Mondal, P K

Sarma, B S

Swamy, C V K

English

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Visualization of refractive index gradients in a fluid-stream by carrier-restoration in dark-ground techniquesB. S. Sabma, P. K. Mondal and C. V. K. SwamyPhysicfi Department, Jawaharlal Nehru Technological University,BEG Campm, Warangal (A.P.){Received 18 April 1973, revised 25 September 1973)The usual sot-up for tli,o dark-p;rmmd tochniqno has boon utilised for visualizing tlio refractive index gradients in a fluid medium. The carrier beam is first suppressed by an opaque stop and is subsequently restored in the field of vi(‘.w due to beam dt f^lection on account of refractive index gradients, ft is shown that a marked improvement in the qualita­tive visualization of the phenomena can be obtained by taking photo­graphs of the image plane. As relatively short exposure times are required for recording this type of transient phenomenon, a 15 mw Spectra Physics laser has been used in our experiment.1. IntroductionWlten a beam of liglit passes through a fluid medium of varying density and therefore of varying refractive index, it is subjticted to phase changes and to small deflections from its original direction of propagation. Various techniques are employed both for (qualitative and for quantitative analysivS of fluid flow to determine the deflections and to ealculaU  ^ the ndractivo index gradients set up within the medium. The Schlieren technique originally proposed by Toeplor (1866), but more popularly known as the Foucault test, is the most common method for displaying tJie refractive iiidex gradients in a fluid stream. This technique is particularly advantageous for measuring large deflections and cal­culating the refractive index variations from them. The shadowgraph mctliod of Dvorak (1880) is (essentially qualitative and exhibits the changes in the density gradient. Finally, there are interferometric methods (Kraushaar 1950, Sten'ott & Erwin 1952) for regions of small gradients, which measure the density variations directly. Technique's for obtaining simultaneous records of a combination of Schlieren and interferometric methods have also been reported (Zobel 1947, Blue & Pollack 1952). All these techniques have widespread applications in diverse fields of aerodynamics, liquid diffusion, sedimentation, combustion re­search, heat transfer etc.The above three fundamental techniques supplement each othor and the ideal procedure will be to use all the three methods to obtain qualitative as well as quantitative information. There are again a few special variations to im­prove the performance of Schlieren systems by the use o f graded filters (NorthIndian J. Phyt. 48, '428-431 (1974)428Visualization of refractive index gradients 4291952), coarse grid (Darby 1946), colour filters (Holder & North 1952), Savart plates (Savart 1840) and phase retardation plates (Zomike 1934). Tn this paper, we describe a qualitative method of visualizing the rofraotivo index gi‘a<lients in a fluid stream. In our experimental set up, the direct light coming from tjie object specimen in the tost space is obstructed by an opaque stop placed in the back focal plane of the imaging lens. Thus in this 4ase, unlike in a pui'o Schlioren system, only the carrier beam and no side band is suppressed. Incidentally, the same method is known as the dark-groimd ziiotliod in microscopy, diffrimoscopy in optical imago formation, carrier suppx'ossion in communication tlieory and removal of zero frequency in spatial filtering tccluiiquos or optical data processing. Tjio carrier beam can bo subsequently restored by setting up a refractive index gradient w]\ich causes a deflection of the direct lij^t from its original direction.2. Principle ok the MethodFigure 1 illustrates the principle of tlie method. A parallel beam of light is allowed to pass througJi the test space T <*ontaini)ig a fluid viz. a liquid of suitableFIG 1Fig. 1. Ilhisiration of the carrier shift duo to deflection.refractive index. In the absence of any refractive index gradient within the medium, the parallel beam of light is focussed by the imaging lens i  to a point 8 in its back focal plane. The point 8  is thus the image of the source of light. I f an opaque stop is placed at 8, no direct light will reach the imago plane. Under these conditions, it is said that the carrier beam is suppressed and only the side­bands which correspond to the liglxt diffracted by the object plane are allowed to pass. In a pure Schlieren system, a knife-edge is placed at S so that the carrier beam as well as side-band on one side are suppressed.Lot now a refractive index gradient bo set up within the tost space in the region indicated by P  by dropping another liquid of slightly different refretive index. The beam of light passing through this region will be deflected from its original direction. There will be a shift of the carrier beam and the image of the source will now be formed at 8\ The carrier beam can thzis bo restored in the image plane due to the variation of refractive index in the object space. This principle is utilised in the following experimental arrangement fpr fluid flowinvestigation.3. Experimental Set upAb a powerful Bonrco of light is more oonveniont for recording this type of phenomena requiring relatively short exposure times, we employed a 15 mw. He-No Spectra Physics laser for our experiment. Figure 2 shows a schematic430 B . S. Sarma, P. K . Mondal and C* V* K . SwamyFIG. 