. ,.

PEN00186 OPTICAL METHODS IN AERONAUTICAL RESEARCH

By J~ R. sta.'l'Tett, and C. Bo Palmer

The purpose of this report is to describe optical methods used

in aeronautical research (shadowgraph, schlieren, interferom.eter, ~.)

in terms as nearly non-technical as ,possible. It is ho:redthat by so ~, ,

doing, those who are not specialists,.in the field of optics may be

, assisted in the design and operation of these systems.

The report will nOt employ-referenees, because any1;hing which ,Will < ., • •

be ,written fn1J:y' be f'ounuin text books on optics or in a: volnminou.s·, lit ...

erature to which it is irnpraet~cable to refer.

The repOrt will first present the' pbysicalprinciples underl;r...ng

these optical methods,am. thenea:ch system will be diseussedseparately. , ,

Tnetreatment of the various systems will include design" operation of

tne apparatUs"and typl-call"ecords from each system.

Physical Principles

,I' ' / • diOfJ Aeronautical research on high speed fl'QW. irlvolv~s the deternu.L~ ,

of' press-ure ,and veloCity :fields about ~frp1anes 01" missiles. usue.llY

the problem. is simplif't~,(thy ,stud:r'ing the V'arions eonn:onents of these , .<:;"

in tl:.'E: region adjacent to the body because ~oi' the distUrbance' ~ 'the . ,

flow caused by tbe introduction oflli-easuring probes, and be~use such '. I ,,'

devices require elaborate instr-umentatioT! and tedious testing •

•

stl"'~a!l1, application of . -?)d

energy~ mOmentum,

., 'ttne

2

(conservation of

two of these. state :factors at a!1Y pei:nt intbe flew;, where the third is

: .. ';

'. entroPY' to Increase.

Opt.ica.l methods utilize the fact that t1ce index of ref'rf:ction-oi'

the medium is related to the density. it s:inrple relation for air is

A .. 1 =/(;0

where

.4 is the iMex of 1"'efracticm

;0. "is .}~he air densi tv

a constan:, 1r ,,'IV:~~~~<(;:(,Lr~ ~t'iJ:l£ti~_t:le- . k is

The index oi' refraetioll is s~mp1y the ratio is

the velocity of light ::..n va.cuum, a.nd

tb.e particular medium at, a giv€ll der:si ty •

in tte err flow .. q- '>:/

~1.._1.1. 8. snaIl

tt~otlgb

the ws.vefroll'(,s , '

ine from B i~tead ,of frola A.

Figure lG~ Refraction of wave fronts< '<

l~ ~7Bl?city "

Para.llel light J ~.,~., •••• --~~~ ••• '.~/.:~.:~:-jj!1i

-·;z

-

~~--. ~--

-~----

-- -~-- - ---~ ---- --

Figure 3 c - Shadowgraph SY8 terns.

method. 3:1. -tb!s!" pe.re,llel light ~'S it:: figure 3a .or divergent light a.s in

h5Ir-J-~'3b tf:,iB,~t be uss-j, ?!ld give equ&.11y good results.

/'. . "" "6, 2-~. h . ThS .. Bb.ru:-:p41e3s of def'ini~.:.io:fl in the image obtained by the shadow ... nPi1 f ;.,.-'? ~ . Ir .

v graph method b strongly- d.eperoer:t upon tihe ra.tio of: light source diameter

"(,0 sOlrr-ce-dis'wrbar;ce dist;ancl8 {A/B in figure ~.

Disturbance Screen

D B ---------------------?):~(----------- c

:-,

i'i,"N! ~.-

into

~ .

_i __

-1+ -1+

'-~----------C2-----------+'

£>.-

of tb.$ dist:L!!"oancs,

~

"A'I "' ....

.",iv·en, a,ne extrert8ly ncn-unifoz-r;:"

)

desired only for "l6ry

Sl10uld in'~-

ifl:EX'efor·e the most that cae. be said

7

adjustments, with different A/B, should lead to an arrangement givirlg

rt{!ll is not crlticaJ, but the screen is usually set at the mst conven-

ient place ;;i thin several .feet o.f the disturbance, and A/B reduced as

much as the .film sensHivit:.r wiD permit.

not ori tical, and the final arrangement will be a. system of compromises-

I based on experience and t-l"::<.~ type of results desired.

Jtigure 6 oi':a 3~~~ moving at ---.- ., ,- .

..

\ IHa..y ~ t~~re se\.""cre.l 0.1 ff"'Jl~Bct a.l~i~ang€me!1t~s.

Th~

ion this focal point,

Divergent

)

~"'.:.~~.~ -"" ;';',.,:- "'~"

11gh t o;;-:':{ -~~""ji ~~=" 44;\iZi:~~~~Jl-·

)

Figure -r:T.= i \' "Schllerer;. systems.

) ./

_"I--,

... 8 ..,

of lighifa.lling on a screen t,laced behind the :point e:tperiences an

overall-uniform reduction .. ThE') action is ana1.ogous to that of an .iris

di~phragm ill a camera lens, which decreases the a."lloun~ -of light; trans­

¢ t ted:11ith~t in the least decreasing -the area of filJ'!l covered.

