JPH0329860A - Method for digitally processing schlieren image - Google Patents

Abstract

PURPOSE:To quantitatively measure density distribution by giving the density value of one arbitrary point in lattice rows, integrating a density gradient value centering around said point over the distance in the direction vertical to be edge ridge line of a knife edge, and calculating the density value of each micro-region. CONSTITUTION:Since a schlieren image is a variable density image proportional to the density in the direction vertical to the edge ridge line 15 of a knife edge 7, when the density gradient value obtained by the conversion of the brightness distribution of the image is integrated in the direction vertical to the ridge line 15, density distribution is obtained. At this time, the density value of one arbitrary element of each integral line is given and the direction going toward the arranging position of the ridge line 15 from said element is set to negative and the direction reverse thereto is set to positive to calculate the product of the value obtained by applying a code to the distance DELTAx in the direction vertical to the ridge line 15 of a micro-element subjected to lattice division with respect to each of the directions and the density gradient value of each element. Then, the calculated value is successively added from the element to which a reference density value is given with respective positive and negative directions to calculate the density value of each element of lattice rows in the direction vertical to the ridge line 15 and density values are processed with respect to all of lattice rows to calculate the density value of the entire region of the image.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an image processing method for determining a density distribution by numerically processing an image measured by the Schlieren method.

[Conventional technology]

The Schlieren method is one of the representative methods conventionally used for visualizing flows. In other words, light passing through an area with a density gradient is refracted. This property is utilized. In particular, it has been used as a visualization method for flow fields where density changes exist, such as thermal convection fields and compressible flow fields.

FIG. 3 shows a basic configuration diagram of the Schlieren method. That is, it is a cross-sectional view perpendicular to the flow direction of gas flowing inside the measuring section 1. A light source 2 with a finite length is placed at the focal point of a lens 3, and an image of the light source 2 is placed at the position a′ b′ in the figure using a lens 4 placed on the opposite side of the measurement unit from the lens 3. tie. Further, an image of the straight line gi in the measuring section is formed on the screen 6 by a lens 5 and a screen 6 placed behind a' and b'. Here, the knife 7 is placed at the position of the light source image a'b'.
If it is installed so as to cover part of 'b' and there is a density gradient in the flow within the measuring section, an image showing a density distribution proportional to the density gradient will be obtained on the screen 116.

FIG. 4 is a basic layout diagram of the Schlieren method using a concave mirror instead of a lens. Light from a light source 2 is collected by a condenser lens 8, a pinhole 9 is placed at its focal point, and the light passing through the pinhole 9 is converted into parallel light by a concave mirror 10.

The light passing through the measurement unit 1 is reflected by the other concave mirror 1↓, and further reflected by the plane mirror 13, and is reflected by the camera l.
A knife edge 7 is installed at a position corresponding to the focal length of the concave mirror, and when a part of this focal point is covered with the knife edge, the camera 14 detects a density distribution proportional to the density gradient at the measuring section. images can be recorded. Here, the grayscale image recorded by the camera has a brightness distribution proportional to the density gradient of the flow within the measuring section in the direction perpendicular to the cutting edge line of the knife edge.

Regarding this type of measurement method, please refer to Aerospace Technology Research Institute report TR-964. (1988), it has already been introduced in many textbooks on fluid mechanics (for example, Mechanics of Compressible Fluids, co-authored by Ikui and Matsuo, published by Rigakusha).

[Problem to be solved by the invention]

In the conventional technique described above, the image obtained is the density gradient distribution of the flow state in the measuring section, and the density distribution of the flow field cannot be directly obtained from the image.

Further, although it is possible to know how the state of the flow field changes based on the gray scale distribution of the image, it is not possible to know the quantitative distribution state of the changes.

An object of the present invention is to provide an image processing method for obtaining a density distribution of a flow field in a measuring section from a grayscale image obtained by the Schlieren method, and furthermore, to provide an image processing method for converting the Schlieren grayscale image into a density gradient value. The purpose of this invention is to provide a calibration method.

Furthermore, another object of the present invention is to provide a measurement system configuration suitable for digitally processing Schlieren images.

