Patent Application
20070177650
1. Field of the Invention
The present invention generally relates to a system for optical pyrometry for use in combustion devices.
2. Description of Related Art
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.
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.
In view of the above, it is apparent that there exists a need for an improved system for video pyrometry.
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.
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.
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.
Referring now to
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.
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.
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.
The dual band pass filter 58 is designed to pass two narrow bands, as denoted by reference numerals 70 and 72 in
The spectral responses for an RGB color camera are provided in
W(λ, T)=ε*C1/(λ5*(exp(C2/λT)−1)) (1)
Where,
W(λ, T)—spectral radiant emittance of object or medium,
ε—emissivity of object or medium,
λ—wavelength of radiation,
T—temperature of object or medium, and
C1, C2—constants
For two-color pyrometry, two different wavelengths are selected where the emissivities are either equal or have a constant ratio, yielding two equations:
W1(λ1,T)=ε1*C1/(λ25*(exp(C2/λ1T)−1)) (2)
and
W2(λ2,T)=ε2*C1/(λ25*(exp(C2/λ2T)−1)) (3)
Where W1 and W2 are the measured spectral emittances at the selected wavelengths λ1 and λ2 and ε1 and ε2 are the emissivities at each respective wavelength.
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.
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=(C2*((1/λ2)−(1/λ1)))/In((1/λ1)/(1λ2)5*(W1/W2)) (4)
Noting that (C2*((1/λ2)−(1/λ1)))/In((1λ1)/(1λ2)5 is constant for any wavelength pair at all temperatures, the ratiometric equation can be further simplified to:
T=K*(W1/W2)) (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.
The spectral responses for a CyGrMgYe complementary color camera are provided in
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*(W1/W2). 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.
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.
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.