-If,the -disturbance is such that light is bent away from t~e 00-

st:r.llCtiOll, aseconda!""J image is formed displaced slightly from the orig-

£'~l~e~- A smaller fr~~cti6n of this disturbed bean tr.,an of an uri-- .

-disturbed Peamwi1,1 be ·stopped/-and more light will reach ti1e came..-..a

fllm-f':rOm::the. ;dlstu~bance r:~t of the field tha..l1 j)-om a.Y'J.';f other. part of

the- f1eJil.- -The image of the d.isturbancew:ill then appear as a bright ....

regiotlqD.the film. Similarly,. . light bent towa.."'d the obstruction will

callse' thedist')rbance to appea-r as a dark region .. :"-~_~4""-~

Thef'r8.ction~df the undisturbed ligbt which may be cut off at the

focal<poin-t is 'determined by ~a rumber of factors.. These include inten ...

-sity of,tne disturbance, size of' the photograph, cha:racteristics of the

All these factors

will.be considered in the section on schJieren~sensitivity. ~-

For greates~sensitivity, a ?traight edge of tbe light source, and ., _.":...::' ....

the direction of cut-off by the-knife edge should _ be perpendicular to . . .' .,,~

the density gradient of the disturbance. In nany disturbances this may

reqnirevar101is orienta.tions. The knife edge must also be adjusted for

the proper degree- of cut .. off. Fignr_e~ 8a~ and 80, illustrate photo­

graphicaIly the effect of lmi.fe edge orientation. Figures 9a, 90, and 9c

illustrate:photographics.lly the effect of degree of -cut-oi'f for a given

distUl'!bance.

_ L

, )

)

AD_ schlieren photographs presented itl this Teponta,re of flow through a two dimensional n022'.le" FigUre 9 was taken under slightly

, . ," 'if" t .,..' . ". . . .!-'- ..... c' do -~ 11 Q. reran ae ... oa..,vnam.:i...C conO:1.>:'lons ~.!lenJ.:lgures. O.a.l.!U .' • It g:.s' easier

3 to observe the boundary layer when the birre ~ge :is ~pemicul.at' to . . ' . ,.,.-. .,.. ~ - , ,'. '. ..'

the boun::3.ax-y layer density gradient (figureaafthi9D.when thekiiife

edge is pa.ra.J~el to the bouuda.::-y layer- density graC!.ient(figure $b).

,Similar comparisons can be, made in these'figures to :the relat.ipnsh1p

betwi?en shock density gradients a'1d knife edgeol"ientations. A cer ...

tain amount of shadow graph effect is evident in these photographs be-

coosa the c1ist1.1rbance has depth$ and it-is ,impossible to have all

parts of the disturbance exactly in focus on the photographic plate.·

Thus strong disturbances are evident in the' photographs when the' knire

edge is parallel to the density gradient •.

Adjustment of the' schlieren apparatus>is essentiall~r a Problem. ()f-

t placing the knife edge 8....l1d the: light- SOlU"C~ in theriglit pOsitions, and­

of adjusting the camera. to photograph the disturbance.. The ,knife edge

must be located eJrectly at the image of the light source. The posi~ion

of the image is determined by th~ focaJ.~ length of' the mirrors and' the

distance from light source to first -l11irror.FigureslOa and1Obshow ,

diagr8.!-a.atical1y the effec·tof a.."'dal :misadjui:ltment .. ,Fig1.u-e 1l shows

photographically the ap);.eara."'1ce of s'D_ch misadjustmentoas compai-ed with

. fig'J:re!{ffiich ha.s the mife edge in the prOpel" position .. :

r

Figure 6 .. - Shadowgre.ph of' a sphere, M = •

(7J )l n //e e 1 e ~ f' 7'" /l ~ev /6? r- /J? ~h ~

Fi) t/ /e . cr. - C t?;1. ;/;'/7 ~ e ,R('

-'

(Cl) /mIle t'1~ cpm/kkjr Ob5!/'UC/;r ul1u/'s/u/"b<tP/ /fl! 'l e ,

r{ff/l /f 1. - E1/?;c/ IfJ / P{ 11fC &t 1-11 lie e 1Je &ul - Pi!;-A 1'1/' / e c if t! f? 't I" t? ~ / -r,p flP rz/.

{\ I

(7,) f)'l/'{7Y' ofJ In/Ie t'~e ('uf-t?-;f2 !e5~ f),""r/ /)1 ~.II.

fltY1l'~ 9. ~ CtPn/;nht'#

(c) /)1rn' 19/ in/Ie -r1~ cql-o/Y' /e~5 I)qr; /'rItfJ6.

f tj t(l'e 9.- CIJn//nP7t'/

Flgure/ t o- Schlieren photDgreph taken ~lth knife e1E. p no at focus, a~ in fipure 70

I I

") j

)

;<

7 --------'

r"'''''-~'~:-''~':''':-'---::-==,c",=,=~'-C,,~=c-.c;

..

