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

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.

CROSS-REFERENCE TO RELATED APPLICATION 
This application is related to U.S. Patent Application entitled "METHOD AND 
SYSTEM FOR HIGH-SPEED, 3-D IMAGING OF AN OBJECT AT A VISION STATION", U.S. 
Ser. No. 052,841 filed May 21, 1987 now U.S. Pat. No. 4,796,997 and having 
the same Assignee as the present application. The entire disclosure of 
U.S. Ser. No. 052,841 is hereby expressly incorporated by reference. 
TECHNICAL FIELD 
This invention relates to method and system for imaging an object at a 
vision station to develop dimensional information associated with the 
object and, in particular, to method and system for the high-speed, high 
resolution imaging an object at a vision station to develop dimensional 
information associated with the object by projecting a beam of controlled 
light at the object. 
BACKGROUND ART 
A high-speed, high resolution (i.e. approximately 1 mil and finer) 3-D 
laser scanning system for inspecting miniature objects such as circuit 
board components, solder, leads and pins, wires, machine tool inserts, 
etc., can greatly improve the capabilities of machine vision systems. In 
fact, most problems in vision are 3-D in nature and two-dimensional 
problems are rarely found. 
Several methods have been used to acquire 3-D data: time of flight, phase 
detection, autofocus, passive stereo, texture gradients, or triangulation. 
The latter approach is well suited for high resolution imaging and is 
perhaps the most well known technique. 
In the general scanning triangulation method a laser beam is scanned across 
the object to be inspected with a deflector and the diffusely scattered 
light is collected and imaged onto a position sensitive detector The 
scanner can be a rotating polygon, galvanometer, resonant scanner, 
holographic deflector, or acousto-optic deflector Likewise, the position 
sensitive detector can be a linear or area array sensor, a lateral effect 
photodiode, a bi-cell, or an electro-optic position sensing device. 
Sometimes, a pair of position detectors are used to reduce shadowing. With 
linear arrays or area cameras there is severe trade off between shadows, 
light sensitivity and field of view. 
For obtaining very high speed and low light sensitivity, the position 
sensing system described in the above-noted patent application is 
preferred However, if it is not required to detect very low light levels, 
lateral effect photodiodes can be used at data rates up to about 1 MHz and 
are inexpensive, commercially available devices. 
Often triangulation-based methods and systems have used the concept of 
"structural light". As described in U.S. Pat. No. 4,105,925 such a method 
involves projecting a line or multiple lines onto the surface of the 
object to be inspected and detecting the displacement of the projected 
line (or multiple lines) with a video camera. Such systems are now 
available off-the-shelf and are relatively inexpensive. 
The primary disadvantages of such a system are the very low speeds 
(typically 10,000 points/second) and, in the case of multiple projected 
lines in a single image, ambiguous interpretations of the data result from 
overlap of adjacent stripes and multiple scattered light between stripes. 
Both disadvantages can be overcome by replacing (1) the line projector 
with a flying spot scanner and (2) the video camera with one of several 
types of position sensitive detectors, as illustrated in U.S. Pat. No. 
4,375,921. 
Conventional triangulation based scanners or structured light systems often 
utilize conventional imaging lenses (i.e., reduction lenses, 35 mm lenses, 
or cylinder lenses designed for long line detectors) to deliver light to 
large area position sensitive detectors such as area sensor, linear arrays 
or large area position sensitive detectors The large area detectors have 
several limitations: low speed due to large detector capacitance, high 
dark currents, and a much higher noise floor than what is found with small 
area devices. 
For example, a 20 mm.times.20 mm P-I-N lateral photodiode (equivalent to 
the approximate area of a typical 1" video camera tube) has a capacitance 
of several hundred picofarads and a dark current of several microamps. On 
the other hand, a 2 mm.times.2 mm device will have capacitance of about 5 
pf and a dark current of about 50 nanoamps. Both the speed and noise 
performance of the smaller detectors can be orders of magnitude better 
than the performance achievable with large area devices. The improvement 
in speed is directly proportional to the reduction in capacitance and the 
improvement in signal-to-noise is at least as large as the square root of 
the reduction in capacitance. 
