Method of measuring in three-dimensions at high speed

An apparatus with a common aperture and multiple image recording surfaces with individual filters and/or controlled shutters enables the implementation of several methods of 3-D measurement systems. The parallel nature of the device lends to short measurement times suitable for measuring moving objects. A similar apparatus may be constructed to project multiple images simultaneously or in rapid succession. Elimination of mechanical motion within the projectors and cameras of the prior art reduce the measurement time significantly. A new method of 3-D measurement employing a sweeping light plane and time encoded image recording uses the apparatus. An alternate method maintains a stationary projected light plane through which an object moves as it is measured in 3-D. Another method uses simultaneous projection of light patterns at different frequencies. Another method employs the time of flight of a light pulse and time encoded recording of the reflected energy.

BACKGROUND OF THE INVENTION 
It is desirable, in certain applications, to make three dimensional 
measurements at very high speed, especially with fast moving or fast shape 
changing objects. To make three dimensional measurements in the prior art, 
two methods have been used as follows: 
1. A succession of patterns were projected onto the object being measured. 
Corresponding to each of the light patterns projected onto the object, a 
photograph or TV picture was taken. Both digital and analog patterns were 
used. 
2. A succession of pulses of light were transmitted and a gated receiver 
integrated the light reflected from the object to be measured. The gating 
was done in a time-coded manner to encode distance to the object surface 
as a function of the travel time of the light. 
Since both methods involve a sequence of events that have time durations 
that may accumulate to a period of time too great to make an adequate 
measurement, the present invention provides two methods to improve the 
state of the art. The limitations of the prior art come from the time it 
takes to change the projection of one pattern to another and the time it 
takes to record the reflected light and prepare for a subsequent 
recording. 
The present invention provides methods for allowing events on both the 
projector and receiver sides of the measurement system to operate in 
parallel in order to reduce the overall time of measurement. The methods 
can also be applied, in part, to implement other prior art techniques such 
as separating the transmitted patterns by the frequency of radiation and 
parallel gating of received energy from a single projected pulse. 
SUMMARY OF INVENTION 
It is an object of the present invention to provide improved methods of 
projecting and receiving electro-magnetic energy in order to extract 
three-dimensional information in a shorter period of time. 
In keeping with this object, and with still others, which will become 
apparent as the description proceeds, one aspect of the invention resides 
in a method of detecting three-dimensional information comprising the 
steps of projecting multiple patterns of light upon the subject to be 
measured, one pattern at a time, and recording each reflected pattern with 
a separate camera. 
A second aspect of the invention resides in a method of detecting 
three-dimensional information comprising the steps of projecting a moving 
plane of light upon the subject to be measured, and making multiple 
recordings of the reflected light with multiple cameras in a coded manner. 
A third aspect of the invention resides in a method of detecting 
three-dimensional information comprising the steps of projecting a 
stationary plane of light upon a moving subject to be measured and making 
multiple recordings of the reflected light with multiple cameras whose 
views are synchronized to the subject's motion. The multiple recordings 
are made in a coded manner. 
A fourth aspect of the invention resides in a method of detecting 
three-dimensional information comprising the steps of projecting multiple 
patterns of light simultaneously upon the subject to be measured. Each 
pattern is at a different wavelength, and recording each reflected pattern 
with a separate matching filter and camera. 
A fifth aspect of the invention resides in a method of detecting 
three-dimensional information comprising the steps of projecting a pulse 
of light upon the subject to be measured and making multiple recordings of 
the reflected light with multiple cameras in a coded manner. 
The invention will hereafter be described with reference to an exemplary 
embodiment, as illustrated in the drawings. However, it is to be 
understood that this embodiment is illustrated and described for the 
purpose of information only, and that nothing therein is to be considered 
limiting of any aspect of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, projector 12 illuminates subject 14 to be measured with light 
beam 15, causing reflected light 16 which can be recorded by camera 11. 
(Hereinafter the term "light" is to be understood to mean electromagnetic 
radiation including but not limited to, the visible spectrum.) Electronic 
control 13 can synchronize the camera 11 with the light projection 15. In 
one practice of the prior art, projector 12 emits a unique pattern of 
light 15 onto subject 14 and camera 11 records the reflected light 16. 
