MANY AN experimenter has hankered for a way to record sound waves and radio waves as conveniently as he can record light waves with a photograph. It now appears that all that is needed is a new method of processing a material that has been at hand for many years: Polaroid film. The method was discovered in 1968 by Keigo Iizuka, who was then a lecturer at Harvard University and is now associate professor of electrical engineering at the University of Toronto. His procedure opens up several new fields of experimentation, including microwave holography. Iizuka writes:

"Fields of ultrasonic waves and microwaves have ordinarily been investigated by two classical procedures. The wave pattern can be scanned mechanically by a probe that picks up a signal that varies m amplitude with the energy in each increment of the field. The resulting data are plotted to display the distribution of the energy. Alternatively, the field can be simulated m two dimensions by an appropriately shaped film of moving water. Streamers of dye in the water record the force potentials in the field [see "The Amateur Scientist", SCIENTIFIC AMERICAN, July, 1967]. "Both techniques are tedious and time consuming. Moreover, the accuracy of the scanning technique can be degraded to the extent that the probe disturbs the field. The results of the fluid-mapping technique can be no better than the similarity of the fluid model to the field it represents. In contrast, Polaroid film can be employed to map the fields directly in less than five minutes. The method is both convenient and inexpensive. The intensity of a field can be measured simply by holding the film in the path of the waves.

"The procedure is based on the fact that the rate at which an emulsion develops after it has been exposed to light varies with its temperature or with the degree of mechanical agitation of the reagent. A microwave generates heat and an acoustic wave agitates the reagent. To record a wave pattern the emulsion is exposed uniformly to light of a certain color, precooled to an optimum temperature, coated with developing reagent and immersed in the pattern of waves to be recorded. The emulsion darkens at rates proportional to the local heating or agitation to form an image of the wave pattern. Development is interrupted when the darkest parts of the image reach maximum density.

"Polacolor Type 58 film, which is designed for making four-by-five-inch color pictures, is particularly appropriate for recording fields of sound and radio waves because the interval required for developing the resulting image varies with the color of the light to which the film is pre-exposed. The relation between development time and color arises from the structure of the negative. The negative consists of three major layers of dyed emulsion on a plastic base [see illustration lower left]. After exposure to light the film pack is drawn between a pair of rollers that break a pod containing developing reagent in the form of a viscous jelly. This action also distributes the reagent as a thin film between the negative and positive emulsions.

"The layer of the negative emulsion that contains yellow dye is in intimate contact with the positive emulsion. The magenta and cyan layers lie progressively deeper m the sandwich. The time required for the reagent to reach each layer of dye and migrate to the surface for transfer to the positive emulsion varies with the depth of the layer.

"A negative emulsion that has been pre-exposed to yellow light develops in less time than one exposed to magenta or to cyan. For this reason I pre-expose with dark blue when photographing weak fields of microwaves (fields on the order of 60 milliwatts per square inch at a frequency of from one to 10 gigahertz). Conversely, pre-exposure to yellow light is used for relatively strong fields.

"Pre-exposures can be made with any camera that fits the Model 545 Polaroid four-by-five-inch Land film holder. The camera is focused on a screen of white paper that is uniformly illuminated from the sides by a pair of carbon arc lamps. I check the intensity by supporting a Kodak Neutral Test Card at the position of the screen. With an exposure meter I measure the light reflected by the card. The arc lamps are adjusted for a reflected intensity of approximately 50 footcandles. The camera is equipped with a holder for supporting Wratten light filters.

"To make a pre-exposure I set the lens opening at f/9.5 and adjust the shutter for an exposure of 1/2 second. I use a No. 13 Wratten filter for yellow and a No. 35 for magenta. Cyan is made by double exposure. In my experiments I first expose through a No. 47 Water filter (blue) for l/10 second at a lens opening of f/9.5 and then through a No. 61 (green) for 1/5 second at the same lens opening.

"After making the pre-exposure I cool the film either with dry ice or with an instant-freeze aerosol can of Freon. During subsequent handling in air the film warms to a temperature of about 25 degrees Fahrenheit. That is the optimum temperature at which to make exposures of radio or sound waves.

"A small box m which the film can be refrigerated with dry ice can be made from a sheet of foam plastic. Make the box somewhat larger in area than the film. Cut a rectangle of plastic to serve as a cover. Put crushed dry ice in the box. Place the film on top of the ice. Close the box with the cover for a few minutes. Care must be taken to keep the portion of the film packet that contains the reagent pod outside the box. The reagent solidifies at 32 degrees F. Do not freeze it!

"Alternatively, the film pack can be cooled by spraying it with Freon. Aerosol cans of the refrigerant, such as those manufactured by the Cryokwik Company for freezing biological specimens, are available for about $1 per container from dealers in biological and pharmaceutical supplies. Avoid spraying the reagent pod.

