Abstract:
A spectra generator having an electrically programmable diffraction grating. There may be a broad band light source that emits light which is diffracted by the grating. Diffracting elements in the grating may be individually adjustable so that generation of a specific spectrum or spectra may be achieved. The diffracting elements may be adjusted according to electrical signals of a program from a computer. The generated synthetic spectra may be used for testing and calibration of spectrometers or other devices. Synthetic spectra may also be used for scene generation and other purposes.

Description:
BACKGROUND  
       [0001]     The present invention pertains to spectra and particularly infrared spectra of various substances. More particularly, the invention pertains to the generation of synthetic spectra and use of such spectra in testing and calibration of spectrometers.  
         [0002]     The invention may be related to U.S. Pat. No. 5,905,571, by Butler et al. issued May 18, 1999, and entitled “Optical Apparatus for Forming Correlation Spectrometers and Optical Processors”; U.S. Pat. No. 5,757,536, by Ricco et al., issued May 26, 1998, and entitled “Electrical-Programmable Diffraction Grating; and U.S. Pat. No. 6,664,706, by Hung et al., issued Dec. 16, 2003, and entitled “Electrostatically-Controllable Diffraction Grating”; which are herein incorporated by reference. The invention may also be related to U.S. patent application Ser. No. 10/352,828, by Hocker, filed Jan. 28, 2003, and entitled “Programmable Diffraction Grating Sensor”; and U.S. patent application Ser. No. 09/877,323, by Hocker et al., filed Jun. 8, 2001, and entitled “Apparatus and Method for Processing Light”, which are herein incorporated by reference.  
       SUMMARY  
       [0003]     The invention may be an apparatus and method for the testing and calibration of spectrometers using generated synthetic spectra. These generated synthetic spectra may be used for other purposes such as scene generation. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0004]      FIG. 1  is a schematic of a spectra generator having an electrically programmable reflective diffraction grating;  
         [0005]      FIG. 2  is an intensity versus wavelength graph of radiation from an example black body;  
         [0006]      FIG. 3  is an example spectrum that may represent the absorption of infrared light by a particular substance;  
         [0007]      FIG. 4  is a diagram of an actual spectrum of HF, a simulated spectrum of HF and a displaced simulated spectrum of HF;  
         [0008]      FIG. 5  is a diagram of an actual spectrum of TCE, a simulated spectrum of TCE and a modified simulated spectrum of TCE; and  
         [0009]      FIGS. 6   a  and  6   b  show end and top views, respectively, of an electrically adjustable grating.  
     
    
     DESCRIPTION  
       [0010]     Spectrometers may be used to detect molecules in the atmosphere by observing the characteristic spectra of light absorbed by the molecules. Such spectrometers should be tested and calibrated with spectra that resemble the target molecules. Creating test spectra by using samples of the molecules, such particular species of them, may be inconvenient, time consuming and expensive. Additionally, using samples of the actual molecules may be hazardous if the species are toxic. Specifically, military systems used for standoff chemical agent detection need capabilities for test and calibration with actual spectral input representing the chemical agents to be detected, but without the need to use samples of actual toxic chemical agents.  
         [0011]     In  FIG. 1 , a generator  10  of spectra is shown. A light source  11  may output light  12  of a black body, that is, broadband infrared light. A lens  13  may collimate light  12  for impingement on a diffractive grating  14 . Electrically programmable diffractive grating  14  may reflect broadband light  12  as spectra light  15 . The design may instead incorporate a transmissive grating in lieu of the reflective grating. The reflective grating may be generally referred to here.  
         [0012]     Spectra light  15  may be detected by a spectrometer  16 . Light  15  may be a synthesization of a light spectrum resulting from absorption by a specific substance. If spectrometer  16  is functioning properly, then it may identify that that spectrum light  15  to be that of the specific substance. The light  15  beam width may be adjusted with a beam expander or beam compressor so that light  15  is more effectively transmitted and detected by spectrometer  16 .  
         [0013]     The electrically programmable diffraction grating  14  may transform broadband light  12  into spectra light  15  in accordance with a dimension, such as the height of diffraction elements  17 , relative to the base of diffractive gating  14 . These dimensions of electrically programmable diffraction grating  14  in the Figures are not drawn to scale but are illustrative. The actual number of elements  17  may be over 1000, e.g.,  1024 . Also, angle  18  may be a factor affecting the synthesized spectra  15 . The spectra of light  15  generated may be a function of the heights of the elements  17  and of angle  18  of the direction of diffracted light  15  relative to the direction of incident light  12  impinging grating  14 . Each element  17  may have a unique height.  