With typical triangulation-based images it is difficult to deliver light to 
a small area device without decreasing the field of view (and consequently 
the inspection speed). Furthermore, if the field of view is increased the 
height resolution is necessarily decreased in conventional triangulation 
based imagers. Also, if a spherical reduction lens is used to deliver 
light to the detector (with the necessary proportional decrease in 
resolution) the light gathering capability of the system is reduced in 
proportion to the area. These are severe limitations and impose 
undesirable trade-offs which limit the system performance. 
A "synchronized scanning" approach can be used to overcome this problem as 
described in U.S. Pat. No. 4,553,844 to Nakagawa et al. This scanning 
approach is commonly implemented with polygonal or galvanometer driven 
mirrors. However, this approach requires that the sensor head contain 
moving parts in the form of a rotating mirror (for example, in the 
Fournier plane or telecentric stop) or a pair of mirrors. In effect, a 
second mirror is used to follow the spot which is scanning by means of the 
first mirror. These moving parts are often not desirable, particularly if 
the sensor is to be subjected to the type of acceleration found with x-y 
tables and robotic arms in industrial environments. 
A dilemma exists with conventional triangulation imagers: it is desirable 
to use a small detector but unless moving parts are included the field of 
view becomes too small, the resolution too coarse, and the light gathering 
capability poor. Even if the coarse resolution is tolerable, the loss of 
light gathering capability also further reduces the system signal-to-noise 
ratio The signal-to-noise ratio is not good in the first place 
(particularly at high speeds) because of the use of the large area 
detector thereby compounding the problem. 
Many other prior U.S. patent describe various methods for the acquisition 
of 3-D data by means of triangulation. For example, the U.S. Pat. No. 
4,188,544 to Chasson describes a structured light method in which a beam 
expander and cylinder lens is used to project a line of light onto an 
object. The line of light is sensed with an imaging lens and video camera. 
The position of each point is determined with a peak detection algorithm. 
The measurement rate is slow due to the readout of the video camera. 
Multiple lines of light alleviate this problem to some extent. 
In the U.S. Pat. No. 4,201,475 to Bodlaj, an object is scanned in a 
position sensing dimension and the time displacement is detected by a 
single photodetector having a very narrow field of view. The speed of the 
system is limited by the retrace time of the scanning device at each 
measurement point. This method is relatively slow especially for the 
requirements of small part inspection at quasi-video rates (i.e. MHz). 
In the U.S. Pat. No. 4,645,917 to Penny, a swept aperture profiler is 
described. It too measures a time displacement for determining position. A 
galvanometer driven mirror is used to scan a line of data (i.e. x, y 
coordinates). An acousto-optic deflector is used to scan the position 
sensing dimension and the instant at which the light is received by the 
photodetection device indicates depth. The use of the A-O deflector for 
the z dimension scanning represents an improvement over the previous 
technology. Also, the use of a photomultiplier as a detection device 
allows for a much improved dynamic range. 
The U.S. Pat. No. 4,355,904 to Balusubramanian, describes a 
triangulation-based method which incorporates a position sensing device in 
the form of a variable density filter together with a system for sweeping 
the laser beam and controlling the position of the measurement probe. The 
tolerance on the density of typical variable filters, whether fabricated 
with a metallic coating on glass or with photographic film plate, is 
typically +5% at any single point. 
The U.S. Pat. No. 4,589,773 to Satoshi Ido, et al., describes a position 
sensing method and system for inspection of wafers which utilizes a 
commercially available position detector. A reduction lens is used to 
focus the light into a small spot on the surface of the object with a 10X 
reduction. A magnification lens is used in the receiver (10X) to deliver 
light to a detector. The triangulation angle is 45 degrees with the 
receiver and detector at complementary angles (90 degrees). This is fine 
for wafer inspection. However, the method is deficient for several other 
types of inspection tasks because (1) unacceptable shadows and occlusion 
effects would occur for tall objects; (2) the field of view of the probe 
is very small; (3) a reduction of the angle to 15 degrees (to reduce 
shadows) would degrade the height sensitivity significantly; and (4) the 
detector area is relatively large which limits speed and the signal to 
noise ratio as the speed of the system is increased. 