2Fig. 2. Schematic diagram of l.ho oxporimontal set up ; Code ; L-Laser; -fiJ-boam expander;0-collimating lens; T-test space; B-babinet Compensator; -4-Polaroid; /-imaging Ions; jS-Opaque stop; P-Image plane; O-Camera.diagram of the exj)orimental set up. The narrow beam of light from the laser L is expanded by the beam expanding unit E consisting of a microscope objective and a pinlrole. The collimating lens 0 gives a beam of parallel light to iluminato the test space T which consists of a liquid cell containing pure castor oil. The refractive index gradient is set up by allowing drops of glycerine from the top. A Babinet compensator B and a polarioid A are used in between the test space T and the imaging lens L  The purpose of interposing the Babinet eompensator with the Polaroid will be explained later. However, as the laser beam is polarised, a separate polariser is not required before the Babinet compensator. An opaque stop made of an Indian ink dot on a good quality glass plate tested by a Fizeau interferometer is placid at S. A Zeiss recording camera model PP1021 with automatic control for single frame exposure is focussed on the image plane.4. Results and DiscussionsPandya et al (1972) has recently suggested the use of Babinet compensator for measuring the deflections which are related to the refractive index gradients set up in the test space. Wo also chose a Babinet compensator with a different view so that it would be possible to visualize the system of sharp fringes within th© gly- cerine drops whenever th© carrier beam would be restored in the field of view. Thus, whereas their aim was to make a quantitative analysis of the phenomenon by deflection mapping, we hftve turned our attention towards an improvement of its qualitative visualization. To achieve this we focussed the recording earners' on the image plane, whereas Pandya et al focused the camera on the test spaceB, S. Sarma, et al. Indian J. Phys. Vol. 48 No. 5, 1974Pig. 3(a) Photograph of tho imago f)la!U‘ when the carri<‘r b(*am is not suppressed.*,> Voyif't _ ." 1. 1 !■ '* j, ' ' , ’AFig. 3(b) Photograph of the image plane w hen the earner beam is suppressed.3(c) Photograph of the image plane as the glycerine drop enters the field of view.Visualization o f refractive index gradients 431thereby reducing the magnification to a great extent and losing much of the in-formation as to what is happening within tho drop.Figures 3(a), 3(b) and 3(c) show the photographs of the imago plane under different conditions. These photographs were taken on ORWO 35mm films with exposure times of 1/125th o f a second. The Agfa-Gevaort fine grain developer was moderately diluted and the exposed film was developed for 25 minuites at Figure 3(a) shows the photograph of the image plane when the opaque stop is not used. The Babim^t compensator wias rotated about its horizontal axis and was fixed in a convenient position to give an inclined system of fringes. Figure 3(b) shows the photograph of the image plane wlion tho carrier beam is suppressed by placing the opaqxio stop at S. Tha illumination on the imago plane Jias iwidontly decreased due to the obstruction of the carrier beam. The factor by which the illumination on the carrier suppresstxl image plane is reduced, can bo determined from a knowledge of the illuminance of the sourcjo, the height and t]i(^  width of the illuminating beam, magnificantions and focal lengths of the col­limating and the imaging lenses and the area of the opaqxte stop. Figure 3(c) sjiows the photograph of the image plane when the carrier beam is restored as the glycerine drop moves through the castor oil and enters the field of view. The distrotions in the rostoretl fringe system nivoal tho presence of local variations in tho refractive index within the drop5. ConclusionTho conclusion from tho present study is that one can record what is happen­ing in the test space with a marked improvement in its qualitative visualization by taking photographs of tho image plane.AcknowledgmentThe authors wish to thank Prof. T. P. Pandya, Department ov Physics, Luck­now University, for giving some valuable information to them through private communication.ReferencesBlue R. & Pollack J. 1962 Rev, Sci. Inst, 23, 764.Darby P. 1946 NAVORD Report 74-46.Dvorak V. 1880 WiedemanrCa Ann. Phys, Chem, 9, 502,Holder D. & North R. 1962 Nature 169, 466.Kraushaar R. I960 J. Opt. Soc. Am. 40, 480.North R. 1962 Brit. A.R.O. No. 16099.Pandya T. P., Saxena A. K. & Srivastava B. C. 1972 Rev. Sci. Inst. 43, 1186. Savart M. 1840 Ann. Phyaik. 49, 292.Sterret J. & Erwin J. 1962 NACA TN 2827.■Toepler A, 1866 Pogg. Ann. Phys. Chem. 127, 666.Zemike F. 1934 Physioa 1, 689.Zobel T. 1947 NACA TM 1184.

Visualization of Refractive Index Gradients in a Fluid-Stream by Carrier-Restoration in Dark-Ground Techniques

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Visualization of Refractive Index Gradients in a Fluid-Stream by Carrier-Restoration in Dark-Ground Techniques

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