After a period of time sufficient for both projector 12 and camera 11 to 
set up for a second unique pattern, the procedure is repeated. It is not 
uncommon for many unique patterns to be required to obtain all the desired 
3-D measurement data, so the process can take significant time. The 
present invention eliminates the time delay in moving film if camera 11 
uses film or in reading out the recorded video image if camera 11 is a TV 
camera. This is accomplished as shown in FIG. 2 where it is shown that, 
internal to camera 11, a lens 21 can be placed, through which the 
reflected light is imaged onto multiple light-sensitive surfaces 22 
(photographic film or light-sensitive electronic receiver such as a 
vidicon or a TV semiconductor chip). The light exiting lens 21 first 
encounters beam splitter 24 which reflects a fraction, 1/N, of the light 
toward light-sensitive surface 22. An optical path length adjustment 25 
can be made to make a more compact design and avoid moving surface 22 for 
focusing. A controlled light shutter 23 (such as a Pockels cell) allows 
the light to reach the surface 22 only during the projection of one of the 
unique patterns. Similar paths split the light onto N equal intensity 
images on the N surfaces 22; only one path of which is not blocked by its 
shutter 23. Each shutter 23 only opens during its respective unique light 
pattern projection period. The ratio of light split at each beam splitter 
24 is shown to vary as 1/N:(1-1/N)=1:N-1 for the first, 1/N:(1-2/N)=1:N-2 
for the second . . . 1/N:(1-(N-1)/N)=1:1 for the (N-1)th and 1/N:0 (total 
reflection) for the Nth. 
Thus the minimum time in which the sequence of the patterns can be recorded 
is no longer limited by the time in which a camera can store the data away 
and prepare for the next recording. The limit now is the time needed to 
record the reflected light intensity pattern and the speed of the shutter 
23. 
A similar problem may exist with projector 12 not being able to change 
projected patterns quickly enough. This being particularly true if 
mechanical means are employed to change patterns. FIG. 3 shows a means 
analogous to that of FIG. 2 except applied to projector 12. In each of N 
paths, within projector 12, a light source 32 may be placed, whose light 
is concentrated by lens 37 into the center of lens 31 after passing 
through mask 33, optical path length adjustment 35 and beam splitter 34. 
Lens 31 images the light passing through mask 33 onto the subject to be 
measured, forming the unique pattern required for 3-D measurement. 
The beam splitter ratio of the first path (nearest the lens 31) is chosen 
to be 1/N:(1-1/N)=1:(N-1) with (1-1/N) of the light passing through and 
absorbed by light trap 36. The second path (next furthest from lens 31) 
ratio is chosen to be 1/(N-1):(1-(1/N-1))=1:(N-2). Thus, 
1/(N-1).(N-1)/N=1/N of the light of the second path also reaches lens 31. 
By similar proper choice of beam splitter ratios such as 1/2:1/2 for the 
(N-1)th path and 1:0 (total reflection) for the Nth path, each path will 
project 1/N of the source light on the subject. It will be noted these are 
the same ratios as in camera 11 of FIG. 2. 
Light sources 32 can be flashed sequentially for the N unique patterns to 
be sequentially projected upon subject 14 and the electronic control 13 
can simultaneously open corresponding shutters 23 in camera 11. 
Alternately, light sources 32 may be constant with a controlled shutter 
mechanism to release the light at the prescribed moment indicated by 
control 13. 
Thus the minimum time in which the sequence of patterns can be projected is 
no longer limited by the time required to change patterns. The limit now 
is the time to reach the required light intensity-time product and the 
flash turn on/turn off or light shutter times. 
A second preferred embodiment of the invention, shown in FIG. 6a, allows 
acquiring the required information to make a 3-D measurement in the time 
it takes to sweep a plane of light 65 across the subject 64 at a speed 
that provides the required light intensity-time product of reflected light 
66 to be reliably recorded by camera 61 having the same internal 
construction as previously described for camera 11 in FIG. 2. Shutters 23 
controlled by controller 63 are opened in a time encoded manner in order 
to provide unique time image patterns. 
FIG. 6b illustrates the principle of the invention for a simplified case of 
N=3. As light plane 65 sweeps across subject 64, assumed to be a cone for 
this example, the first light path shutter 23 is opened for the first half 
of the sweep, producing an image 67 on the image recording surface 22. 
During the same sweep of plane 65, shutter 23 in the second light path is 
opened for the first and third quarters of the sweep, producing an image 
68 on recording surface 22 of that light path. Likewise shutter 23 in the 
third light path is opened for the first, third, fifth and seventh eighths 
of the sweep time producing image 69 on the respective recording surface 
22. 
At the time of system calibration each resolvable element (pixel) on the 
recording surface 22 of the first path is associated with every other 
pixel on the other recording surfaces 22 that images the light entering 
the lens 21 from the same point on the subject surface. That is, each 
pixel of surface 22 of the first light path records light from a unique 
bundle of light rays entering camera 61 (11) through lens 21 and light 
from that bundle also images on a pixel (ideally) or cluster of four 
adjacent pixels on each of the other light path recording surfaces 22. 