         [0014]     The heights of the elements  17  may be adjusted in order to generate various spectra in diffracted light  15 . The heights of the diffractive elements  17  may be set with electrical signals from a computer  19  via a connection  21  to an interconnection base  22  attached to grating  14 . Computer  19  may be programmed to provide ready-made settings for the elements  17  to generate specific spectra of respective substances. Background about an electrically programmable diffraction grating may be disclosed in U.S. Pat. Nos. 5,905,571 and 5,757,536, and U.S. patent application Ser. No. 09/877,323.  
         [0015]     For instance, if a request is input to computer  19  for a spectrum of CO, than an element pattern may be sent to grating  14  which may result in an adjustment of elements  17  so as to result in a spectrum of CO being in diffracted light  15  sent to a receptor  16  such as a spectrometer. Elements  17  may be adjusted so as to reflect light  15  having spectra of more than one substance. Also, background may be added to the spectra of light  15 . Spectrometer  16  may be tested with the reception of light  15  to determine detection capability of various substances among various backgrounds. Device  16  may be tested for identifying a spectrum of a particular substance or several substances buried in noise at one level or another. Computer  19  may provide spectra settings to elements  17  in a sequential fashion over a given period of time. Spectra for calibration of spectrometer  16  or other instrumentation may be provided via light  15 . Further, a detection mechanism may be used added to device  16  to identify and verify the spectra being used for testing and calibrating spectrometers and the other instrumentation. Also, spectra may be generated for scene generation and the testing and/or calibration of microbolometers and other detection mechanisms.  
         [0016]      FIG. 2  is a graph of intensity versus wavelength of infrared light. Curve  31  reveals a spectrum of black body source which may be light source  11  of generators  10  and  20 . However, source  11  may emit other wavelengths of the like, such in the ranges of visible or UV light.  FIG. 3  is an example of a spectrum  32  of light  15  or  24  after the light  12  is transmitted through a region containing a specific molecule. Light may be absorbed in spectral wavelengths in a pattern characteristic of that molecule.  
         [0017]      FIG. 4  shows a graph of three spectra of HF. The top spectrum may be regarded as an actual spectrum curve  51  of HF. Curve  52  may be a synthetic spectrum of HF as provided by spectra generator  10  or  20 . Curve  53  is the same as curve  52  except that it is shifted to the right about 50 wave numbers. If the spectrum  52  is compared with the actual HF spectrum  51 , and spectrum  52  is delayed periodically to the position of spectrum  53 , spectrum  51  may be easier to detect when a comparison is done for calibrating the generator. Generators  10  and  20  may generate both a spectrum  52  and a displaced spectrum  53  of HF. This spectra displacement shifting may accommodate AC detection of an actual spectrum.  
         [0018]      FIG. 5  shows an actual spectrum  61  of TCE and a synthetic spectrum  62  of TCE. Spectrum  62  may be provided by generators  10  and  20 . Another provided spectrum  63  may effectively be spectrum  62  of TCE with the 850 cm −1  absorption line removed by generator  10  or  20  due to an adjustment of the elements  17  in diffractive grating  14  or  23 , respectively.  
         [0019]      FIGS. 6   a  and  6   b  show aligned end and top views of the adjustable grating  14  that may be used in generator  10 . A basic structure of this adjustable grating  14  may be like that of grating  23 , except that grating  23  may have a transparent property rather than a reflective one. Elements  17  may be pulled down electrostatically by elements  71 . One polarity of a voltage source may be connected to all of the elements  17  at support  73 . The other polarity of the voltage source may be connected to an individual element  71  positioned relative to its corresponding element  17 . Elements  17  may be like flexible tongs that have a natural resting position close structure  72  and a fixed structural connection to structure  73 . A magnitude of a voltage applied across element  17  and  71  may determine the position of element  17  relative to element  71 . The greater the voltage magnitude, element  17  may be drawn closer to element  71 . Thus, all elements  17  may be individually adjusted to achieve a particular and unique diffractive grating  14  setting for providing a desired spectrum from generator  10 . The voltage inputs to elements  71  may be individual and different from one another. The base  74  is insulated so that elements  71  may be electrically isolated from one another and connected to an external signal source such as computer  19 . The various sets of voltage inputs with their respective combinations of magnitudes may be programmed in computer  19 . The positions of elements  17  and consequently grating  14  may be dynamically changed in a manner to get the effect of going from spectrum  52  to spectrum  53  of  FIG. 5  or from spectrum  62  to spectrum  63  of  FIG. 6 .  
         [0020]     Although the invention has been described with respect to at least one illustrative embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.