[Means to solve the problem]

In order to achieve the above purpose, the Schlieren image is divided into micro regions that are grid-divided in two directions: the direction of the cutting edge line of the knife edge and the direction perpendicular thereto, and the average brightness within each micro region is calculated in advance. Convert the luminance and density gradient of the Schlieren image into a density gradient value using a calibration curve, and for each grid row aligned in the direction perpendicular to the cutting edge line of the knife edge, give the density value of an arbitrary point in the grid row,
By numerically integrating the density gradient value over the distance perpendicular to the cutting edge line of the knife edge around that point, the density value of each minute region divided into grids is determined, and the density distribution over the entire image area is determined. This is what I did.

In addition, in order to convert the a-grain distribution of the Schlieren image into a density gradient value, we will compare the output of the image input device when a knife edge is not installed and no fluid is flowing through the measurement section, and the output of the image input device when a knife edge is installed but no fluid is flowing through the measurement section. The output value of the image input device and the output value of the image input device are calculated using the output of the image input device when no fluid is flowing, and the output of the image input device when the knife edge is installed in the same state as above and the fluid is flowed to the measuring section. By creating a calibration curve with the density gradient value, the gray-scale brightness distribution of the Schlieren image is converted into the density gradient value.

In order to achieve other purposes, a measurement system is constructed by directly connecting a Schlieren image input device and an image processing device.

[Effect]

Since a Schlieren image is a grayscale image proportional to the density gradient in the direction perpendicular to the ridgeline of the knife edge, the image's brightness distribution is converted into a density gradient value, which is then integrated in the direction perpendicular to the ridgeline of the knife edge. For example, the density distribution can be obtained. At this time, give the density value of any one element of each integral line, use this element as the starting point of integration, and subtract the direction from this element toward the installation position of the knife edge cutting edge line, and vice versa. For each direction, the distance ΔX in the direction perpendicular to the cutting edge line of the knife edge of the minute element divided into bean paste is given this sign, and the density gradient value of each element, with the direction away from the position toward the space side being a plus. By finding the product of , and sequentially adding them together from the elements that have given the standard density 15 value for each of the positive and negative directions,
The density value of each element of the grid row in the direction perpendicular to the cutting edge line of the knife edge is determined, and the density value of the entire image is calculated by performing the same process for all the grid rows in the direction perpendicular to the cutting edge line of the knife edge in the image. is found.

Furthermore, in order to calibrate the gradation luminance distribution and density gradient value of the Schlieren image, it is theoretically possible to use an input image with no knife edge installed and no fluid flowing through the measuring section, since this is the maximum value of this training measurement system. Gives the brightness and minimum brightness (minimum brightness appears when there are walls, objects, etc.). Within this range, images are captured with the γ characteristic of the camera set to γ=1, that is, the relationship between brightness and camera output is directly related.

Next, an image is captured when the knife edge is installed without flowing fluid through the measuring section, and the camera output value at this time is the output value when the density gradient of the flow corresponds to zero. Furthermore, if there is another amount of information, the calibration straight line can be found, so
The calibration straight line can be defined by measuring a flow with a known density change and obtaining the camera output value at this time.

In addition, by directly connecting the image input device and the image processing device, the output value of the camera that captures the image in the measurement section can be sent directly to the image processing device, reducing processing time. Processing errors are reduced.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. No.l
Figure (a) shows how a Schlieren image is divided into grids. Furthermore, Fig. 1-(b) shows the position of the knife edge when the same Schlieren image was taken, and the diagonal line m5 in Fig. 1(b) is the knife edge, and in the figure, the cutting edge ridge of the knife edge The y-coordinate is taken in the direction along, and the The lattice division in Figure 1(a) is
It is divided in the I mark direction, rn divisions in the X direction, and n divisions in the y direction.

FIG. 1(c) shows the density gradient value in an arbitrary j column of the lattice partition diagram of FIG. 1(a). The gray-brightness distribution of the Schilleren image is converted into a density gradient value using a calibration straight line prepared in advance, and the density gradient at the center of gravity of each divided element is taken as the density gradient value for the entire area within that element. The broken line in FIG. 1(a) represents a line segment connecting these center of gravity positions. Also,
The grid division elements shown in FIG. 1(a) correspond to pixels when a Schrillen image is taken into an image processing device or the like. In FIG. t(c), when the density value of the element in direction io is known, the density gradient value may be integrated in the X direction based on this value.

That is, for i ) io and for i < io . Here, ΔX is the length of the grid dividing element in the X direction.

Figure 1 (d) is the density distribution obtained by numerically integrating the density gradient distribution in (c) using equations (1) and (2) with reference to the density value ρ0 at number i = jo. be.