These associated pixels can be used to decode the image patterns 67, 68, 
69 to reveal the time and hence the plane 65 spatial position for the 
recorded data. 
For example, if a pixel is illuminated in images 67 and 69 but not in image 
68, then the third eighth time slot is indicated as the time source of the 
images. Since by triangulation, a bundle of rays piercing a plane of light 
uniquely determines the three-dimensional co-ordinates of the point of 
intersection, it is possible to relate the decoded image to 
three-dimensional position information for the inferred plane position. To 
resolve ambiguities arising at the transition region between the on and 
off regions of an image caused by the non-ideal alignment of the pixels of 
one surface 22 and the pixels of another surface 22, additional patterns 
may be recorded with transition regions not overlapping the former 
transition regions. 
The scanning plane of light can be provided by a rotating mirror apparatus 
as shown in FIG. 4 in which light source 41 is imaged on the subject by 
lens 42. The width of the light plane is then governed by the width of 
light source 41 and the magnification of the lens 42. Light plane 44 is 
caused to sweep across the subject by the action of rotating mirrors 43. 
Electronically generated scanning planes of light can also be used. 
It is also possible to employ this method with but one light-sensitive 
surface 22 and N sweeps of light plane 65 which takes longer but does 
automatically associate the light ray bundles since the same pixel is used 
for each code. The accuracy is adversely affected by any error in the 
repeatability of the sweeps. 
A third preferred embodiment is shown in FIG. 5. Projector 52 projects a 
plane 55 of light onto conveyor belt 57 which transports an object 54 to 
be measured through plane 55. Plane 55 may be perpendicular to conveyor 57 
as shown or at any arbitrary angle. Camera 51, with internal construction 
as given in FIG. 2, receives the reflected light 56 from the intersection 
of plane 55 and object 54 and records the image of the reflected light for 
short time intervals on each of the N light-sensitive surfaces 22 in an 
encoded sequence. This is the same principle as just described for FIG. 6 
except that now the camera's 51 direction of view 58 is moved in 
synchronism with the conveyor 57 so that as object 54 passes through plane 
55, the intersection of plane 55 and object 54 appears to sweep across the 
view of camera 51. One method of sweeping the view of camera 51 would be 
to employ a rotating mirror as depicted in FIG. 4, although other methods 
could be employed. By monitoring the speed of conveyor 57 and supplying 
the controlling electronics of the camera 51 sweep mechanism, the sweep 
can be made synchronous with the motion of object 54 and thus produce an 
accurate measurement of its surface. 
A fourth preferred embodiment is similar to the first and can be envisioned 
with FIG. 1. The difference is that projector 12, possibly using the 
arrangement of FIG. 3, projects all N patterns simultaneously. Each 
pattern, however, is transmitted with a different frequency of radiation 
as derived from different frequency sources 32 or subsequent filtering 
such as by mask 33. Camera 11 with arrangement of FIG. 2 are provided, 
except that the N controlled light shutters 23 are replaced with N filters 
that only pass the frequency of radiation for the corresponding projected 
pattern to be recorded on the N surfaces 22. If a shutter is required, a 
common shutter can be placed by lens 21 to expose all paths 
simultaneously. This arrangement reduces the time of acquiring the 3-D 
measurement since all patterns are simultaneously recorded. 
A fifth preferred embodiment employs the time of travel of light to make 
the 3-D measurements as in U.S. Pat. No. 4,199,253. Referring to FIG. 7, 
projector 72 emits a pulse of light whose time duration is no greater than 
the two way travel time of light across the distance of the desired depth 
resolution. A separate projection and receiver lens may be used or a 
common lens 71 as shown in FIG. 7 with beam splitter 74 allowing part of 
the projected light to reach the subject 75 and reflecting the rest of the 
light to light trap 76. Light reflected by subject 75 is imaged on 
light-sensitive surfaces within box 11 which is identical in construction 
as given in FIG. 2 with the exception that lens 21 is now external to the 
box in the form of lens 71. Beam splitter 74 reflects a part of the imaged 
light toward box 11. System losses are minimized if beam splitter 74 
passes half of the incident energy and reflects the other half. Shutters 
23 time encode the light reflected from subject 75 as described in U.S. 
Pat. No. 4,199,253. The novelty of the present invention is in providing 
the means for allowing the several parallel cameras required, to share a 
common aperture and imaging lens which provide benefits of view angle, 
cost, size and weight. 
The invention has been described and illustrated with reference to 
exemplary embodiments. It is, however, not to be considered limited 
thereto, inasmuch as all modifications and variations which might offer 
themselves are intended to be encompassed within the scope of the appended 
claims.