  104  displays a representative series of electrical square-wave generating patterns (i.e., alternating black and white lines) of low to high spatial frequency (i.e., from coarse to fine line sizes and spacing as are represented at  106  and  108  for example). 
   The patterns of the  FIG. 1  chart  104  allow the observer  100  to make fine changes to the several adjustment controls of the night vision goggles  102 . These adjustments may include objective and eyepiece focusing, interpupillary distance adjustment and tilt of the goggles axis with respect to the observer&#39;s line-of-sight for examples. After performing these adjustments, the highest observable pattern, the finest of the gratings  106 ,  108  and so on resolved, is then an indication of the goggle&#39;s ability to resolve fine detail. The performed adjustments may achieve as much as a 20/30 Snellen acuity or a 1.5 minutes of arc degree of resolution capability for example. Any goggle performance problems such as less than optimum focusing or loss of resolution are usually discernible during one or more tests performed in the  FIG. 1  manner. 
   As may be understood by the presence of the measuring device  112  in the  FIG. 1  scene the measurements accomplished in this scene include resolution determinations accomplished at several different irradiance levels of the chart  104 . Notably a night vision goggle or other night vision device tested in the  FIG. 1  manner may perform adequately at full moon irradiance level but fail under a lower clear starlight irradiance level for example. Such performance gaps or lapses inevitably adversely affect night vision device usage during a low light level mission. 
   Typically the  FIG. 1  chart  104  is dimly irradiated by variable incandescent lamps comprising the source  110  in the  FIG. 1  drawing. The U.S. Army and other users for example employ a standard lamp of 2856 degrees Kelvin color temperature for the irradiation source  110 . Such a source of known color temperature is needed for performing the  FIG. 1  test because the goggles being adjusted are inherently wavelength sensitive devices, i.e., are expressly arranged to be responsive to a specific band of input wavelengths, such as 665 to 930 nanometers, while largely rejecting wavelengths outside of this band. A shift of input energy toward or away from this band of wavelengths thus has a direct effect on goggle output level. 
   During the  FIG. 1  chart usage the measuring device  112  provides an indication of the level of night vision goggle-sensitive irradiance received by the chart  104 . A series of drawing figures and equations as follows may be used to describe the underlying concepts upon which this irradiance-measuring device  112  operates.  FIG. 2   a  in the drawings for example shows the spectral irradiance distribution A(λ), i.e., spectral irradiance plotted as a function of wavelength, for representative ambient nighttime irradiance conditions such as a full moon, quarter moon, clear starlight or overcast starlight condition. Wavelength based plots for other elements of the  FIG. 1  apparatus appear in the  FIG. 2   b  and  FIG. 2   c  drawings. In order to better appreciate the present invention and its differences from other light measuring arrangements, such as may be used in the photography or television fields for examples, it is perhaps informative to realize that the light levels under consideration in the  FIG. 1  drawing are extremely low and involve wavelengths that are essentially invisible to the unaided human eye. 
     FIG. 2   b  shows the spectral sensitivity G(λ) of a given set of night vision goggles, for example Generation III types A, B, or leaky green goggles, also as a function of wavelength.  FIG. 2   c  shows the spectral irradiance distribution of a variable intensity variable color temperature, incandescent light source (L). Equation [1], shown below, uses the  FIG. 2   b  and  FIG. 2   c  data to define the integrated, night vision goggle-weighted irradiance Q AMBIENT , for a given goggle and nighttime irradiance. Q AMBIENT , is equal to the definite integral with respect to wavelength from 600 to 1000 nanometers of the product of goggle spectral sensitivity and ambient illumination spectral irradiance distribution. Equation [2] defines Q LAMP  for a goggle and a given variable intensity (K) incandescent light source (L) used to irradiate the resolution target. Q MMP15  is equal to the definite integral with respect to wavelength from 600 to 1000 nanometers of the product of goggle sensitivity and the incandescent lamp spectral energy distribution. 
