Abstract:
A system and method for determining the temperature of an object without physically contacting the object. The method involves reading a spectral radiation of the object over a plurality of wavelengths to obtain a set of radiation data related to a temperature of the object. A known characteristic of a black body is determined at a plurality of predetermined, different test temperatures. The spectral data and the characteristic of the black body at the various test temperatures are used to calculate a temperature of the object.

Description:
FIELD 
     The present disclosure relates to temperature measurement systems and methods, and more particularly to a spectral radiometer system and method that is able to determine a temperature of a high temperature object without making physical contact with the object. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     Various system and methods have been employed where the need has existed to determine the temperature of object. Such systems and methods have typically involved the use of optical pyrometers, laser assisted pyrometers, multi-spectral pyrometers and thermocouples. In the case of thermocouples, there has been a need to make physical contact with a portion of the object whose temperature is being sensed. This is less desirable since it can affect the measurement by way of the physical contact. Pyrometers, on the other hand, do not involve physical contact of the sensor with the object whose temperature is being sensed. However multi-spectral pyrometers tend to be complex in construction. 
     SUMMARY 
     The present disclosure relates to an apparatus and method for determining a temperature of an object without the need to physically contact the object, and which has a straightforward implementation. In one implementation a method is provided that involves reading a spectral radiation of the object over a plurality of wavelengths to obtain a set of radiation data related to a temperature of the object. A known characteristic of a black body is determined at a plurality of test temperatures. The characteristic is used at each of the test temperatures, along with the set of radiation data, to determine a temperature of the object. 
     In one particular implementation the method involves determining the temperature of an object without physically contacting the object. First a reading of the spectral radiation of the object over a plurality of wavelengths is taken to obtain a set of radiation data related to a temperature of the object. A characteristic of the black body is then determined at a plurality of test temperatures. The characteristic is then analyzed, at each of the test temperatures, relative to the set of obtained radiation data, to generate a set of ratios, one set for each test temperature. Thus, each set of ratios is uniquely associated with a respective one of the test temperatures. A standard deviation is then determined for each set of ratios, to thus create an array of standard deviations. The minimum standard deviation is selected from the array of standard deviations. The minimum standard deviation serves to identify that specific set of ratios that deviates the least from the characteristic of the black body at a specific test temperature. Using the minimum standard deviation, the specific test temperature associated with the set of ratios that produced the minimum standard deviation may be deduced, and subsequently used to calculate an actual temperature of the object. 
     Thus, by using a known characteristic of a black body at a plurality of different test temperatures, and by analyzing the ratios created when comparing the spectral data obtained with the black body characteristic at a plurality of different known test temperatures, the actual temperature of the object can be obtained without any physical contact with the object. A particular advantage is that by using the known characteristic of a black body, in connection with the analysis of the ratios obtained, the system and method is not affected by (nor does it require knowledge of) the emissivity of the object whose temperature is being measured. 
     In one form the apparatus includes an etalon filter, and in one specific implementation a Fabry-Perot filter combined with a linear variable filter. The filter receives spectral radiation from the object whose temperature is to be determined. The filter has a pair of optical elements, one of which may have its angular position adjusted relative to the other, to sweep an optical signal across a linear array detector arranged adjacent an output side of the linear variable filter. The output of the linear array detector is analyzed by a processor using an algorithm that creates the sets of ratios and the array of standard deviations described above. The processor and the algorithm determine, from the minimum standard deviation, the specific test temperature associated with the ratio set that produced the minimum standard deviation. The processor and the algorithm use this information to then calculate the actual temperature of the object. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a high level block diagram of one embodiment of an apparatus in accordance with the present disclosure for determining the temperature of an object without physically contacting the object; 
         FIG. 2  is a prior art diagram that helps to explain the nature of the output of an etalon filter; 
         FIG. 3  is a graph showing an exemplary spectral radiation input signal and an exemplary output from the etalon filter of  FIG. 2 ; 
         FIG. 4  is an enlarged view of a portion of the output waveform shown in  FIG. 3  as well as a characteristic (in this example radiance) of an ideal black body; 
         FIG. 5  is a flowchart illustrating operations performed by the apparatus of  FIG. 2 , and more particularly the operations performed by the processor of  FIG. 2  using an algorithm, that enables the processor to analyze the spectral data collected against the test temperatures; and 
