Abstract:
A three-dimensional shape measuring apparatus having a high performance and a reduced drive energy is disclosed. At least a portion of an in-focus state detection optical system having an internal light source is movable while maintaining a light path near an object lens of the optical system. A distance of movement of the movable portion which is moved with auto-focusing operation for an object is measured and the three-dimensional shape is measured precisely with a high stroke. An inclination angle measuring optical system which shares the light path near the object lens with the in-focus state detection optical system and has an internal light source is provided in a common casing. Thus, the distance of movement of the movable part of the in-focus state detection optical system and an inclination angle are simultaneously measured so that the three-dimensional shape can be more precisely measured.

Description:
This application is a continuation-in-part continuation of application Ser. No. 07/289,534 filed Dec. 27, 1988, which is a continuation of application Ser. No. 07/135,462 filed Dec. 21, 1987, which is a continuation of application Ser. No. 06/750,281 filed July 1, 1985, all of which are now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a three-dimensional shape measuring apparatus, and more particularly to a non-contact and fast three-dimensional shape measuring apparatus. 
     2. Description of the prior Art 
     In the past, various methods have been used to non-contact measure a three-dimensional shape. They include an interference measuring method which uses a coherent light, and a method of reading a light sectional image with slit light. The interference measuring method has an advantage in that all surfaces of an object can be simultaneously measured with a high precision but a disadvantage in that the measurement of the shape is difficult to attain when an irregularity on the surface of the object is much larger than a light wavelength. In the light sectional image reading method, it is difficult to measure an irregular shape of an order of the light wavelength and hence a high precision measurement is not attained. In order to resolve the above disadvantages, a three dimensional shape measuring method has been proposed in which an in-focus state detection optical system having an internal light source is mounted on a carriage which is moved such that the optical system is focused to an surface of an object. (Japanese Patent Publication No. 40231/1971). In this method, the shape is measured with a high precision irrespective of the irregularity on the surface of the object. However, in the prior art method, a large quantity of energy is required to move the carriage which carries the in-focus state detection optical system because of the large weight of the movable portion. 
     When a fine irregularity is included on the surface of the object, more exact information is obtained by expressing the irregular shape by height or distance as well as an inclination angle than by expressing it only by the distance. In the prior art methods, exact information cannot be obtained by this method. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a three-dimensional shape measuring apparatus having a high stroke and a low drive energy. 
     It is another object of the present invention to provide a three-dimensional shape measuring apparatus having a high precision. 
     The three-dimensional shape measuring apparatus of the present invention has an in-focus state detection optical system having an internal light source, at least a portion of the optical system is movable while keeping a light path near an object lens in order to bring the optical system into an in focus position. A distance of movement of the movable portion is measured. Thus, the apparatus with a low drive energy is attained. 
     To attain a higher precision measurement, the three-dimensional shape measuring apparatus of the present invention has the in-focus state detection optical system having an internal light source. At least a portion of the optical system or a casing thereof is movable while keeping a light path near an object lens in order to bring the optical system into the in-focus position. A distance of movement of the movable portion is measured. An inclination angle measuring optical system having an internal light source which shares the light path near the object lens with the in-focus state detection optical system is provided. 
     The three-dimensional shape measuring apparatus of the present invention has the in-focus state detection optical system or that optical system and the inclination angle measuring optical system. A distance of movement of the movable portion is measured so that a position is measured with a low drive energy. By adding the measurement of the inclination angle, the three-dimensional shape measurement with high precision, high speed and high stroke is attained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an embodiment of a three-dimensional shape measuring apparatus of the present invention. 
     FIG. 2 illustrates an in-focus state detection method. 
     FIG. 3 is a plan view of a sensor. 
     FIG. 4 shows a sensor output. 
     FIG. 5 shows another embodiment of the three-dimensional shape measuring apparatus of the present invention. 
     FIG. 6 shows an other embodiment of the three-dimensional shape measuring apparatus of the present invention having an inclination angle measuring optical system. 
     FIG. 7 illustrates an inclination angle measuring method. 