The U.S. Pat. No. 4,472,056 to Nakagawa et al., describes a method which 
involves projection of a line of light and the use of a rectangular CCD as 
the position sensor. This represents a significant improvement in speed 
over the method described in the above noted U.S. patent to Chasson and is 
good for inspection of parts with a relatively limited height range (i.e. 
16 levels). Logic and hardware is included for peak detection which can be 
related to the depth of the object. 
In the U.S. Pat. No. 4,650,333 to Crabb et al., a method of structured 
light projection is described which is somewhat complementary to the 
method described in the Nakagawa patent noted immediately above. A stripe 
of light produced with a cylindrical lens is swept across the object with 
an acousto-optic deflector in such a way that a single CCD line array can 
be used. This is a less expensive way of implementing the structured light 
method which does not require a custom CCD. Again, the speed and stray 
light rejection capabilities of the probe are limited which restrict it to 
depth measurement of objects (like traces) which are not very tall. 
Nevertheless, the method is suited to the inspection task of trace height 
measurement. 
The U.S. Pat. No. 4,593,967 to Haugen assigned to Honeywell describes a 
triangulation-based scanning system utilizing a holographic deflection 
device to reduce the size and weight of the scanning system and a digital 
mask for detection of position. The digital mask is in the form of binary 
grey code and requires a detector for each bit (i.e. 8 detectors for an 8 
bit code). A single cylinder lens is used in the receiver to convert a 
spot of light into a thin line which must be sharply focused onto a series 
of photodetectors. In other words, the spot is converted into a line to 
deliver the light to the series of long thin detectors. Spatial averaging 
is not performed in the system nor is the centroid of the light spot 
determined. 
U.S. Pat. No. 4,634,879 discloses the use of optical triangulation for 
determining the profile of a surface utilizing a prism and two 
photomultiplier tubes in a flying spot camera system. These are arranged 
in a "bi-cell" configuration. The bicell, however, does not compute the 
centroid of the received light spot and is therefore sensitive to the 
distribution of intensity within the received light spot. As an anti-noise 
feature, amplitude modulation is impressed upon the laser beam and a 
filter network is used to filter photomultiplier response so as to exclude 
response to background optical noise. 
DISCLOSURE OF THE INVENTION 
An object of the present invention is to provide an improved method and 
system for high speed, high resolution 3-D imaging of an object at a 
vision station wherein high speed and sensitivity can be obtained by using 
a flying spot laser scanner with a light deflector and an optical system 
to deliver the light reflected from an object to a single, small area 
position detector such as a photodetector to develop dimensional 
information associated with the object while substantially reducing 
ambient and multiple reflected light. 
Another object of the present invention is to provide a triangulation-based 
method and system for imaging an object at a vision station which 
overcomes many of the limitations of the prior art methods and systems by 
achieving excellent height resolution at a narrow triangulation angle 
wherein shadow and occlusion effects are reduced while having a relatively 
large field of view. 
Yet still another object of the present invention is to provide a method 
and system for high speed imaging of an object at a vision station to 
develop high resolution, dimensional information associated with the 
object and having a high signal-to-noise ratio in a relatively inexpensive 
and compact fashion and which system can be interfaced with standard, high 
speed apparatus. 
In carrying out the above objects and other objects of the present 
invention, a method is provided 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 includes 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 the reflected light signal at a second 
angle with a set of optical components, including first and second lenses 
and filtering the received signal with the set of optical components. The 
method further includes the steps of measuring the amount of radiant 
energy in the reflected light signal with a small area position detector 
having a position-sensing direction, 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. The 
method is characterized by the steps of delivering the filtered light 
signal to the small area position detector with an anamorphic 
magnification and field lens system. The lens system includes a third lens 
for increasing the filtered light signal in the position-sensing direction 
of the position detector and a fourth lens having a relatively short focal 
lens for delivering the light signal to the position detector. 
Further in carrying out the above objects and other objects of the present 
invention, 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 is provided. The system includes 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 and a set of optical components 
including first and second lenses for receiving the reflected light signal 
at a second angle and for filtering the reflected light signal. The system 
further includes measuring means including a small area position detector 
having a position sensing direction for measuring the amount of radiant 
energy in the reflected light signal and producing at least one electrical 
signal proportional to the measurement. Signal processing means computes a 
centroid value for the reflected light signal from the at least one 
electrical signal. An anamorphic magnification and field lens system 
includes a third lens for increasing the filtered light signal in the 
position-sensing dimension of the position detector and a fourth lens 
having a relatively short focal length for delivering the light signal to 
the position detector. 
