A drift compensated and intensity averaged extensometer for measuring the diameter or other properties of a substantially cylindrical sample based upon the shadow of the sample is described. A beam of laser light is shaped to provide a beam with a uniform intensity along an axis normal to the sample. After passing the sample, the portion of the beam not striking said sample is divided by a beam splitter into a reference signal and a measurement signal. Both of these beams are then chopped by a light chopper to fall upon two photodiode detectors. The resulting ac currents are rectified and then divided into one another, with the final output being proportional to the size of the sample shadow

Marcus, H. L.

Stocker, P. J.

English

NASA Technical Reports Server

United States Patent [I91 
Fletcher et al. 
1541 LASER EXTENSOMETER 
[76] Inventors: James C. Fletcher, Administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention of; Phillip J. Stocker, 
South Pasadena, Calif.; Harris L. 
Marcus, Austin, Tex. 
[21] Appl. No.: 732,630 
[22] Filed: Oct. 15, 1976 
[51] Int. C l . 2  .............................................. GOlB 11/08 
[52] U.S. Cl. .................................... 356/159; 250/571; 
356/160; 356/199 
[58] Field of Search ............... 356/159, 160, 199, 200; 
250/559, 560, 571, 572 
1561 References Cited 
U.S. PATENT DOCUMENTS 
2,670,651 3/1954 Burns et al. .......................... 356/160 
3,905,705 9/1975 Petrohilos ............................ 356/160 
3,947,129 3/1976 Wiklund ............................... 356/159 
[I11 4,OQ 1,427 
Dee. 6,1977 [451 
4,021,119 5/1977 Stauffer ................................ 356/159 
Primary Examiner-John K. Corbin 
Assistant Examiner-R. A. Rosenberger 
Attorney, Agent, or Firm-L. D. Wofford, Jr.; George J. 
Porter; John R. Manning 
1571 ABSTRACT 
A drift compensated and intensity averaged extensome- 
ter for measuring the diameter or other properties of a 
substantially cylindrical sample based upon the shadow 
of the sample. A beam of laser light is shaped to provide 
a beam with a uniform intensity along an axis normal to 
the sample. After passing the sample, the portion of the 
beam not striking said sample is divided by a beam 
splitter into a reference signal and a measurement signal. 
Both of these beams are then chopped by a light chop- 
per to fall upon two photodiode detectors. The result- 
ing AC currents dre rectified and then divided into one 
another, with the final output being proportional to the 
size of the sample shadow. 
14 Claims, 2 Drawing Figures 
el OUTPUT 
https://ntrs.nasa.gov/search.jsp?R=19780006437 2020-03-20T12:56:55+00:00Z
U.S. Patent Dec. 6, 1977 4,06 1,427 
4,061,427 
1 
LASER EX'IENSOMETER 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 5 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 435; 42 U.S.C. 2457). 
FIELD OF THE INVENTION 
The present invention relates to a laser extensometer 
which is capable of measuriug the cross-section of a 
sample, such as its diameter. This device can conduct 
this measurement without actually physically contact- 15 
ing said sample, and while said sample is moving. 
10 
BACKGROUND OF THE INVENTION 
All laboratory light sources are, to some extent, non- 
uniform across their beam front, both in time and in 20 
space. Therefore, a light source imaged upon a screen 
exhibits both hot and cold spots which fade and 
brighten, semi-uniform gradiations which fluctuate with 
time, and a general waxing and waning of overall inten- 
sity. In an incandescent lamp, the variations are due to 25 
asperities of the glass envelope, the shape and position 
of the filament, and time changes in the filament mate- 
rial such as oxidation or crystal growth. A laser suffers 
from defects in its optics, contaminants on its optical 
surfaces, and lasing fluctuations. While the degree of 30 
these problems varies over orders of magnitude, they 
are always present to some extent and must be consid- 
ered if one is attempting to use a light source at its 
highest resolution. 
ing (i.e., reference) signal is obtained from one part of 
the beam front, and a mezxsurement signal is obtained 
from another part, then there will be inherent errors in 
the normalized output due to the time and spatial varia- 
tions alluded to above. 40 
There are three ways around this problem. The first 
solution is to somehow homogenize the beam in time 
and space. The second solution is to obtain both the 
