JPH0375505A - Extraction of interference fringe - Google Patents

Abstract

PURPOSE:To facilitate measurement by comparing the difference in lightnesses of corresponding picture elements on the picked up images of interference fringes with a reference value, and identifying the picture element as a stereoscopic image or the interference fringes in correspondence with the result of the magnitudes in said comparison. CONSTITUTION:The image of a surface to be measured 6 and the image of the interference image caused by the reflected light from a reference reflecting surface 8 are picked up with an industrial TV 10 before and after the minute movement of an optical system in the direc tion of the focal depth of the optical system. The two picked-up images are stored in an image memory 11. The minute amount of the movement is imparted on the reflecting surface 8 positively by using a vibrator 9. In both picked-up images which are picked up before and after the minute movements continuously, the interference fringes in the vibrating state are overlapped on the steroscopic image of the surface to be measured 6 in the stationary state. A part where the change in lightnesses between both picked-up images is less is the stereoscopic image. The part where the change is much is the interference fringes. Then, in an image processing device 12, the difference in lightnesses of the corresponding picture elements of both picked-up images is compared with a preset reference value. When the difference in lightnesses of the corresponding picture elements is less than the reference value, it is judged as the stereoscopic image. When the difference is higher than the reference value, it is judges as the interference fringes.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for extracting interference fringes by leaving a solid image on the imaging screen through image processing after imaging a surface to be measured in an optical interference measurement system. .

[Conventional technology]

Optical interferometric measurement systems, such as the surface accuracy measurement method, use equal thickness interference to observe the number and contours of interference fringes on a stereoscopic image and their distribution, and are used to evaluate the fine shape of the surface. . All of these optical measurement methods have characteristics such as being able to measure the object to be measured without contacting it, and being able to directly observe the surface state with the naked eye.

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

C. Object of the Invention] Therefore, an object of the present invention is to provide an image processing method for separating a solid image and interference fringes from an imaging screen of a surface to be measured while maintaining the same positional relationship and extracting interference fringes in an optical interference measurement system. The objective is to provide a method that facilitates measurement.

[Means for solving the invention]

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

Focusing on this point, the present invention moves the surface to be measured or the reference reflective surface by a minute amount in the direction 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. do. After that, the present invention uses image processing to compare the brightness difference of corresponding pixels on both imaging screens with a reference value, and identifies the pixel as a solid image or an interference fringe depending on the magnitude of this comparison. To go.

In this way, by using the unchanged portions of both shooting screens as the solid image and the changing portions as the interference fringes, the solid image and interference fringes can be separated from the shooting screen through image processing. .

〔Example〕

FIG. 1 shows the configuration of an optical interferometric measurement system 1. As shown in FIG.

A light source 2 generates measurement light 3 with a wavelength suitable for optical interference, for example, 5000 (A), and a beam splinter 4 and an objective lens 5.
The light is irradiated onto the surface to be measured 6 via the objective lens 7, and is also irradiated onto the reference reflective surface 8 as reflected light. When the reflected lights overlap on the beam splitter 4, they interfere and generate interference fringes on the surface. In this embodiment, the reference reflecting surface 8 is attached to a fixed part by a vibrator 9, and can be moved by a small amount of movement by vibration in the direction of the depth of focus of the optical system.

The amount of movement is set to a value of approximately 1/4 wavelength of the measurement light 3 at the time of measurement.

A solid image of the surface to be measured 6 and its interference fringes are captured by an industrial television 10 and stored in an image memory 11.

The image processing device 12 has a built-in program based on the present invention, and in addition to reading data from the image memory 11 and displaying the captured screen on the image monitor 13, it also processes images based on the image processing program according to the present invention. Processing is performed to separate the solid image and the interference fringes from the image capture screen, and store them in the respective memories 14 and 15.

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

First, the industrial television IO captures the image of the surface to be measured 6 and the interference fringes caused by the reflected light from the reference reflective surface before and after a small amount of movement of the optical system in the depth of focus direction. 1
Two imaging screens such as 1 and 2 are stored in the image memory 11. Before and after the two images are taken, the optical system moves minutely according to the wavelength of the measurement light 3! (about 1/4 wavelength), and is moving in the direction of the focal depth of the optical system. This small amount of movement can be provided by actively applying vibration to the reference reflecting surface 8 using the vibrator 9, or by passively using the weak external vibration of the normal measurement environment. Of course, these small amounts of movement are within the range of the depth of focus of the optical system, so they do not affect the imaging state of the surface to be measured 6 and can be ignored in terms of image quality. Note that the two imaging screens are taken in consideration of the vibration period so as to be before and after the movement, and are approximately continuous if the frame rate of the industrial television 10 is used as a measure.

