JP2847536B2 - Interference fringe extraction method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an image including interference fringes obtained by irradiating a measurement surface with a measurement light through a beam splitter, and applying the measurement light to the beam splitter. In an optical interference measurement system that measures an image including interference fringes obtained by irradiating a reference reflection surface movable within a range of the depth of focus with a small amount of movement via a lens, after imaging the surface to be measured, The present invention relates to a method for extracting interference fringes while leaving a real image from an imaging screen by image processing.

[Conventional technology]

Optical interference type measurement system, for example, surface accuracy measurement method, is used to observe the number of interference fringes on a real image and the distribution of those contours by using equal thickness interference, and to evaluate the fine shape of the surface. . All of these optical measurement methods have features such as being able to measure the object to be measured in a non-contact state and being able to directly observe the surface state with the naked eye.

Normally, the interference fringes appear as a brightness difference on the real image of the surface to be measured, but since they are not clear, distinguishing between bright and dark finishes is a difficult task in this type of measurement.

[Object of the invention]

Therefore, an object of the present invention is to provide an image processing method for separating a real image and interference fringes from an imaging screen of a measured surface in an optical interference measurement system and extracting the interference fringes,
This is to facilitate measurement.

[Solution of the Invention]

The optical interference measurement system generates interference fringes according to, for example, the principle of a Michelson interferometer, and observes the degree of surface finish based on the degree of disturbance of the interference fringes. In this case, the interference fringes are overlaid on the surface to be measured. At the time of this observation, if a small movement amount is given to the measured surface or the reference reflecting surface in the direction of the depth of focus of the optical system, the interference fringes move in the direction of the fringe pitch in the viewing plane.

Focusing on this point, the present invention moves the reference reflecting surface by a small amount within the range of the depth of focus of the optical system, and images the surface to be measured and the interference fringes thereon before and after the movement.

Thereafter, according to the present invention, the brightness difference between corresponding pixels on the two imaging screens before and after the movement of the reference reflection surface is compared with the reference value by image processing, and the pixel is determined according to the magnitude of the comparison. It is identified as a real image or interference fringes.

As described above, in the two imaging screens before and after the movement of the reference reflecting surface, a portion having a small difference in brightness is defined as a real image, and a portion having a large difference in brightness is defined as an interference fringe. The real image and the interference fringes can be separated by image processing.

〔Example〕

FIG. 1 shows the configuration of the optical interference measurement system 1.

The light source 2 generates a measurement light 3 having a wavelength suitable for light interference, for example, 5000 [Å], irradiates the measurement target surface 6 via the beam splitter 4 and the objective lens 5, and transmits the measurement light 3 via the objective lens 7 to the reference reflection surface. 8 and irradiates the measured surface 6 as reflected light. At this time, when the reflected light from the measured surface 6 and the reflected light from the reference reflecting surface 8 overlap on the beam splitter 4, both images interfere with each other and, for example, black interference fringes are generated on the surface of the beam splitter 4. In this embodiment, the reference reflecting surface 8 is attached to the fixed portion by the vibrator 9 so that the reference reflecting surface 8 can be moved by a minute movement amount by vibration in the direction of the depth of focus of the optical system. Then, this minute movement amount is set to a value of about / 4 wavelength of the measurement light 3 at the time of measurement.

The real image of the measured surface 6 and its interference fringes are picked up by the industrial television 10 and stored in the image memory 11. The image processing device 12 incorporates a program according to the present invention, reads data from the image memory 11, displays a captured image on the image monitor 13, and executes image processing based on the image processing program according to the present invention. Performs processing to separate the real image and interference fringes from the imaging screen,
4 and 15 will be memorized.

 FIG. 2 shows the construction sequence of the present invention.

First, the industrial television 10 images the image of the surface to be measured 6 and the interference fringes due to the light reflected from the reference reflecting surface 8 before and after a small amount of movement within the depth of focus of the optical system. 3
The two imaging screens before and after the movement as shown in FIGS. 1 and 2 are stored in the image memory 11. The optical system moves within a range of the depth of focus with a small movement amount (about 1/4 wavelength) corresponding to the wavelength of the measurement light 3 in two imagings before and after the movement. The minute movement amount is obtained by applying vibration to the reference reflecting surface 8 by the vibrator 9. Of course, the small amount of movement
Since it is within the range of the depth of focus of the optical system, the position change is negligible in terms of image quality of the surface 6 to be measured. The two imaging screens are imaged before and after the movement of the reference reflection surface 8 in consideration of the vibration cycle.

Assuming that the wavelength of the measuring light 3 is 5000 [Å] and the moving amount of the optical system in the depth of focus direction is 0.1 [μm] as the total amplitude of the vibration.
In the case of, since the depth of focus is sufficiently larger than the amplitude of the vibration, the in-focus state of the imaging screen does not change before and after the movement of the reference reflecting surface 8. However, the interference fringes are shown in FIG.
As illustrated in (2), it moves close to half the fringe pitch in the plane of the field of view. Assuming that the number of interference fringes in the field of view was imaged at an optical magnification that appears several to ten or more in the field of view,
Movements close to half the field pitch appear as very large changes on the image. Here, as described above, 2
Since it is considered that it is best for the interference fringes after the movement to be between the interference fringes before the movement on one imaging screen, the optimal movement amount is about / 4 wavelength of the measurement light 3.