normalizing signal and the measurement signal from the 
same portion of the beam, and to alternate samplings at 45 
intervals much shorter than the time variations of the 
light source. The third solution is of course to just deter- 
mine that the errors are below the required system 
accuracy. 
ant energy to inspect the diameter of a moving cylindri- 
cal object. One of these patents is U.S. Pat. No. 
3,604,940 issued to David R. Matthews. As shown in 
FIG. 1, two highly collimated beams of light are di- 
rected tangentially along diametrically opposite surface 55 
portions of the object so as to be partially intercepted 
thereby. These partially intercepted beams are focused 
on two detectors and then compared to determine the 
size of the sample. Therefore, since two light beams are 
used, each signal would fluctuate differently with re- 60 
spect to time and space and errors in the readings would 
result. In addition, even though a beam splitter is used in 
FIG. 6 to determine the diameter of very small objects, 
two different light beams are still employed and, simi- 
larly, errors would result. 65 
U.S. Pat. No. 3,856,412 issued to Carl A. Zanoni 
describes an electro-optical sensor for providing an 
output proportional to the cross-section of an object, 
If a light beam is used in such a way that a norm&- 35 
Several U.S. patents exist in the field of utilizing radi- 50 
such as the diameter of a cylinder which utilizes only a 
single beam of laser light. The diameter is determined 
by photoelectrically sensing when the laser beam is 
modulated on or off by the edge of the object, using the 
first and second derivatives of the output of the sensor. 
An auxiliary photoelectric output is obtained by passing 
a part of the scanning laser beam onto a sensor over a 
precision grading or scale with markings. This opera- 
.tion generates a modulated signal whose spatial fre- 
quency is independent of variations in the speed of 
scanning and intensity of the laser beam. Therefore, it 
can be seen that the Zanoni patent calculates the diame- 
ter of the signal using a time duration of the signal 
rather than the signal's intensity. 
Therefore, a review of the prior art has failed to 
discover any patent or other reference which deter- 
mines the diameter of a moving cylindrical object utiliz- 
ing only a single beam of laser light with the measure- 
ment being based upon the shadow diameter of the 
sample. 
SUMMARY OF THE INVENTION 
The present invention overcomes the defects of the 
prior art by placing a cylindrical member, such as a thin 
wire or filament, whose dimension is to be measured in 
a beam of coherent, monochromatic light, the beam 
being obtained by oscillating a galvanometer mirror on 
which a laser beam is incident to produce a beam whose 
cross-section is oblong in a direction normal to the axis 
of the cylinder. Light that passes said cylindrical mem- 
ber is incident on a beam splitter having two channels. 
In the first, or measurement channel, light passing the 
cylinder passes through a pair of knife edges and then 
into a photodetector after passing through a light chop- 
per. The knife edges in this channel are spaced apart a 
distance slightly less than the beam height to block the 
outer periphery of the beam, allowing a pair of narrow 
bands of light to be incident on the detector. One edge 
of each band is established by a longitudinal edge of the 
member, and the other edge of each band is established 
by a knife edge. 
Light passing through the second, or reference, chan- 
nel of the beam splitter passes through two pairs of knife 
edges and into a second photodetector after passing 
through the light chopper. One set of knife edges is 
spaced apart a distance just larger than the dimension of 
the member, while the other set of knife edges is spaced 
apart a distance substantially equal to the distance be- 
tween the knife edges in the first channel. As a conse- 
quence, two narrow bands of light pass into the second 
photodetector, such bands being slightly smaller than 
the bands passed into the first photodetector. 
The AC outputs of the photodetectors are respec- 
tively rectified to establish a pair of DC signals which 
are divided one into the other to establish a normalized 
signal that is directly proportional to the size of the 
member and independent of laser intensity, detector 
dark current, and slight lateral motion of the member. 
Other features and advantages of the invention will 