   In order for a resolution test condition as represented in  FIG. 1  to be equivalent to an actual nighttime condition, i.e., a low-light level condition with a specific spectral distribution, Q LAMP  must equal Q AMBIENT . (Q is a constant for any given illumination and goggle combination). It follows, that when Equations [1] and [2] are equivalent, equality is achieved by varying the intensity K of the irradiation falling on the resolution chart as is shown by Equation [3]. In Equation [3] an expression of equality of the two integrals is rearranged to define K. Note that in order to vary the intensity of the irradiance falling onto the  FIG. 1  chart  104  while still maintaining a preset blackbody temperature of about 2856 K, the light source  110  is moved closer to or farther away from the chart or an illumination controlling device such as the aperture of an iris is changed in diameter. Dimming of a lamp by changing its operating voltage cannot be used for intensity change purposes since this results in a change of lamp operating temperature and spectral distribution. 
   In the  FIG. 2  waveforms and in Equations [1], [2] and [3] 
   Q=integrated NVG-weighted irradiance 
   A(λ)=spectral irradiance distribution of a given nighttime ambient condition 
   G(λ)=spectral sensitivity of a given set of night vision goggles 
   L(λ)=spectral irradiance distribution of a variable light source (L) 
   K=irradiance level adjustment constant 
   
     
       
         
           
             
               
                 
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   Q, the integrated night vision goggle-weighted irradiance, is therefore a weighted constant for any specified goggle and irradiance combination. The response of a radiometric-based device such as that disclosed by the present invention can be tailored to detect a level of irradiance present and provide an indication that proper irradiation level is achieved. The measuring device  112  of the present invention contains a radiometric energy detector with accompanying electronic circuitry providing a range of selectable, preset (weighted) Q&#39;s, allowing the re-creation of commonly encountered lighting conditions for a given piece of night vision equipment and irradiating lamp spectral distribution. 
     FIG. 4  and  FIG. 5  in the drawings show electrical schematic diagrams for a preferred arrangement of apparatus usable for irradiance measurement in the present invention irradiance testing. The  FIG. 5  drawing consists of the portions  FIG. 5   a ,  FIG. 5   b ,  FIG. 5   c ,  FIG. 5   d  and  FIG. 5   e . Thus  FIG. 5  shows an overall diagram of the signal processing circuits and electrical signal flow used in the invention and  FIG. 4  shows details of one portion of the  FIG. 5  apparatus, the photodiode preamplifier appearing at  400  in the  FIG. 5  drawing. Significant portions of the signal processing accomplished in the  FIG. 5  measurement circuit occur in the form of software embodied into the microprocessor  518  in the  FIG. 5  diagram. This software is disclosed in the form of a code listing contained in the compact disc appendix identified above and incorporated by reference herein. 
   The Silicon photodiode used to generate an electrical signal proportional to the level of irradiance received at the  FIG. 1  chart  104  appears at  500  in the  FIG. 5  drawing. This photodiode is identified as preferably being of the type PIN IODP devices available from UDT Sensors Incorporated of Hawthorne, Calif. The photodiode is operated in the photoconductive mode of operation by direct coupling to the input terminals of the operational amplifier  404  in the  FIG. 4  preamplifier circuit. The operational amplifier  404  is preferably of the low offset voltage type and when provided with the DC offset adjustment of the potentiometer  406  provides a ground referenced output signal from the preamplifier. 
   The operational amplifier  404  provides a large signal voltage gain by way of the Tee connected feedback network indicated at  408  in  FIG. 4 . The magnitude of this gain is determined by the activated one of the field effect switching transistors  410  and  412  and the selected resistance of the associated 100 K transistor drain-connected potentiometer. The shielded input network  402  of the operational amplifier  404  includes two oppositely connected diode elements to provide excessive voltage and static electricity protection. Signals in the range of 0 to +10 volts occur at the output of the operational amplifier  404 . The needed bipolar supply voltages for the low offset operational amplifier  404  preamplifier are provided by the DC to DC inverter circuit shown at  516  in the  FIG. 5   a  drawing. 