As described above, in the two imaging screens imaged before and after the slight movement amount of the reference reflecting surface 8, the black interference fringe overlaps the real image, and the brightness at the corresponding position of both imaging screens The portion with a small difference is a real image, and the portion with a large brightness difference is interference fringes before and after movement.

Therefore, the image processing device 12 compares the brightness difference between the corresponding pixels on the two imaging screens before and after the movement with a reference value set in advance according to the brightness characteristics of the image to be imaged.

As a result of the comparison, if the brightness difference between the corresponding pixels on the two imaging screens before and after the movement of the reference reflecting surface 8 is equal to or smaller than the reference value, the pixel is determined to be a pixel of the real image. For this reason, the image processing apparatus 12 stores the value of one pixel of the two images of the imaging target before and after the movement as the real image level in the real image memory 14 for the two compared pixels,
Alternatively, the average value of the two pixels of the two images of the imaging target before and after the movement is stored as the real image level. Further, the image processing device 12 substitutes, for example, a value corresponding to white as a level without interference fringes into the memory 15 for interference fringes.

Note that, since imaging is performed before and after the movement of the reference reflection surface 8, even if the corresponding pixels in the two images before and after the movement represent the same pixel, a certain brightness difference can be avoided due to a change in the imaging environment. Therefore, there is a problem as to which one of the two pixels is stored as the value of the real image level. Therefore, in this embodiment, 2
In one image, the value of one of the two corresponding pixels compared is stored as a real image level,
Alternatively, the average value of the compared two corresponding pixels is stored as the real image level.

As a result of the comparison, when the brightness difference between the corresponding pixels on the two imaging screens before and after the movement of the reference reflection surface 8 is equal to or larger than the reference value, the pixel is determined to be an interference fringe. Therefore, the image processing apparatus 12 stores a memory 14 for a real image with respect to the compared pixels.
A value brighter than the black interference fringe. Further, the image processing device 12 substitutes dark level values into the interference fringe memory 15 in order to represent black interference fringes.

In this way, the image processing device 12 performs
All pixels of the two imaging screens before and after the movement of the reference reflecting surface 8 are processed in, for example, the scanning direction, and as shown in (3) and (4) of FIG. And the separated image. As a result of this processing, a state in which the real image and the interference fringe image are separated from the two imaging screens before and after the movement of the reference reflecting surface 8 is obtained.

In the above description, it is assumed that the interference fringe appears as a black level line on a white background, and when the black and white level is reversed in image processing, the light / dark determination is set in reverse. It is also possible to binarize the interference fringes as they are when extracting the image of the interference fringes. However, the interference fringes are unclearly distinguished from the real images, and are considered in combination with the characterization by other image processing methods. The method of extracting interference fringes with the same brightness has been described. However, it is also possible to perform binarization as required.

When the interference fringes are extracted from the two imaging screens before and after the movement when the interference fringes are extracted, the number of the interference fringes may be nearly doubled. Therefore, if necessary, for example, the screen after the movement is better among the two imaging screens. By taking out to the interference fringe memory 15 only when it is dark, it is possible to extract only one of the interference fringes.

〔The invention's effect〕

According to the present invention, the actual image and the interference fringe of the surface to be measured are formed based on the comparison result between the brightness difference between the corresponding pixels of the two imaging screens before and after the movement of the reference reflection surface, which is captured almost continuously, and the reference value. It can be separated by processing, and because it is a comparison of images of almost the same quality, the processing can be streamlined, and since the real image and the corresponding interference fringe can be extracted as separate images, the number of interference fringes, contours and their pitch, etc. Measurement becomes easy.

In addition, in the imaging process, since the reference reflecting surface is minutely moved in the direction of the depth of focus, the separation between the real image and the interference fringes can be performed if the amount of movement is within the range of the depth of focus.

[Brief description of the drawings]

FIG. 1 is a block diagram of a measurement system, FIG. 2 is a flowchart of a measurement method of the present invention, and FIG. 3 is an explanatory diagram of two imaging screens, a separated real image and interference fringes. DESCRIPTION OF SYMBOLS 1 ... Interferometric measuring system, 2 ... Light source, 3 ... Measurement light, 4 ... Beam splitter, 5 ... Objective lens, 6 ...
... Measurement surface, 7 ... Objective lens, 8 ... Reference reflection surface, 9
…… Vibrator, 10 …… Industrial TV, 11 …… Image memory,
12 ... Image processing device, 13 ... Image monitor, 14, 15 ... Memory.

Claims (1)

(57) [Claims] An image including an interference fringe obtained by irradiating a measurement light to a surface to be measured via a beam splitter, and the measurement light is moved in a depth of focus direction by a small amount of movement through the beam splitter and a lens. In an optical coherence measurement system that measures an image including interference fringes obtained by irradiating a movable reference reflecting surface, an image captured before moving the reference reflecting surface within the range of the depth of focus and a post-movement image Compare the brightness difference between the pixels corresponding to the two images before and after the movement with a predetermined reference value, and determine a portion having a small brightness difference between the two images as a real image, and a portion having a large brightness difference between the two images. A method for extracting interference fringes, comprising determining an interference fringe and extracting the interference fringes from a real image.