     FIG. 8 shows an other embodiment of the three-dimensional shape measuring apparatus having the inclination angle measuring optical system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a first embodiment of a three-dimensional shape measuring apparatus of the present invention. Numeral 2 denotes an in focus state detection optical system, numeral 4 denotes a light source, numeral 6 denotes a collimator lens, numeral 8 denote a knife edge, numeral 10 denotes a half-mirror, numeral 12 denotes an object lens, numeral 14 denotes a lens, numeral 16 denotes an optical sensor, numeral 20 denotes an object whose shape is to be measured, numeral 22 denotes a casing of the three-dimensional shape measuring apparatus, numeral 24 denotes a distance measurement means and numeral 26 denotes a corner cube. 
     Light emitted from the light source 4 is collimated by the collimator lens 6, and the collimated light is reflected by the half-mirror 10 toward the object lens 12. The collimated light from the collimator lens 6 is particularly screened by the knife edge 8, and only one (upper) of two zones divided by a boundary surface passing through an optical axis X impinges on the object lens 12. The light focused by the object lens 12 forms a spot image on the surface of the object 20. The reflected light from the spot again passes through the object lens 12 and the half-mirror 10 and is focused by the lens 14 and reaches the sensor 16. The light reaching the sensor 16 changes depending on the distance between the surface of the object 20 and the object lens 12. This change of the light is detected to discriminate the in-focus state. The in-focus state detection method in the present embodiment is explained with reference to FIG. 2, in which the like elements to those shown in FIG. 1 are designated by the like numerals. X denotes an optical axis, (a) shows an in-focus position, (b) shows a rear-focus position and (c) shows a fore-focus position. An area below the optical axis X is called a zone A and an area above the optical axis X is called a zone B. 
     When the surface of the object 20 is at the focal point of the object lens 20 ((a) in FIG. 2), the spot on the surface of the object 20 is centered on the optical axis X and the reflected light is centered on the optical axis X at the sensor 16. When the surface of the object 20 is further than the focal point of the object lens 12 ((b) in FIG. 2), the spot on the surface of the object 20 is positioned in the zone A deviated from the optical axis X. Accordingly, the reflected light is positioned in the zone B in the sensor 16. On the other hand, when the surface of the object 20 is closer than the focal point of the object lens 12 ((c) in FIG. 2), the spot on the surface of the object 20 is deviated from the optical axis X and positioned in the zone B. Accordingly, the reflected light is positioned in the zone A in the sensor 16. By using a sensor array such as CCD sensor array as the sensor 16 and arranging sensor segments of the same size over the zones A and B and the optical axis X on the surface of the sensor 16, the focus state can be detected by checking a difference between the outputs of the sensor segments in the zones A and B. 
     FIG. 3 shows a plan view of the sensor 16. It is viewed from the left side of the sensor 16 shown in FIG. 2. Hatched areas show channel stoppers which isolate the sensor segments. The spot positions and the light intensities when the surface of the object 20 is at (a), (b) and (c) in FIG. 2 are shown. A sum of the outputs of the sensor segments in the zone B of the sensor 16 is represented by I B  and a sum of the outputs in the zone A is represented by I A . Δ I  =I B  -I A  changes depending on the focus state of the optical system 2 relative to the object 20. A relation therebetween is shown in FIG. 4. In the vicinity of the in-focus state area ((a) in FIG. 4), ΔI changes in a substantially linearly manner. From this characteristic, it is possible to check whether the optical system 2 is in the fore-focus state, in-focus state or rear-focus state. 
     Thus, auto-focusing operation can be attained by servo-driving a portion of the optical system 2 to bring the difference ΔI to zero. 
     In the present embodiment, the elements of the optical system 2 except the object lens 12 are mounted in the casing 22, and the object lens 12 is movable relative to the casing 22. The object lens 12 is moved along the optical axis and it may be driven by an actuator (not shown) attached to the casing 22. The actuator preferably has a hydro-dynamic bearing slide mechanism to attain a high precision control of the distance of movement. 
