Abstract:
A method involves determining multiple temperatures of an object from spectral data collected from the object. The spectral data covers a plurality of wavelengths. The method comprises using a computer to (a) assign an initial value for residual radiation; (b) identify a black body profile that best fits the spectral data over the plurality of wavelengths; (c) remove radiation corresponding to the identified profile from the residual radiation; and (d) return to (b) until the residual radiation reaches a termination criterion.

Description:
BACKGROUND 
       [0001]    A pyrometer may be used to measure temperature of an object without making physical contact with the object. For instance, radiation from an object may be focused onto a detector, whose output is related to irradiance of the object. The temperature of the object may be inferred from the measured irradiance and known emissivity of the object. 
         [0002]    A multi-spectral pyrometer eliminates the need to know the emissivity of an object. Temperature measurements by a multi-spectral pyrometer are based on a gray body assumption whereby emissivity at different wavelengths is assumed to be constant. However, the gray body assumption is not valid for metals and other materials that do not have the same emissivity at two different wavelengths, thus leading to erroneous results if used to measure a multi-temperature profile. 
         [0003]    Consider the example of low emissivity glass manufacturing, where different grades of low emissivity glass have very low, yet different, emissivity values. These emissivity values may not be known, especially at their respective processing temperatures. A multi-spectral pyrometer would produce an erroneous profile. 
       SUMMARY 
       [0004]    According to an embodiment herein, a method involves determining multiple temperatures of an object from spectral data collected from the object. The spectral data covers a plurality of wavelengths. The method comprises using a computer to (a) assign an initial value for residual radiation; (b) identify a black body profile that best fits the spectral data over the plurality of wavelengths; (c) remove radiation corresponding to the identified profile from the residual radiation; and (d) return to (b) until the residual radiation reaches a termination criterion. 
         [0005]    According to another embodiment herein, a method of determining multiple temperatures of an object from spectral data covering a plurality of wavelengths comprises using a computer to set residual radiation to total radiation in the spectral data; compute a black body profile for each of a plurality of temperatures for each one of the wavelengths; select one of the profiles that best fits the spectral data; infer a temperature from the selected profile; and remove radiation contributed by the selected profile from the residual radiation. Control is returned to computing another black body profile for each of a new plurality of temperatures. The new plurality doesn&#39;t include any inferred temperature. 
         [0006]    According to another embodiment herein, an apparatus comprises at least one thermal sensor for capturing spectral data over a plurality of wavelengths, and a processor programmed to assign an initial value for residual radiation; identify a black body profile that best fits the spectral data over the plurality of wavelengths; remove radiation corresponding to the identified profile from the residual radiation; and return to identifying a black body profile for another temperature until the residual radiation reaches a termination criteria. 
         [0007]    These features and functions may be achieved independently in various embodiments or may be combined in other embodiments. Further details of the embodiments can be seen with reference to the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is an illustration of a method of determining multiple temperatures of an object. 
           [0009]      FIG. 2  is an illustration of a method of computing black body power per unit area. 
           [0010]      FIG. 3  is an illustration of spectral data covering a plurality of wavelengths, where some of the wavelengths are not useful. 
           [0011]      FIG. 4  is an illustration of a method of determining multiple temperatures of an object. 
           [0012]      FIG. 5  is an illustration of a system for collecting radiation data from an object and processing the data to produce a multi-temperature profile for the object. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Reference is made to  FIG. 1 , which illustrates a method of determining multiple temperatures of an object from spectral data collected from a spot on the object. The spectral data is collected by one or more sensors that are sensitive to a plurality of wavelengths. Therefore, the collected spectral data may cover a plurality of wavelengths. 
         [0014]    At block  110 , an initial value for residual radiation is assigned. For example, the residual radiation may be set equal to the total radiation collected by the sensors. 
         [0015]    At block  120 , a black body radiation profile that best fits the spectral data over the plurality of wavelengths is identified. A black body radiation profile, as used in this application, refers to an emitted radiation profile according to Plank&#39;s law, where an ideal emitter (a black body) has emissivity of 1 and a non-ideal emitter (gray body) has an emissivity of less than 1. That is, the black body profile as used in this application covers black body radiation and gray body radiation. 
