Patent Document

BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to a system for optical pyrometry for use in combustion devices.  
         [0003]     2. Description of Related Art  
         [0004]     Optical pyrometry is a measurement technique in which the temperature of an object or medium is determined based on the spectral radiant emittance of the object or medium. Such techniques are used in various applications, including evaluation of combustion processes and the state of fouling of surfaces within a large scale combustion device. Typically, video pyrometers for such applications utilize two optical paths such that one wavelength band of light is processed down the first optical path and a second wavelength band of light is processed down the second optical path. Each optical path creates two separate images that are focused onto two monochrome video cameras or on two non-overlapping areas of a single monochrome video camera. One such design is provided in U.S. Pat. No. 5,225,893.  
         [0005]     In the case of the above-referenced prior art, the coincident optical paths require very precise spatial alignment of the images on the camera or cameras as well as optical path length equalization to ensure proper convergence and focus of the images for dual wavelength pyrometry calculations. Variations in the spatial alignment or optical path length due to misalignment, vibration, and thermal expansion result in large temperature measurement errors and poorly defined images.  
         [0006]     In view of the above, it is apparent that there exists a need for an improved system for video pyrometry.  
       SUMMARY OF THE INVENTION  
       [0007]     In satisfying the above need, as well as overcoming the enumerated drawbacks and other limitations of the related art, the present invention provides an improved system for video pyrometry for use in combustion devices.  
         [0008]     The system of this invention uses a color camera and an optical system to map two colors emitted from an object such as a furnace, boiler combustion zone, or burner flame into a temperature image. The color camera utilizes a color video chip with interspersed pixels for each color to reduce alignment issues and utilize the same optical path. An RGB (red-green-blue) or CyGrMgYe (cyan-green-magenta-yellow) color video camera may be readily utilized in the system. In addition, the optical system utilizes a single dual band pass filter thereby eliminating the number of optical elements and minimizing radiation loss through the optical system thereby improving the dynamic range of the system.  
         [0009]     Further objects, features and advantages of this invention will become readily apparent to persons skilled in the art after a review of the following description, with reference to the drawings and claims that are appended to and form a part of this specification. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a schematic view of a video pyrometry system in accordance with the present invention;  
         [0011]      FIG. 2  is a graph illustrating the transmission characteristics of a dual mode band pass filter in accordance with the present invention;  
         [0012]      FIG. 3  is a graph of the peak spectral responses for an RGB color camera in accordance with the present invention; and  
         [0013]      FIG. 4  is a graph of the peak spectral responses for a CyGrMgYe four color camera in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     Referring now to  FIG. 1 , a system embodying the principles of the present invention is illustrated therein and designated at  50 . As its primary components, the system  50  includes an optical system  57  and a color video camera  62 .  
         [0015]     The system  50  provides for remote viewing and an isothermal contour temperature mapping of an object  52 , such as a furnace, boiler combustion zones, and burner flames. Although primarily intended for fireside furnace or boiler temperature measurements, the system  50  can also accurately measure temperatures of any object or medium that are radiating within the spectral and illuminance ranges of the color camera  62 . The object  52  emits optical radiation as denoted by line  54 . The optical radiation  54  is transmitted from the object  52  and is received by the optical system  57 .  
         [0016]     The optical system  57  includes an objective lens  56  that forms a focused image of the object  52  on the color detector  60  of the color camera  62 . The objective lens  56  is in optical communication with a dual band pass filter  58 . The dual band pass filter  58  transmits two wavelength bands of light but blocks other wavelengths of light. Light that is transmitted through the dual band pass filter  58  reaches the color detector  60  where it is sensed by the color camera  62 . Accordingly, the system  50  does not require two separate optical paths, instead it uses the dual band pass filter  58  and a single optical path to form an image on a single color detector  60  of the color camera  62 . Since the two colors are inseparably focused on each pixel of the color camera  62  there is no need for spatial alignment of multiple CCD arrays. Further, since two colors use the same optical path, there is no need for path length equalization.  