     Distance measurement means is provided in the present apparatus to measure the distance of movement of the object lens which is moved by the actuator. As shown in FIG. 1, the distance measurement means 24 of a type which counts the number of waves of a laser interferometer is attached to the casing 22, and the corner cube 26 which is a part of the distance measurement means is mounted on the object lens 12. The laser beam emitted from the means 24 is reflected by the corner cube 26 and directed to the means 24. 
     In the auto-focusing operation, the distance of movement of the object lens 12 is measured by the distance measurement means so that a position on the surface of the object 20 at which the optical axis crosses is determined. By carrying out the position measurement on the entire surface of the object, the three-dimensional shape can be measured. 
     The performance of the three-dimensional shape measuring apparatus of the present embodiment is evaluated below. 
     The precision of the position measurement is determined by both the resolution of the in-focus state by the optical system 2 and the precision of the distance measurement means. For example, when the object lens 12 has a focal distance f=2.1 mm and NA=0.9, the lens 6 has a focal distance f 1  =6.6 mm, the lens 14 has a focal distance f 2  =85 mm and the sensor 16 is a CCD array sensor, a gradient in the linear area in the graph of FIG. 4 is 200-1,000 mV/μm and a noise in the output ΔI is smaller than 1-2 mV. Thus, the in-focus state discrimination resolution of the optical system 2 is 0.01-0.02μm. When the distance measurement means of the type which counts the number of waves of the laser interferometer is used, precision of 0.1-0.01μm is achieved. The distance measurement means may be a light heterodyne interference system (e.g. Hewlett Packard Laser Distance Measurement Device, O pulse E, p 82, December 1982) or a grid interference distance measurement system (O pulse E, p 84 -, April 1981), and similar precision can be achieved. 
     The stroke of the position measurement is determined by the stroke of the actuator and the stroke of the distance measurement means. The grid interference distance measurement system, light heterodyne interference system and laser interferometer wave counting system can attain a high stroke of longer than 100 mm. The actuator can also attain a similar stroke. 
     A diameter of the light spot is determined by the NA of the object lens 12. When the object lens 12 has NA=0.8, the diameter φ of the light spot of the optical system 2 is φ=2.44 Fλ≈2.38μm (where ##EQU1## λ=0.78μm) and the spot of approximately 2 μm in diameter can be measured. When a larger spot diameter is desired, the effective light beam diameter of the optical system 2 is reduced to reduce the effective NA of the light beam. 
     In the present embodiment, since the point on the optical axis X on the surface of the object 20 and the sensor 16 are conjugate in the in-focus state, the measurement is attained when the surface of the object 20 is either mirror-finished or irregular, if the NA of the object lens 12 is large. 
     FIG. 5 shows a second embodiment of the three-dimensional shape measuring apparatus of the present invention. Like elements to those shown in FIG. 1 are designated by like numerals. The present embodiment differs from the first embodiment only in that the object lens 12 is fixed to the casing 22 and the light source 4 is movable relative to the casing 22. The light source 4 is moved along the optical axis of the collimeter lens 6 by an actuator (not shown) arranged in the casing 22. The distance measurement means 24 is mounted in the casing 24 and the corner cube 26 which is a part of the distance measurement means is mounted in the light source 4. The auto-focusing operation is carried out by moving the light source 4 and the distance of movement of the light source 4 is measured by the distance measurement means so that the position on the surface of the object 20 at which the optical axis X crosses is determined. In the present embodiment, it is necessary to convert the distance of movement of the light source 4 to the distance of movement of the focusing position of the light beam transmitted through the object lens 12. 
     The embodiments described above have the in-focus state detection optical systems. An embodiment of the three-dimensional shape measuring apparatus which also has an inclination angle measuring optical system to enable more precise measurement is now described. 
     FIG. 6 shows a third embodiment of the three-dimensional shape measuring apparatus of the present invention. Like elements to those shown in FIG. 1 are designated by like numerals Numeral 5 denotes an inclination angle measuring optical system. The optical systems 2 and 5 are assembled in the casing 22. 