         FIG. 6  is graph illustrating test results showing the accuracy of the apparatus and method in computing the temperature of a black body object. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
     Referring to  FIG. 1 , there is shown an apparatus  10  in accordance with one embodiment of the present disclosure. The apparatus  10  includes a lens  12  that receives spectral radiation  14  from an object  16 . The apparatus  10  is not in contact with the object  16  whose temperature is being measured. The lens  12  may be an anamorphic lens. The spectral radiation  14  is focused by the lens  12  onto an etalon filter, in this example a Fabry-Perot filter combined with a linear variable filter  18 . For convenience, the filter  18  will be referred to simply as the “etalon filter”  18 . 
     The etalon filter  18  includes a first substrate  20  and a second substrate  22  that is spaced apart from the first substrate. Spacer elements  24  may be positioned between the substrates  20  and  22  to maintain a minimum spacing, and thus limit to set the minimum wavelength that may be transmitted by the etalon filter  18 . The second substrate  22  may include a broad band reflective coating  26  on a first thereof that faces the lens  12 . A second (i.e., opposite) surface of the second substrate may include a linear variable transmitting filter  28 . The wavelength of the linear variable transmitting filter  28  may be set to any desired range for a specific application. 
     The first substrate  20  may include a broad band anti-reflective coating  30  on a first surface thereof that faces the lens  12 , and a broad band reflective coating  32  on a second (i.e., opposing) surface thereof. Substrates  20  and  22  may be comprised of a material that transmits the set of wavelengths that are received at the linear array detector  34 . Examples of suitable materials are Germanium, Zinc, Selenide, and Sapphire. Each may have a thickness typically within a range of about 0.125_inch-0.25 inch (3.175 mm-6.35 mm). 
     A linear array detector  34  is disposed adjacent the etalon filter  18  to receive spectral information being output from the etalon filter. The detector  34  includes a plurality of independent detector segments  34   1 - 34   n  that each are tuned to a specific wavelength. Each detector segment  34   a - 34   n  generates an electrical output signal in relation to the energy of the spectral radiation that impinges it. The detector  34  output signals are denoted in simplified form by reference number  36 . In one specific form the linear array detector  34  forms a linear array of pyroelectric type sensors. 
     The electrical output signals  36  from the detector  34  are fed to a processor  38  that is adapted to execute an algorithm  40 . The processor  38  generates output signals that may be used by a controller  42  to control an actuator  44 , such that operation of the actuator  44  is synchronized to the output of the linear array detector  34 . When the processor  38  receives a synchronizing pulse from the linear array detector  34  within the output signals  36 , it transmits the signal  41  to the controller  42 . The controller  42  applies a swept voltage, linear in time, to the actuator  44  which responds by changing the angle that the substrate  20  makes with respect to substrate  22 . The angle is also changed in a linear fashion with time, after which the controller  42  resets the actuator  44  to its initial position, thereby resetting the substrate  20  to its initial angle of zero degrees. The synchronization of the actuator  44  comes about from signals that are internally generated by the array detector  34  that are sequentially sent to the processor  38  at a rate of preferably about ten times per second or greater. Thus, movement of the first substrate  20  effectively causes the focused spectral radiation from the lens  12  to be “swept” across the surface of the detector  34  that faces the etalon filter  18 . 
     The algorithm  40  is used by the processor  38  to analyze known, predetermined characteristics of a black body, at various test temperatures, in relation to the spectral data provided by the linear array detector  34 . This will be explained in greater detail in the following paragraphs, but in brief the algorithm  40  operates to obtain a set of ratios at each one of the test temperatures, and to generate an array of standard deviations therefrom. A minimum one of the standard deviations is identified. The minimum standard deviation identifies the set of ratios that varied the least from its associated test temperature. From this information the actual temperature of the object  16  can be determined. The actual temperature may then be displayed on a display device  46  such as an LCD display, a CRT display, or otherwise printed using a printer (not shown) or stored using a memory device (not shown) 
     Referring briefly to  FIG. 2 , background information on the characteristics of the linear variable transmitting filter  28  will now be discussed. One may assume that for a given wavelength, the transmission from the filter  28  will be a Gaussian shape, with σ=0.1 micron. 
     Also assume center or average wavelength, λ, is proportional to the “x” position. Thus, the output of the linear variable transmitting filter  28  may be expressed as: 
     
       
         
           
             
               
                 T 
                 IVF 
               
               ⁡ 
               
                 ( 
                 x 
                 ) 
               
             
             = 
             
               ⅇ 
               
                 
                   - 
                   
                     
                       ( 
                       
                         x 
                         - 
                         λ 
                       
                       ) 
                     
                     2 
                   
                 
                 
