JPS6010105A - Measurement of shape of body of low coefficient of reflection - Google Patents

Abstract

PURPOSE:To measure a surface condition as a clear image, by taking an image element difference of image wherein interference fringes are existing by projection of two fluxes on the faces of a low coefficient of reflection and image wherein it is lighted by the other flux. CONSTITUTION:A flux of rays issued from a laser source 2 is bisected by a beam splitter (BS)4, a flux of rays L1 passed through BS4 and a flux of rays L2 reflected by a mirror 5 are incident on a work 2 on a measuring bench 1 and its interference fringes are formed. An image P1 taken by a video camera 6 is stored in a memory in an image element decrement unit 7. Then the flux L2 is interrupted by a shutter and work surface 2 is photographed in a condition that it is illuminated only by the flux L1 and an image P2 is obtained. Then, an image P3 is obtained by interruption of the flux L1. By an image decrement of P1-P2-P3 by the unit 7, a clear image of surface condition becomes available.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for measuring the shape of low reflectance objects.

To measure the unevenness of machined surfaces by various processing methods, generally a surface roughness measuring device, that is, a diamond probe with a radius of several ILffl, is used to trace the measurement surface, and the amount of minute unevenness is converted into voltage using a differential transformer. and display devices are used. In addition, for inspecting polished surfaces with a good finish or optical components, there is a method in which the surface to be inspected and a reference mirror are assembled in a predetermined state to form an interference tool 1, and the difference in unevenness between the two is determined by the degree of curvature of the interference fringes. is being used.

Although the former method is very simple, it has the biggest drawback of damaging the surface depending on the measuring pressure of the probe, and is not suitable for inspecting all types of objects. On the other hand, the latter method has the problem that interference fringes do not occur unless the finished surface has a high reflectance and is as good as possible.

In recent years, with the invention of holography, it is not impossible to measure the unevenness of rough surfaces with low reflectance, but since it is a dry plate image, there is inevitably deformation of the image due to expansion and contraction, and advanced and complicated operations are still difficult. remains as. Furthermore, although practical equipment for the moire topography method is rapidly becoming widespread,
Even if a grating with an interval of several p or m is used to measure fine irregularities, the resolution is said to be at most 20 gm because of diffraction phenomena.

The present invention attempts to detect interference fringes using a low-reflection measurement method in which the 1 reflectance is smaller than 5z, and to measure the surface shape of the interference fringes obtained by using the pixel difference method as a clear image. be.

The shape measurement method of the present invention creates interference fringes by projecting two light beams onto the surface of an object to be measured with low reflectance, and creates a state in which these interference fringes exist on the surface of the object to be measured, and a state in which the interference fringes exist on the surface of the object to be measured. The method is characterized in that the surface is simply illuminated with only one of the light beams, and the surface is imaged, and a clear image of only the interference fringes on the surface of the object to be measured is obtained by the pixel difference between these two images. It is said that

The pixel difference here means that when you take an image of a test object onto which interference fringes are projected, both the interference fringes and the front surface of the test object are projected, making the fringes unclear. This is a method of obtaining a clearer image by taking an image of only the interference fringes and subtracting it from the interference fringe projection image to obtain a $A calculation image that shows only the interference fringes.

According to the method of the present invention, when measuring an object having large irregularities, widely spaced interference fringes are projected onto the surface of the object, and the present invention will be further described with reference to the drawings below. Explain in detail.

FIG. 1 shows a measuring device for measuring the surface shape of an object to be measured by the method of the present invention using a computer, in which 1 is a measuring table, 2 is an object to be measured placed on the measuring table, and 3 is a laser beam. A light source, 4 is a beam splitter that divides the laser beam from the laser light source into two, and 5 is a mirror that reflects the laser beam from the beam splitter. L2
An optical path difference is applied to the object to be measured, and these light beams are projected onto the object to be measured 2 so as to coincide with each other on the surface of the object to be measured, thereby creating interference fringes thereon.

As shown in FIG. 2, the video camera 6 photographs both the base material of the surface of the object to be measured and the interference fringes projected thereon through a microscope, and records the photographed images in the memory in the pixel subtraction device 7 connected to the video camera. memorize it, and display it on a black and white monitor if necessary. In this case, the illumination obtained by the double illumination is performed, and in this state, the surface of the object to be measured is imaged by the video camera 8. At this time, an image as shown in FIG. 3 is obtained, and this image P2 no longer contains interference fringes, but only the basic surface of the object to be measured.

Further, the light flux L1 is blocked by a shutter, and the light flux L1
The length of the shadow also differs by the difference in direction.

The pixel difference device 7 connected to the video camera 6 performs pixel difference of p+ −(P2 + P:l) or P, −P, −P3,
Through this series of operations, the base, shadow 8,
Then, speckles 8 and the like are removed, and only the interference fringes corresponding to the irregularities of the object to be measured remain as a clear image on the monitor. Therefore, subsequent image processing becomes extremely easy.

Although the interference fringes sharpened in this way also have a width, resolution and measurement accuracy can be improved by performing line thinning processing to select the area with the deepest width.