   Unity signal gain, buffering and signal inversion are provided by the operational amplifier shown at  502  in the  FIG. 5   c  drawing; this is followed by two stages of low pass waveform filtering provided in the form of active filtering by the operational amplifiers shown at  504  in  FIG. 5   c  and  FIG. 5   d . Low pass filtering is desirable in the  FIG. 5  circuit especially in order to remove components of alternating current ripple possibly existing in the photodiode signal as a result of the  FIG. 1  lamp source  110  being energized with alternating current energy. In differing arrangements of the invention, a higher voltage lamp with its resulting small filament and small thermal mass is likely to provide such ripple; a direct current energized lamp source at  110  may generate so little of this ripple as to enable omission of the filter at  504 . 
   The analog-to-digital converter circuit shown at  508  in the  FIG. 5   d  drawing provides digital output signals from the offset and adjusted full-scale analog photodiode signal achieved with the four operational amplifiers at  506  in the  FIG. 5   d  drawing. Digital output signals from the analog-to-dial converter  508  are received into the microprocessor  518  by way of one input port signal path of the micro controller or microprocessor  518 . A second input channel on pin  3  of the  FIG. 5   d  analog-to-digital converter  508  provides warning of a low battery or low power supply voltage condition at  514  in  FIG. 5   a  by way of the resistor divider and buffer amplifier at  510  in  FIG. 5   e  and a warning alert light emitting diode driven from the microprocessor  518 . The intentional appearance of a strong warning input signal thus discourages further operation of the  FIG. 1  apparatus until battery or voltage correction occurs through operator intervention. 
   In other words, the summing amplifier at  506  in  FIG. 5   d  drawing combines the photodiode low pass filter signal from the filter  504  and an offset adjustment voltage from the potentiometer at  507  for amplifier circuit biasing. The summing amplifier&#39;s 0 to +10 volt analog output signal is reduced by the full-scale adjustment amplifier to a 0 to +5 volt range so the signal can be handled by the analog-to-digital converter circuit  508 . The pin  2  first input channel of the two input channel analog-to-digital converter circuit shown at  508  provides a digital output from the adjusted full-scale analog photodiode signal achieved with the four operational amplifiers at  506 . The two input channel analog-to-digital converter circuit shown at  508  communicates with the  FIG. 5   b  microprocessor  518  by way of a single bidirectional port of the microprocessor. The second input channel of the analog-to-digital converter  508  responds to battery and power supply voltage condition at  514  in  FIG. 5   a  by way of a microprocessor  518  driven flashing “too low” detection range blue light emitting diode at  520  in  FIG. 5   e ; this warning also appears physically at  312  in  FIG. 3 . The appearance of a flashing “too low” detection range blue light emitting diode on the measurement device  112  in  FIG. 1  suggests discontinued operation of the apparatus until battery or voltage correction occurs. 
   The illumination range setting switches used to accommodate the ambient light conditions and viewed object reflectivity variables encountered by a tested night vision device appear as the centermost and lowermost switches  306  and  308  in the  FIG. 3  drawing and are represented at  512  in the  FIG. 5   a  drawing. Two switches are used in this function in order to accommodate several night vision apparatus input conditions; either centermost switch  306  setting being usable with either lowermost switch  308  setting. The three position lowermost  FIG. 3  switch  308  elects starlight, one-quarter moon and overcast starlight input conditions for the measuring device  112 . These settings correspond to night vision goggle-weighted irradiance levels of 5.85×10 −9 , 2.08×10 −8  and 5.85×10 −10  watts per centimeter squared (w/cm 2 ) respectively for a Class A night vision goggle. Additional information regarding the illumination levels encountered by night vision apparatus is contained in the military standard MIL-L-85762A the most current version of which is dated 24 Jan. 1986. This standard is also hereby incorporated by reference herein. 
