Method and system for high-speed, high-resolution, 3-D imaging of an object at a vision station

• US 5,024,529 A
• Filed: 01/29/1988
• Issued: 06/18/1991
• Est. Priority Date: 01/29/1988
• Status: Expired

• Current Assignee: Electro Scientific Industries Incorporated (MKS Instruments Incorporated)
• Original Assignee: Synthetic Vision Systems, Inc.
• Inventors: Doss, Brian L., Svetkoff, Donald J.
• Primary Examiner(s): Rosenberger, Richard A.
• Application Number:
  US07/150,135
• Time in Patent Office: 1,236 Days
• Field of Search: 356/1, 356/376, 350/358
• US Class Current: 356/608
• CPC Class Codes: G01B 11/02 for measuring length, width or thickness G01B11/08 takes precedence
• CPC Class Codes: G01B 11/24 for measuring contours or curvatures

Abstract

A method and system for high-speed, high-resolution, 3-D imaging of an object including an anamorphic magnification and field lens system to deliver the light reflected from the object to a small area position detector having a position-sensing direction. Preferably, an acousto-optic deflector together with associated lens elements scans a beam of modulated laser light across the object to produce a telecentric, flat field scan. The deflector has a feedback loop to enable uniform illumination of the object. The light scattered from the object is collected by a telecentric receiver lens. A combined spatial and polarization filtering plane preferably in the form of a programmable mask is provided to control the polarization and acceptance angles of the collected light. A reduction or focusing lens is positioned immediately behind the filtering plane and is utilized as a telescope objective. The lens system includes a negative cylinder lens having a relatively large focal length and a field lens having a relatively small focal length. The cylinder lens and the reduction lens magnify the image in the position sensing direction of the detector and the field lens delivers the magnified light to the detector. The detector is a photodetector such as a lateral effect photodiode or a rectangular lateral effect detector. A pre-amplifier provides a pair of electrical signals which are utilized by signal processing circuitry to compute the centroid of the light spot.

Claims

1. A method for the high-speed, high-resolution, 3-D imaging of an object at a vision station to develop dimensional information associated with the object, the method including the steps of:

scanning a beam of controlled light in a scanning direction at the surface of the object at a first predetermined angle to generate a corresponding reflected light signal;

receiving said reflected light signal at a second angle with a set of optical components for creating a relatively small focused spot of light from the reflected light signal, the set of optical components including first and second lenses;

filtering the received light signal with the set of optical components;

measuring the amount of radiant energy in the reflected light signal with a small area position detector having a detector area and having a position-sensing direction substantially orthogonal to the scanning direction and producing at least one electrical signal proportional to the measurement; and

computing a centroid value for the reflected light signal from the at least one electrical signal;

wherein the improvement comprises;

converting the focused spot of light into an enlarged, elongated spot of light and imaging the filtered light signal to the small area position detector with an anamorphic magnification and field lens system, including a third lens for magnifying the focused spot of light in the position-sensing direction of the position detector and a fourth lens having a relatively short focal length for reducing the focused spot of light in the scanning direction to form the elongated spot of light, wherein the position detector comprises a single detector element including the detector area of less than 20 mm² and having a relatively small capacitance and dark current and wherein the entire elongated spot of light is uniformly imaged to a discrete extended region of the detector area, to obtain a relatively high signal to noise ratio, the extended region being elongated in a scan dimension substantially orthogonal to the position-sensing direction, wherein spatial noise resulting from variations in sensitivity from point to point in the detector area are averaged.

4. The invention as claimed in claim 1 or claim 2 wherein the fourth lens has a focal length in the range of 20 to 30 mm.

5. The invention as claimed in claim 4 wherein the fourth lens has a speed in the range of f/0.4 to f/0.7.

6. The invention as claimed in claim 5 wherein the fourth lens is a double convex field lens.

7. The invention as claimed in claim 4 wherein the third lens is a negative cylinder lens having a focal length in the range of 200 to 1,200 mm.

8. The invention as claimed in claim 1 or claim 2 wherein the set of optical components includes a programmable mask correlated to a height profile of the object for filtering the received light signal.

9. The invention as claimed in claim 8 wherein the first lens has a first focal length and wherein the first lens is adapted to be located a distance approximately equal to the first focal length from the object.

10. The invention as claimed in claim 9 wherein the mask is located a distance from the first lens approximately equal to the first focal length.

11. The invention as claimed in claim 1 or claim 2 wherein the set of optical components includes a mask having a fixed aperture for filtering the received light signal.

12. The invention as claimed in claim 1 or claim 2 wherein the position detector includes a single photodetector to measure the amount of radiant energy in the reflected light signal to thereby convert the radiant energy into an electrical current.

13. The invention as claimed in claim 12 wherein the photodetector is a semiconductor device having a detector area of less than 1 cm².

14. The invention as claimed in claim 12 wherein the photodetector has a detector area of less than 0.1 cm².

16. The invention as claimed in claim 1 or claim 15 or claim 2 wherein the beam of controlled light is a laser scanning beam.

17. The invention as claimed in claim 16 wherein the laser scanning beam is provided by an acousto-optic deflector.

18. The invention as claimed in claim 17 wherein the light deflector further provides a D.C. beam, the amount of radiant energy in the D.C. beam being measured to produce a control signal proportional to the measurement, the control signal being utilized to control the light deflector so that the light deflector illuminates the object in a substantially uniform fashion.