Suppose that the wavelength of measurement light 3 is 5000 (A), and the amount of movement of the optical system in the depth of focus direction is 0.1 [μ
m], the depth of focus is sufficiently larger than the amplitude of the vibration, so the in-focus state of the imaging screen does not change before and after the movement. However, as illustrated in FIGS. 3(1) and 3(2), the interference fringes move approximately half the fringe pitch within the plane of the field of view. If the photograph is taken at an optical magnification such that the number of interference fringes within the field of view is several to ten or more, a movement of nearly half the field of view pitch will appear as a very large change in the image. Here, as described above, it is considered best for the second interference fringe to be between the first interference fringes, so the optimal movement amount is about 1/4 wavelength of the measurement light 3.

In this way, in the two images taken almost continuously before and after a small amount of movement, the interference fringes in the vibration state are superimposed on the solid image of the measured surface in the stationary state, and the two images are The portion of the screen with little change in brightness is the real image, and the portion with much change in brightness is the interference fringes before and after movement.

Therefore, the image processing device 12 compares the difference in brightness between the corresponding pixels of the two imaging screens with a reference value set in advance according to the brightness characteristics of the images. If the difference in brightness between corresponding pixels on both imaging screens is less than the standard, that pixel is determined to be a real image. Therefore, the image processing device 12 causes the memory 14 for stereoscopic images to store one of the stereoscopic image levels or their average value, and also stores the memory 15 for interference fringes.
For example, a value corresponding to white is substituted into the interference fringe level. Furthermore, when the brightness difference is equal to or greater than the reference value, the pixel is determined to be an interference fringe. Therefore, the image processing device 12 inputs one of the bright level values into the memory 14 for the stereoscopic image, and also substitutes the dark level value into the memory 15 for interference fringes.

In this way, the image processing device I2 performs the steps described above.
All pixels of both imaging screens are processed, for example, in the scanning direction, and a solid image without fringe and a screen mainly composed of interference fringes are reproduced in a separated state, as shown in (31 (41) in Fig. 3). go.

In this way, the real image and the interference fringe image are obtained in a separated state from both imaging screens.

The above explanation assumes that interference fringes appear as black lines on a white background, and when the black and white level is reversed in image processing, the determination of brightness and darkness is set to be reversed.

It is also possible to binarize the image as it is when extracting the interference fringe screen, but as mentioned above, many of the interference fringes are unclear, so considering the combination of characterization using other image processing methods, We have explained the method of extracting interference fringes with their original brightness. However, if necessary, it is also possible to perform binarization in advance.

When extracting interference fringes, if you take out each piece of both screens,
Since the number of interference fringes may nearly double, if necessary, by retrieving them to the interference fringes memory 15 only when, for example, the rear screen of both imaging screens is darker, it is possible to eliminate which one is the only one. It is also possible to extract interference fringes.

〔Effect of the invention〕

In the present invention, the solid image of the surface to be measured and the interference fringes can be separated in the field of image processing based on the results of comparison between the brightness difference of corresponding pixels of two screens imaged more or less continuously and a reference value.
In addition to streamlining the processing since the comparison is made between images of substantially the same quality, the actual image and the corresponding interference fringes can be observed as separate images, making it easy to measure the number of interference fringes, their contours, and the pitch of their fringes. becomes.

In addition, because the method uses minute movements in the direction of the depth of focus in the optical system, reference reflective surface, or measured surface during the imaging process,
Even in the case of external vibrations, which are generally disliked in the measurement of interference fringes, it is possible to separate the real image and interference fringes as long as the amount of movement is within the depth of focus.

[Brief explanation of drawings]

FIG. 1 is a block diagram of the measurement system, FIG. 2 is a flowchart of the measurement method of the present invention, and FIG. 3 is an explanatory diagram of two imaging screens, a separated solid image, and interference fringes. 1. Optical interference measurement system, 2. Light source, 3. Measurement light, 4. Beam splitter, 5. Objective lens, 6.
・Measurement surface, 7. Objective lens, 8. Reference reflective surface, 9.
... Vibrator, 10... Industrial TV, 11... Image memory, 12... Image processing device, 13... Image monitor, 14
．． 15...Memory. Patent Applicant Nippei Toyama Co., Ltd. Agent Patent Attorney Kunio Nakagawa Figure 2 Figure r7, r2, r3, r4

Claims (1)

[Claims] In an optical interferometry measurement system, images of the surface to be measured including interference fringes are captured before and after a minute movement according to the wavelength of the measurement light in the focal depth direction of the optical system, and the corresponding pixels on both imaging screens are The brightness difference is compared with a reference value, and pixels with a brightness difference less than the reference value are determined to be a solid image, and the same pixels are stored as a solid image level on the screen for the solid image, and the pixels are stored on the screen for the interference fringe. The same pixel is stored as an interference fringe level, and a pixel with a brightness difference of more than a reference value is determined to be an interference fringe, and the value with higher brightness is stored as a solid image level for the same pixel on the screen for a solid image, An interference fringe extraction method characterized in that the lower brightness value on the screen for interference fringes is stored as the interference fringe level for the same pixel.