WHEN a moving object is photographed in ordinary light by time exposure, the resulting image is more or less blurred. Usually such photographs are of little interest to experimenters even though the fuzziness of the image varies with the amplitude of the movement. A comparable photograph made with coherent light, however, yields an accurate measurement of the displacement of the object. For example, a photograph of a vibrating tuning fork that is made with the coherent light of a helium-neon laser can be used for determining to less than a thousandth of an inch the amplitude of movement of the tines.

Displacements can be measured by making a double exposure of an object on a single sheet of film. One exposure is made before the object moves and one is made after it moves. The technique, which is known as speckle interferometry, can also be used to map local deformations in stressed mechanical parts such as the components of telescope mountings, seismometers, optical benches and similar devices. Ulrich Köpf of Fleischmannstrasse 7 in Munich explains the procedure as follows:

"Speckle interferometry resembles conventional holography in some respects but is much simpler to do. LeRoy D. Dickson has presented in these columns several ingenious hints for reducing the sensitivity of holographic apparatus to vibrations [see "The Amateur Scientist; Scientific AMERICAN, July, 1971]. Nevertheless, good holograms are not easy to make. In contrast, a reasonably careful beginner who undertakes an experiment with speckle interferometry can expect success on the first try.

"The technique is based on the speckling or granularity that appears when laser light is reflected by a dull surface such as a painted wall, a sheet of white paper or a roughened piece of metal. The uniform illumination produced by an incoherent source, such as an incandescent lamp, is replaced by a pattern of dazzling granules when a coherent source is used. Each speckle marks a point of constructive interference between waves of coherent light. The speckles are of high contrast. The diameters of the speckles vary statistically, but the mean diameter is determined by the resolving power of the optical system with which the speckles are observed and is approximately equal to the product of the aperture of the pupil of the eye (in the case of a camera, the focal ratio, or f number) multiplied by the wavelength of the light. The wavelength of the coherent light emitted by a helium-neon laser is 6.33 x 10 meter (6,330 angstroms).

"The pattern of speckles is caused by the diffuse surface texture of the object. When the surface is displaced at a right angle to the direction from which it is viewed, the pattern of speckles moves with it, much as if the speckles were minute spots of paint. The pattern can be recorded on photographic film as a double image. The amplitude of the displacement can be determined by examining the pattern of speckles.

"With these principles in mind, one begins the experiment by flooding an object with the expanded beam of a helium-neon laser. The object might be a simple metal clamp similar to the one shown in the accompanying illustration [at left]. The object can be supported by any reasonably stationary base, such as a brick. The beam of laser light can be expanded into a cone with a lens that has a focal length of some four to eight millimeters. I use a 40 power objective lens from a microscope, but a simple plano-convex lens will work as well. The distance between the laser and the object is not crucial, but it should be adjusted so that the cone of coherent light floods all parts of the object. The lens can be held in position by any improvised support that is reasonably rigid. The camera is placed at a distance where the sharply focused image of the object occupies a substantial portion of the photographic negative. The negatives should be of the high-resolution type. I use four-by-five-inch Agfa Gevaert Scientia plates, which are similar to Eastman Kodak 649F plates.

"My exposures are made with a studio camera. After the camera has been focused the aperture is set at a focal ratio, or f stop, that varies according to the resolving power of the photographic emulsion and is equal to 1 divided by the product of the resolving power of the emulsion multiplied by the wavelength of the coherent light. (The resolving power of the emulsion is specified by the manufacturer.)

"The exposure interval is determined by flooding the object with laser light and measuring the intensity with a commercial exposure meter. Make the first exposure. Displace the object or otherwise alter its position in whole or in part at a right angle to the direction of the camera. With an object such as the clamp used in my experiment the metal can be warped by heating one side of the device with a small torch. The second exposure is then made on the same negative. The negative records two displaced but otherwise identical patterns of speckles. The negative is developed and fixed according to the procedure recommended by the manufacturer.