be apparent from the following description, the accom- 
panying drawing and the appended claims. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a schematic drawing of the laser extensome- 
FIG. 2 is a cross-sectional view of the lasing beam 
ter according to the present invention; and 
taken through 2-2. 
4,06 1,427 
3 
DETAILED DESCRIPTION OF A PREFERRED 
EMBODIMENT 
FIG. 1 is a schematic drawing of the laser extensome- 
ter according to the present invention. A beam of radi- 
ant energy produced by, for example, a helium-neon 
laser 10, is directed upon the mirror of a galvanometer 
12 oscillating at approximately 25Khz. This galvanome- 
ter mirror 12 provides a beam of radiant energy 14 with 
a uniform intensity along its x-axis (i.e., normal to the 
sample axis). Beam 14 is reflected by mirror 16 and is 
then directed into an optics system including lenses 18, 
20 and 22. Lens 18 collimates the beam along the x-axis, 
lens 20 diverges the beam along the x- and y-axis and 
lens 22 collimates the beam along both the x- and y-axis. 
Beam 26 now has an oblong cross-sectional area as 
shown in FIG. 2 and is then directed off of mirror 24 
toward the sample 28. 
After passing the sample, light beams 30 and 32 (the 
light which is neither reflected nor absorbed by the 
sample) are incident upon a beam splitter 34 having two 
channels. In the first channel, beams 36 and 38 pass 
through a pair of knife edges 40 such that the remainder 
of the beams 42 and 43 are only slightly wider than the 
sample shadow. These knife edges eliminate the non- 
uniform outer fringes of the laser beam. These beams 
control the sensitivity of the measurement. 
Light beams 48 and 50 passing through the second 
channel of the beam splitter pass through knife edges 52 
so that the sample shadow is no longer retained, form- 
ing beams 54 and 56. These beams provide a reference 
carrying only information of the beam intensity. This is 
necessary since lasers drift in intensity as much as 5%. 
The resulting beams 54 and 56 are reflected from mirror 
57 and then are trimmed by a second pair of knife edges 
58 to also eliminate the non-uniform outer fringes of the 
beams. , 
The beams of both channels are now chopped by a 
4 
width of said beam being larger than the cross-sec- 
tional width of the sample; 
beam splitter means for dividing the radiant energy 
passing around the sample into a measurement and 
a reference beam of radiant energy, the sample 
located between said source of radiant energy and 
said beam splitter; 
first pair of blocking means spaced apart a distance 
larger than the dimensions of the sample, said 
blocking means disposed in the path of said refer- 
ence beam and trimming said reference beam so 
that said reference beam carries only information 
of said reference beam intensity; 
means for receiving said reference and said measure- 
ment beams, and for producing an electrical signal 
for each of said beams; 
circuitry means for dividing said electrical signals 
into one another; and 
output means for displaying the resultant signal pro- 
duced by said circuitry means, whereby said resul- 
tant signal is directly proportional to the size of the 
sample shadow. 
2. A laser extensometer according to claim 1 addi- 
tionally including shaping means for providing a beam 
25 of energy having uniform intensity normal to the sam- 
ple axis, said means located between said source of 
radiant energy and the sample. 
3. A laser extensometer according to claim 2 wherein 
illating galvanometric mir- 
ror. 
4. A laser extensometer according to claim 2 includ- 
ing an optic system located between said shaping means 
and the sample for shaping the radiant beam of energy. 
5. A laser extensometer according to claim 1 further 
including a second pair of blocking means disposed in 
said measurement beam between said beam splitter and 
said receiving means, and a third pair of blocking means 
dhosed in said reference beam between said beam 
5 
10 
20- 
3o said shaping means is 
35 
light chopper 46 at a frequency of approximately 500hz, 40 spitter and said receiving means, both said second and 
pass through opal glass diffusers 60 and 62 respectively, third pairs of blocking means spaced apart a distance 
and then fall upon two photodiode detectors 64 and 66. less than the height of said measurement and reference 
The AC current from the detectors is rectified by recti- beams respectively, said second and third blocking 