In Figure ↓, element i gives the density as the reference position for integration.
The density gradient value at o is zero, but if the grid division elements are sufficiently fine and the difference between the position that gives density and the element's centroid position can be ignored, then the density gradient of the element that gives density as the reference position for integration is It does not necessarily have to be zero. Also,
If the coordinate in the direction X perpendicular to the cutting edge of the knife edge is set as positive in the direction away from the cutting edge of the knife edge toward the spatial side, and the numbers of the dividing elements are assigned so that they increase sequentially in the positive direction of the X coordinate, the formula is obtained. Using (1) and equation (2), the density value in the j column of the grid division can be found. In this case, the element that provides the density as the integration reference position can be arbitrarily selected.

FIG. 2 shows a flowchart when the numerical integration process shown in FIG. 1 is performed over the entire image area. According to this embodiment, it is possible to quantitatively measure the density distribution of the flow field in the measurement target from the Schlieren image.

1st. In the embodiments shown in FIG. 2 and FIG. 2, one point with known density was always required as an integration reference point for each column a. In other words, for the entire image area, a condition is required that the density values for one row in the direction parallel to the cutting edge line of the knife edge are known.

On the other hand, in another embodiment of the present invention shown in FIG. 5, if the density value of one point within the image area is known, the density of the entire area can be measured. In the embodiment shown in FIG. 5, the density distribution of the entire area is obtained from two Schlieren images in which the cutting edge line of the knife edge is set at a different position for the same flow state of the object to be measured.

Two Schlieren images of the same object are used in which the set positions of the ridge lines of the knife edges are orthogonal to each other.

FIG. 5(a) shows the case where the cutting edge line of the knife edge is set in the X direction, and now the element i=j. o,j=
When the density of the Jo-th element is known, a process similar to the integration process shown in FIG. t is performed. In other words, 1=] For column o, if j > Jo, then 20. Also, if j < j o, integrate column J as Seek.

Next, from the Schlieren image in which the blade surface of the knife edge in Figure 5(b) is set in the y direction, using the density values in the i = i o column determined in Figure 5(a), Using the method shown, the density of the entire area can be determined by performing integration processing for each column from j=1 to j=n. Fifth
In the illustrated embodiment, the y-direction integration is first performed, and then the X-direction integration is performed, but this order may be reversed. Also, the fifth
In the figure, the knife edges are set in directions perpendicular to each other, but the set positions may be set at any angle.

However, if the set angle is arbitrary, the coordinates of the grid divisions created on the two Schlieren images do not match, so coordinate transformation is required when transmitting the data of both images. In addition, by using the method shown in Figure 5, even when there is an object in the flow path, the density value can be obtained by performing integration processing in two directions alternately on the back side of the object. can be found. According to this embodiment, if the density value of any one point in the flow field to be measured is known, the density value of the entire area can be found, so there is an effect that the scope of application of this method can be expanded.

Another embodiment of the invention is shown in FIG. Figure 6 shows the a of the Schlieren image required when performing the integration process of this method.
This shows a method for calibrating the brightness and density gradient of a pale distribution. (a) to (c) of FIG. 6 each show an image of the measuring section. In the example shown in Figure 6, the measuring section has flat walls on two sides, and the lower wall has a wedge-shaped slope in the downstream area.
The flow path becomes narrower as it goes downstream. Figure 6 (
A) is an image when there is no flow in the measuring section and the image of the light source is not blocked by the knife edge, and the output of the image input device at this time is the maximum brightness of the recessed part of the measuring section. In addition, the brightness of the dark area corresponding to the wall portion of the measuring section is set to be the minimum.

Subsequent measurements are performed without changing these set values (for example, the aperture of the camera for image input, the illumination of the measuring section, the camera position, etc.). Furthermore, objects brighter or darker than the brightness at this time will not appear in subsequent shooting. Note that the so-called γ characteristic of the camera for image input is set as γ=k, and the relationship between the camera input luminance E and the camera output current 1 is set as follows: l=kE·(5). Here k is a constant.

Next, as shown in FIG. 6(b), the knife edge is pushed in with no gas flowing through the measuring section.