     In the in-focus state detection optical system 2, numeral 4 denotes a light source, numeral 6 denotes a knife edge, numeral 15 denotes a polarization beam splitter, numeral 17 denotes a half-mirror, numeral 19 denotes a quarter wavelength plate, numeral 12 denotes an object lens, numeral 23 denotes a band-pass filter, numeral 14 denotes a lens and numeral 16 denotes an optical sensor. 
     In the inclination angle measuring optical system 5, numeral 28 denotes a light source, numerals 30 and 32 denote lenses, numeral 34 denotes a polarization beam splitter, numeral 36 denotes a band-pass filter and numeral 38 denotes an optical sensor. The optical system 5 shares the half-mirror 17, quarter wavelength plate 19 and object lens 12 of the optical system 2. 
     The casing 22 is connected to an external actuator 40. The casing 22 is moved along the optical axis X of the object lens 12 by driving the actuator 40. The actuator 40 preferably has a hydrodynamic bearing slide mechanism in order to achieve a high precision control of the distance of movement. 
     The distance measurement means 24 for measuring the distance of movement is arranged in the casing 22. The distance measurement means 24 may be of grid interference distance measurement system (O pulse E, p 84 -, April 1981). In FIG. 6, numeral 44 denotes a reference grid fixed to the casing 22 and numeral 46 denotes an external grid pitch reader. 
     The light emitted from the light source 4 is collimated by the collimator lens 6 and the collimated light beam passes through the polarization beam splitter 15, is reflected by the half-mirror 17, passes through the quarter wavelength plate 19 and is directed to the object lens 12. The collimated light beam from the collimator lens 6 is partially screened by the knife edge 8 and is directed to only one (upper) of two zones in the object lens 12 divided by a boundary plane passing through the optical axis X. The light focused by the object lens 12 forms a spot on the surface of the object 20. The reflected light from the spot again passes through the object lens 12 and the quarter wavelength plate 19 and is reflected by the half-mirror 17 and the beam splitter 15, passes through the band-pass filter 23 and the lens 14, and reaches the sensor 16. The in-focus state, fore-focus state or rear-focus state of the optical system 2 is thus determined in accordance with the principle shown in FIGS. 2-4. The auto-focusing operation is carried out by servo-dividing the actuator 40 to bring αI(= I B  -I A ) to zero. In the present embodiment, neither the object lens or the light source is moved; rather, the entire casing 22 is moved along the optical axis X to focus the optical system. The position on the surface of the object 20 at which the optical axis X crosses can be determined by measuring the distance of movement of the casing 22 by the distance measurement means 24. The three-dimensional shape can be measured by carrying out the position measurement for the entire surface of the object. 
     An inclination angle measurement method by the inclination angle measuring optical system 5 of the present invention is now explained. 
     The light emitted from the light source 28 passes through the lenses 30 and 32 and is collimated, and the collimated light beam is directed to the polarization beam splitter 34 and reflected thereby, and the reflected light passes through the half-mirror 17 and the quarter wavelength plate 19 and is focused by the object lens 12. In the optical system 5, the light beam directed to the object lens 12 is centered at the optical axis X and directed in parallel to the optical axis X. The light focused by the object lens 12 forms a spot centered on the optical axis X on the surface of the object 20. The reflected light from the spot again passes through the object lens 12, quarter wavelength plate 19, half-mirror 17, polarization beam splitter 34 and band-pass filter 36 and reaches the sensor 38. 
     The light reaching the sensor 38 changes depending on the inclination angle of the surface of the object 20. Referring to FIG. 7, if the surface of the object 20 inclined relative to a plane normal to the optical axis X by an angle α at a position on the optical axis X, the reflected light from the spot is directed to the object lens 12 with an inclination angle of 2α. In FIG. 7, the light beam directed to the object lens 12 travels in parallel with the optical axis X and the center of the light beam is spaced from the optical axis X by h=f sin 2α (where f is a focal distance of the object lens 12). 
     The sensor 38 may be a light beam center detection sensor or a position sensor, which measures the distance h, from which α is determined. 
     In the inclination angle measurement, it is necessary that the surface of the object 20 is at the focal point of the object lens 12. This condition is normally met by the focusing function of the optical system 2 and the actuator 40. 