fiers 68 and 70, thus eliminating the dark current of the means trimming said beams to eliminate non-uniform 
detectors, with the resultant DC signals being divided, 45 outer fringes. 
one into the other by a divider 72. The final output 6. A laser extensometer according to claim 5 wherein 
signal is directly proportional to the size of the sample said first, second and third blocking means are knife 
shadow, and is independent of laser intensity, detector edges. 
dark current, and slight lateral motion of the sample. 7. A laser extensometer according to claim 1 further 
This final output can be exhibited by any standard out- 50 including a light chopper in the path of said measure- 
put means 74, such as an oscilliscope. ment and reference beams between said beam splitter 
While this device has been described with particular and receiving means. 
reference to the configuration shown in FIG. 1, said 8. A laser extensometer according to claim 1 wherein 
invention should not be construed to be so limited. For said means for receiving radiant energy are first and 
example, the source of radiant energy 10 need not be 55 second photodetectors, said first photodetector receiv- 
limited to a helium-neon laser, since many other laser ing said reference beam and said second photodetector 
producing devices may be utilized. Additionally, the receiving said measurement beam. 
particular optic system for collimating and diverging 9. A method for measuring the cross-sectional dimen- 
the beam of radiant energy need not be exactly the same sion of a sample, comprising the steps of: 
as depicted in FIG. 1. It will also be obvious for those 60 producing a coherent beam of radiant energy having 
who possess ordinary skill in the art to make other a cross-sectional width larger than the cross-sec- 
changes and modifications to this device without de- tional width of the sample; 
parting from the scope of the invention. 
I 
directing said beam of energy toward the sample; 
. splitting the energy beam passing around the sample 
into a measurement beam and a reference beam; 
trimming said reference signal so that said reference 
beam only contains information relating to said 
coherent beam intensity; 
What is claimed is: 
1. A laser extensometer for measuring the cross-sec- 65 
a source of radiant energy for directing a beam of 
energy incident to the sample, the cross-sectional 
tional dimension of a sample, comprising: 
4,061,427 
5 6 
directing said measurement beam and said reference 
beam to photodetector means, thereby producing 
an electrical signal for each of said beams; 
dividing said electrical signals into each other pro- 
ducing a resultant signal proportional to the size of 5 
the sample shadow; and 
observing said resultant on an output means. 
10. A method according to claim 9 further including 
the step of positioning a shaping means between said 
beam of radiant energy and the sample for providing a 10 
beam of energy having uniform intensity normal to the 
sample axis. 
11. A method according to claim 10 further including 
the step of placing an optic system between said galva- 
nometric mirror and the sample for shapiug the radiant 15 
beam of energy. 
12. A method according to claim 9 further including 
the step of trimming said reference and measurement 
signals for eliminating non-uniform outer hges.  
13. A method according to claim 9 farther including 20 
ment beams before said signals are directed into said 
photodetector means. 
14. A method for measuring the cross-sectional di- 
mension of a sample, comprising the steps of: 
producing a coherent beam of radiant energy having 
a cross-sectional width larger than the cross-sec- 
tional width of the sample; 
directing said beam of energy toward the sample; 
splitting the energy beam passing around the sample 
into a measurement beam and a reference beam, 
trimming said reference beam so that said reference 
beam only contains information relating to said 
coherent beam intensity; 
directing said measurement beam and said reference 
beam to two separate photodetectors, thereby pro- 
ducing an electrical signal for each of said beams, 
dividing said electrical signals into each other pro- 
ducing a resultant signal proportional to the size of 
the sample shadow; and 
observing said resultant on an output means. 
the step of chopping said reference and said measure- + * * * I  
25 
30 
35 
40 
45 
55 
65 


Laser extensometer

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