At this time, the brightness of the recesses shown in FIG. 6(a) uniformly decreases. Set the insertion position of the knife edge to the state at the time of measurement,
The output of the image input device of the brightness state of the measurement space at this time is recorded. In other words, this output value is the same as the output when the density gradient of the flow field at the measuring section is zero. The calibration straight line of the brightness distribution of the Schlieren image is lower than this output value (l
li'l' state), the density gradient becomes negative, and
Make the density gradient positive in high (bright) conditions. If the camera output when this density gradient is zero (the output is obtained by converting the current value in equation (5) into a voltage value) is Vo, then the relationship between the density gradient and the camera output is α is determined by substituting the camera output value Vc at the # wave attack section in the state shown in FIG. 6(c) into equation (6).

a η . Here, η is the coordinate in the direction perpendicular to the blade surface of the knife edge. Further, α is a proportionality constant between the density gradient and the camera output. FIG. 6(c) is an image when a uniform high-speed flow is caused to flow into the measuring section, and oblique shock waves are generated from the wedge-shaped inclined portion of the lower wall surface. At this time, the density change before and after the shock wave is determined by assuming that the angle of the generated oblique shock wave is β, where k is the specific heat ratio of the fluid, and M1 is the Matsuha number of the inflowing fluid. Using equation (7), the density gradient in the α may be calculated by the square method.

In the example shown in FIG. 6, density changes due to shock waves of high-speed airflow flowing on the inclined wall surface are used for calibration, but other flows may be used for calibration. In that case, the density of the flow field used for calibration is
It is necessary to be able to clearly define it by calculation etc. In this way, the gray scale distribution of the Schlieren image is changed to the output voltage value of the image input device (camera), and furthermore, the equation (6
) can be converted into a density gradient by the calibration straight line.

According to this embodiment, since the gray scale distribution of the Schlieren image can be converted into a density gradient value, there is an effect that the numerical processing of the Schlieren image can be performed accurately.

As shown in Fig. 4, the Schlieren image can be taken using a normal photographic camera, and the film of the camera is photographed and processed in batch processing by an image processing device. Alternatively, a CCD camera or the like may be used to integrate the camera and an image processing device. FIG. 7 shows a method in which a CCD camera is used as a Schlieren image input device, the output of the camera is directly input to the image processing device 20, and integration processing is performed while observing the monitor 21 of the image processing device. At this time, the image processing apparatus incorporates the integral processing software 1 shown in FIG. According to this embodiment, since the image input device and the image processing device are integrated, the density distribution of the flow field in the measuring section can be determined while performing Schlieren measurement, and it is possible to speed up data processing and reduce errors. .

〔Effect of the invention〕

According to the present invention, since the schlieren image can be numerically integrated, the density distribution of the flow field in the measuring section can be quantitatively determined from the schlieren image.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of a processing method according to an embodiment of the present invention, FIG. 2 is a flowchart of a processing method according to an embodiment of the present invention, FIGS. 3 and 4 are diagrams showing a conventional measurement method, and FIG. FIG. 6 is an explanatory diagram of a processing method according to another embodiment of the present invention, FIG. 6 is an explanatory diagram of another embodiment of the present invention, and FIG. 7 is a system diagram of the measurement system of the present invention. DESCRIPTION OF SYMBOLS 1... Measuring part, 2... Light source, 3, 4.5... Lens, 7... Knife edge, 10.11. Concave mirror, 12,
↓3...Plane mirror, 14...Camera, l5...Knife edge ridge line, 16...Image area, 18...Wall,
19 CCD camera, 20・Image processing device, 21・Moq 1 Figure 6゛(a-) JP-A-3 29860 (11) Figure 7

Claims (1)