                   
                     2 
                     · 
                     
                       σ 
                       2 
                     
                   
                   ⁢ 
                   
                       
                   
                 
               
             
           
         
       
     
     The transmission of the etalon filter  18  “T” also varies across the aperture of the first (i.e., tilted) substrate  20  as a function of tilt angle, wavelength, I, and Reflectance, R. Thus, the transmission function for the etalon filter  18  with a spacing of t is, for normal incidence, 
     
       
         
           
             
               T 
               etalon 
             
             = 
             
               1 
               
                 1 
                 + 
                 
                   F 
                   · 
                   
                     
                       sin 
                       2 
                     
                     ⁡ 
                     
                       ( 
                       
                         δ 
                         2 
                       
                       ) 
                     
                   
                 
               
             
           
         
       
       
         
           where 
         
       
       
         
           
             F 
             = 
             
               
                 4 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 R 
               
               
                 
                   ( 
                   
                     1 
                     - 
                     R 
                   
                   ) 
                 
                 2 
               
             
           
         
       
       
         
           and 
         
       
       
         
           
             δ 
             = 
             
               
                 4 
                 · 
                 t 
               
               λ 
             
           
         
       
     
     The spacing “t” used in the transmission function varies across the length of the etalon plate “x” and depends upon the tilt or wedge angle “q” for small angles as represented in  FIG. 2 . Higher orders of etalon transmission, for example where λ desired=10 micron, and λ=5 micron is also transmitted, are rejected by the linear variable filter transmission function. 
     For a given tilt angle “θ”, the total transmission of both filter elements (i.e., substrates  20  and  22 ) is given as a function of x and λ by: 
                 T   total     ⁡     (     x   ,   λ     )       =       ⅇ       -     (     x   -   λ     )         2   ·     σ   2           ·       [     1   +     F   ·       sin   2     ⁡     (       4   ·   θ   ·   x     λ     )           ]       -   1               
In general, the algorithm  40  may be used to find the minimum difference between a set of test functions and the measured data (represented by output  36 ) obtained from the linear array detector  34 . The main assumption about the data is that it has been acquired from a source with constant emissivity (a gray body). For an ideal gray body source, the ratio of an ideal black body (I BB ) to the measured gray body characteristic (I data ), at the same temperature, will be a constant:
 
     
       
         
           
             
               
                 
                   
                     I 
                     BB 
                   
                   ⁡ 
                   
                     ( 
                     λ 
                     ) 
                   
                 
                 ⁢ 
                 
                   ❘ 
                   
                     T 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                 
               
               
                 
                   
                     I 
                     data 
                   
                   ⁡ 
                   
                     ( 
                     λ 
                     ) 
                   
                 
                 ⁢ 
                 
                   ❘ 
                   
                     T 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                 
               
             
             = 
             
               
                 R 
                 ⁡ 
                 
                   ( 
                   λ 
                   ) 
                 
               
               = 
               
                 
                   constant 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   when 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   T 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
                 = 
                 
                   T 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
           
         
       
         
         
           
             where T 1 =temperature  1 =T 2 =temperature  2 . 
           
         
       
