Sales Service Division

EASTMAN KODAK COMPANY • ROCHESTER 4, N. Y.

Schlieren Photography

Kodok Pamphlet No. P·11

3-61 Minor Revi.ionL·KP·E

LITHOGRAPHED IN THE

UNITED IITATES 0,. A.MERICA

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Eastman Kodak Company gratefully acknowledges the assistancein initial preparation of this pamphlet given by Mr. William T.Reid, Battelle Memorial Institute, Columbus, Ohio. Also, apprecia-tion is expressed to Comell Aeronautical Laboratory, Inc.,Buffalo, N. Y., for technical assistance and information on high-speed schlieren photogra phic techniques.

TABLE OF CONTENTSIntrod ucrion

General Principle of the Schlieren Method

Optical Details of Schlieren Equipment

Formation of the Schlieren Image

Location of Image Plane

Size of the Image Field

Arrangement of Schlieren Components

Single-Element Systems

Two-Element Systems

Accessory Equipment

Light Sources

MOU/lting Supports for Equipment

Knife-Edges

Cameras

Adjustment of Schlieren System c

Photographic: Materials

Color Schlieren Photography

Examples of Schlieren Photographs

© Eastman Kodak Company, 1960

Punched to fit the Kodak •Pholographic Notebook.See your Kodok dealer.

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SCHLIERENPHOTOGRAPHY

ISchlieren photography, a procedure unknown to most technologists a few years ago,is fast becoming a riearly indispensable tool for investigating the flow of gases. Par-ticularly is this true in aeronautical engineering, where a knowledge of the flowpatterns of air over surfaces becomes increasingly important as aircraft speedsapproach and surpass the speed of-sound. In the study of ballistics; schlieren photo-graphy discloses valuable information about shock waves accompanying projectiles.The combustion engineer uses the schlieren method in studying how fuels burn, andinvestigations of heat transfer are aided by the ability of schlieren photography toshow the paths taken by air passing over a hot surface. In general, the schlierentechnique can be used to advantage whenever it is desirable to visualize the flowof gases. .

Being optical, schlieren methods do not interfere with the subject being observed.Normal motion of gases is not impeded, as is the case when pitot tubes or yaw headsare inserted in the gas stream to detect flow direction. This is particularly valuable athigh gas velocities, where shock waves set up by probes in the stream may seriouslydistort the data.

The sensitivity of the schlieren method can be made surprisingly great. It caneasily detect temperature differences as small as 10 degrees Fahrenheit in an airstream. This is adequate to disclose the currents of heated air rising from a person'sfingers. Conversely, the sensitivity can be reduced to the point where the exhaust ofa liquid-fueled rocket with a total temperature of more than 5,000 degrees Fahrenheitcan be recorded to show the presence of shock waves and other flow phenomena.

Other optical methods also are commonly used for visualizing gas flow. Most im-portant of these are the interferometer method and shadow photography. Neither is sowidely used for visualizing gas flow as is the schlieren method. The interferometer'has the characteristic of producing an image in which the differences in density areproportional to the differences in refractive index in the field. Thus, it is adaptableto quantitative measurements. With shadow photography, the differences in densityof the image are proportional to the derivative of the gradient in refractive index. Itis most useful in cases where the gradients are numerous and changing rapidly.Schlieren photography, intermediate between these two extremes, indicates thegradient in refractive index. Its capabilities are adequate for the majority of caseswhere flow patterns are of interest. Combinations of the three methods sometimesare used.

A major disadvantage of the interferometer for investigating gas flow is its greatcost to assemble. Also, much care must be taken in adjusting the instrument, and theresults are usually difficult to interpret. Shadow photographs, on the other hand, areeasily taken with a minimum of equipment, but the results are not very useful unlessthe subject has strong gradients in the index of refraction.

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_-·--The schlieren method offers a good compromise. The equipment needed is rela-.-- tiyely simple and inexpensive, it can be used under widely varying experimental

conditions, the sensitivity of the system can be varied at will to suit test requirements,the results are not difficult to interpret, and quantitative measurements can be ob-tained under idealized test conditions.

