CN107084794B - Flame three-dimensional temperature field measuring system and method based on light field layered imaging technology - Google Patents
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Abstract
The invention discloses a flame three-dimensional temperature field measuring method system and method based on a light field layered imaging technology. The system comprises a precision optical platform and a slide rail; the light field camera is used for acquiring four-dimensional light field information of the flame; and the computer is used for storing information and calculating and analyzing. The invention obtains the three-dimensional flame light field information by using the light field camera, obtains the flame radiation intensity value on each layer by carrying out digital refocusing processing and deconvolution processing on the image, and further obtains the temperature distribution of the flame. The measuring system can obtain the information of each focusing plane by taking a picture once without zooming or changing the position of a camera, and can realize high-time-precision optical non-contact measurement on flame which changes constantly.
Description
Technical Field
The invention belongs to the technical field of flame temperature measurement, and particularly relates to a combustion flame three-dimensional temperature field measurement system and method based on light field layered imaging.
Background
The light field camera is provided with a micro lens array between the main lens and the image sensor, and the imaging system can simultaneously distinguish the intensity information and the direction information of light rays, so that the sensor can collect four-dimensional light field data of a shot object. The light field camera has the characteristics of photographing first and focusing later, can effectively capture scene depth in a wider range and display the three-dimensional structure of a real scene, and the light field imaging technology is gradually applied to the fields of aerospace, three-dimensional reconstruction, safety monitoring and the like.
The flame temperature field distribution is an important standard for judging the flame combustion state. The measurement of the three-dimensional flame temperature field distribution is realized, and the method has great significance for revealing the nature of the combustion phenomenon and the law of the combustion process and promoting the development of the combustion theory. At present, the flame temperature field measuring method can be mainly divided into a contact type and a non-contact type. The non-contact measurement method has the advantages of wide measurement range, fast dynamic response, small influence on a flow field and the like. With the development of CCD image sensor technology and computer vision theory, non-contact optical measurement methods are used by more and more researchers.
The following methods are mainly used for the non-contact measurement of flame at present. The method comprises the steps of obtaining a flame section from top to bottom based on a CT chromatography method of projection, carrying out three-dimensional reconstruction on a flame temperature field by utilizing an interpolation algorithm, reconstructing the flame temperature field by utilizing a holographic interference CT method and an iterative reconstruction technology, utilizing a flame temperature field CT measurement method of Moire deflection and the like. However, these chromatographic measurement methods generally require many devices, have complicated optical paths and long time consumption, and are not easy to be popularized in the measurement of industrial flame temperature fields, and the CT chromatographic method has a large error in reconstructing flame information in the absence of large-range angle information. Researchers also use a single-camera single-light-path system to reconstruct the cross-sectional temperature distribution of the gas flame, but in the experiment, the flame shape and the structure are assumed to be axisymmetric and not in accordance with the actual condition of the flame. The method is simple in equipment and high in calculation speed, but after flame is shot each time, a motor is used for driving the camera to change the shooting position of the camera or drive a lens to zoom, the flame changes every moment, and a large delay error exists in an image sequence acquisition mode of the method.
Disclosure of Invention
The invention utilizes the characteristics of a light field camera, can store the four-dimensional light field data of the space on the same original light field image by one-time shooting, obtains flame image sequences focused on different fault planes of flames by combining a refocusing technology, obtains a point spread function of the images by an experimental method, and finally can realize high-precision and real-time flame layered imaging by utilizing an image inversion algorithm so as to realize the reconstruction of three-dimensional temperature field distribution.
The invention discloses a flame three-dimensional temperature field measuring system based on a light field layered imaging technology, which comprises a flame generating device, a light field camera and a data analyzing and storing device, wherein the flame generating device is used for generating flame; wherein:
the flame generating device is used for generating flame;
the light field camera is used for acquiring four-dimensional light field information of flames on different geometric faults and transmitting the acquired four-dimensional light field information to the data analysis and storage device;
the data analysis and storage device adopts a refocusing technology to refocus the four-dimensional light field information of the flame fed back by the light field camera to obtain flame image sequences focused on different geometric fault planes of the flame; then, performing inversion calculation on the obtained flame image sequence by adopting a deconvolution technology, inverting the original information of each fault, and further obtaining the flame radiation intensity value on each fault so as to obtain the temperature distribution of the flame;
the flame generated by the flame generating device is positioned in the shooting visual field of the light field camera, and the light field camera is in signal connection with the data analysis and storage device.