~E-:-:':-~=== - _____ _ - ----~- --------- ---- -.-: ----

~~~"\::~l _______ r~' ,,- "'!:~. Between i.ens ]"'

------------ -- ------- ------ --------

R~tween fO~:lS anC'':-screeI!r~ t, ",'

___ -'-_________ -I.i!~~_-__'CS.-.:. .. ~-_ .. ~~ . • __ ~_._.~ ___ .-'t· . _____ _

, ' ~ , "~ .... ~ +,L"'--.1r~~-_'P_ ....... .:.'"'-~o:.i·.·.· AX1 .. BL !r:l sp.-::: • ..1 'JS \ ... r':.en ~.~ 01 ..... :--i.~ .C\.,d1.. .. ~ ~-.:1~~

.~. I

Schlieren Sensitivity

of this anelysis, sensitivity will be defined

as a meaS\12e of t~le smallest dens:1ty Gradient in t~e distlF'oanne

to do v;:' th resollJ.t:ion vvhioh 18 defihed e.s the 8bi1i t:r to sep8::'s

distlJ.roances from '38ch other., H0vJ 6ver, :it mB·Y happen tt2t sensitivit~

..... J .. " • .." -'-. l '7 IS neSl.T8Q..L6 u()

of the sch:l:i~ren syst.ems that changes sensitivitYe

Sen2- i ti vity is e :funct ioT.! of thp rj::ttio of the unstopped illumine V,a

that passes thrQu£h th ~. +- ' " • f - . (. "e 018,,111'0-:30- n:nl e eage }mage lrD."ge of source

formed at t!H'J kni fa edge) +,0 the' unst:::rp:ped illumi:'1atiol1 that uass!">s

ret 1 0 vrUl he:ces.fter be rei'Brred to e Q, the knjfe edge illuminetion

) retic. The sensitivity may be incre2sed by incre~sf4g the knif~

i'Jto the

..

l.Y,!C.-:-[~Fi~3f!:::, "J'"'et th~ k::-ii.f& edge 8_J.l{)'":.vs sll:f':fiei.eht ilI.1.11TIina. i.:.io!l to • :-;# ,,!<

" .

10 Jf?

./";'. In ordi~18_:r'y p]J.oiJo,~,?rC?phy J t,j:.? J:at.ic:: of· c8.:;(:m'1TI1 7.10 Yairlirrrtl:TI e):~pc.'::~!}.l>e ~ , -

) is: <ieterm.ined ent i rely by condi t5,nn;:; 17} thE> fis}d 2nd :Ls not

)

. ~'t . d' b' 'h ' +- 1 '1' t" 'lS u8,erm-lne 7 0.J.8 "to'.R .. !. rac1.8. ~on

i:mages and the TeJative o.is:p18cement of t1-;.'?se imae;es" The:L ... ef0r!~tE?;

in SC~~,ierenith.e Y'at:\..0 of: maxim.um to min5..:rr.li'Tl E'~rposu!'e is improved

by (}u.tt1ng off more light at the knife edge i,,8"$incr~8.sing the

:kni'fe edge illum5net.i,on ratto. In prt:nt,1ng out the piC~1J.re8:'

,.the absolute difference of the rre;.;:c t·i ve diL.~rit]a (lens it j. $8

c

..

.. 11 ~

In this analysis, each elemer,t of the ~~stem VJil1 be treated O:'f

it~~lf to show its effect upon the sensitivity.

of' the except

knife edge remain fixed unless otherwise stated. It is a.l1'iIlYs assumed

that the k1'l.ife edge is reset to give the e-reatest sensitivity.

. disC'J.s13ed is the partiC",;,l;<..r arrar..gement sto'?Jn in figm-e 7c. Similar- re- .. .,' .

sults can be shmm for any arrangement. For simplicity, -the d.isturba..~ce

is asSumed to be a weak prism and the refracted rays are bent a71ay from I

-A-/lic/lf} a.. d;t-~t:fi~1i 1:.~rpr?lidic,u.I4. ..... /;1> f~(J'I1;'t<fJf~ e.,~/t?' .. the-knife ed~eJ\ In general, strong ~ustm~oances p!'Bser:" no prob.i.em, ana.

therefore this discussion will be limited to weak disturbances where it

. . . d.l.'" t . t.. " • , d" '.', -.:" . t 0,~""rle""+''''rl of'f' +.i-;iO- 1_P')r.~'"l_· str,..'hed;-.-.. :LS aSSll.m6 . v_,a t,"le QJ..sp .... ace ::trnage '],8, .no-", .., _'-'V. ~ _ _J.l_ "'-'-- -'-'-'- ~ ... 'd';' " -, .... [

or. off the k:nife edg-~'"fi§;t'~~s- otherVlise st9 ted ~ It • ;....:.;_. "~r