In one construction of the imaging system, the source preferably includes a 
solid state (i.e. acousto-optic) laser light deflector and the set of 
optical components preferably includes a mask to control the polarization 
and acceptance angles of the collected light. 
Also, preferably, the measuring means includes a highly sensitive 
photodetector such as a lateral effect photodiode for converting the 
radiant energy into at least one electrical current. 
Still, preferably, 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. 
The advantages accruing to the method and system as described above are 
numerous. For example, such an imaging system can be incorporated into an 
inspection/gauging product wherein both range and intensity data are 
acquired. 
Also, such a method and system provide high resolution, quasi-video rate, 
full 3-D imaging at a relatively low cost. A long scan line (i.e. field of 
view) is achieved as well as a high signal-to-noise ratio, height 
sensitivity and light gathering capability and low capacitance and "dark 
current". 
The present invention overcomes many of the problems of the prior art by 
utilizing an anamorphic magnification and field lens system to deliver 
light to a small area position sensor in conjunction with the benefits of 
utilizing an all solid state light deflection system (i.e. compact, 
rugged, easy to interface with, etc.) 
The objects, features and advantages of the present invention are readily 
apparent from the following detailed description of the best mode for 
carrying out the invention when taken in connection with the accompanying 
drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to FIG. 1, there are illustrated the major components of a 
3-D imaging system constructed in accordance with the present invention 
and generally indicated at 10. The system 10 is positioned at a vision 
station and includes a controlled source of light such as a laser, 
modulator and optical feedback circuit 12. A scanner in the form of an 
acousto-optic deflector 14 and beam shaping and focusing optics in the 
form of various lens elements 16 produce a telecentric, flat field scan by 
projecting a series of laser beams at the reflective surface 18 of an 
object, generally indicated at 20. The object is supported on a reference, 
planar surface 22 at the vision station. 
Within the block 12 a laser is coupled to a modulator to shift the 
information to a higher frequency where system noise characteristics are 
better. The modulator may perform one of many types of modulation, 
including sine wave, pulse amplitude, pulse position, etc. Preferably, the 
laser is a solid state laser diode and is "shuttered" with a TTL signal 
(i.e. TTL modulation). In this way, the laser signal is encoded so as to 
allow separate signal processing functions to be performed during "on" and 
"off" intervals as described in detail in the above-noted application. 
Typically, power levels are 20-30 mW (Class III-B) which are well suited 
for machine vision applications. 
A solid state acousto-optic (i.e. A-O) deflector 14, such as one 
commercially available from Newport Electro-Optics, is preferably used. 
The deflector is easy to interface with, is very rugged and compact. This 
presents numerous advantages. The size of the system 10 can be about the 
size of a video camera. No moving parts are present in the system 10. Long 
term stability is easy to maintain. The system 10 can be made rugged 
enough to mount on a translator like an x-y table or robotic arm with 
relatively little effort. Therefore, producing the unit in large 
quantities is relatively easy. Most A-0 deflectors produce about 500 
spots/scan line which provides a very convenient interface to digitizers 
and image processing equipment. The duty cycle is also very high compared 
to other types of scanners (95% vs. 50%). 
The A-O deflector 14 has the advantage of being all solid state as 
previously discussed. However, due to the nature of diffractive scanning, 
a smooth illumination gradient of about 10-30% of the average value in the 
field of view results. Although this type of gradient can sometimes be 
tolerated, it is undesirable because it offsets a potentially large 
advantage of laser scanning in general: the ability to deliver the same 
quantity of light at the same angle of incidence to every point in the 
field of view. 
An optical and electronic feedback loop generally indicated at 24, is 
utilized to correct this slowly varying gradient (i.e. for flat field 
correction). The A-O deflector 14 produces both a scanning beam and a "DC" 
beam which is normally blocked with a spatial filter. This DC beam will 
contain about 30% of the laser power. By sensing the variations in this 
beam it is possible to infer the variations in the illumination because 
the total light is the sum of the .canning (i.e. 1st order) light and the 
DC beam (0th order). 
