Coherent light beam device and method for measuring gas density in vacuum chamber

Barrington, A. E.

English

NASA Technical Reports Server

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
WASHINGTON, D.C. 20546 
TO: USI/Scientific c Technical Information Division 
Attantion: MPbs Winnis M. Morgan 
GP/Office of Assistaot General Counsel for 
Patent Matters 
F ROM : 
.( I 
SUBSECT: Announcement of NASA-Owned U. S. Patents i n  STAR 
I n  accordance w i t h  the procedures agreed upon by Code GP 
and Code USI, the  attached NASA-owned U. S. Patent is being 
forwarded bor abstracting and announcement i n  NASA STARI 
The following information i s  provided: 
U. S. Patent No. 
Government or 
Corporate Employee 
f d  7 
Supplementary Corporate 
Source (if  applicable) t /d . 
NASA Patent Case No, 
NOTE - If 
employee 
Pursuant 
t h i s  patent covers an invention made by a corporate 
of a NASA Contractor, the following is applicabler 
Y e s  0 No V 
t o  Section 305(a) of the National Aeronautics and 
Space Act, the name of the Administrator of NASA appears on 
the f irst  page of the patent; however, the name of the actual 
inventor (author) appears at the heading of Column No. 1 of 
the Specification, following the words . . with respect t o  
Elizabeth A; Cagter 
Enclosure 
Copy of Patent aited abc 
>- c 
=! " ( N A M  CR OR TMX OR AD NUMBER) 
I2 
https://ntrs.nasa.gov/search.jsp?R=19710019518 2020-03-17T03:11:33+00:00Z
States Patent [ I 1 1  3,578,867 
Inventor Alfred E. Barrington 
Appl. No. 815,366 
Filed Apr. 11,1969 
Patented May-l8,197!- 
Assignee 
Lexington, Mass. 
The United States of America as represented 
by the A d m i i a t o r  of the National 
Aerdutics and Space Administration. 
1561 References Cited 
UNITED STATES PATENTS 
2,080,613 511937 Lange ........................... 3 5 61206 
Primary Examiner-Ronald L. Wibert 
Assistant Examiner-Orville B. Chew II 
Attorneys-John R. Manning, Herbert E. Farmer and Garland 
T. Mc Coy 
DEVICE FOR MEASURING LIGHT SCATTERING 
WHEREIN THE MEASURING BEAM IS 
SUCCESSIVELY REFLECTED BETWEEN A PAIR 
OF PARALLEL REFLECTORS 
1 Claim, 3 Drawing Figs. 
U.S. C1. ....................................................... 3561103, 
. 2501218 
Int.CL ........................................................ GOln 21/26 
Field of Search ............................................ 356/102, 
103,201,206,207,244,246; 2501218 
ABSTRACT: A method and device is disclosed for detennin- 
ing the gas density within a vacuum chamber by measuring the 
scattered radiation produced when a gas passes through an ap- 
parent single coherent beam of light. The beam is directed 
onto the first of a pair of highly reflective, parallel surfaces 
and caused to be successively reflected therebetween, the 
reflecting surfaces being located within the vacuum chamber. 
The invention described herein was made by an employee 
of the United States Government and may be manufactured 
and used by or for the Government for governmental purposes 
without the payment of any royalties thereon or therefor. 
I I 
PATENTED 1 8 1971 
F f  ' ' I t  v -  
3,578,867 
GAS 
IN i 
SOURCE FIG. 3, 
UTtLlZATlON 
ALFRED E BARRWGTON 
IN VENTOR. 
3,578,867 
T SCATTERING 
IS SUCCESSIVELY 
EFEECTED BETWEEN A PAIR OF PARALLEL 
ty of the resultant coherent li t beams that are produced by 
such an array of multiple reflected beams, all lying in the same 
plane, is determined in accordance with the following formu- 
REFLECTORS la: 
Io 8 ,  =- =KZo 
1-7 
5 
BACKGROUND OF THE INVENTION 
This invention relates to pressure measurement and more (2) 
particularly to a pressure measuring device capable of obtain- 
ing measures of the instantaneous pressure within an enclo- 
sure without mechanical or fluid connection to the volume l o  
within the enclosure. 