         [0016]    At block  130 , a temperature of the object may be inferred from the identified profile. That temperature is considered to be one of the temperatures of the object. 
         [0017]    At block  140 , radiation corresponding to the identified profile is removed from the residual radiation. For instance, if the profile indicates spectral radiance per unit area, then the radiation to be removed is a product of the spectral radiance per unit area and an area-emissivity factor. 
         [0018]    At block  150 , a termination criterion is tested. If the residual radiation does not satisfy the termination criteria (e.g., the residual radiation is above a level of noise), it is assumed that one or more additional temperatures of the object have not yet been identified. Therefore, control is returned to block  120 , and another black body profile is identified from the spectral data. 
         [0019]    If, however, the termination criterion has been satisfied, then all temperatures of the object are assumed to have been identified, and the method is terminated (block  160 ). For example, the method is terminated if all that remains in the residual radiation is noise. All saved temperatures may be displayed. 
         [0020]    Reference is now made to  FIG. 2 , which illustrates an example of a method of identifying a black body profile that best fits spectral data over a plurality of wavelengths. At block  210 , a black body profile is computed for each of a plurality of different temperatures for each of the plurality of wavelengths. For instance, a first set of profiles is computed for N temperatures at a first wavelength; a k th  set of profiles is computed for N temperatures at a k th  wavelength; and so on until a last set of profiles is computed for N temperatures at a last wavelength. 
         [0021]    At block  220 , the black body profile that best fits the spectral data over the plurality of wavelengths is selected. This selection may be made by computing a ratio of residual radiation to black body radiation for each of the temperatures, and then identifying the global minimum of standard deviation of the ratio. The black body profile corresponding to the global minimum is deemed the best fit. The identification of a black body profile that best fits the spectral data is described in greater detail in assignee&#39;s U.S. Pat. No. 7,891,866. 
         [0022]    Multiple sets of discrete wavelengths improve the accuracy of the calculation. With the method utilizing a statistical approach for selecting black body profiles, accuracy may be increased as the number of wavebands (or discrete wavelengths) increases. The inventors have found that a minimum of ten discrete wavelengths should be considered. 
         [0023]    Reference is now made to  FIG. 3 , which illustrates an example of spectral data covering a plurality of wavelengths. In this example, some of the spectral data might not be useful. Consider the situation where there is strong atmospheric absorption in several regions (R 1 , R 2  and R 3 ) of a spectrum. Those absorption regions (R 1 , R 2  and R 3 ) would not be useful for calculating temperature since they do not look like a black body. 
         [0024]    Discrete wavelengths within those absorption regions (R 1 , R 2  and R 3 ) may be excluded from consideration. Thus, black body profiles are not computed at those discrete wavelengths, and the radiation collected at those discrete wavelengths is not added to the total collected radiation. Black body profiles may be computed for some or all of the discrete wavelengths outside of those regions (R 1 , R 2  and R 3 ). That is, black body profiles may be computed for some or all of the wavelengths within regions R 4  to R 7 . For instance, black body profiles are computed for wavelengths at 4, 5, and 8 to 14 microns. 
         [0025]    Reference is now made to  FIG. 4 , which illustrates an example of a method of determining multiple temperatures of an object. At block  405 , spectral data is collected from the object. The collection of spectral data is not limited to any particular type or number of sensors, so long as the spectral data covers the requisite wavelengths. In some instances, a single sensor may be capable of detecting radiation over the requisite wavelengths. However, a greater number of sensors and their associated wavelength resolutions may improve accuracy by providing more data samples for the statistical analysis 
         [0026]    At block  410 , residual radiation (I residual ) is set equal to the total spectral radiation collected by the sensors (I collected ). Radiation from any non-useful wavelengths is not added to the total collected spectral radiation. 
         [0027]    At block  415 , a range of temperatures may be identified for each of the wavelengths to be considered. The range may be defined by a maximum temperature (Tmax) and a minimum temperature (Tmin) for each wavelength. The range may exclude those wavelengths that are not useful. Use of the range is more efficient, as it reduces the temperature search space. 
         [0028]    At block  420 , the black body radiation profile (I BB ) is computed for the absolute temperature T=Tmin for each wavelength λ. For instance, each black body profile may be computed as follows: 
         [0000]    
       