The DC beam is sensed by a photodetector 26 of the loop 24. The resulting 
electrical signal is used by an automatic gain control circuit 28 (i.e. 
including an amplifier and an integrator) of the loop 24 to attenuate or 
amplify the RF power applied to the A-O deflector 14 at a balanced mixer. 
The resulting intensity distribution is flat to about 1% which provides a 
significant advantage for greyscale inspection and a modest dynamic range 
improvement for 3-D inspection. 
There is generally indicated at 38 an optical system for use in optically 
processing the light signal reflected from the object 20. The optical 
system 38 includes a set of optical components, including a telecentric 
receiver lens 40 to collect scattered light from the object 20 at a 
position approximately one focal length from the object 20. A reduction 
focusing lens 42 operates as a telescope objective. The lenses 40 and 42 
operates as a preferred conjugate. The reduction lens 42 can be 
interchanged to accommodate various reduction and magnification ratios. 
The reduction lens 42 is placed directly behind a mask 44. 
The mask 44 is located at one focal length from the receiver lens 40 and 
functions as a telecentric stop to provide a spatial and polarization 
filtering plane. In one embodiment, the mask forms a rectangular aperture 
(i.e. spatial filter) positioned at the intermediate spatial filtering 
plane to reject background noise (i.e. stray light) which arises from 
secondary reflections from objects outside of the desired instantaneous 
field of view of the system 10. The mask 44 may be a fixed aperture 46 or 
electromechanical shutter, or, preferably, is a liquid crystal, binary, 
spatial light modulator or valve which is dynamically reconfigured under 
software control. Such a configuration is useful for inspection of very 
shiny objects (reflowed solder, wire bond, loops, pin grids, etc.) which 
are in close proximity from which multiple reflections will be created. 
Consequently, both the angle (through stop size) and polarization of the 
input light can be digitally controlled prior to delivery to a detector. 
If desired, the spatial filter or strip can be programmed in a chosen 
pattern of opaque and transmissive patterns correlated with the height 
profile of the object to be detected. For example, a height measurement of 
shiny pins placed on a shiny background will be more reliable if only a 
narrow strip corresponding to the height range over which properly 
positioned pins is viewed. Multiple reflections may produce a signal 
return which is significantly larger than the return produced by useful 
light. If properly placed, the position of the pin will be reported If 
defective, no pin will be found. 
When a conventional triangulation-based scanner is used (i.e. a solid state 
device having no moving parts but an area detector) the aperture 46 of the 
mask 44 is no larger than necessary for detection of a specified height 
range, but is still preferably programmable. 
The optical system 38 further includes an anamorphic magnification and 
field lens system, generally indicated at 48. The lens systems 48 includes 
a pair of anamorphic elements or lenses 50 and 52. The lens 50 is a very 
long focal length, precision negative cylinder lens to magnify the image 
in the position-sensing direction. The focal length of the lens 50 is 
typically between about -300 mm and -1000 mm and may have a focal length 
in the range of -200 to -1200 mm.. 
The lens 52 is a custom short focal length cylinder lens having a speed of 
about f/0.5 or f/0.6 and may have a speed in the range of f/0.4 to f/0.7 
which is used to expand the field of view and light gathering capability 
of the system 38. The lens 52 has a preferred focal length of about 25 mm 
and may have a focal length in the range of 20 to 30 mm. 
FIG. 2a illustrates the profile of a "step object" wherein several 
positions on the stop object are labelled. 
FIG. 2b illustrates the labelled positions of FIG. 2a as seen in a large 
area detector as a laser spot is scanned along the object. This represents 
the prior art. 
FIG. 2c shows the same labelled positions of FIG. 2a, and also shows the 
effect of using the pair of lenses 50 and 52. The lenses 50 and 52, 
convert a small focused spot of light into a smooth, enlarged rectangular 
or elliptical spot which uniformly illuminates an extended region of a 
single position sensitive detector 53 and averages spatial noise resulting 
from variations in sensitivity from point to point. 