The prior art is replete with pressure sensitive measuring 
devices designed and used for the measurement of reduced Thus, if the reflective efficiency of each mirror were equal to 
pressures within an enclosure. However, these devices usually 15 about 90 percent, then S,would be equal to about 100 times 
either require that a pipe be connected to the enclosure or the incident intensity. 
must employ some mechanical device that is positioned within To illustrate, it has been calculated that if a coherent beam 
the enclosure and connected to external indicating means. of light, having a cross-sectional area of about 1 mmz (about 
Many of these prior art devices, particularly the more sensitive 10-%n2 ) and a length of 1 meter, were projected from a ruby 
ones, also require a cyclic manipulation by an operator before 20 laser having one joule of energy at 6943 A., when passing 
the pressure can be measured and, therefore, a significant through an enclosure containing a permanent gas at a pressure 
time may elapse before a pressure sample may be taken and of about 1 torr and a molecular density of about 3 X lozs 
the reading determined. The present invention requires no molecules per cubic centimeter, there will be produced a scat- 
physical connection between the gas inside the enclosure and tering of about los photons. Accordingly, if the threshold of 
the measuring device, nor does it require any mechanical 25 detection is taken as one photon, then the minimum detecta- 
device within the enclosure that will be acted upon entirely ble pressure determinable from the above-mentioned arrange- 
upon the amount of photon scattering from a beam of light ment would be about 10’8 torr. 
that has been projected into the enclosure. The scattered It is, therefore, one object of the present invention to pro- 
photons within the chamber may be detected by suitable light vide an improved gas pressure measuring device which avoids 
detecting instruments placed outside the chamber to give an 30 one or more of the disadvantages and limitations of prior art 
indication of the pressure of the gas within the enclosure. arrangements. 
Another object of the present invention is to measure the 
gas pressure within an enclosure without any mechanical or 
This invention provides a device for determining the instan- fluid COnnection to the e ~ c b s e d  VOhne. 
taneous gas density within an enclosure by measuring the scat- 35 Another object of the present invention is to provide a gas 
tered radiation generated when multiple passes of a coherent pressure measuring device which responds instantly to varia- 
light beam pass through an evacuated chamber. The reflected tions of gas pressures. 
coherent beam of light has an intensity equal to just less than The features of my invention which are believed to be novel 
the intensity of the incident beam. The device utilizes a pair of are set forth with particularity in the appended claims. The in- 
planar, highly reflective mirrors placed within the enclosure of 40 vention itself, however, both as to its organization and method 
the chamber; a window for allowing a beam of coherent light of operation, together with further objects and advantages 
into the enclosure; and, photodetector means for measuring thereof, may best be understood by reference to the following 
the scattered radiation produced when the beam passes description taken in conjunction with the following drawings. 
Where: 
S-=the total intensity of all the beams of light: 
I,= the incident intensity; 
r) = the reflective efficiency of each mirror; and 
K=aconstant equal to 1 / 1 7 ,  
SUMMARY OF THE INVENTION 
45 BRIEF DESCRIPTION OF THE DRAWINGS 
through any gas that may be in the chamber. 
It is a well known fact that a beam of light passing through a 
gas will be scattered at random by the gas atoms in accordance 
with the following formula: FIG. 1 is an elevational view of the device adapted for use in 
FIG. 2 is a cross-sectional view of the embodiment of FIG. 