be, noted th8.t all distr~.rbanc""8 which deflecttneir

sensiti.V'~~; A rrL1.nlmlmi'

.Width o.fthe l:igbt source is necessary to give sufficient light £'01" photo--

graphic res.pqnse ... The size and v.n;it intensity of tbe kl1ife edge image

isp,roportional to t~e source's size ar~ intensity. If the width of the

. light source increases, the degree of overlapping of dist"v.rbed and un-

o.isturbedimge increases (see :fig-ure 13a) & I::..s t11€- angu.lar .deflE~ction

of th.e l'igt~t re6ains cc~nsta:"':t for any p~ticula.r rlisturba:r~ce, a char~ge

light source effect

the sat!le sen-

This can be dcne by increas-1ng-'the 1-1idth of the

source:'t'4~til t?-ie cistureen 2sno longer deflected off the

.. ., POSSlO..L8 tl:e a great.er

f

(a) Stralg~t knife edge and straight l'ight source image.

(b) Stralght knife edge .and c<.lrvec. light source image 0

,Figure Ii.- Rela tioD bet,,;een the shape of the lmlfe edge

t-~-: .~-,-_a_r:_d_'_ the l~~_e_.:~ __ ~::_!~_~~~ __ s_o_~r~~~__________________ __________ l,::~. ~o, .• ;1 The foeaf"°leniih ot the first ~ may also be 1ta!"ied to ch.anJ' ;,~"t \'::'~::--t,;;!;~-:--A .

the sensitivity. Sv.ppose the focal length oithe first mirTor reduced-,' .,<" •

. .

so that a. larger sol'id angie of' the~ light from the sow.·ee is eolleet&.'\ '" .:7

by the mirror (see f'i~~ 18:. :g; tbe secoro mirror remai.ns lJpclwnged, .,.. :;"'1- _.;~#:-~~'. .

the iJllage- at theknif'e edge inereasesin size.

are prop)l."tional to the ratio of 'the focal lengths 01' the·two mirrors.

~he.§.~tualuni t intensity of the image remains the "same ,'because the

"-light rays frat} thesOUTce;--'ftf~:~l:ight---rays---retai-n-±'he_ sallle._~r:s~~_~e'"

f'leetiOll, _due to the disi.;urbance, aftE:r tbe focal length is changed..

the gl~e8. t . he

(

- ",,.~.

Sout'-c e

(a) Focal length, F. -~---~.- .. --.---~~--.

~<-------2{~--~7~ r -,

w 1,--' ___ --'

Source

(b) Foeal length,:Fi2.

.FiO:1.j re "'I'e": E'ff'e .... +-.oW .!::.... .LJo- .. ..... 1,..1. of decrease in 'the foc~I length of the

first mirro!'6 -------_ ..... _-

It should be noted that, _ t11e effectso.l'-lieCireb.sing '~i1e focal. TEingr..n-

light source aI'G identical. In geneI"al, it isecollor:iic,tl, t6 have a

J6io~icu1ar- e ppl~ca ti<.ro. . ~$'-

TIbene'.rer any }art of the ligi:.t source is oi':t:5€d, it"om t:'~e

-t,be se:usi ti vi t.y • Optical s:berrations increase' VlteeL the off set. e..ngle of

-·t:~e light 30lit~Ce i11Crta~~s. rto;y'€ver, certain tu~&!lgBlferlts elf" the

. :j

,. '.-

)

, " ,

! I

i i : j . I

;- \

I I I

.. 15 -

schlieren el.ements t1ill minL"llze the distortion effects at the knife

edge. One example of _this is to use mITOr5 of the same focal length

and to ,e,;r'range mirrors,light source, and knife edge so that the system.

is ~orn.p1etely Sj'T!;"TI.etrj.cal (see -figure 16)" Arrarlgements for nrl:n,l¢zing -:;.-,~~; -.

optical aberrations vary "?lith d.ifferent S",{stems ar.-.d are generally deter ...

mined by experiment.

As the a-~1ar deflection of the lieht rays remains the same for-

any particular disturbance, the distance- fro? dis~urbance )

nUrror has no efi'ect on sensitivity (see figure 17).

to second

---