GENERAL PRINCIPLE OF THE SCHLlEREN METHODBased on an optical principle generally credited to Toepler(1) and amplified exten-sively by Schardin , (2) the schlieren method depends u\pon refraction of the narrowlydefined edge of a light beam by gradients in the refractive index of the g-as throughwhich the beam of light passes. Thus, it receives its name "schlieren," which istranslated as "optical inhomogeneity." In a typical system, a limiting diaphragm,usually a straightedge, is so adjusted with respect to the edge of the light beam thatrefraction in one direction adds to the total illumination, and refraction in the otherdirection subtracts from it. Thus, an image is formed wherein the variations in bright-ness depend upon differences in the gradients of refractive index in the light path.

Figure 1 shows the simplest type of schlieren system. Light from the source, whichis preferably a line rather than a point, is focused by the condenser to form animage. A limiting d iaphragm, generally straight rather than circular, is placed parallelto this image of the light source and intercepts part of it so that the resultant beamhas a sharply defined edge. This beam then passes through the Schlieren Head, whichfocuses it through the Schlieren Field onto the second knife-edge. By adjusting the posi-tion of this second knife-edge so that it is exactly parallel with the first knife-edge, andby inserting it partially into the beam of light, a gate is provided that can intercept alarge part of the luminous flux. This attenuated beam of light then passes to a photo-graphic film where it can be recorded.

(1) Toepler, A.. "Beoboetungen Nech Einer Heuen Optischen Methode," Q,twald, Kla"ikor der exaelenW;,..n,chaften, Ho. 157, Leipzig, 1906.

(2) Schardin, H. "DOl Toeplersche Schlieren-Verfahren," Ver. Oeubch. Ing. Forsehung.ehoft, 367, July-August,193·1-

SCHLlERENHEAD IMAGE

PLANE

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Figure I-Basic Schlieren System

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If there are no gradients in refractive index within the Schlieren Field, theamount of light reaching the film is fixed by the relative position of the two knife-edges. However, if a gradient normal to the plane of the knife-edges exists, the beamwill be refracted so that it either adds to or subtracts from the light normally presenton the screen, Thus, a Schlieren Field involving a pattern of gases is reproduced invarious tones on the film. Opaque objects appear in silhouette.

OPTICAL DETAILS OF SCHLlEREN EQUIPMENTMost literature on the schlicrcn method describes only specific systems or its use [ora given problem. Liepmann and P~ckett(3) and Lewis and van Elbe(4) review schlierensystems in general and point out their application for general scientific purposes.Barnes and Bellinger(5) summarize schlieren methods in considerable detail andprovide an extensive bibliography on the subject.

Optical equipment not commonly available commercially is needed in designingschlieren systems. Therefore, technologists usually design and construct schlierenapparatus for their own particular needs. The following design considerations willbe helpful in deciding on components for specific applications.

Formation of the Schlieren Image

Formation of the schlieren image depends essentially upon two superimposed opticalsystems. One provides general illumination of the field and forms a silhouette imageof opaque subjects. The other produces variations in light intensity within the subjectarea for transparent subjects, depending upon how the light is refracted by gradientsin the index of refraction in the Schlieren Field.

Figure 2 illustrates how this double image is produced. The Schlieren-Head lens Hforms an image of the light source S at SI, passing through the Schlieren Field F_The objective lens 0 then illuminates the screen I.' At the same time, the objectivelens 0 forms a real image of the point A at AI, the planes F and I being conjugateabout O. Most important is the fact that all rays of light passing through any point inplane F will form a real image at I, irrespective of the angle at which the rays pass throughplane F. Thus, any object in the SchIieren Field, plane F, will produce a real imageat plane 1.

Because the presence of a gradient in refractive index at plane F will cause therays of light to be refracted, say upward as at A, the image which they form at S"will be displaced from the normal position at S', This displacement d will dependupon the angle a, known as the "angular deviation," through which the rays werebent. Thus, all rays passing through A intersect the image plane I at AI to form areal image, and A also receives light from all points of the source S so that lightpassing through A produces an image of the source.

