Optics: measuring and testing – Shape or surface configuration
Reexamination Certificate
2000-03-17
2002-05-28
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
Shape or surface configuration
C356S603000, C356S607000
Reexamination Certificate
active
06396589
ABSTRACT:
This application is based on patent application No. 11-72522 filed in Japan, the contents of which are hereby incorporated by references.
BACKGROUND OF THE INVENTION
This invention relates to an apparatus and method for measuring a three-dimensional shape of an object such as a machine part in a non-contact manner.
Conventionally, optical measuring methods have been known as means for measuring a three-dimensional shape of an object in a non-contact manner. These methods are roughly classified into four categories as follows.
First methods are so-called “light Cutting Method” which adopt the trigonometric measuring method as a basic principle and measure a three-dimensional shape of a measurement object by detecting a direction and a distance from a light projecting section to the measurement object. These methods enable a relatively highly accurate measurement with a simple construction comprised of the light projecting section and a light receiving section. On the other hand, they have disadvantages of a dead spot in measurement depending on the shape of the measurement object and a likeliness to be influenced by the surface condition of the measurement object.
Second methods measure a three-dimensional shape of a measurement object by projecting a specified regular optical pattern to the measurement object and detecting a variation degree of the optical pattern. These methods also have a disadvantage of a large dead spot in measurement despite its advantage of being capable of a highly accurate measurement with a simple construction.
Third methods measure a three-dimensional shape of a measurement object by projecting a ray to a measurement object as a radar does, calculating a distance to the measurement object by calculating a time required for reflected light to reach a light receiving section, and measuring the three-dimensional shape based on the calculated distance and the light projecting position. Although these methods are suited to measuring a long distance, they are not suited to measuring a shape of a small object, such as mechanical parts for a small machine, with high accuracy.
Fourth methods measure a three-dimensional shape of a measurement object by projecting a ray to a measurement object, calculating a distance to the measurement object by detecting a peculiar response of reflected light at a focus of an optical system, and measuring the three-dimensional shape based on the calculated distance and the light projecting position. These methods include a knife-edge method, an astigmatism method, and a confocal method.
The knife-edge method takes advantage of the inversion of, for example, the shadow of the edge of a knife across a focus in a detector. The astigmatism method takes advantage of a change of oval spot light in orientation across a focus using an optical system having an astigmatism.
According to the confocal method, as shown in
FIG. 14
, a measurement object
100
is moved within a specified range of a focus of a confocal optical system
101
, first and second pinholes
102
and
103
are arranged in conjugated positions of the focus, an illumination ray emitted from a light source
104
through the first pinhole
102
is focused by the confocal optical system
101
and projected onto the measurement object
100
to be reflected or diffused, and a ray reflected on the measurement object
100
is caused to transmit through the confocal optical system
101
again and is detected by a light receiving section
105
provided immediately after the second pinhole
103
.
Since the fourth methods are based on the change of an optical response which occurs before and after the focus position, measurement can be conducted with a higher resolution as compared with the first to third methods. Further, the confocal method can conduct measurement with a considerably high resolution since the light is most strongly observed in the light receiving section when the surface of the measurement object
100
corresponds with the focus of the confocal optical system
101
.
According to the first to third methods, information of shape is obtained when the measurement light reflected on the measurement object is received. On the contrary, according to the fourth methods, shape information cannot be obtained until the measurement object is entirely scanned and a position where the intensity of the measurement light is at maximum is detected. Accordingly, the fourth methods disadvantageously require a longer time than the first to third methods. The fourth methods have additional disadvantages of a complicated optical system and a necessity for a highly precise optical design in order to suppress an aberration to a lower level.
Further, in a conventional confocal detection method shown in
FIG. 14
, the optical aberration doubly influences in the fourth methods since the light emitted from the light source
104
transmits through the confocal optical system
101
, is reflected and diffused on the surface of the measurement object
100
, and is received by the light receiving section
105
after transmitting through the confocal optical system
101
again. Therefore, the optical aberration doubly influences.
FIG. 15
shows a light intensity distribution (dotted image distribution) when the light from the light source
104
transmits through the optical system
101
having a certain wave aberration once.
FIG. 16
shows a light intensity distribution (dotted image distribution) when the light having transmitted through the optical system
101
is regularly reflected on the measurement object
100
and then transmits through the optical system
101
again, i.e., when the light transmits through the optical system
101
twice. A peak of the dotted image distribution of
FIG. 16
is largely lowered as compared to that of FIG.
15
. This shows that the image is extremely degraded if the light transmits through the optical system having an aberration.
It has been very difficult to realize an optical system having a high optical performance over an entire range of light when the light transmits through the optical system
101
twice. For example, Japanese Unexamined Patent Publication No. 5-332733 discloses a confocal detection method provided with a confocal optical system. However, since light from a measurement object transmits through the optical system twice, this method also has the aforementioned problem.
Further, in the case that the confocal optical system has a zooming function, an extremely highly precise optical design and a complicated and highly precise optical system are required in order to suppress the optical aberration to a low level over the entire zooming range.
Furthermore, in the confocal optical system
101
used in the conventional confocal detection method shown in
FIG. 14
, measurement light of various intensities such as specular reflection and diffuse reflection from the surface of the measurement object
100
are incident on the light receiving section
105
. Accordingly, the variation range where the intensity of light incident on the light receiving section
105
becomes considerably wide, thereby sometimes exceeding the dynamic range of the light receiving section
105
.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an apparatus and method for measuring a three-dimensional shape of an object which are free of the problems residing in the prior art.
According to an aspect of the present invention, a three-dimensional shape measuring apparatus comprises an illuminator for illuminating a measurement object by two luminous fluxes in different directions. The two luminous fluxes intersect each other at a specified position. The apparatus further comprises a light receiver including a plurality of photoelectric conversion elements for receiving light to generate an electric signal in accordance with an intensity of received light, and an objective optical system for transmitting light reflected from the measurement object to the light receiver.
According to another aspect of the present invention, a three-dimensional sh
Minolta Co. , Ltd.
Pham Hoa Q.
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