Flgur~ IJG= Effect of change lnloca~ionof the disturbanceG .J The l:ocal len""h of th~ ~OOnd m;o,ror is imwrt:':t-o~~:::-r:,~~~~cc:~'11

5 \J • .. .,.;:..."J..;

. ~ "';' ';', :::";1

to the l::nife edge. Suppose the focal length 0 f the second mirror is in--

creased (see figure IS). The displacement of the disturbed image in-

creases, the size of the images inoreases, and the unit intensity of ~he

images decreases. The relative separatio~ and size of the images remain

the sa.me.. Slr£e thetota.1 light of the S",fstem remains the same, the

knife edge illumination ratio cannot be i..'1cr~ased. However, tht:) setting

of the knife edge at the proper degree of cut-eff becomes easier as the

focal length .of the second nUTTor increases 6

\ j

--{; j;-:-:.:-

~ . (b)

I .-0----.--~

I Figure 18,,- Effect ri"t

j'

... 16 co

f 2

increase In- th€foCEl.ll€ngt~, ot second mit-ror. . .. -

-;",.' -"'-.

the-

l. R~iatiV'£ vioraif6iii"bitwooii--ihe knife edge and other elsl!!el'xts (jlf

on sensitivity ..

---------- ------------ ------------- --~-----~---- -------

'i,w::..w. t-.... ·-., ..... r

then for

-\

)

---r;lenis ---tha-'i"; lL>Q.ke~-it--Of-t-erf!1ee.e-SSary- :"-0- -eorn.~:OJnI-s-e-(jn~tlfe --e-~_TCilc12 -~-f-fi:tm ~ --'-

increased when the size of tr-;e photograph is decreased ..

, Below ~~3 giv\~n a srtHIi~J of"_ t~tle l~OSt .impOrtant iacts ooncerning

J .• . " l.ncreasec.. oy:

\

)

{b)

"'" 18 ...

deer-Base itt th~ f~el length of' the firstmirrol"

(optical abenations and imper.f'eetions of: optical

elements determine practical li!!'.i't; of these changes)

(d) A decrease in the size o;t t1:te photograph

and the relative vibration oetl1ieen the knife edge aoobhe oth~eleme!lt,.::.

,.. .. ,., > ", _ op!J"''::7I1 '" K .~. f!I .. ~"' 01 ~he s'OnLlersn sys .. err~ ue;:;errru.ne vne""I.OCai .Leng",n ~ seCOnd I;:l::trror~

. \

- - - - ----- - --4.,- -{ie~e~al1;r,--i-n-rooGe~--sehJ_i~~n -system.s--tn.'w-tiJtst -ru!€1--sesolri--- - ------

~", .. j

/

"',.------_ ..

r.

D1sturballoe

I

f1rst Surfaoe Mirror's I

, I

----------~------ --,- -------' -- -r-

---- ,

---------'---" .

~-" .. -

- '. I 1-1 ,,~

L.

, '-.../

v r --1

I I

-'

1 ~;::;'~q;

I '" , I

,-~~----

Soreen ... Vd.rh (iJ I f:I .s 1 7. c:'­J All Ii (J:.!!>...s

--~ I" ,I

--,:-::-:::-~"-..:c_:::-___ .---+t-- ~ ~,_,;-~'-:::::~ ~~: ____ ~-h---:--_-~--=-=====-=---- U h -~ -~ V J!:):L~.k..~ I

Condenser ~ . ~ -) ~"Fr- ~li t Knife Edge I! Rotatable

I¥" ! I II V- L1~ht-!l~~~~~

;<1

/J-i i'" (:,~ I~ }-Y ~-"P.- M P - ' - -----"- -- -----t-----

PI!A t fA. 6-4 J, '" Bphepl~Ql, Mirrors F = 90 in.

I -~~------~- ~-----------~----------------------C--------_h-

: rIi. ' , Flg'uJ:'e 110"" SCh(;mattc dia/?;ram Of" NACA schlt"erer1 apparatus.

I

!

'~;:~

.f" .

- 19

'.<, The. '~chll~ensysteJn elllPlor~~:the al.1.thorsis. shom) in ~~. 'C

.... In:$?;.~se· ~ it.bee1aimed tha't thtsis theidea1 system under .$: .. -"""': '". ~-;.; .... -., '. . . ." .'~ " , ,.-"~' .... ~-:t~ . . ,", ~ '\>-':'-.-:; : •

., ',-:;,. '::';"~:-. '-,- ... ,: - "... -" "., " , . ," .-.:' , ";'; ; . ,'.' .,,--.y~,- ... '

.c~~~~s.lfowever,it doesJ>fferto thereader,.1mf'~~1iJ.th a.