(3) lIepmann, H_ W., and PuckaH, A. E. Introduction to Aerodynamie. of a Compressible Fluid. John Wiley andSons,lnc., New York, 19<17, pp 89-101. -

.(4) Lowi., B., cnd vcn Elbe, G_ Combustion, flame. and Explos;on, of Go,e s. Academic Press lnc., New York,1951, pp 211-218.

(5) Bames, N. F., ancl Bellinger, S. l. "Schlieren and Shaclowgraph Equipment for Air Flow Analysis," J. OptSoc. Amer., VoL 35. Ha. 8, pp <197-509.

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Figure 2-Formation of Schlieren Imoge

The two independent modes of formation of the schlieren image, therefore, are:(1) displacement of the image of the source by the angular deviation produced by arefractive-index gradient in the Schlieren Field, and (2) formation of a real image ofthe Schlieren Field at the film plane. For the first case, the displacement is unaffectedby the position of the point in the Schlieren Field; and for the second, the image formedis independent of the angular deviation.

The average intensity of illumination of the image field is fixed by the position ofthe second knife-edge normal to the bundle of light rays at S'. Usually, the knife-edgeis adjusted so that the brightness is about midway between full illumination and com-plete extinction. Varying the position of this knife-edge also affects the sensitivity ofthe system. Generally, as sensitivity is easily judged visually, no critical adjustmentsare involved. However, in extremely high-speed applications, a micrometer-typeadjustment mechanism for the knife-edge should be used.

5CHLlEREN 5CHLlERENFIELD HEAD

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Location of Image Plane

Simple optical relationships fix the location of the image plane. (6) For simplicity,assume a two-element schlieren system where a parallel beam of Iizht passes throughthe Schlieren Field. 0

Figure 3 shows the optical paths. If the objective lens were not present, the SchlierenHead H would form a primary image PI of the field F. From the thin lens formulathe distance x of this image would be equal to the Field distance s times the focallength of the Head FH, divided by their difference-

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When the objective or camera lens is added to the system, this primary imagebecomes its subject which, in turn, is imaged on the film. The lens-to-film distance orcamera extension a can be found by

F.·(d-x)a= d-x-Fo (2)

where d is the separation between the Schlieren Head and the objective lens.It should be noted that the position of the primary image PI, and thus the sign

of x, will depend on the relation between sand F[{. When s is greater than FFl PIis a real image formed somewhere to the right of H, and x is positive. When s islessthan FIl, PI is a virtual image to the left of H, and x is negative.

(6) Keogy, W. R., Ellis, H. H., end Raid, W. T. Sch/ieren Techniques For the Quantitative Study of Gas Mixing.Report R-164, The RAND Corporation, Scntc Monica, Colif., 1949,57 pp.

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Figure 4-Size of Imoge Field in Schlieren System

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Size of the Image Field

The magnification of the image, or the ratio between the size of the illlage and thesize of tile field, is equal to the magnification between the field and primary image(m,,) multiplied by the magnification between the primary image and the cameraimage (mo), Figure 4_ These can be found from the following relations:

P"m,,=------ and (3).1'-1'"

F,mo=----I':,+x-ri

Since x cannot be measured directly, it is more convenient to substitute its valuein terms of sand Fu from equation (1)- With this substitution, and multiplying(3) by (4),

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As an example, assume:Focal length of Schlicrcn Head =F/I = 100 inches,Focal length of Objective Lens =F, = 10 inches,Distance from Schlieren Head to Field=s=200 inches, andDistance between Schlieren Head and Objective Lens = d = 120 inches.

Calculate the position of the image plane and the relative size of the image com-pared with the field:

The position of the primary Image

s·FH 200·100 200' hx=--= = Inc ess-FH 200 - 100

Oblectlve lens-to-fllm distance

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. = -SOO =8.9 inches.-90

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Thus, for the stated conditions, the image will be 11 percent of the size of the object,and a field of, say, 12 inches in diameter would be recorded in a circle 1.3 inches indiameter. It should also be noted that the camera extension, or objective lens-to-filmdistance, of 8.9 inches is less than the focal length of the lens. Therefore, the lensboard or the camera back will have to be racked in from the normal infinity position,a situation that is not encountered in conventional photography.