The combination of the lenses 42 and 50 serve to provide magnification in 
the position sensing dimension. The magnification in the position sensing 
direction is usually greater than 1:1, thereby yielding microscopic 
magnification. 
The lens 52 serves as an anamorphic field lens into which the scan line is 
imaged. The length of the imaged scan line can be almost as large as the 
lens 52 (i.e. -40 mm) but is clearly much larger than the dimension of the 
detector 53. Hence, it serves as the reduction optic. The lens 52 can be 
fabricated in the form of a double convex singlet, a plane convex 
"hemi-cylinder" or with a gradient index optic having a radial gradient or 
a combination thereof. A double convex design, however, is preferable. 
In order to extend the depth measurement range of the system 10, a 
translating tracking mirror 54 is included and can be placed at any of 
several convenient positions provided it is behind the mask 44 to maintain 
telecentricity. Alternatively, a small angle deflector can be used but 
will deviate rather than translate the light beam. 
The translating mirror 54 is mounted on a precision miniature translation 
stage which is displaced under software control via a control or 
controller 56 which, in turn, is coupled to a signal processing circuit 
58. 
The mirror 54 is useful because it can significantly extend the measurement 
range of the system 10. For example, the position sensor or detector at 
any instant can discriminate about 256 levels or height. Several 
inspection tasks may demand an extension of this height range. For 
example, it may be desirable to measure the height of solder on pads which 
requires depth sensitivity of about 0.0004 inch. On the other hand, it may 
be desirable also to measure the position and geometry of component leads 
which are to be aligned with the pads. The leads may extend upward about 
0.25" or more to the body of the component. This exceeds the linear 
measurement range of lateral photodiodes. Also, wire loops are very thin 
and require high spatial and depth resolution for an accurate measurement. 
However, these wires may also extend up to 0.25" and a sensor which is to 
accommodate this entire range at the required 0.0002" inch height and 
spatial resolution is not practical. 
The translating mirror 54 alleviates this problem. The only requirement is 
that the lens 40 receive the light. The lens 40 can be expected to provide 
an image size (in the position sensing dimension) which is somewhat larger 
than the detector 53. Displacing the mirror 54 has the effect of 
translating the total field of view (constrained by the lens 40) across 
the detector 53 so that many more levels of height can be sensed while 
still utilizing the small area detector 53. 
Preferably, a single detector element is utilized as a small area position 
sensitive detector 53 of the system 10. The system 10 can obtain quite 
accurate z (i.e. height) measurements with a lateral effect photodiode 
(LEP), the internal resistance of which provides the depth sensing and 
centroid computation capability through attenuation of signal currents. 
The position detector 53 is preferably a lateral effect photodiode like 
the Si-Tek 2L2 or 2L4 or a special rectangular lateral effect detector. 
These position sensitive devices have substantial speed and depth range 
advantages over linear arrays. Bi-cells or digital masks (i.e. optical 
encoder) are not preferred. 
The detector 53 is coupled to a pre-amplifier 58 which, in turn, is coupled 
to the signal processing circuit 58 which computes the centroid of the 
light spot thereby allowing for non-uniform and directional intensity 
distributions. 
The signal processing circuit or unit 58 expands/compresses the variable 
data in order to obtain the proper Z value, grey scale information and 
special values indicating incorrect height information. The signal 
processing circuit 58 is described in greater detail in the above-noted 
application. 
Although the system 10 is designed to support a scanning mechanism with no 
moving parts, it can also be used in the synchronized scanning geometry 
approach to provide additional benefits, namely increasing resolution 
using a very small point detector and spatial averaging over the detector. 
The above-described imaging method and system present numerous advantages. 
For example, imaging can be performed at high resolution and at 
quasi-video rates to obtain full 3-D information. A large scan line (i.e. 
field of view) is achieved as well as a high signal-to-noise ratio, height 
sensitivity and light gathering capability and low capacitance and "dark 
current". Also, such a method and system offer the potential of accurate, 
quasi-video frame rate depth sensing at low cost. 
While the best mode for carrying out the invention has herein been 
described in detail, those familiar with the art to which this invention 
relates will recognize various alternative designs and embodiments for 
carrying out the invention as defined by the following claims.