~~~f~·~~m~ .. a })as is fromwbich .other systems eafi::'~ built."

Interferometer

,. ..~ ~zehllder InterierO~~rigthe t,.pegra11y used for .·.s~i$~~phenomena in aerO~uticali"es~rch (~flgure 20). Light . , ,'.: .,; "'-.;.~~>;;~ "... -: . '. ." .

'!romJ~~~gie so~'ce pas~s' ~rotUid t~e' 1;w6 ~ths of the. interferometer .

and is reeomb;tned to give interference £ringes. MonochrOmatic light,

whieh wIll be discussed latter, is assumed •. If the tw,"\;'6mbinSd beams

-are exactly congruent' (COincide exaotly at all points) ootringes. will

) be formed.. Eo11€ver, if instead of being exactly eon.:,~uent, the two com-

t

- -

j

bining beams cross each other at a' very small angle , £ringes are formed

(see figure 21). nilen light waves are in phase, they recombine -:to give . --- ----~-----

---------~------

bright line fringes (for example on t~e center line of figure2J.).

Similarly dark line fringes are -formed .'then the two ""$eye trai~s are 1800

out of phase. . Atri density disturbance..in e1 ther beam increases or de-

creases the veJ,.oriit.y of tr-..e wave front ,Jhile the wave front passes through

it. ;Tpis results iDfI-inge shifts pro.Portional to the depth ar.d density '~"' ..

cha.-ige of the d.is~b~nce ,,'Accordi!:l~ly, the optical elements in the two ~iL" pa:t),. '"

~~must be almost perfect, or the fringes will be distorted •

.. TJ:-ds is t!;e general prinoiple of· tte interferometer" Severs1

"',"" ...

I ~

First Surface

Light ':;)ource (\ , r

'-.."..1

I

441 !~- --~l Splitter Plate. i)

Mirror " , - -:.: ':" -' '0 " • '.., ,,', -... '"

j I ' - - . 't f I ,,' 1 ' ','.' "':" " ': f';' =-:' "'0'·I' --'-1. t':')"'" , ~",:, , ,I -1 , v ; : D sturbance i ' I : I

I ' J I I ! I ! I I

'I . .., ~"i ~ " .-.1,,·r" -t' "'cqrnp,ansatine' ,

I Plates

\

I

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i ,Y V First Jurf'ace Mirror'

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Splitter Pla~e

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F,~gur+ 18 .. - Interferometer ..

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important conditions rOT ii;.t~r.cerBnGi; a,y's not sBlt<-evicier.:t. In order /,~(J.'WJ .s/;,,"-I" <¥l'r4 x/";'a.h'f

to have interference fringe:s, each li>:;ht~~ 1£'':0''.: be divided~equal1:r, .

The reaSon

case of such an arrangement. It is apparent tbat each poiyt of the light

... lWLthematieal roint source of light -1.5 usc"!,:; ~ A light SOn1"CB wLi.o!} is

'. . '.' a./r this smaJ..1. In the r~~Zehnder IrrlJ<Sl":!:erometer, .~ wav,? frt)~\-;:,s fl,l"~

. ~/)~rjlHa/~l ~"'SaA'l" .5~ell(}~J ,,~r,,;to War~ ftlJAI/.s ar~ "1P~"'h.",:~/ al>;ays divided4equ8J~y andJpil'.:- €:met":;! f?1;.r>:S%"!.ffl~~.:~. 'rhus i!iteri'erence

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Interferenc€e-frcm an extended light source .. . . ------

20 1/2

If strictly \i10l .. ochrom~tlq~ight is uie~ t~e .. twc :J.ight -pathsthrouP'oh , ". ," .~,. - , . ' ..' , . "t. IU:;;' .:. .:;, . •

"the inter:ferc.me*~. to the:'.:sOTe~nmay differ by;anY amount and fringes , . . ~~~ . '. .' ~~.

will dUl a~~Y'~h c~ior"of light has it sown seri e~ ,of fringes • . '~ ... ~:".:~y~. .

whicli fcl"~ givB;].a.rrangement have a spacing propcrtionaJ.'to the wa.ve

lenght o:f- the,U.@t.Thus. if the light is not strictly- monochromatic, , :.' . . ~ ... ~

the fringos'o¢come ind~st111ct as the difference inpathl.enghtincI"ea.s~,B. t • . • ,_.. ~ ." ," . :'

" .' "'-:'.' . _ ,.,,:,.. -'- " . - -." . - " ,

oru.yaf,e*f~1IigeSmay be.formed whvnwhit~ light i~ ~~d. and'these w-i~

be-formed dose to the position where the .path lenghts "are exaCtly.~~l.

Because s~,rictly monoci1.romatic light of ~flcient intensitY is not .' •. ? . . 4-' .,7."._,_--. ''''. ", :.~.:'~'. _, .. : (:.,' ,

obtainable. 1 ti S cu,stomary to adj"o.st the tW01.1ghtyathS'ti'o exact.,

eqy.ali t";- ( to g!. ve', whi te light' fringes) • and then use light as nea.rl.y ".>

monochromatic as !s practicable. With this arrangement th~ma.Xlll.Utl· range of,,,;

disturbance intc~3ity may be studied ~~thout serious less of distinctness

of the fr:Luges.

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USUs;1ly'the' . light· s0l.!!'cehas.a li~~:.·~rit~if:;~§-&l.;~~:~Y.:the<.~ .' .... " .... ':",' ./ .. ~ ., .,',e,'o-.':' '~':" ":';-. ..".::.;:!~ " '~.'. ,,,~,', -..... _.. r;

risU,l& range eJld ~ ~;~~.J1:~;~;~"~~~,¢~;F ". ., the. des~ .llne.The requfrelnentot~llroimi't~e'll gh~)~orinter.f~~nee

<.~. . ." ">-:;';", y. ~ • • ;';:'-;';~":~ .';-" .~.:,

means, that only ·aslatlll·!ractio:4 of ~',YO~~).i~~~;·~in .• 4 .. ~~!-~:· , ~. "0'· . _. ,", . . ' ... ~..... ".) ",- - '. .