It is also possible to operate a schlieren system of this type without an objectivelens by placing the film at the position of the primary image PI. In this case, onlyequations (1) and (3) are involved. .

ARRANGEMENT OF SCHLlEREN COMPONENTS

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Single-Element Systems

Many possible combinations of elements can be assembled to formschlieren systems.The simplest would be a system arranged according to Figure 1, with a lens of goodoptical quality serving as the Schlieren Head. Because lenses of adequate qualitygenerally are not available in sizes more than a few inches in diameter, it is moreconvenient to use first-surface concave parabolic mirrors. Such mirrors should befigured accurately so that imperfections in the surface are not confused with theimage produced by the Schlieren Field. Usually, mirrors of a grade suitable for tele-scopes are satisfactory .. Common specifications call for mirrors figured within 0.1wavelength of sodium light.

Figure 5 shows two typical schlieren systems using a single mirror as the SchlierenHead. That on the left is essentially the system of Figure 1, with a mirror substitutedfor the lens. However, because the light source and the camera cannot both be normalto the mirror, some coma is introduced by the skewness of the components. Anotherminor shortcoming is that the Schlieren Field must be placed well out in the con-verging beam from the mirror, thus limiting the size of the field as compared withthe size of the mirror.

The schlieren system on the right in Figure 5 uses a prism-a first-surface mirrorwould do equally well-to bring the central axis of the light source nearer to that ofthe camera. Although coma is reduced by this expedient, its main purpose is to permit

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Figure 6--Two-Mirror Schlieren Systems

the light beam to pass twice through the Schlieren Field, thus doubling the sensitivityof the system over the more conventional arrangement at the left. Because'the twolight paths through the Field are not exactly coincident, the resolution of the imageis somewhat impaired.

Many different modifications of high-sensitivity schlieren systems can be devised,with the light beam passing through the Schlieren Field as often as desired. As thenumber of paths becomes greater, however, the resolution of the image usually ispoorer.

Two-Element Systems

Figure 6 illustrates schlieren systems based on the use of two Schlieren Heads. Al-though mirrors are shown, good-quality lenses could be used.

In the upper system, the light beam passesfour times through the Schlieren Field,producing high sensitivity. Except for the use of two prisms and the second SchlierenHead, this system is essentially the same as the double-sensitivity system shown inFigure 5.

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The lower system of Figure 6 is the most popular of all schlieren systems. It has theunique advantage that parallel rays of light pass through the Schlieren Field, thusproducing an image of superior resolution. Further, because the light source and thecamera arc placed on opposite sides of the light path between the mirrors, coma iscancelled. Another advantage is that the Schlieren Fi.eld is located away from themirrors.

In fact, the least permissible separation of the mirrors is about twice their focallength in order to provide space for the Schlieren Field between the entrance andexit cones of light. On the other hand, since the light beam is parallel between thetwo mirrors, they can be separated as far as desired. This is particularly useful withwind tunnels where it is not convenient for the Schlieren Head to be near the Field,or with explosion or high-temperature combustion phenomena where the SchIierenHead must be protected from damage.

Figure 7 shows a typical two-mirror parallel-path schlieren system in operation.The mirrors used here were 12 inches in diameter and had an equivalent focus of96 inches. Spacing them 28 feet apart provided ample space for locating convenientlythe phenomena to be studied.

Figure 7-A Two-Mirror Schlieren Sy.tem in U.&--Courl",y Bottell" Memorial InstiMe

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ACCESSORY EQUIPMENTlight Sources

Any light source of adequate brightness can be used for schlieren systems. Dependingon the application, either intcrmittcnt or continuous ,(JUI·CCS of light can be employed.Generally, the source should be of high brightness and small dimensions so that itcan be focused accurately at tile first knife-edge. Sources as small as a 25-wattzirconium arc lamp may be used, but more intense sources are generally desirable.