The invention also discloses a flame three-dimensional temperature field measuring method based on the light field camera layered imaging technology, which comprises the following steps:
the method comprises the following steps: according to the size of the flame, carrying out geometric stratification on the flame, and calibrating the distance from each geometric fault position to a light field camera;
step two: setting a calibration plate at a corresponding position of each flame geometric fault, and acquiring an image of the calibration plate to obtain a point spread function at the corresponding position;
step three: photographing the flame to obtain four-dimensional light field information of the flame, and performing refocusing processing on the four-dimensional light field information of the flame to obtain a flame superposition image sequence of the flame focused in different geometric faults;
step four: combining the point spread functions of the flames on different geometric faults obtained in the step two, carrying out inversion calculation on the flame superposed image sequence on the corresponding geometric fault, and inverting the original light brightness distribution information of each fault;
step five: and solving the temperature field distribution of each geometric fault of the flame according to the corresponding relation between the image gray scale and the radiation intensity and the temperature, thereby realizing the reconstruction of the three-dimensional temperature field distribution.
Has the advantages that: compared with the prior art, the method has the following advantages: the light field camera can acquire four-dimensional light field information by taking a picture once, the flame image of each focusing plane can be acquired by utilizing a refocusing technology without zooming a lens or changing the position of the camera, and the flame image has extremely high time resolution for real-time changing flames; the calibration method of the flame position and the point spread function is simple, the operation speed is high, and the flame can be separated from the flame for independent calibration; the measuring system adopts a single light path, has less equipment, simple operation, less limitation on measuring environment and small influence of a non-contact device structure on the temperature distribution of flame.
Drawings
FIG. 1 is a refocusing distance calibration schematic.
Fig. 2 is a sharpness profile.
Fig. 3 plots straight edge regions.
Fig. 4 a grey scale fit plot.
Fig. 5 a schematic diagram of imaging of a transparent luminophore.
FIG. 6 flame light field raw image.
Fig. 7 is an apparatus diagram of the present measurement method.
In fig. 7: 1-candle; 2-a light field camera; 3-computer.
FIG. 8 flame refocused image.
FIG. 9 is a graph of the original luminance of the flame.
FIG. 10 is a flame temperature field profile.
Detailed Description
The invention is further illustrated with reference to the following figures and specific examples. It is to be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present invention, which is to be given the full breadth of the appended claims and any and all equivalent modifications thereof which may occur to those skilled in the art upon reading the present specification. In the present embodiment, a light field camera of R29 model from raytrix, germany, was used as the light field camera.
The invention relates to a flame three-dimensional temperature field method based on a light field camera layered imaging technology, which comprises the following steps of:
the method comprises the following steps: refocusing the inclined scale with scales by using the method shown in fig. 1 to obtain a picture containing distance information and with distinct definition, dividing the taken picture of the scale into different parts by equally dividing the picture into N parts along the scale direction, and calculating the definition of the EVA point sharpness function for each part, wherein the formula is as follows:
in the formula: df represents the amplitude of the gray level change of the image, dx represents the distance between the phase elements, df/dx takes 8 neighborhoods of the pixel when calculating, and M represents the pixel number of the image.
And (3) obtaining a definition curve chart, as shown in fig. 2, wherein the part with the maximum definition is a refocusing focus plane, and converting the scale corresponding to the focus plane into a horizontal distance D to realize the calibration of the refocusing position and the distance.