aV!1ilable fOr. phQto~e.~therriJiges •.. ~ere:·~s,ot'.c6Jli~J. the'~" . , .-.' '; .' - '._' " .~ ·-._:r~''';'-· -.:: .. " < :,' ,_,,: ;~', .'::'::.~.' "'0' "<-_', ," . ; ... ,.-

: necessity ro~a Shorli,time~~;eJtpoSUre~';stGP~,}mo.t'i~n ()t .. ·~·~~tur--" :-'\ .', "; ,<'::. . .,' '.. ".'.' :"';">~~»>/'>;';.;:<:.

b~ce aswasrequire<lfol":'#hado'lrgrapll~d scbliereg.~·ll:J3o't~~,is~ ... '

samepro~i~in ~ge.rd\o ~h~:.~J.m.!The·fil~I{~~.~.;~;'~~~ir~r(~ , ~. . .-,".,' ". ~.

photofP:"aph~c reriponse:,but mud have ,small gr.aintoal,l~.e1l1arg~nt~ ..•.... " . ".

wer:~:k~t3w7f~-:f;;,iA :~:,"I'~'::;§/r!Ki::::J7~tr~*rA~;¥Y;::P,"r~ ~cLYa-1 NR;?-Zo/C-. '. Interferometer Adjustment " '.. .'.' '. . . >;' .' "

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. The interferometer shoUld be' des:i.gne.~ rigidiY-~cons.tructed ~ith • , '" .

as nearlyequa}. opti,cai' path lengths a~ mec~*callypo~sible. The plates

and mirro;'s, must b~ mounted rigid~ ~ 'and so ~.ged that they are ~er

no strain; The mountings should be such that the llli.ITors and p1a.tes are . '

parallel to each other with the light ~ound eaehpath being approximately

equal. . F'Urther, mountings should, be capable of giVing the .lnirrors and , .'

plat~s angu:lar nntion about two axes perpeooicularto eaoh other. and to

·.the normal to the sur,t:ace of the mirror. ~ .' # . .~'-'

First, the plates and. mirrors s~ou1d be arranged s~ t..~tthe recom-

~ined light beams are appro:xiJiJately congruent. ~itp.a 'beam'of parallel ,... ....... li~bt entering the - ,.;. " interf'ero~ter, rough adjustments are made by v;isual

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inspedtion to give 8. single emergent beam';" Finl';r ad:justmentsare mad~

wben viewing the recombined beams with a telesoope. Two images of the

. -light source will appear when the recombinea beams are not congruent.

The more nearly' congr~.ent tbe berun~ are, the closer these images appear.

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if~' .Irt6.n~·c}Tr_oFli'tlc·:~.":1 .. fght . 'is. '-)-~ '. - ~-;-:: :~.:, -. . :' ,""""':" .;

the interferometJ!~r ..

),,_~;i£Ii:_~l'tO~h~blypr().duce. t~;i~~~s os" tt' .. e '-path lenght.sof" th~~ two, be9~s ar~:"~:ppi-?xj;:ma.~elyequ,a.l!'lr'f'rInges are not pr;auceq, ~ne mmrrcr

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'.' o:ri-'p;~~!e'~~f~t~~~rst~~~~ii\be~bve'1 in ,a di1Jection_~erpehdi0Ular t6 th~::~Urf,aee;...of<;"t~e,~m,i,fi;"0r:'~'ciritil fri ngesare PI:oc. uced 0 The

'> ••• ~:;:',.;:~'~_';>:.: :,"\ ~ -~\:'_ -1-.",.: 'r~':"~"'''- ":'. .. ,\, .... ~_ .... :J ~. ~ __ , '; .::"

,spa61~f{::'9~,~'ii::' ft'ing~~'depen~; UDon thew8ve' lengh~ of: the -light

,"nd.;t;~;;t1ie~~eYi~ti~n1'l'QJll 'exact cong""] enc... The 01'1 ent~. t i on o~ ~s' .ean ,pe-. chang~~t;.ny :r:otating~,.ei ther thepletes or

•. -.? . ..;~. :~: . <:·~i..;:;ifi-: \~.' -" ~ ~ ~-->..~. ,~:': ~. '" ~~...,. ,.

lub ... r'ohs$;G,that the plan~'"~of .the ,angle of deviation of ':; ~-

\)eams::'~:4~ Chafl?8d .. .' ." ::.~(.<]:~-.;, . ,~~-.-~~-~

"Th-etwo beams should be so

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erranged thpt they a:ppea:r; to

cross ,at the disturbflnce 4 The angular deviation of the bea!.'1S sno their

the locat,ion of -tl1Hxapparent intersection can be Ch8:"lEed. by

adjnsttng' th'1 plates and mirrors~ The location 0f thesPlJ8 "!'&nt

on the fri'1ges end then I1leaS 11ring the <

r8.DGf3 :for vv'rric'lf. trte teJ,esc0pe

is adjusted."