Next, a method of performing shape measurement similar to the above using a color film will be described.

In this case, a single-lens reflex camera is used in place of the video camera 6 and pixel difference device W7 shown in FIG. When a color photograph is taken after a measurement is taken, only the base material of the object to be measured is photographed. This corresponds to the above-mentioned image P2 or P3. In the image P1, speckles 8 due to the roughness of the substrate or its surface appear, or shadows 8 of the unevenness of the object to be measured appear, which are mixed with the projected interference fringes, making it impossible to obtain a clear image. I can't do it. Therefore, from image P1 to image P2 or P,
It is necessary to obtain information about only the irregularities of the object to be measured, that is, the projected interference fringes, by subtracting the . Therefore, a method for obtaining clear interference fringes from the above-mentioned color photograph will be explained.

Figure 4 is a diagram to explain the complementary colors of each color when three filters of yellow, magenta, and cyan are superimposed. For example, since the complementary color of red is cyan, a red subject is photographed using a color negative. When shooting with film, the color becomes cyan. In other words, the color of the subject and the color of the negative film on which it was photographed are complementary colors. Therefore, by applying this fact, if you shine white light on a negative image of only the base material of the object and map it to the same type of negative color film, you will obtain a positive color film image of only the base material of the object. It will be done.

Therefore, by combining the negative image of the interference fringe projection image Pl and the newly obtained positive image of only the base material, it becomes black and appears white on the photographic paper, and only the projected interference fringes excluding the base material are obtained as a clear image. I can do it. However, the reversal negative film used to convert a negative image of only the substrate into a positive image must have a γ value close to 1.0, just as it was used to obtain the negative image. Otherwise, the negative and positive films that capture the base material will not have a complementary color relationship, and the colors will remain.

Further, similar shape measurement can be performed using a black and white film. In this case, first, an image in which interference fringes are projected onto the base of the object to be measured is photographed using a black and white negative film to obtain a negative master plate A1.

Next, using the same type of negative film, the base of the object to be measured illuminated by one beam of light is photographed. At this time, the exposure time is set twice as long as that used to obtain the above-mentioned negative A, so that the same density as that of the negative original plate A is obtained, thereby obtaining the negative original plate A2. Furthermore, the negative original plate is subjected to contact printing on the same type of negative film used in the above operation.
Positive original plate A.

get.

When the film surfaces of the negative original plate A1 and positive original plate A3 obtained in this way are aligned and the images are superimposed, the base image part becomes almost grayish with poor gradation, whereas the interference fringes are clear. Therefore, by fixing the two original plates and printing them on photographic paper, only the projected interference fringes can be obtained.

In the above operation, it is necessary to use a negative film with γ close to 1.0 as the negative film used and the negative film from which the positive image is obtained. If a negative film with γ of 0.5 is used, when obtaining a positive image from the negative original plate on which the base image has been copied, the negative original plate and negative film are printed in close contact, so the γ of the positive image is 0.5 Xo , 5'=0.25, resulting in an extremely soft tone. Therefore, γ is 0.5
A negative master plate of 0.25 and a positive master plate of 0.25 are superimposed and fixed, and then printed on photographic paper.In this case, since the γ of the base image portion of both master plates is different, the base image is completely erased. It will remain unused.

Next, a method for measuring the absolute value of the amount of unevenness on the surface of the object to be measured from the interference fringes obtained as described above will be described.

Regarding the amount of unevenness, place an objective micrometer (for example, a glass scale with graduations of 1+amlOO) under the microscope instead of the object to be measured, and calibrate the magnification, that is, measure the stripe spacing a in Fig. 2, and measure the unevenness. The absolute value can be found by measuring the lb of lb and taking the ratio b/a of the two.

Further, in FIG. 1, when the angle between the light beam L1 and the light beam is θ, and the wavelength of the laser light source is λ, the interval d between the interference fringes is expressed as d = λ/(2 sin θ/2). When the oblique incidence angle of the laser beam is η, the fringe interval a on the object to be measured is a=(λ/(2si
nθ/2)) cosη.

Therefore, the fringe spacing is wider than ^/2 obtained by a Michelson type interferometer. A Michelson interferometer is used to measure the absolute length of the block gauge, and three or more laser beams with different wavelengths are used. The so-called matching method is applied. Since the interference fringe projection method described here also only widens the spacing between the fringes, the matching method can be applied in the same way as the block gauge, and in this case, there is no need to calibrate using the aforementioned glass scale or the like.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an apparatus for implementing the method of the present invention, FIGS. 2 and 3 are explanatory diagrams of interference fringes to be imaged in the present invention, and FIG. 4 is an explanatory diagram of complementary colors. 2. Object to be measured, 3. Laser light source, 6. Video camera, 7. One pixel difference device L, L? ... Luminous flux. Designated Agent 4th

Claims (1)

[Claims] 1. A method for measuring the shape of a low reflectance object, which is characterized by creating interference fringes by projecting two light beams onto the surface of a low reflectance object to obtain a clear image of only the interference fringes on the surface of the object.