Step two: and dividing the flame area into 4 layers, wherein the position of each fault plane is obtained by calculation in the step one. The calibration plate is respectively placed at the positions of flame layers 1, 2, 3 and 4, and the camera acquires 4 images of the calibration plate at each layered position. Adopting a Gaussian point spread function, and expressing the formula as follows:
in the formula, σ is a parameter of the gaussian defocus model.
The integral of the point spread function along the line direction is a line spread function, the physical meaning of which is the brightness distribution imaged by the line light source. Integration of equation (2) along the y-direction yields a line spread function l (x), which also follows a gaussian distribution:
two adjacent bright and dark image areas with straight edges and a certain contrast are selected for operation, as shown in fig. 3. A straight-sided light source can be represented by a step function:
step (x) can be written as the integral of line source δ (x), i.e.:
the straight edge gets a blurred image after the imaging system is out of focus, and the gradual transition from one gray level to another gray level is called as a knife edge function. According to the linear superposition principle of the system, this edge function e (x) can be written as the integral of the line spread function, i.e.:
conversely, the line spread function can also be determined from the edge function differentiation.
And performing least square fitting on the gray value of the straight-edge image area according to a step function (x) of a straight-edge light source to obtain a fitting curve as shown in FIG. 4, and determining a fitted edge function so as to obtain a Gaussian defocusing PSF function (Gaussian point spread function).
Step three: the flame is photographed to obtain the four-dimensional light field information of the flame and the refocused image E of the flameF(x, y) and light LαFThe following relationship exists between:
α is a coefficient for adjusting the size of the image distance l'. And changing the position parameter alpha to obtain image sequences on image surfaces of different focusing planes. (u, v) represents a point on the lens plane of the light field camera, and (x, y) represents a point on the image plane of the focal plane, the points (u, v) and (x, y) corresponding one-to-one.
Step four: the flame, as a three-dimensional translucent emitter with a thickness δ, can be seen as a combination of a sequence of N emission planes, as shown in fig. 5. The optical system focuses the z ' plane to image, and the obtained image g (x, y, z ') is the superposition image of the focused image of the z ' plane and the out-of-focus image of other planes. According to the Fourier optics theory, for a linear shift-invariant optical imaging system, the brightness distribution function g (x, y, z') at the image plane is the actual intensity function f (x, y, z) at the corresponding object plane and the point spread function h of the imaging optical systemz′-z(x, y), and thus g (x, y, z') can be expressed as:
hz′-z(x, y) is the point spread function when the optical system is focused in the z' plane and imaged in the z plane, and f (x, y, z) indicates the point spread function hz′-z(x, y) an actual light intensity function at the corresponding object plane;
discretizing equation (9):
in the formula, the number of layers N is δ/Δ z, and Δ z is a distance between each layer, that is, a three-dimensional light-emitting object is regarded as a combination of N parallel two-dimensional light-emitting planes.
The refocused flame image sequence can thus be represented in the form:
the light field camera takes the original light field image of the flame as shown in fig. 6, and the device diagram is shown in fig. 7. Digital refocusing was performed to obtain a flame sequence image g (x, y, j Δ z) as shown in fig. 8. After the PSF information of the optical system under different refocus planes is calibrated in the third step, only N unknown quantities f (x, y, i Δ z) are obtained in the formula (7), the equation set is solved by combining deconvolution, and the solved f (x, y, i Δ z) is the original luminance distribution of the i fault plane of the flame, as shown in fig. 9.
Step five: for a luminous flame, the intensity L of the monochromatic radiation of the flame in the visible rangeλThe relationship with the temperature T satisfies the plating law, and the formula is as follows:
in the formula: ε is the blackness: c. C1Is a first radiation constant; c. C2Is a second radiation constant; λ is the wavelength; t is the temperature;
according to the gray level g and the intensity L of monochromatic radiation of the flameλThe relationship of (1):
g＝ΦtLλ(D/f)2 (13)
where D/f is the lens aperture ratio, phitIs the instrument constant.