The system shown in Figure 7 used a B-H6 air-cooled mercury-vapor lamp madeb)' the General Electric Company. Because the light-emitting area of this lamp isabout two centimeters long and only one millimeter in diameter, it provides anintense line source of illumination. By using cylindrical lenses as part of the con-densing system, a good line image of this source can be produced at the knife-edge,thereby utilizing the light most effectively. For cooling, this light requires a. sourceof clean compressed air at 30 psi or higher.

An additional advantage of this light source is that it can be used for continuousoperation or it can be pulsed for very short intervals. Using a single light source forsuch dual purposes is desirable when flash exposures must be made, because the systemcan be focused and adjusted with the light burning steadily. Then, when all is ready,the light is extinguished, the camera is loaded in darkness, and a single £\ashexposureis made with assurance that the system is still in alignment.

Relatively simple electronic switching devices can be constructed to provide singleshort-duration exposures. A typical system consists of an FG-95 thyratron connectedin. series with the B-H6 lamp across a capacitor. With a high-voltage charge on thecapacitor, a positive pulse on the- grid of the thyratron causes it to conduct, dischargethe capacitor across the B-H6larop, and produce a brilliant pulse of light. A 3-micro-farad capacitor charged to 1350 volts produces a flash lasting about 13 microseconds-short enough to stop roost motion and yet long enough to deliver enough luminousflux to give a satisfactory exposure- and high schlieren sensitivity with KodakSuper-XX Panchromatic Film. Decreasing the size of the capacitance decreases theflash duration, but at the expense of decreased light output.

For extremely high-speed schlieren photography, a unique light source has beendeveloped by Cornell Aeronautical Laboratory, Inc., Buffalo, New York. This is ahigh-speed, multiple-spark system capable of producing high-intensity light flashesof 0 .l-rnicrosecond duration at any preset time interval of more than 10 microsecondsbetween sparks (see Fig. 8). Using a 10,000-yolt power source, barium titanatecapacitors are charged to slightly less than the breakdown voltage of the spark gaps(850() volts). This system, in conjunction with a mum-type camera loaded, forexam pie, with Kodak Royal-X Pan Film, can provide a- series of schlieren photo-graphs of phenomena lasting, say, as little as four milliseconds in a hypersonic shocktunnel. (7) Exposure brevity in this excellent system. is limited only by camera drumspeed. '

For simpler schlieren systems where flash exposures are not necessary, a slide pro-jector can be used as the light source.

C7J WilSon, M. R., and liiemenz, R. J. "Light Source for High-Speed Photography:' Research Trends, Vol. VII,No. 3, Fall, 1959. Copies Clvailable on re quest from Publication .• B.onch, Cornell Aeronautical Laboratory, Inc.,Box :23~, Buffolo 21, New Tork.

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Figure Sa-Cutaway of Electrod e As.embly-Courte.y Cornell Aeronautical Le boretory, Inc.

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Figure 8b---lighl Source Optical System-Courtesy Comel! Aerenautieol laboratory, Inc.

Figure Bc-Schlieren Photos Taken with ·Multiple-spark light Source of Supersonic AirstreamFlowing OYer Slender Cone--Courtesy Cornell Aeronautical laboratory, Inc.

Mounting Supports for Equipment

Because the alignment of schlieren systems is critical, it is generally necessary toprovide solid supports for the components. Where the systems are relatively smalland the componen ts can be mounted rigidly on a steel framework or a rigid table, itis usually necessaq' to provide for only minor adjustmen s. For larger systems,however, where the spacing between components is relatively great, the equipmentshould be located in the basement or in a location where building vibration is notserious. The components shown in Figure 7 were mounted on pedestals weighing700 pounds and fitted with heavy-duty casters to permit locating them where required.Once in position, three jackscrews lifted the pedestals from the casters, firmlyanchoring the components in place. Once properly aligned, this system could be usedindefinitely without further adjustment.

Knife-Edges

!he fir~t ~nife-edgc in a schlieren system serves mainly to define a sharp boundaryIII the l~ght beam. It must be placed approximately parallel to the image of the lightsource if a rectangular source is used. Generally, it intercepts about half the lightbeam, to assure.~ well defined boundary. It should be mounted on a firm support.