"The developed plate resembles an ordinary photographic negative, but the exposed area shows speckles. To detect the displacement and measure its amplitude the experimenter must analyze the complex speckle pattern into its local components by means of a Fourier transform. Do not be dismayed by the prospect of having to carry out this mathematical procedure with paper and pencil. It can be accomplished by an inexpensive analogue computer: a simple lens.

"To make the analysis support the film in the vertical plane by an improvised clamp and direct the laser beam through part of the image. Intercept the transmitted beam by the transformation lens adjusted to focus an image on a distant screen [see illustration at left]. The image consists of parallel fringes of light and shade. The spacing of the fringes varies inversely with the displacement of the object at the point on the negative through which the laser beam is transmitted.

"Measure the distance (d) between any pair of adjacent fringes. It is easy to calculate from this known distance the displacement (s) of the object. The calculation includes the wavelength of the laser light (l), the focal length of the transformation lens (f) and the magnification of the photographic image (m). Usually the image is smaller than the object, hence the magnification is represented by a fraction. To determine the magnification divide the width of the image, as measured with a ruler, by the width of the object. The amplitude of the displacement is calculated by means of the formula s = l x f / m x d.

In my experiment with the metal clamp the laser beam was directed through the image at two points: the upper corner, where maximum displacement was expected, and at the thin edge on one side [see illustrations on page 00]. Fringes associated with the upper corner of the part were spaced 6 x 10^-3 meter apart. Fringes associated with the thin edge measured 3.8 x 10^-2 meter. The focal length of my transformation lens was 3 x 10^-1 meter and the magnification of the image was 0.7. The displacement of the upper right corner of the clamp was therefore 6.33 x 10^-7 x 3 x 10^-1 / 0.7 x 6 x 10^-3 0.0045 centimeters or 0.0018 inchs. The displacement of the edge of the clamp was similarly calculated: 0.00071 meter, or 0.00028 inch.

"The method is particularly easy to apply in the case of vibrating objects such as a tuning fork because the motion can be photographed by a single exposure. The amplitude of vibration can be measured throughout the length of the vibrating part by directing the laser beam through the photographic negative at a series of points. The laser need not be adjusted for operation in a single mode, as in making holograms, but can oscillate simultaneously in many transverse modes. Local deformations in an object that is stressed but not moved between two exposures are measured by scanning the photographic negative with the laser beam and monitoring the changing fringe distances and directions.

"Displacements occur at right angles to the direction of the fringes. Incidentally, if the transformation lens is removed, the fringes can still be observed provided that the diameter of the beam is less than the fringe separation."

A warning: Laser light is hazardous, particularly to the eye. Never look into the beam. When you are using a laser, confine the beam and all targets within an opaque housing.

ROGER BAKER (Box 7854, University of Texas Station, Austin, Tex. 78712) submits a simple apparatus for viewing images formed by infrared radiation. In principle his device resembles the Czerny evaporograph that was developed at the University of Frankfurt before World War II. In the Czerny apparatus a lens of rock salt focuses an image in infrared radiation on the blackened side of a plastic membrane that is enclosed in a sealed box from which air can be pumped. Rays from the lens enter the box through a window of rock salt. Volatile oil in a heated container at the bottom of the box condenses on the opposite surface of the membrane in the form of a transparent film of iridescent color. Local differences in the temperature of the membrane induced by the focused radiation cause the oil to evaporate from the film nonuniformly.