FIG. 3 is a transverse sectional view of another embodiment 
the exhaust line of a system being evacuated; 
k(l+cosZ e)n 
50 1, taken along line 2-2 of FIG. 1, and, 
( ) A4 #J 
i= Io 
of the present invention. where: 
i = the intensity of the scattered light into a solid angle @ 
lo= the incident intensity; 
k = a constant which depends upon the nature of the gas 55 
r )  =the angle between the incident and scattered light; 
DETAILED DESCRIPTION OF THE PREFERRED 
EMBODIMENT 
particles. 
= the density of the gas particles; and, 
of the incident light. 
As will hereinafter be used, the terni “transducer” refers to 
a device which receives light energy and transforms it into 
electrical energy. In the exegisis which follows, it is to be un- 
This formula holds if the dimensions of the gas particles are 60 derstood that this designation shall include all transducers 
small compared with A, the wave ]en@ of the light. When a which transform light into electrical energy such as, but not 
mixture of gases is used or when the gas is composed of large limited to, photoconductive cells, photoelectric cells, 
molecules, the scattering will still be proportional to the gas photomultiplier tubes, and all types of crystal and semicon- 
density. ductor elements whose electric properties vary with incident 
used, the total measured intensity of the scattered light Referring now to FIGS. 1 and 2 there is shown an enclosure 
produced would be equal to about the sum of the intensity of 10, coupled to an exhaust line 12.1 and 12.2 of an evacuated 
scattered light produced by each beam. An infinite number of system. The shape and dimension of the enclosure 10 may 
coherent beams could be approached by directing a single vary, depending on the intended use of the device in ac- 
coherent beam of light obliquely onto the surface of a first 70 cordance with the needs of the user. First and second mirrors 
planar mirror having an extremely highly reflective surface 12 and 14 are mounted within the enclosure 10 and form a 
and then directing the resulting reflected coherent beam portion of the inner walls thereof. It is preferred that the 
obliquely onto the surface of a second planar mirror disposed reflective surfaces of the mirrors 12 and 14 be highly planar 
parallel to the first mirror, the reflecting surfaces of both mir- and that they be disposed parallel with each other, spaced 
rors having about equal reflective efficiency. The total intensi- 75 apart at a fixed distance. While the dimensions of the mirrors 
=the wave length 
In those cases where an infinite number of beams could be 65 light energy. 
3,578,867 
12 and 14 may vary in accordance with the intended use, may easily serve as a continuous monitor in a system being 
rectangular shaped mirrors are preferred. The device 10 is evacuated to indicate when the desired pressure within the 
provided with a window 16 that is preferably formed near one system is reached. 
end of the long axis. Referring now to the FIG. 3, there is shown another em- 
As will be seen in FIG. 2 a window 16 is provided in the en- 5 bodiment of the present invention suitable for use in evacu- 
closure 10 so as to provide a means for communicating with ated system wherein extremely low gas densities will be en- 
the interior for projecting a beam of light into the enclosure. It countered. The relative arrangement of elements illustrated in 
is preferred to locate the window 16 near one end of the long FIG. 3 is substantially identical to the arrangement illustrated 
axis of the mirror 14 as to be centered on the midline of the in FIGS. 1 and 2. However, the embodiment of FIG. 3 is 
short axis thereof. A light source 18 is located adjacent to the 10 shown as having an elongate, tubular enclosure, with .x Pair of 
window 16 and may be any conventional light source capable elong substantially rectangularly shaped planar m'rrOrS 12 
is preferred for use with the present invention. A conventional ment length of the 10% axis of the mirror 12' and mirror 
light for collimating 14' is greater than twice the length of their respective short 
the beam projected from the light source. A high intensity 15 axis. This serves to reatly increase the beam path b\ provid- 
ing an opportunity $or a larger number of reflectio is to be 
coherent beam of light I,, is then directed obliquely into the en- produced in the area being viewed by the photodet :ctor 22 
strike the extremely reflective surface of the mirror 12 causing the single coherent beam being viewed. Somewhat ,'le Same 
the incident beam I, to be reflected the chamber 2*t 20 effect can be produced by increasing the distance between the 
formed between the mirrors 12 and 14 onto the extremely parallel disposed mirrors (12' and 14') without increasing the 
reflective surface of the mirror 14. The reflected beam IRI is long axis. However, the extent to which this can produce an 
then reflected off the mirror 14, and beam ]IRs which is then increase in intensity of the beam being viewed by the 
reflected as shown, is directed obliquely Onto the surface by photodetector 22 is limited by the sensitivity of the device. For 
the beam I R P  This Process of multiple reflections continues as 25 a given gas density optimum scattering of the coherent beams 
I , ,  IRJ etc. until such time as the beam is completely dissipated of light is produced when the greatest number of high intensity 
OrtraPPed. It is considered an important feature ofthe Present beams are combined to be viewed as a single coherent beam 
invention that the reflected beam IRI and each subsequently confined in a space having the smallest practical volume. 