The second knife-edge is used to adjust the sensitivity of the system. Because itmust be able to rotate around the beam, move transversely with the beam, or inter-cept the beam at different positions, its mount must be adjustable in three directions.Slides with gibs, as on the tool carriage of lathes, together with fine-pitch screwsusually are provided to furnish the necessary precision of adjustment. Rotationaround the light beam may be done with a simple worm-gear arrangement.

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New safety-razor blades make excellent schlieren knife-edges, and are commonlyused. Although straight knife-edges are often employed because they permit obscrva-tion of schlicrcn effects predominantly in a single direction, circular diaphragms canbe used where polarization of the schlieren effects is not important. In this type ofsystem, a circular diaphragm defines the edges of the light beam passing to the firstSchlieren Head. Another circular diaphragm at the focal point of the secondSchlieren Head serves to obstruct rays having sufficient angular deviation. Beingcircular, the diaphragm intercepts strongly diffracted rays in any direction. In somesystems, the objective lens of the camera may serve as the second diaphragm, butsuch systems usually are not very sensitive.CamerasAny type of lighttight box can serve as a camera for schlieren photography. Becauselenses of relatively long focal length are often necessary to obtain large images, viewcameras are commonly used because they can be provided with long-extensionbellows. Such cameras are ·recommended where the .schlieren system is used for awide variety of problems.

In cases where a schlieren system is designed to work with a single piece of equip-ment, such as a ballistics gallery or a wind tunnel, it is usually simpler to construct afixed box with a lens mount at one end and a focusing back at the other. A lensequipped with a shutter is generally used so that the exposure can be synchronizedreadily with the phenomena being observed.

A negative size of 5 by 7 inches is useful for most schlieren work. Larger imagesare generally difficult to obtain with available objective lenses, and smaller negativesoften have limited usefulness.

The objective lens is generally not the limiting factor in getting images of goodquality. An anastigmat lens will provide more than enough resolution to get sharpimages. The aperture of the lens need be large enough only to admit the bundle oflight rays diverging from the second knife-edge. Thus, lenses of relatively smallworking aperture will be suitable if they have long enough focal length to give thedesired image size and are placed reasonably close to the knife-edge.

ADJUSTMENT OF SCHLlEREN SYSTEMGood schlieren photographs of high sensitivity can be made only when the systemhas been properly aligned and adjusted. Although this is not as difficult as adjustingan interferometer, for instance, care taken here will be repaid with photographs' ofbetter definition, improved contrast, and adequate sensitivity.

The first step is to obtain a sharply focused image of the light source on the firstknife-edge. This knife-edge is inserted far enough into the beam at the point of bestfocus so as to intercept about half the incident light. For the case of the two-elementparallel-beam system, the first schlieren mirror is then placed exactly one focallength away and so positioned as to receive all the diverging beam from the knife-edge. The reflected beam from the mirror then will have uniform, parallel rays. Ifthe emergent beam is not parallel, the mirror must be repositioned. This mirrormust- also be adjusted so that the reflected beam makes the smallest possible angle withthe diverging beam from the first knife-edge.

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The second schlieren mirror is then placed so as to receive the parallel bundle ofrays from the first mirror, and at least two focal lengths away from the first mirror'for convenient field location. By tilting this second mirror slightly, the convergingbeam coming from it is deflected .slightly to one side, the angle preferably being thesame as that between tile first knife-edge and the first mirror. The sn:ol\<l kuilc-cclg«is then placed precisely at the focal point of the second mirror and rotated so that itsedge is exactly parallel to the sharply defined edge of the light beam.

Proper positioning of the second knife-edge can be checked by observing the imageon the ~ound glass while moving the knifc-cclg-c" slightly. When propnly Ine,llt'd,lII()Villg IIIC'kllilt:-ccl~'c W;ldll;llly illlo llil: lighll,c;wl will produce a uniform darkening:of the image area. If the image area darkens nonuniformly in the direction of motionof the knife-edge, the position of the knife-edge should he shifted along the beamuntil uniform darkening is obtained. If darkening is nonuniform in a direction per-pendicular to the motion of the knife-edge, the edge must be rotated until thiscondition is corrected.