         [0017]     In addition, the color camera  62  may be a conventional three color RGB (red-green-blue) type camera or the color camera  62  may be a newer four color complementary CyGrMgYe (cyan-green-magenta-yellow) type camera. Each color represents a set of pixels that are sensitive to a certain wavelength band of visible light. Each set of pixels are interspersed in an alternating pattern on the color detector  60  of the color camera  62 . Other single detector color cameras having multiple color pixels interspaced may also be substituted for the above-mentioned cameras. However, the above referenced cameras provide a standard interface allowing the two colors to be easily displayed and processed with a variety of hardware and software packages. Although the spectral responses may be different for each type of camera, the dual band pass filter  58  can be designed for the selected camera. In addition, using commonly available color cameras and visible spectrum optics allow low cost and readily available components to be used providing an elegant commercial solution.  
         [0018]     The dual band pass filter  58  is designed to pass two narrow bands, as denoted by reference numerals  70  and  72  in  FIG. 2 . Each wavelength band  70 ,  72  may correspond to the sensitivity band of a set of pixels. Further, each band  70 ,  72  may be more narrow or restrictive than the corresponding sensitivity bands of each set of pixels. Band  70  has a minimum cutoff wavelength of WL1 and a maximum cutoff wavelength of WL2. Accordingly, the bandwidth of band  70  is the range between WL1 and WL2, namely BW1. Similarly, band  72  has a minimum cutoff wavelength of WL3 and a maximum cutoff wavelength of WL4. Accordingly, the bandwidth of band  72  is BW2. The dual band pass filter  58  can be implemented by constructing a special optical filter that passes only the selective wavelength bands or by integrating three separate optical filters into a single optical device, such as a short pass filter, a long pass filter, and a notch filter to generate two modes according to band  70  and band  72 . When fabricating the dual band pass filter  58  from three overlaying filters, the short pass filter is selected to pass wavelengths up to the longest wavelength of band  72  (WL4) and the long pass filter is selected to pass wavelengths down to the shortest wavelength of band  70  (WL1). The two filters together form a very wide band pass filter passing all wavelengths between WL1 and WL4. The notch filter is selected to block wavelengths between the longest wavelength of band  70  (WL2) and the shortest wavelength of band  72  (WL3). As such, the notch filter passes wavelengths up to WL2, blocks wavelengths between WL2 and WL3, and passes wavelengths above WL3. The spectral response is the product of the three filters with the center wavelengths of (WL1+WL2)/2 for band  70  and (WL3+WL4)/2 for band  72 . Further, the band width BW1 of band  70  is WL2−WL1 and the band width BW2 for band  72  is WL4−WL3. Further, the dual band pass filter may also be fabricated using two filters. For example, one very wide band pass filter may be utilized to pass wavelengths between WL1 and WL4 and a notch filter used to block wavelengths between WL2 and WL3.  
         [0019]     The spectral responses for an RGB color camera are provided in  FIG. 3 , the spectral response for red is denoted by reference numeral  80 , while the spectral responses for green and blue are denoted by reference numeral  82  and  84 , respectively. In order to obtain the best optical signal and most accurate color to temperature calculation, the two bands BW1 and BW2, of the dual band pass filter should closely match any two of the color camera spectral peaks. In the case of an RGB type color camera, the peak spectral responses are centered at approximately 470 nanometers for blue, 540 nanometers for green, and 650 nanometers for red. Therefore, the dual band pass filters should be centered at 470 nanometers for band  70  and 540 nanometers for band  72 , 470 nanometers for band  70  and 650 nanometers for band  72 , or 540 nanometers for band  70  and 650 nanometers for band  72 . By limiting the spectral response to the narrow band wavelengths, Plank&#39;s law, provided in equation 1 below, may be used to solve for the temperature at each pixel on the color detector  60 .
 