The distribution of the temperature field can be obtained by combining the formulas (12) and (13), as shown in FIG. 10. As can be seen from the figure, in appearance, the image areas of the 1 st layer and the 4 th layer are smaller, the areas of the 2 rd layer and the 3 rd layer are larger, and the height and the width of the inversion image both accord with the structure of the flame layered appearance. In temperature, the low-temperature regions of the 1 st and 4 th layers are low, and the low-temperature regions of the 2 nd and 3 rd layers are high; the flame distribution of each layer is the highest temperature at the edge of the middle part, the flame distribution of each layer has obvious difference, the combustion condition and the layered temperature structure in the flame are reflected, and the temperature value obtained by inversion also accords with the real temperature of the flame. The result shows that the inversion result of the method basically accords with the real flame condition, and the measurement of the flame three-dimensional temperature field can be realized.
Claims (5)
1. A flame three-dimensional temperature field measuring system based on a light field layered imaging technology is characterized by comprising a flame generating device, a light field camera and a data analyzing and storing device; wherein:
the flame generating device is used for generating flame;
the light field camera is used for acquiring four-dimensional light field information of flames on different geometric faults and transmitting the acquired four-dimensional light field information to the data analysis and storage device;
the data analysis and storage device adopts a refocusing technology to refocus the four-dimensional light field information of the flame fed back by the light field camera to obtain a flame superposition image sequence focused on different geometric fault planes of the flame; then, carrying out inverse calculation on the obtained flame superposition image sequence on each geometric fault plane by adopting a deconvolution technology, inverting the original brightness information of each fault, and further obtaining the flame radiation intensity value on each fault so as to obtain the temperature distribution of the flame;
refocusing image of flame EF(x, y) and light LαFThe following relationship exists between:
in the formula: (u, v) represents a point on the lens plane of the light field camera, and (x, y) represents a point on the image plane of the focal plane, the points (u, v) and (x, y) corresponding one-to-one; α is a coefficient for adjusting the size of the image distance l'; acquiring image sequences on image surfaces of different focusing planes by changing the position parameter alpha;
the refocused flame superimposed image sequence is a superimposed image of the focused image of the focal plane and the out-of-focus images of other planes, and is:
wherein: i is the ith fault in the N equally divided faults of the flame; j is a fault corresponding to the ith fault after being focused by the optical system; Δ z is the interlamellar spacing of N equal fault layers of the flame;
the expression of the gaussian point spread function h (x, y) is:
in the formula: sigma is a parameter of the Gaussian defocusing model;
the flame generated by the flame generating device is positioned in the shooting visual field of the light field camera, and the light field camera is in signal connection with the data analysis and storage device;
the flame generating device and the light field camera are respectively installed on the sliding rail at intervals through a precise optical platform, and the flame generating device is connected with the light field camera in a sliding way; the slide rail is provided with a scale ruler which can calibrate the distance between each geometric fault of the flame and the light field camera;
the distance between each geometric fault of the flame and the light field camera is calibrated in the following mode: firstly, a scale with scales is obliquely placed in a shooting visual field of a light field camera, then refocusing is carried out on the scale to obtain a scale picture which contains distance information and has distinct definition, then the scale picture is equally divided along the scale direction N of the scale, EVA point sharpness function definition calculation is carried out on each part of the equally divided scale picture to obtain a plurality of corresponding definition point values, a definition curve graph is obtained after point value fitting, the part with the maximum definition in the definition curve graph is a refocusing focus surface, the scale scales corresponding to the refocusing focus surface are converted into a horizontal distance D, and the distance between each geometric fault of flame and the light field camera can be obtained.