19. The invention as claimed in claim 1 or claim 15 or claim 2 wherein the field of view of the filtered light signal is translated across the position detector by translation means to expand the range of dimensional information associated with the object.

20. The invention as claimed in claim 19 wherein the translation means includes a tracking mirror for reflecting the filtered light signal and a controller for controlling movement of the tracking mirror.

2. An imaging system for the high speed, high-resolution 3-D imaging of an object at a vision station to develop dimensional information associated with the object, the system including:

a source for scanning a beam of controlled light in a scanning direction at the surface of the object at a first predetermined angle to generate a corresponding reflected light signal;

a first set of optical components for creating a relatively small focused spot of light from the reflected light signal, the set of optical components including first and second lenses for receiving the reflected light signal at a second angle and for filtering the received light signal;

measuring means, including a small area position detector having a detector area and having a position-sensing direction substantially orthogonal to the scanning direction for measuring the amount of radiant energy in the reflected light signal and producing at least one electrical signal proportional to the measurement; and

signal processing means for computing a centroid value for the reflected light signal from the at least one electrical signal, wherein the improvement comprises;

an anamorphic magnification and field lens system for converting the focused spot of light into an enlarged elongated spot of light, the lens system including a third lens for magnifying the focused spot of light in the position-sensing direction of the position detector and a fourth lens having a relatively short focal length for reducing the focused spot of light in the scanning direction to form the elongated spot of light, wherein a position detector comprises a single detector element including the detector area of less than 20 mm² and having a relatively small capacitance and dark current and the lens system uniformly imaging the entire elongated spot of light to a discrete extended region of the detector area, to obtain a relatively high signal to noise ratio, the extended region being elongated in a scan dimension substantially orthogonal to the position-sensing direction, wherein spatial noise resulting from variations in sensitivity from point to point in the detector area are averaged.

3. The invention as claimed in claim 2 wherein the second and third lens cooperate to magnify the filtered light signal in the position-sensing direction of the position detector.

15. A method for the high-speed, high-resolution, 3-D imaging of a relatively small object at a vision station to develop dimensional information associated with the object, the method including the steps of:

scanning a beam of controlled modulated light in a scanning direction at the surface of the object at a first predetermined angle to generate a corresponding reflected light signal;

receiving said reflected light signal at a second angle with a set of optical components for creating a relatively small focused spot of light from the reflected light signal, the set of optical components including first and second lenses, filtering the received light signal with the set of optical components;

measuring the amount of radiant energy in the reflected light signal with a small area position detector having a detector area and having a position-sensing direction substantially orthogonal to the scanning direction and producing at least one electrical signal proportional to the measurement;

demodulating the at least one electrical signal; and

computing a centroid value for the reflected light signal from the at least one demodulated signal;

wherein the improvement comprises;

converting the focused spot of light into an enlarged, elongated spot of light and imaging the filtered light signal to the position detector with an anamorphic magnification and field lens system, including a third lens for magnifying the focused spot of light in the position-sensing direction of the position detector and a fourth lens having a relatively short focal length for reducing the focused spot of light in the scanning direction to form the elongated spot of light, wherein the position detector comprises a single detector element including the detector area of less than 20 mm² and having a relatively small capacitance and dark current and wherein the entire elongated spot of light is uniformly imaged to a discrete extended region of the detector area, to obtain a relatively high signal to noise ratio, the extended region being elongated in a scan dimension substantially orthogonal to the position-sensing direction, wherein spatial noise resulting from variations in sensitivity from point to point in the detector area are averaged.

21. An imaging system for the high-speed, high-resolution 3-D imaging of a small object at a vision station to develop dimensional information associated with the object, the system comprising:

a flying spot laser scanner including a light deflector scanning a beam of controlled light in a scanning direction at the surface of the object at a first predetermined triangulation angle of less than 20 degrees to generate a corresponding reflect light signal;

a first set of optical components for creating a relatively small focused spot of light from the reflected light signal, the first set of optical components including first and second lenses for receiving the reflected light signal at a second angle and for filtering the received light signal;

measuring means, including a single, small area position detector having a detector area and having a position-sensing direction substantially orthogonal to the sensing direction for measuring the amount of radiant energy int he reflected light signal and producing at least one electrical signal proportional to the measurement;

signal processing means for computing a centroid value for the reflected light signal from the at least one electrical signal; and

an anamorphic magnification and field lens system for converting the focused spot of light into an enlarged elongated spot of light, the lens system including a third lens for magnifying the focused spot of light in the position-sensing direction of the position detector and a fourth lens having a relatively short focal length for reducing the focused spot of light in the scanning direction to form the elongated spot of light, wherein the position detector comprises a single detector element including the detector are of less than 20 mm² and having a relatively small capacitance and dark current, the lens system uniformly imaging the entire elongated spot of light to a discrete extended region of the detector area, to obtain a relatively high signal to noise ratio, the extended region being elongated in a scan dimension substantially orthogonal to the position-sensing direction, wherein spatial noise resulting from variations in sensitivity from point to point in the detector area are averaged.