[Scope of Claims] 1. Lenses arranged on both sides of the measurement section flow path, a light source of finite length that enters light from one side through the lens into the measurement section, and a light source that passes through the measurement section and enters the measurement section on the opposite side from the light source. Position to,
A Schlieren measurement method comprising a knife edge placed at a position to focus the image of the light source, and a lens installed behind the knife edge to focus the image of the measuring section on a screen, or a light source and light from the light source. a condenser lens that collects the light, a pinhole or slit placed at its focal point, a plane mirror placed to reflect the light from there to a concave mirror, and a plane mirror placed on both sides of the measuring section to direct parallel rays into the measuring section. a concave mirror, a plane mirror that reflects light that has passed through the measuring section and reflected by the concave mirror toward an image input device, the knife edge placed at the focal position of the concave mirror, and an image input device installed behind it. A Schlieren measurement method consisting of a light source, a condenser lens that collects light from the light source, a pinhole or slit placed at its focal position, and the light emitted from this passes through a measurement section and then returns to A Schlieren measurement method comprising the concave mirror that reflects light toward the light source, a knife edge placed at the concave mirror focal point of the reflected light, and an image input device installed behind the knife edge, or a light source and the light from the light source. The condenser lens that collects the condenser lens, the long hole or slit placed at its focal position, and the concave mirror that reflects the light emitted from this lens back to the light source after passing through the measuring section, the light beam that enters the center position of the concave mirror and a semitransparent mirror placed between the measuring section and the pinhole or slit position so that the reflected light from the concave mirror passes in a direction different from the light source. A schlieren image is obtained by a schlieren measurement method in which a knife edge is installed at the concave mirror focus position of the reflected light beam, and an image input device is placed behind the knife edge. In this case, the knife edge is divided by a grid in two directions: the direction of the cutting edge ridge line and the direction perpendicular to it, and the area is divided into minute elements surrounded by the grid lines. Convert the brightness into a density gradient value using a calibration curve obtained in advance, take the coordinate in the direction perpendicular to the ridgeline of the knife edge cutting edge, and take the positive direction of the coordinate in the direction away from the cutting edge of the knife edge toward the space side, Furthermore, the numbers of the elements arranged perpendicular to the cutting edge line of the knife edge are numbered so that the element numbers increase sequentially in the positive direction of these coordinates, and the density value is given as the standard value for integration. For an element with a large element number, the average value of the density gradient value of that element and the density gradient value of the element with one smaller element number multiplied by the length of the side perpendicular to the knife edge edge line of the element of,
Find the density of an element by adding it to the density of the element whose element number is one smaller. Also, for elements whose element number is smaller than the element whose density value is given as an integration standard,
The average value of the density gradient value of that element and the density gradient value of the element whose element number is one larger, multiplied by the length of the side perpendicular to the knife edge edge line of the element, is the value of the element whose element number is one larger. A method for digitally processing a Schlieren image, characterized in that the density is calculated by subtracting the density from the density of the schlieren image. 2. A lens placed on both sides of the measurement section flow path, a light source of finite length that enters light from one side through the lens into the measurement section, and a light source located on the opposite side of the light source that passes through the measurement section and creates an image of this light source. A Schlieren measurement method consisting of a knife edge placed at a position where the knife is tied, and a lens placed behind the knife edge to focus an image of the measuring part on a screen, or
A light source, a condenser lens that collects the light from the light source, a pinhole or slit placed at its focal point, a plane mirror placed to reflect the light from there to the concave mirror, and a condenser lens placed on both sides of the measuring section. A concave mirror that lets parallel light rays in, a plane mirror that reflects the light that passes through the measuring section and is reflected by the concave mirror toward the image input device, a knife edge that is placed at the focal point of the concave mirror, and an image input device that is installed behind it. The Schlieren measurement method consists of a light source, a condenser lens that collects the light from the light source, a pinhole or slit placed at its focal position, and the light emitted from this passes through the measurement section and then returns to the light source side. A Schlieren measurement method consisting of a reflecting concave mirror, a knife edge placed at the concave mirror focal point of the reflected light, and an image input device installed behind it, or a light source and a condenser lens that collects the light from the light source, and its A pinhole or slit placed at the focal point and a concave mirror that reflects the light emitted from the pinhole or slit back to the light source after passing through the measuring section, the light ray that enters the center of the concave mirror and the reflected light pass on the same line. Furthermore, the reflected light from the concave mirror is reflected in a direction different from the light source by a semi-transparent mirror placed between the measuring part and the pinhole or slit position, and the reflected light is reflected from the concave mirror. A schlieren image obtained in a schlieren measurement method in which a knife edge is installed at the focal position and an image input device is placed behind it is calculated by projecting the knife edge onto the image plane. By dividing the grid in two directions, the direction and the direction perpendicular to it, the area is divided into minute elements surrounded by the grid lines.For each element surrounded by the grid