2. A flame three-dimensional temperature field measuring method based on a light field layered imaging technology is characterized by comprising the following steps:
the method comprises the following steps: according to the size of the flame, carrying out geometric stratification on the flame, and calibrating the distance from each geometric fault position to a light field camera;
the calibration method for the distance from each geometric fault position to the light field camera specifically comprises the following steps: firstly, obliquely placing a scale with scales in a shooting visual field of a light field camera, then refocusing the scale to obtain a scale picture which contains distance information and has clearly distinguished definitions, then equally dividing the scale picture along the scale direction N of the scale, calculating the definition of an EVA point sharpness function of each part of the equally divided scale picture to obtain a plurality of corresponding definition point values, obtaining a definition curve map after point value fitting, wherein the part with the largest definition in the definition curve map is a refocusing focus plane, and the scale scales corresponding to the refocusing focus plane are converted into a horizontal distance D, so that the calibration of a refocusing position and an interval can be completed;
step two: setting a calibration plate at a corresponding position of each flame geometric fault, and acquiring an image of the calibration plate to obtain a point spread function at the corresponding position;
step three: the flame is photographed by adopting a light field camera, four-dimensional light field information of the flame is obtained, refocusing processing is carried out on the four-dimensional light field information of the flame, and a flame superposition image sequence of the flame focused on different geometric faults is obtained;
refocusing image of flame EF(x, y) and light LαFThe following relationship exists between:
in the formula: (u, v) represents a point on the lens plane of the light field camera, and (x, y) represents a point on the image plane of the focal plane, the points (u, v) and (x, y) corresponding one-to-one; α is a coefficient for adjusting the size of the image distance l'; acquiring image sequences on image surfaces of different focusing planes by changing the position parameter alpha;
step four: combining the point spread functions of the flames on different geometric faults obtained in the step two, carrying out inversion calculation on the flame superposed image sequence on the corresponding geometric fault, and inverting the original light brightness distribution information of each fault;
the original luminance distribution is obtained by the following steps:
according to the Fourier optics theory, for a linear shift-invariant optical imaging system, the brightness distribution function g (x, y, z') at the image plane is the actual intensity function f (x, y, z) at the corresponding object plane and the point spread function h of the imaging optical systemz′-zConvolution of (x, y):
the refocused flame image sequence is:
wherein: i is the ith fault in the N equally divided faults of the flame; j is a fault corresponding to the ith fault after being focused by the optical system; Δ z is the interlamellar spacing of N equal fault layers of the flame;
hz′-z(x, y) is the point spread function when the optical system is focused in the z' plane and imaged in the z plane, and f (x, y, z) indicates the point spread function hz′-z(x, y) an actual light intensity function at the corresponding object plane;
f (x, y, i delta z) obtained by solving in a deconvolution mode by combining flame superposed image sequences focused by flames on different geometric fault surfaces is the original light brightness distribution of the i fault surfaces of the flames;
step five: and solving the temperature field distribution of each geometric fault of the flame according to the corresponding relation between the image gray scale and the radiation intensity and the temperature, thereby realizing the reconstruction of the three-dimensional temperature field distribution.
3. The flame three-dimensional temperature field measurement method based on the light field layered imaging technology as claimed in claim 2, wherein the expression of the point spread function is as follows:
in the formula: sigma is a parameter of the Gaussian defocus model.
4. The flame three-dimensional temperature field measurement method based on the light field layered imaging technology according to claim 3, wherein the point spread function is obtained by: selecting two adjacent bright and dark image areas containing straight edges and having certain contrast for operation; the two adjacent bright and dark image areas which contain straight edges and have certain contrast are blurred images obtained by the straight-edge light source through the focusing of an imaging system; performing least square fitting according to a step function step (x) of the straight-edge light source to obtain a fitted edge function e (x), wherein the edge function e (x) can express an image gradually transited from one gray level to another gray level; from the fitted edge function e (x) to obtain a gaussian off-focus spread function h (x, y):
wherein: sigma is a parameter of the Gaussian defocus model.
5. The flame three-dimensional temperature field measurement method based on the light field layered imaging technology according to claim 3,it is characterized in that in the fifth step, for the luminous flame, the monochromatic radiation intensity L of the flame is in the visible light rangeλThe relationship with the temperature T satisfies the Plank law:
in the formula: ε is the blackness: c. C1Is a first radiation constant; c. C2Is a second radiation constant; λ is the wavelength; t is the temperature; and the image gray g and the flame monochromatic radiation intensity LλSatisfies the following conditions:
g＝ΦtLλ(D/f)2
where D/f is the lens aperture ratio, phitIs the instrument constant.
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