PHOTOGRAPHIC MATERIALSIn general, photographic emulsions having moderate sensitivity and contrast servenicely for recording schlieren images, Either Kodak SuperPanchro-Press Film,Type B, or Kodak Super-XX Panchromatic Film will meet the needs for most cases.For Hash schlieren photographs of a duration of only a few microseconds, KodakRoyal Ortho or Royal Pan Film, Kodak Tri-X Panchromatic Film or Tri-X Pan-chromatic Plates, Type B.' should be used to assure adequate exposure. In extremelyhigh-speed schlieren photography, such as that conducted at Carnel! AeronauticalLaboratory, Inc. (see page 12), more sensitive film, such as Kodak Royal-X Pan,mus t be used. .

Preferably, negatives are developed to moderate contrast, but the requirementshere will depend largely on the nature of the subject. In general, the darkest part ~fthe field should provide enough exposure to show a definite tone in the developednegative image.

COIOR SCHLlEREN PHOTOGRAPHYA new technique in schlieren photography was reported from England in 1952. (8)

Using a two-mirror parallel-beam schlieren system, a constant-deviation dispersionprism is placed between the white-light source and the first Schlieren Head. Theusual second knife-edge is replaced with a slit at the focus of the second SchlierenHead. This slit is adjusted so as to pass only light of a particular color. The remainderof the schlieren system is conventional.

In operation, the slit is adjusted with respect to the main beam so that a mono-chromatic, uniformly illuminated field is produced when there is no gradient in

(8) Holder, D_ W_, cnd liorth, R. J_ "A Schlieren Apparatu. Givins on Imoge in Color," Nature, Vol. 169,March 15, t952, p 446.

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refractive index in the Schlieren Field. For example, the slit may be adjusted so thatthe background is yellow when no gradients are present. Then a compression wavewould appear red and an expansion wave green, the shift from the background calorbeing dependent upon the angular deviation caused by refractive-index: grildients.

Tlu: iJll:I),il' produced llY litis SYSll'lIl with rclat ivcly IOIl!{ exposure times is C011l-

parable with that of the conventional schlieren system, except that the usual differ-ences in illumination intensity are replaced with changes in calor. Kodak HighSpeed Ektachrome Film, Type B, is ideal for recording such images.

The rr.il ~('iel\tili(' V:t1IIC' of nlior s('hliC'l'('1I l'ltolof.:\'al'hy i~ d(,ll:lialllt'. /\1 It',lsl unrexpert ill lite field makes quautitativc analyses 01' his schlicren results by measuringthe spectral calor bands reproduced on a color film. However, the same result can beobtained, it is generally felt, by observing density differences produced by usingblack-and-white films. Because of its spectacular appearance, perhaps the best useof a calor schlieren photograph would he for illustrative reproduction in a periodicalor for advertising purposes.

0.00

0.20

0.05

0.30MILLlSECOIiDS

0.15

0.55

Interaction between a shock wave ond aflame In-butane-air mixture ignited ot bottomof combustion chamber! that is initially dis-torted by passage through a wire grid_~Caurlesy G. Markslein, Cornell AeronauticalIcborcrory for Office of Naval Research,Project SQUID.

Turbulenl Bunsen flame, exposure 13 mlcroseconds,-Courtesy BaUell. Memoria/lnsfirure.

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Worm oir currents rising from flngers in stillair. Temperature difference estimated to be100 F.-Courtesy Battelle Memorial Institute.

Typical heated air flow pattem from hotsoldering iron.-Court ••y Battelle MemorialInstitute.

18

J [

:J [

o [::J [

CJ [

w [u c

::J C

[Stream turbulence from four impinging jets ofhelium.-Courtesy Battelle Memorial Institut,;<

[

[ By measuring photographic density at various points in the image of the helium11IIed soap bubble, it Is possible to compute the percentage of helium in the freelyexpanding jet os it mixes with the surrounding oir.-Courtesy The Rand Corporation.

19