"The evaporation alters the thickness of the oil film in a pattern that corresponds to the image, which then appears in contrasting color. The colored image can be observed visually or photographically. An exposure of about 10 seconds registers the image of a human being after sunset at a range of 100 yards. Images can be erased by altering air pressure inside the box and thus controlling the rate at which oil is exchanged between the reservoir and the membrane. Baker's evaporograph is much simpler to construct and operate. He discusses it as follows:

"The device consists of a wide-mouthed jar partly filled with ink stained water and closed by a diaphragm of thin plastic of the kind used for wrapping food. Like the Czerny evaporograph, the device can be used to record and display images formed by heat. Water vapor condenses on the inner surface of the plastic cover. The opaqueness of the condensation varies with its thickness and hence with the temperature of the plastic. An image in infrared radiation that is focused in the plane of the plastic becomes visible in the condensation as a negative image when viewed against the background of water colored with ink. The image can be erased by tipping the jar to wash away the condensation.

"The sensitivity of the film to small changes in temperature varies with the way the device is used. For maximum sensitivity I partly fill the jar with a mixture of black ink and water at room temperature and let it stand until the temperature of the solution reaches equilibrium with that of the room. I then cool the metal handle of a kitchen knife or a rodlike piece of metal in ice water and pass it slowly across the top of the jar about an inch above the plastic. Currents of cold air flow down from the metal and cool the plastic to form an even coating of condensation. The temperature of the film changes only slightly because heat liberated by the condensing vapor is largely absorbed by the plastic.

"As in the case of photographic emulsions the density of the condensed film of water droplets varies with both temperature and time. For example, a film of plastic that is cooled one degree Celsius for one second will condense a given amount of water. The same amount of water would condense if the film were cooled .1 -degree C. for 10 seconds or 10 degrees C. for .1 second. Conversely, the identical quantity of water will evaporate from the plastic if the temperature is raised .1 degree C. for 10 seconds or .01 degree for 100 seconds.

"The film tends to oppose any change in temperature. The sensitivity of the device to temperature changes in either direction can be demonstrated by coating the film as described and passing a finger lightly but rapidly across the top of the plastic. You can hardly stroke the plastic quickly enough to avoid making a visible streak across the fogged surface where the flow of heat from you finger evaporated the water.

"Removing heat from the plastic similarly encourages condensation. Drop a cool penny or something like it on the plastic. If the plastic is stretched tightly like the head of a drum, and is incline slightly, the coin may bounce rapidly across the surface. Each bounce will deposit an image of the coin, including many details of the bas-relief.

"The condensation will store an image for 10 minutes or more if the room temperature is fairly constant, although the droplets gradually increase in size As the droplets grow, color will appear as a result of optical refraction. The vividness of the color depends somewhat on the angle from which the image is observed.

"The plastic film is transparent an therefore relatively insensitive to infrared radiation. The sensitivity can be increased by covering with a second film that has been blackened on one side with a coating of soot. I deposit the soot by wrapping a clear sheet of wet plastic tightly around a smooth bottle filled with ice water and passing it through the flame of a candle. The layer of water between the plastic and the glass increases the transmission of heat through the layers and prevents the plastic from reaching its melting temperature. Pinholes will appear in the plastic if it overheats. The smoked plastic is stretched across a supporting ring, such as a small embroidery hoop, and placed black side up over the jar. Air pressure in the jar causes the plastic cover to bow upward slightly, which ensures intimate contact between the two films.

"I have recorded impressions of distant incandescent lamps and other hot objects by focusing the rays on the blackened film with an ordinary magnifying glass. The image is observed by removing the blackened film. The images have not been of high quality. Glass is rather opaque to infrared radiation. I cannot afford a lens of rock salt.

"A concave mirror with a reflecting front surface would work well. I tried to make one of gold resinate, but it cracked as a result of my impatience during the firing procedure. Still, the method is sound. Gold resinate is available from dealers in ceramic supplies. A brilliant front-surface mirror can be made by flowing a thin layer of the resinate over a clear concave lens and baking it. Inexpensive lenses of this type up to two and a half inches in diameter are available from the Edmund Scientific Co., Barrington, N.J. 08007.