     Since the in-focus state detection optical system 2 and the inclination angle measuring optical system 5 share certain portions thereof, the wave-lengths of the light sources of the respective optical systems or the polarizations are changed from each other to prevent crosstalk. To this end, the band-pass filters 23 and 36, the polarization beam splitters 15 and 34 and the quarter wavelength plate 19 are used. 
     The performance of the three-dimensional shape measuring apparatus of the present embodiment is essentially identical to that of the embodiment of FIG. 1 as to the precision of the position measurement. 
     The precision of the measurement of the inclination angle is determined by the precision of the position detection of the sensor 38. For example, when the detection precision of the sensor 38 is 0.3 μm and the focal distance of the object lens 12 is f=3.3 mm, an inclination angle measurement precision of approximately 9&#34; is attained. An inclination angle measurement range is 10-30° when the object lens 12 has NA=0.5-0.9. 
     FIG. 8 shows a fourth embodiment of the three-dimensional shape measuring apparatus of the present invention. The present embodiment is different from the third embodiment only in the construction of the inclination angle measuring optical system 5. In the optical system 5 of the present embodiment, numeral 60 denotes a light source, numeral 62 denotes a collimeter lens, numeral 64 denotes an aperture, numeral 66 denotes a half-mirror, numeral 67 denotes a bandpass filter, numeral 68 denotes a lens, numeral 70 denotes a mirror, numerals 72 and 74 denote a lens, and numerals 76 and 78 denote optical sensors. In the present optical system, the mirror 70 is located only in one (lower) of two zones divided by the boundary plane passing through the optical axis X. The aperture 64 and the mirror 70 are conjugate with respect to the object lens 12 and the lens 68. Namely, the aperture 64 is positioned at the focal point of the object lens 12. 
     In the optical system 5 of the present embodiment, if the surface of the object 20 is not inclined to the plane normal to the optical axis, the light reflected by the surface of the object 20 reaches the lens 68 without a shift from the optical axis X at a pupil position of tho object lens 12. Under this condition, the light intensity of the light which is reflected by the mirror 70 and reaches the sensor 76 through the lens 72 and the light intensity of the light which reaches the sensor 78 through the lens 74 without routing the mirror 70 are equal. The output of the sensor 76 and the output of the sensor 78 under this condition are set to be equal. When the surface of the object 20 inclines to the plane normal to the optical axis X, the light reflected by the surface of the object 20 is directed to the lens 68 with a parallel shift from the optical axis at the pupil position of the object lens 12. Thus, the light intensity of the light which is reflected by the mirror 70 and reaches the sensor 76 through the lens 72 and the light intensity of the light which reaches the sensor 78 through the lens 74 without routing the mirror 70 are different. The inclination angle is therefore determined based on the difference between the outputs of the sensors 76 and 78. 
     In the present embodiment, the entire in-focus state detection optical system 2 is moved. Alternatively, only a portion of the optical system 2 may be moved. For example, the object lens or the light source may be moved as shown in FIGS. 1 and 5. The size of the actuator can be reduced by reducing the size of the movable portion. 
     In the present embodiment, a so-called TTL-A 2  F (through the taking lens - active auto-focus) system (Journal of Television Institute of Japan, Vol. 35,  No. 8, 1981, p 637 - ) is used for the auto focusing system, although other auto-focusing systems such as a system used for a video pickup and a system used for camera auto focusing may be used. 
     The inclination angle measuring optical system is not limited to the illustrated optical system but various modifications thereof such as combinations of the in-focus state detection optical system may be used. 
     Various modification of the three-dimensional shape measuring apparatus may be made without departing from the concept of the present invention. 
     As described hereinabove, the three-dimensional shape measuring apparatus of the present invention can measure a three-dimensional shape using a fine spot with high precision, high speed and high stroke. The precision of the measurement of the shape is further increased by the provision of an inclination angle measuring optical system. The drive energy is reduced by reducing the size of the movable portion, and a very practical apparatus is thereby provided.