         
           
             
               I 
               BB 
             
             = 
             
               
                 
                   2 
                    
                   π 
                    
                   
                       
                   
                    
                   
                     hc 
                     2 
                   
                 
                 
                   λ 
                   5 
                 
               
                
               
                 1 
                 
                   
                      
                     
                       hc 
                       
                         λ 
                          
                         
                             
                         
                          
                         
                           k 
                           B 
                         
                          
                         T 
                       
                     
                   
                   - 
                   1 
                 
               
             
           
         
       
     
         [0000]    where h is the Planck constant, c is the speed of light, and k B  is the Boltzmann constant. Since the profile is for black body radiation, the pattern of the intensity of the radiation over wavelengths depends only on the absolute temperature T. 
         [0029]    At block  425 , a ratio (r) is computed for each profile as 
         [0000]    
       
         
           
             r 
             = 
             
               
                 
                   I 
                   residual 
                 
                 
                   I 
                   BB 
                 
               
               . 
             
           
         
       
     
         [0030]    At block  430 , a standard deviation (σ r ) of each ratio (r) is computed. Each standard deviation (σ r ) represents a variation of the spectral data from the profile of the black body over the set of temperatures. The standard deviation (σ r ) may be computed as a square root of variance, where variance is the value minus the average quantity squared. 
         [0031]    At blocks  435  and  440 , the temperature (T) is incremented (T=T+ΔT) and another black body profile (I BB ), ratio (r) and standard deviation (σ r ) are computed (according to blocks  420 - 430 ) for each wavelength. As a result, additional profiles (I BB ), ratios (r) and standard deviations (σ r ) are computed for additional temperatures up to, and including T max . 
         [0032]    At block  445 , the temperature (T) corresponding to a global minimum of ratio standard deviation is identified. This may be done by analyzing first and second derivatives of the ratio standard deviations. Blocks  435  and  440  produce a discrete function of temperature (T) versus standard deviation (σ) for each of the black body profiles. The first derivative may be computed as change in temperature over a change in standard deviation (dT/dσ) and the second derivative may be computed as dT 2 /dσ 2 . This temperature corresponding to the global minimum is saved as one of the temperatures of the object. 
         [0033]    At block  450 , the residual radiation is reduced by removing the contribution from the temperature T found in block  445 . For example, the residual radiation may be reduced as follows: 
         [0000]        I   residual   =I   collected   −r ( I   BB ). 
         [0000]    The residual radiation may be reduced in this manner to ensure that the profile corresponding to the found temperature is properly removed at its given weight (r) within the total intensity spectra that was measured. 
         [0034]    At block  455 , a termination criterion is tested. For example, if the sum of the residual intensities is greater than a pre-defined percentage of the sum of the collected intensities, then it is assumed that additional temperatures can be found. Therefore, blocks  415 - 455  are repeated for a different profile, except that profiles are not computed for any of the identified temperatures identified at block  445 . 
         [0035]    If the termination criterion has been satisfied, then all temperatures are assumed to have been found. At block  460 , the temperatures may be displayed. 
         [0036]    Thus, a method herein measures multiple temperatures of an object without physical contact of the object. Moreover, the temperatures are measured without knowledge of the object&#39;s emissivity and without having to make assumptions about the emissivity. 
         [0037]    The method is not limited to collecting data from a single spot on an object. Spectral data may be collected from multiple spots, and the spectral data from each spot may be processed. 
         [0038]    Reference is now made to  FIG. 5 , which illustrates an example of a system  510  for collecting radiation data from an object. The spectral data is collected from a “spot” on the object. The system  510  of  FIG. 5  includes first, second and third thermal radiation sensors  520 ,  522  and  524 . These sensors  520 ,  522  and  524  provide direct values for detected spectral radiation. 
         [0039]    The system  510  further includes optics for collecting spectral radiation from a spot on an object, and focusing the spot onto the sensors  520 ,  522  and  524 . Spot size is determined by the combination of sensor size and sensitivity, the optics, and the distance from the sensors  520 ,  522  and  524  to the object. Generally a larger spot is more desirable than a smaller spot. More energy is collected, which results in a better the signal-to-noise ratio. 
         [0040]    The example of  FIG. 5  includes the following optics. Radiation is admitted through an aperture stop  530  and focused by a collection lens  540  at a field stop  550 . (The field stop  550  ensures that sensor collection area is filled.) Radiation passing through the field stop  550  is collimated by a collimating lens  560 , and focused by a cylindrical focusing lens  570 . The radiation focused by the lens  570  is then separated into first, second and third bands by optical elements  580  and  582 . The first band is impinged onto the first sensor  520  (which is sensitive to wavelengths in the first band), the second band is impinged onto the second sensors  522 , (which is sensitive to wavelengths in the second band), and the third band is impinged onto the third sensor  520  (which is sensitive to wavelengths in the third band). There may be overlap between the first, second and third bands. 
         [0041]    In some embodiments, a scanning spectral filter may be used instead of multiple sensors. For example, a multi-element sensor may be used with a linear variable filter to obtain the spectrum. 
         [0042]    The system  510  further includes a machine  590  such as a computer for processing the spectral data to determine a multi-temperature profile of the object. The machine  590  may include a processor  592 , machine-readable memory  594 , and data  596  stored in the memory  594 . When executed, the data causes the processor  592  to process the sensor data to produce a multi-temperature profile of the object as described above. 
         [0043]    Non-contact measurement based on a method herein may be used in manufacturing environments to accurately monitor temperatures during processing. It may also be applied to remote sensing devices to determine temperature profiles of distant objects of interest. 
         [0044]    As a first example, the non-contact measurement herein may be applied to low emissivity glass manufacturing involving grades of low emissivity glass with very low yet different emissivity values. These emissivity values may not be known especially at the respective processing temperatures. Even though the emissivity values are not known, the non-contact measurement herein still provides a multi-temperatures profile. Moreover, the profile is accurate so as to maintain tight temperature controls of the glass manufacturing. 
         [0045]    As a second example, the non-contact measurement herein may be applied to steel manufacturing. The non-contact measurement herein may be used to monitor temperatures of individual materials, iron ore, lime, coal, and coke to ensure quality. In addition, the non-contact measurement herein may be used to measure maximum processing temperature to prevent damage to critical manufacturing equipment. 
         [0046]    As a third example, the non-contact measurement herein may be applied to high temperature processing in refractory lined vessels. The non-contact measurement may be used to detect hot spots. The hot spots may indicate where the refractory lining is worn away or damaged. 
         [0047]    Another example is thermal monitoring systems in plastic extrusion and plastic thermoforming processes. Still another example is plasma temperature measurements.