    
     In the case where T 1 =T 2 , R (λ)=constant=1/emissivity for all λ. In this case, the standard deviation taken over the set of ratios taken across the measurement band is zero, since             (the constant)=0. For cases where T 1  and T 2  are different, the standard deviation is always greater than zero.
     The algorithm  40  compares the data set (i.e., collection of spectral output signals from the detector  34 ) to the black body characteristics at incremental test temperatures and computes an array of standard deviations from the resulting curves. The minimum standard deviation of this array occurs at the test temperature closest to that corresponding to the spectral output data  36 . 
     Absorption mechanisms such as atmospheric C O2  and water vapor, however, can cause the resulting spectral data set to deviate considerably from the ideal gray body characteristic. This is illustrated in  FIG. 3 , where atmospheric transmission  48  is represented in dashed lines and represents the spectral signal  14  from the object  16 , and the solid line graph  50  represents the output of the linear array detector  34 . To circumvent this drawback, only data in wavelength bands outside the known absorption regions need be considered to get accurate results. In this example, the wavelength bands of graph  50  indicated by circled area  4  may be used, because at these wavelengths the portions of the graphs  48  and  50  are substantially unaffected by the atmospheric absorption. Because of this, an accurate measurement may be had by using only a portion of the available data set, if absorption or other perturbing processes are present.  FIG. 4  illustrates the circled portion of graph  50  from  FIG. 3 , together with the radiance of an ideal black body at various wavelengths over the wavelength spectrum between from 8 microns to about 10.2 microns. These curves are substantially unaffected by atmospheric absorption, meaning that no significant ambient influences will affect the computation of the temperature of the object  16  in this wavelength region of interest. 
     With the foregoing overview of the algorithm  40 , reference will now be made to  FIG. 5  for a more detailed discussion of the sequence of operations performed by the algorithm. Referring to the flowchart  100  of  FIG. 5 , which represents the operations of one implementation of the algorithm  40 , at operation  102  the spectral radiation data from the object  16  is obtained using the apparatus  10  shown in  FIG. 1 . More specifically, the output  36  from the linear array detector  34  is obtained by the processor  38 , which represents a set of spectral data taken at the different wavelengths of the individual detector segments  34   1 - 34   n  of the detector. At operation  104 , the test temperatures and a counter are initialized. The term “T=T 0 ” indicates that the initial test temperature is set equal to zero. The term dT=Tmax/N−1 indicates that first test temperature used will be the maximum (highest) test temperature divided by “N−1”, where “N” is the total number of test temperatures to be used by the algorithm  40 . The term “I=0” sets a counter to zero. 
     At operation  106 , the characteristic of an ideal black body, at a given wavelength, is calculated for the first test temperature. At operation  108 , the algorithm  40  calculates a set of ratios for the first test temperature. This set of ratios is formed by using the outputs from the various segments  34   1 - 34   n  of the detector  34 , along with the characteristic calculated for the ideal black body (for example radiance) at the selected test temperature. Thus, this initial set of ratios is uniquely identified with the initial test temperature. 
     At operation  110 , the standard deviation for the ratios just calculated at operation  108  is determined. A check is then made if the just used test temperature is the last temperature to be used, as indicated at operation  112 , and if not, then the next test temperature is obtained as indicated at operation  114  and operations  106 - 112  are repeated. For example, the first test temperature used may be 300° C., the next one may be 400° C., and so forth. 
     Each cycle through operations  106 - 114  creates a set of ratios, with an associated standard deviation, that are both uniquely associated with a specific test temperature. Thus, the repeating of operations  106 - 114  creates an array of standard deviations, with each standard deviation being uniquely associated with a specific set of ratios, which in turn relate to only one of the test temperatures. Each standard deviation essentially represents the variation of the spectral data from the characteristic of the black body at a specific test temperature. 
     When the check at operation  112  indicates that there are no additional test temperatures to use, then the array of standard deviations is searched to determine the minimum standard deviation, as indicated at operation  116 . The minimum standard deviation identifies the set of ratios, for a specific test temperature, that deviate the least from the characteristic of the black body at the same test temperature. At operation  118 , the minimum standard deviation is used together with the minimum test temperature to compute the temperature of the object  16  ( FIG. 1 ). The array index, “M”, is determined from the set of standard deviations as follows. M is assigned to the index of the first element, i.e. M=1. Each element in the array of standard deviations, beginning with the first, is compared with the previous element. If that element is less than the previous element, then the array index of the lesser element is assigned to M. This process is repeated until all the indices are exhausted. The resulting value for M is the index of the minimum standard deviation element. 
     Referring briefly to  FIG. 6 , a comparison of a computed temperature of a black body to its ideal temperature, using the apparatus  10  and methodology described herein, is shown. The In this example it can be seen that the computed temperatures at each of the frequency spectra 2.1-2.5 microns, 8-10 microns, and 2.1-2.5 and 8-10 microns all closely track the “ideal” (i.e., actual) temperature of the black body. 
     The apparatus  10  and method of the present disclosure may be used to measure the temperature of an object over a wide temperature range, and over 1200° C. The apparatus  10  is relatively compact, does not require an active laser source, and is able to operate on a broad range of materials that do not exhibit strong wavelength dependent surface emissivity. Moreover, the apparatus  10  does not require a mechanical light chopper, and performs its temperature measurement without the need to make physical contact with the object being measured. Still further, the linear array detector  34  may be operated without the need for active cooling. 
     A particular advantage of the system and method of the present disclosure is that by using known characteristics of a black body, in connection with the analysis of the ratios obtained, the emissivity of the object becomes immaterial to the determination of its actual temperature. Put differently, the present system and method does not require advance knowledge of the emissivity of the object being measured, nor is the outcome of the measurement determination made by the present system and method affected by the emissivity of the object being measured. 
     While various embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the present disclosure. The examples illustrate the various embodiments and are not intended to limit the present disclosure. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.