,Cit""\l; 1"1 "".,,""" .:- "!")r-1f. .. '1;1.1·"...... "m 11"1 f'1"":~"""""':1~+" n- "1.~ ":t- .... j~r. _", ... ,rve .,'0-. 8.. L,-~ .-,,~\jc; ~tCl~Ov,'-'-".·",_,.""]_,, ·1...:.17n", 80',_ ........ F'J • ",_ ,Y'. II -. , "-" .........

4il1B.l' ;:;:.d jl)stTJlen~" . In"pri nc ipl.e l' "tr,e" ~)F' th' ler;_ght~

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t)8c[:tlSe ::t·1--~B re~'.1i ~'~8ct ;'\0 v e:r s rtt. 'hecc;rn8~2 ·v·e~:.-'~·'{ sro.p 11$ f' ", :-:-

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Gf 2nS1$:; ul=~se~"'~~:lg, t;~,,? ·.~C!~l~sr1.ler.tc:~

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~/11 k(-> perpetiiicularto-the light beam-., The path lengths -6::!"e made exactlY

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~he' fringes~dono_t. ~s~ppear. When' center:f:::;:'in2'6 S 6.ppear with a~bi tEi"-ti

light ~ce,,_ :the :int~rf'erometer is ready' for use ..

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The plates ShO';21d be of unti'orm optiea.l thiek!wss, i.oe., uniform 7J;~ r~fgJr~t( "!'Ni/n'Mif)' ",;/1 d.~lPh/ ~ ~'" t:Jf> ~t!'6;r~~ a&U~.J-4t:, tnickness ana lrAex of r-efract~on. ljev~atLOns ~n either of these fa~t-

ors will cause devlations in the fringes" If the plates are otheri'Tis e

uniform but slight~:r wsdge-snapiet!, thoy m.'3:y be used without ai'feetlng

the straightness of the • ~ I~ /f)ifl,/VJf,/A~/H,rl>

ach:J.evedof'Uniform to one

fringes. UsualJ.y optical thickness ean be

The splitter

for this purpose,

The plates should also be

plates.

fOl~S.

a great amourrt of light.

of' the s11 ts.

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any attempt to r-educe the ba.nG. width resu .. lts in lC!l1fSr e".;,:pOSu.rB 'cimes.

~his :eesttlts "in a corrrprcmise· which is deterEilloo by the size 0:- tria :fir.zal

pi~ture~ photographic speoo and grain size of -the film, and rnh'1l.ber or

Because the -bi>..a.m of light leaving ihe interferomertel:' is apr-Toxi-" 4ia../Ht'ter ..

:ma+.-ely parallel, th13 a:~ t'!S"€ of the optica.l system used £'01" photographir..g ... ! 'I 1 SIl;::Y.-f 4

the fringes ~ be a. t lea3t as large as the dia.::Ieter 01' thefie:d to be

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Theirprincips,l advant.ages and shortcolrtings areas follows.

The shadoogra.pb. is simpl:e, inexpenslve, ani not critical in adjust-

mente It may be adapted to difficult space limitations without serious

The pattern

s:rstem, are independerl1~ of 'the 0:::-.<~~'.

The

sensitivity is looe:ct.han "tr:at, of eit),er t.he sch1ieren or interrerometsl'"

systems ..

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factor of littl€ importanoe,

Relatl~·a stabiJ .. ity· of tb.e eOillpjnents llust be-

sU.:.:n iwa.ge on th.e

is srrall ~el.at1.ve "'to

stop. This system is adjust~

5CD.lieren ma.? be tlse:j~ 'Io conrpensat.e Zor tilis c:riticalness, hOli'6VSr, ti1e

. . cAt~"ir~~..Lj'" .!11.g.:.1, poirrc of showing

l

Because tbe schlieren methoci

kr..if'e edge • . ~ '; .. ~-

as light intensities on the screen are not generally used. c;"uantita:tivBly ..

the .first space derivative of dansi ty in. the field, within the ra.nge of

incre£iseci there is a. c.ecreasein the range of disturbance ir.rter.s~ tY i'oJ:

which proportionate light intensi.i"y is obtained.

in '-

length of ligh.te Accord:irig~.y,. ~p~)lica tions to situations irr!oJ:-; i:r:.g

'lribra tion,

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test area.

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In selecting an optioal t·e~t ~Yiet~hod one sho'!1J..c~ ~onsid::;;~r ~i5lI t:l"}8

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OPrICAL BTHODS m AERONAUTICAL RESEARCH

B7 J. ll. sterrett, aDd C. B. Palmer

Stibjeet

~ .;. !a.boratoq Beseareh Equipment - W1nc1 ,....ls Research Equipment -~ 1ie~ Teelmiqae

wet 8-.2

9.1..1 '.1.2 9.2

This report describes .tical _thode (shadowgra.r.41" ,

,schlieren" hterfe:rometer) lise« in ~utical Nsearch m

terms as nearly-..... technieal as p!.)ssible... .~ report fir-st.

pnsents th$ phJ1deal pri.rtoip1es ·1l!.'1derlyh3g these methOOs~

and then ~ s;rstem is diScussed separatel,.. The ~ . ,

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