   In the “actual” setting of the centermost switch  306  in  FIG. 3 , the measuring device  112  is conditioned for response to the input levels previously noted. In the “tree bark” setting of this switch  306  the expected input levels are one-tenth of these “actual” levels. Tree bark is a low light reflectivity natural material commonly used by convention as a lower limit extremity example in the night vision art and is generally associated with a reflection of about 1/10 of the ambient illumination. The purpose of this setting is to adjust for the fact that the resolution chart  104  in  FIG. 1  is typically 90% reflective instead of the 10% reflectivity of tree bark. This produces an apparent night vision goggle radiance of the 90% reflective chart that simulates the radiance level that would be obtained from the 10% reflective tree bark. By way of the lowermost three-position switch  308  and the centermost two position switch  306  the  FIG. 3  apparatus is therefore provided with ability to respond to the six input irradiance levels of 5.85×10 −9 , 5.85×10 −10 , 2.08×10 −8 , 2.08×10 −9 , 5.85×10 −10 , and 5.85×10 −11  watts per centimeter squared (w/cm 2 ). 
   The switches shown in  FIG. 3  and at  512  in  FIG. 5  provide direct current zero voltage levels that are applied as logic signals to a plurality of different input port nodes of the microprocessor  518 . A reading of these logic signals as well as the analog to digital converter  508  output signals occurs in the software code embedded in the microprocessor  518 ; these readings for example occur by way of the code titled “GNG” and “CalcMoon” in the microprocessor code appearing in the compact disc appendix associated with this document. The two switches shown in  FIG. 3  and at  512  in  FIG. 5  select one of six input irradiance levels for the device to measure. 
   During operation of the  FIG. 5  apparatus the output signal of the silicon detector  500  thus is amplified and then fed to the microprocessor  518 . The microprocessor  518  also decodes the pre-selected binary switch settings at  512  for determining the expected input irradiance level. The detector&#39;s output is compared by the microprocessor to three predetermined weighted constants (Q) representing the desired detection range (high, correct, low) for any received goggle/illumination condition. The microprocessor also activates the appropriate light emitting diodes in the display at  520  in  FIG. 5  to indicate the direction (increase/decrease) in which the current light intensity should be adjusted and when the proper illumination level has been achieved. Preferably the display light emitting diodes at  520  are operated in a cumulative manner with the lower two diodes being “ON” indicating a desired chart illumination level and with all three diodes being “ON” indicating an excessive level of illumination. A decision regarding which diodes are to be illuminated commences at the end of the code titled “CalcMoon” in the microprocessor appendix code listing for example. The “ok” value is stored into memory and the “High” and “Low” values are calculated by the microprocessor to determine which light pattern should be displayed. 
   The microprocessor  518  may be embodied as a “Basic Stamp” type 2e micro controller as is made by Parallax Corporation of Rocklin, Calif., http://www.parallax.com/. Other microprocessors or hardwired logic may be used to achieve the control and memory and other functions accomplished in the Parallax microprocessor with suitable modifications of the software and other microprocessor determined aspects of the invention. 
   An electrical battery, a power-supply connection jack, a fuse and an ON-OFF switch for the  FIG. 5  apparatus also appear at  514  in the  FIG. 5   a  drawing. The power supply connection jack at  514  is preferably made to be of the switching type in order to allow laboratory or other non-portable use of the invention with a power supply and without battery consumption. The diode appearing immediately to the right of the fuse at  514  serves as protection against damaging reverse polarity connection of battery or power supply to the apparatus, current conduction in this diode acts to overload the fuse and open the electrical circuit in crowbar fashion in the event of reverse polarity application. Four alkaline PtAA size batteries serve to operate the 20 milliamp load of the irradiance measuring apparatus for a period of at least sixty hours; Lithium or other high capacity batteries may be substituted for longer operating life. Rechargeable batteries may also be used if desired. 
   The software code disclosed in the compact disc appendix for use in the microprocessor  518  of the  FIG. 5  apparatus cooperates with the electrical circuits shown in  FIG. 5  to achieve functional operation of the present invention. This code consists of seven different areas as are set off by titles having double lines above and below the title words in the compact disc listing. A flow diagram for the software code appears in abbreviated language form in the several parts of the  FIG. 6  drawing herein. 