produced reflected beam be characterized as being coherent While I have described what is presently considered the 
and have an intensity equal to just less than the intensity of the 30 preferred embodiment of my invention, it will be obvious to 
incident beam 1,. Accordingly, the zigzagged pattern of light those skilled in the art that various other changes and modifi- 
produced in the space between the reflective surfaces of the cations may be made therein without departing from the in- 
mirrors 12 and 14 must be kept scatter-free. Hence, the prac- ventive concept contained therein. 
titioner of the invention may find it desirable to employ one or I claim: 
more light traps (not shown) in the enclosure or its environ- 35 1. A device for measuring scattered radiation produced by 
ment. To preserve the integrity of the system, it is desirable the elastic collisions of photons with gas molecules in an 
that portions of the inner walls of the enclosure be provided evacuated system, comprising: 
with a reflective surface, in addition to mirrors 12 and 14, so a shaped enclosure communicating with a system to be 
that when scattering of the beams does occur, the scattered 
light may be reflected back into the space between the mirror 40 a laser beam source of light; 
12 and the mirror 14. a pair of rectangularly shaped planar mirrors mounted 
A photoelectric detector 22 is mounted adjacent the encio- parallel to each other in a fixed spatial relationship within 
sure for communication with a chamber 20 through a window the enclosure such that the direct laser beam can 
26, at the opposite end of the long axis of the mirror 14, and propagate only in the plane orthogonal to the plane of the 
centrally positioned between the mirrors 12 and '64. The de- 45 mirrors; 
tector 22 is positioned so as to collect the scattered radiation a first transparent window in one wall of the enclosure 
produced when the reflected multiple passes of coherent light located at a point near one end of the long axis of the mir- 
pass through the gas in the chamber 20. It is preferred that the ror and adjacent said laser source of light and a second 
photoelectric detector 22 be positioned so as to view the zig- window located at the opposite end and positioned 
zagged arrangement of coherent light beams as a single in- 50 between said mirrors; 
tense beam of tight. Thus, when photons are scattered by a means for directing said laser beam through said first win- 
beam passing through a gas, the scattering effect will be inten- dow and obliquely onto the surface of the other mirror, 
sified, in accordance with the teachings of the present inven- causing the beam of light to be successively reflected by 
tion. A utilization device 24 is connected to the detector 22 to the mirrors in the space therebetween; and 
convert the output signal produced by the detector 22 to data 55 photodetector means mounted axially in a plane parallel 
meaningfully related to the gas density within the system being with the planes of the mirrors for measuring scattered 
radiation produced when the successive reflection5 of the tested. 
Numerous uses of the invention will now become readily ap- beam pass through a gas. 
parent to those skilled in the art. For example, the invention 
of projecting a high intensity coherent beam. However, a laser and I rming the inner walls of the chamber. In this embodi- 
would, of necessity require 
through the window l6 from the source Is* so as to and resulting in a corresponding increase in the int,&ty of 
evacuated; 
60 
65 
70 
75 


Device for measuring light scattering wherein the measuring beam is successively reflected between a pair of parallel reflectors Patent

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server