"Flow a puddle of the resinate over the clean glass. Drain off the excess by letting the glass stand coated side down on a paper towel. Dry the coating under a 100-watt desk lamp or an equivalent heater. As the coating dries, carefully remove the bead of resinate from around the edge of the lens. Bake the resinate by wrapping the lens loosely in aluminum foil and heating it to approximately 500 degrees C. Cool slowly. For a baking oven I use a covered skillet on a stove. Avoid my mistake of uncovering the lens before it has cooled to room temperature!"

AT the beginning of this century the Russian botanist Michael Tswett performed an experiment that was destined to revolutionize the technique of separating mixtures of organic substances into their pure constituents. He extracted a mixture of plant pigments with petroleum ether and poured the solution into a glass tube that had been firmly packed with calcium carbonate. As the fluid trickled through the adsorbent powder each pigment migrated at a characteristic rate. Ultimately the moving pigments separated into distinctly colored zones. By continuing to pass solvent through the powder Tswett could wash the zones completely through the column and recover the pure compounds sequentially. In Tswett's words: "Such a preparation I term a chromatogram, and the corresponding method, the chromatographic method."

Tswett explained that a characteristic force of attraction exists between the particles of each pigment and the particles of calcium carbonate. As a result some pigments are strongly adsorbed by the calcium carbonate and others are adsorbed less strongly. Thermal agitation periodically dislodges the adsorbed particles from the powder. Dislodged particles are swept downstream, readsorbed, dislodged again and so on. A particle that is weakly adsorbed is dislodged more frequently than one that is strongly adsorbed. It therefore migrates through the calcium carbonate at a proportionately higher net velocity than the more strongly adsorbed particle. As Tswett anticipated, the chromatographic technique has been applied for separating organic substances of all kinds.

To assist students in visualizing the mechanism of chromatographic analysis at the molecular level, Ray Parman, Jr., of Central State University in Edmond, Okla., recently developed an apparatus that displays by analogy a greatly magnified version of the action in two dimensions. The apparatus consists of an inclined trough with a flat bottom that is studded with a pattern of protruding brads in hexagonal array [see illustration at left]. The trough functions as the adsorption column. The particles to be separated consist of steel balls in two or more sizes. As the balls roll down the trough they collide with the protruding brads at a frequency determined by the diameter of the balls and the spacing of the brads. The effect is analogous to periodic adsorption. The smaller balls participate in fewer collisions than the larger ones and so migrate at a higher net velocity. As in Tswett's apparatus, the balls become segregated in zones and emerge from the bottom of the trough in lots according to size.

Parman's trough is 36 inches long and two inches wide. It is made of hard fiberboard 5/8 inch thick. As a guide for placing the brads Parman glued graph paper to one side of the strip and coated the paper with clear varnish. Brads were driven into the strip on centers spaced one centimeter apart. Alternate rows were staggered by five millimeters. Parman used 3/4-inch brads driven vertically so that 7/16 inch was exposed. Strips of masking tape 3/8 inch wide were woven between brads at the edges of the array to form the sides of the trough. A few brads were omitted from the pattern at the top of the array to prevent the balls from jamming as they enter the column.

A supplier of balls can be found in larger communities by referring to the classified telephone directory under "Bearings." Parman experimented with balls of four sizes: 1/4, 7/32, 5/32 and 3/32 inch in diameter. The quarter-inch balls cost two cents each; the smaller balls were one cent each. At a brad spacing of one centimeter the cleanest separations are observed with a mixture of 1/4-inch and 3/32-inch balls. A batch of approximately 20 balls can be processed at one time without overloading the column.

Bibliography

ELEMENTS OF INFRARED TECHNOLOGY: GENERATION, TRANSMISSION, AND DETECTION. Paul W. Kruse, Laurence D. McGlauchlin and Richmond B. McQuistan. John Wiley & Sons, Inc., 1962.

LASERS: GENERATION OF LIGHT BY STIMULATED EMISSION. Bela A. Lengyel. John Wiley & Sons, Inc., 1971.

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