   In order to use the present invention an operator may set up a test and adjustment sequence in the manner shown in  FIG. 1 , place the device&#39;s controls in the desired conditions, turn-off the room lights, and then move the light source  110  back and forth to a desired distance with respect to the resolution chart  104 , or adjust the light source aperture size, until the device  112  indicates a proper night vision device weighted illumination is falling on the surface of the resolution chart. The finest resolvable square-wave pattern in the resolution chart  104  then indicates the limiting resolution of the night vision device and identifies an achieved equivalent Snellen visual acuity such as 20/50 or 2.5 arc minutes. Normal limiting resolution for any given night vision device type for a specified nighttime ambient illumination is known and a deviation from the norm thus indicates improper adjustment or below acceptable optical performance necessitating appropriate action. The night vision device at  102  in  FIG. 1  may of course be of the night vision goggle or night vision telescope or night vision periscope or other night vision device types. 
     FIG. 3  in the drawings shows an external overall perspective view of a measuring device  112  made in accordance with the invention. As shown in this drawing the measuring device may be contained in a box-like plastic housing  300  and disposed in the  FIG. 1  scene by either hanging from the lanyard attachments at  320  and  322  or by mounting via the base plate  318  using a ¼-20 threaded member such as the attachment screw of a photographic tripod or by supporting on one of its flat surfaces. Suitable external dimensions for the preferably unreflective black housing  300  appear at  324 ,  326  and  328  in the  FIG. 3  drawing. Typical numeric values for these dimensions are 4 11/16 inches, 4 11/16 inches, and 2⅜ inches respectively. The sensitivity electing switches for the  FIG. 8  irradiance measuring apparatus, i.e., the switches at  512  in  FIG. 5   a , appear at  306  and  308  within the control area face  302  in the  FIG. 3  drawing; the power switch at  514  in  FIG. 5   a  appears uppermost in  FIG. 3  and is made to move in a plane orthogonal to that of the switches  306  and  308  for darkened room use convenience. 
   The photodiode at  500  in  FIG. 5  receives input irradiance via the aperture shown at  304  in the  FIG. 3  view of the measuring apparatus  112 . A protective cap cover and mating receptacle may be provided around the photodiode aperture  304  for physical protection purposes. The light emitting diodes of the output display at  520  in  FIG. 5  appear at  312 ,  314  and  316  in  FIG. 3 . The preferred blue color output of these diodes does not disturb the night vision device under test even though the overall output display  310  is oriented to directly face the night vision device and the lamp  110  in the  FIG. 1  typical irradiance-measuring scene. As indicated in connection with the  FIG. 5  drawing, energization of the light emitting diodes  312 ,  314  and  316  is preferably accomplished in cumulative, multiple light emitting diode energized in unison, fashion for “too low”, “ok” and “too high” measurements of illumination level by the measuring device  112 . 
   A calibration display for the irradiance measuring device  112  appears electrically at  522  in the  FIG. 5   c  drawing. This display  522  is preferably of the liquid crystal type and is a separate external unit from the measuring device  112  in the  FIG. 3  drawing. This calibration display  522  is thus separate and distinct from the blue light emitting diodes circuits at  520  in  FIG. 5   e  drawing. The display  522  is usable for maintenance and adjustment of the correct or center weighted irradiance level constant for each position of the sensitivity switches  306  and  308  in the  FIG. 3  drawing. The display unit  522  indicates which sensitivity setting is currently invoked for the measurement system and the incident irradiance level. The normally open switch associated with the display unit  522  provides a means to adjust or recalibrate the center weight irradiance level constant for each position of the sensitivity switches  306  and  308 . The transistor device connected to the connector pin  1  of the display  522  is a five-volt regulator providing a stable operating supply voltage for the liquid crystal module used in this display. 
   From a perspective view of the present invention it is interesting to note that the measuring device  112  is considered to be of a night vision goggle-weighted nature. This wavelength weighting is accounted for in the described apparatus in the calibration procedure in setting of the electronics. Since the described radiometric detector does NOT include a wavelength filter that properly weights the input irradiance it is necessary that it be calibrated with an irradiance source of known NVIS weighted irradiance. The device will then function properly when the irradiating sources used later have the same color temperature as the source that was used to set the calibration values. 
   While the apparatus and method herein described constitute a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of apparatus or method and that changes may be made therein without departing from the scope of the invention as is defined in the appended claims.