lines, the brightness of the Schlieren image is determined in advance. Convert it to the density gradient value using the calibration curve, take the coordinate perpendicular to the knife edge cutting edge line, take the positive direction of the coordinate in the direction away from the knife edge cutting edge toward the space side, and then take the coordinate perpendicular to the knife edge cutting edge line. The elements arranged in the direction are numbered so that the element numbers increase sequentially in the positive direction of this coordinate, and some elements have larger element numbers than the elements whose density value is given as the reference value for integration. , the average value of the density gradient value of that element and the density gradient value of the element whose element number is one smaller, multiplied by the length of the side perpendicular to the knife edge edge line of the element, is calculated by the element number one smaller. The density of the element is calculated by adding it to the density of the element, and for an element whose element number is smaller than the element whose density value is given as an integration standard, the density gradient value of that element and the element whose element number is one larger are calculated. The method of calculating density is the same by subtracting the average value of the density gradient value multiplied by the length of the side perpendicular to the knife edge edge line of the element from the density of the element with one higher element number. The setting position of the cutting edge line of the knife edge is different with respect to the object to be measured 2
By using two Schlieren images and giving the density of any one element in the image as the reference value for integration, the numerical processing results for each image are exchanged and the density of the entire image area is calculated. A digital processing method for Schlieren images characterized by: 3. Lenses placed on both sides of the measurement section flow path, a light source of finite length that enters the measurement section through the lens from one side, and a light source located on the opposite side of the light source that passes through the measurement section and creates an image of this light source. A Schlieren measurement method consisting of a knife edge placed at a position where the knife is tied, and a lens placed behind the knife edge to focus an image of the measuring part on a screen, or
A light source, a condenser lens that collects the light from the light source, a pinhole or slit placed at its focal point, a plane mirror placed to reflect the light from there to the concave mirror, and a condenser lens placed on both sides of the measuring section. A concave mirror that lets parallel light rays in, a plane mirror that reflects the light that passes through the measuring section and is reflected by the concave mirror toward the image input device, a knife edge that is placed at the focal point of the concave mirror, and an image input device that is installed behind it. The Schlieren measurement method consists of a light source, a condenser lens that collects the light from the light source, a pinhole or slit placed at its focal position, and the light emitted from this passes through the measurement section and then returns to the light source side. A Schlieren measurement method consisting of a reflecting concave mirror, a knife edge placed at the concave mirror focal point of the reflected light, and an image input device installed behind it, or a light source and a condenser lens that collects the light from the light source, and its A pinhole or slit placed at the focal point and a concave mirror that reflects the light emitted from the pinhole or slit back to the light source after passing through the measuring section, the light ray that enters the center of the concave mirror and the reflected light pass on the same line. Furthermore, the reflected light from the concave mirror is reflected in a direction different from the light source by a semi-transparent mirror placed between the measuring part and the pinhole or slit position, and the reflected light is reflected from the concave mirror. Knife edge cutting edge ridge line direction when the knife edge is projected onto the image plane of the schlieren image obtained in the schlieren measurement method, which is configured by installing the knife edge at the focal position and placing an image input device behind it The luminance of the Schlieren image is converted into a density gradient value using a calibration curve determined in advance for each element surrounded by the grid lines, and the knife-edge For each row of elements arranged in the direction perpendicular to the edge of the knife edge, give the density value of any one element in the row of elements, and calculate the density gradient value of each element in the direction perpendicular to the edge of the knife edge using this element as a reference. A schlieren image characterized by calculating the density by numerically integrating , and calculating the density in the entire area of the schlieren image by performing this for all element rows in the direction perpendicular to the cutting edge line of the knife edge. digital processing method. 4. Without covering the image of the light source with the knife edge, and without flowing fluid into the measuring section, set the brightness of the concave captured by the image input device to the maximum output of the camera, and set the brightness of the recess taken into the image input device to the maximum output of the camera, and Set the brightness of the dark area corresponding to the position to the minimum output of the camera, fix the camera state at this time, and calculate the camera output value of the brightness of the measurement part when the knife edge is installed, when the density gradient is zero. With the output value of and the knife edge setting position fixed as it is,
Flow the fluid into the measuring section. At this time, the measurement unit configures the flow path so that the density change is known by calculation etc. in advance, and uses the density gradient calculated at this time and the camera output value to convert the schlieren gray image to the density gradient value. Create a calibration curve to convert to , fix this calibration curve, the camera shooting conditions for image input, and the knife edge setting position,
A method for digitally processing a Schlieren image according to claim 1, 2 or 3, characterized in that Schlieren measurement of an object to be measured is performed. 5. Claims 1 or 2, characterized in that schlieren image processing is performed in a schlieren measurement device in which a measurement system is configured by directly connecting a schlieren image input device and an image processing device. 4. A method for digitally processing a Schlieren image according to item 3 or 4.