 W (λ,  T )=ε* C 1/(λ 5 *(exp( C 2 /λT )−1))  (1)
 
 Where, 
 
         [0020]     W(λ, T)—spectral radiant emittance of object or medium,  
         [0021]     ε—emissivity of object or medium,  
         [0022]     λ—wavelength of radiation,  
         [0023]     T—temperature of object or medium, and  
         [0024]     C1, C2—constants  
         [0025]     For two-color pyrometry, two different wavelengths are selected where the emissivities are either equal or have a constant ratio, yielding two equations:
 
 W   1 (λ 1   ,T )=ε 1   *C 1/(λ 2   5 *(exp( C 2/λ 1   T )−1))  (2)
 
 and
 
 W   2 (λ 2   ,T )=ε 2   *C 1/(λ 2   5 *(exp( C 2/λ 2   T )−1))  (3)
 
 Where W 1  and W 2  are the measured spectral emittances at the selected wavelengths λ 1  and λ 2  and ε 1  and ε 2  are the emissivities at each respective wavelength. 
 
         [0026]     The simultaneous solution (an algebraic operation) of these equations provides the temperature T since all other terms of these equations are either known or equal.  
         [0027]     When relatively short wavelengths are used, such as the visible spectrum (380 to 780 nanometers), the “−1” term can be neglected in both equations allowing a simpler simultaneous solution that yields the single ratiometric equation:
 
 T =( C 2*((1/λ 2 )−(1/λ 1 )))/In((1/λ 1 )/(1λ 2 ) 5 *( W   1   /W   2 ))  (4)
 
 Noting that (C2*((1/λ 2 )−(1/λ 1 )))/In((1λ 1 )/( 1 λ 2 ) 5  is constant for any wavelength pair at all temperatures, the ratiometric equation can be further simplified to:
 
 T=K *( W   1   /W   2 ))  (5)
 
 In the case of two-color video pyrometry, the spatial distribution of temperature can be ascertained by solving for the temperature T for each camera pixel. 
 
         [0028]     The spectral responses for a CyGrMgYe complementary color camera are provided in  FIG. 4 . The spectral response for cyan is denoted by reference numeral  90 , while the spectral responses for green, magenta, and yellow are denoted by reference numerals  92 ,  94 , and  96 , respectively. In the case of a complementary color camera, the peak spectral responses are at approximately 450 nanometers and 610 nanometers for magenta, 510 nanometers for cyan, 540 nanometers for green, and 550 nanometers for yellow. Any two of these peak wavelengths can be used for two color temperature calculations. However, for the best color to temperature measurement accuracy, peak wavelengths pairs that have a large response overlap should be avoided. For example, using green and yellow might be difficult due to the large overlap in peak wavelength of the spectral response. However, the following pairs of wavelengths may be effectively used: 450 nanometers and 540 nanometers (Mg and Gr channels), 450 nanometers and 550 nanometers (Mg and Ye channels), 610 nanometers and 510 nanometers (Mg and Cy channels), or 610 nanometers and 540 nanometers (Mg and Gr Channels). The combination of the dual band pass filter  58  along with the internal color filters of the color camera  62  provide a dual wavelength multi-pixel pyrometer that provides the two radiance values W1 and W2 for the simple radiometric equation T=K*(W1/W2) in a standard color video signal format such as RS-170A for each pixel in the field of view. Where K is equal to a constant to adjust for the sensitivity of the system  50  between the two radiance values.  
         [0029]     The video processor  64  receives the radiance values W1 and W2 as separate colors in the standard color video signal format and calculates the temperature for each pixel using the simple radiometric equation T=K*(W 1 /W 2 ). Accordingly, the video processor  64  provides a real time isothermal contour map of the temperature distribution of the object  52  as a standard color video signal to the video display  66 . Additionally, the video processor utilizes the video signals provided to generate video of the field of view according to one or both of the received colors.  
         [0030]     Further, greater than two wavelengths may be used in the same manner as described above and the results combined to provide a temperature measurement. In the case of an RGB color detector, all three channels would be used and a three mode band pass filter would be substituted for the dual mode filter described above.  
         [0031]     As a person skilled in the art will readily appreciate, the above description is meant as an illustration of implementation of the principles this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from the spirit of this invention, as defined in the following claims.

Technology Category: 3