"I have experimented with several types of black-and-white Polaroid film. Types 52, 57, 55-P/N and 107 work, but they are less sensitive than color film. In cases where interest is confined to a small area of the field the Polaroid eight-exposure color pack can be used to advantage. The image area of this film measures 3-1/4 by 4-1/4 inches. The pack is less convenient to use than the individual packs of four-by-five-inch film because it is difficult to cool the film without freezing the reagent pods that are encased in the pack. Microwave fields of larger area can be mapped with Polaroid Radiographic Packet Type TLX. This emulsion was developed for X-ray work, but it responds well to heat or agitation induced by microwaves or ultrasonic waves. The image area measures 9-3/8 by 10-1/2 inches.

"The pre-exposed and cooled film is placed in the Land film holder and pulled through the steel rollers that break the pod and spread reagent between the negative and positive layers. The pack is promptly removed from the film holder and inserted in the field to be photographed. In the case of microwaves the component of the electric field in the plane of the emulsion generates heat in the silver halide by means of induced current. The current raises the temperature of the emulsion in proportion to the square of the field intensity. A thermal field thus appears in the film. It is a replica of the intensity distribution of the electromagnetic field.

"The localized heating increases the localized rate at which developing reagent diffuses to grain sites of the silver halide and in addition accelerates the chemical reactions of the development. If development continued to completion, the film would turn uniformly dark. The chemical action is interrupted at an intermediate stage by stripping the positive emulsion from the negative emulsion. Development stops at once. The positive emulsion is then insensitive to light. The proper interval of exposure to microwaves or sound waves must be determined experimentally because it depends on the strength of the field.

"Most of my experiments have been made with microwaves, the field of my primary interest. In a typical experiment two beams of microwaves that vibrate at a frequency of about eight billion (8 X 10) cycles per second are projected from a pair of horns. The beams cross at right angles and interfere at the zone of intersection.

"The film pack is placed in this zone at an angle of 45 degrees with respect to each of the beams [see illustration, left]. The pattern of standing waves that results from the interference is a grid made up of alternate strips of warm and cool emulsion. The developed image consists of alternately dark and light fringes comparable to the fringes that appear when two beams of light similarly interfere.

"I have made a microwave hologram of a small coin and a triangle of metal inside a leather purse. Three steps are involved in the technique. The purse is illuminated by microwaves that vibrate in the vertical plane. The waves are projected by a horn, as in the interference experiment. The energy is generated by a klystron oscillator at a frequency of 34.26 gigahertz (equivalent to a wavelength of 8.756 millimeters). The oscillator develops an output power of 10 watts.

"The beam of microwaves is directed at a right angle to the plane of the purse. The pack of Polaroid film, prepared by pre-exposure to cyan light, is placed directly behind the purse at an angle of 45 degrees with respect to the plane of the purse. The time of exposure to the microwaves ranges from 45 to 60 seconds.

"The developed image displays a series of interference fringes. They constitute a hologram of the purse and its contents. The hologram cannot be used directly for reconstructing the image of the object with microwaves because the positive print is transparent to electromagnetic radiation.

"To reconstruct the image a copy of the hologram is made m metal. I use aluminum foil reinforced by a cardboard backing. Slits are cut from the foil that correspond to the darkest portions of the fringes of the image, as judged by eye. A more accurate copy can be made in metal by substituting copper foil for the aluminum and utilizing the photoetching technique employed for making halftone engravings. Replicas made of aluminum foil by hand are adequate, however, for illustrating the procedure.

"The holographic image is reconstructed by illuminating the metallic hologram with the same beam of microwaves that was used to make the original hologram. The image can be made visible by either of two techniques. It will appear on a liquid-crystal film that is placed behind the metallic hologram. The liquid-crystal film consists of four layers: a Mylar sheet, a radio-frequency absorbing layer, an active layer of cholesteric material and a top surface of polythene sheet. A water-soluble carbon paint applied to the back of the liquid-crystal assembly was found to be suitable for absorbing radio-frequency energy.

"The reconstructed image is reasonably good but, as one might expect, the resolution is inferior to that of holograms made with the far shorter wavelengths of coherent light. The image can also be made by substituting sensitized Polaroid film for the liquid crystal, although the film is less sensitive. Even so, the faint image is sufficiently recognizable to demonstrate that microwave holograms can indeed be made by this technique.

"It is well known that a field of sound waves accelerates the development of photographic emulsions. Localized vibration in the emulsion increases the rate at which the developing reagent migrates to the sites of silver halide grains, thus speeding the chemical development in regions of high sound intensity. The effect can be demonstrated with any device capable of generating a loud monotone for a few minutes.

"Interesting patterns can be made in a small area with an ultrasonic generator. For example, I generate sound waves about a third of an inch long with a piezoelectric transducer of the ceramic type that is coupled to an electroacoustic horn [see illustration, right]. The transducer operates at a frequency of about 34,000 vibrations per second and develops a sound volume of 160 decibels at the mouth of the horn. The wave pattern is recorded by placing the sensitized film over the mouth of the horn. Clear images of the acoustic field resulted from exposures of 75 seconds for Type 58 Polaroid film and 10 seconds.

"The minimum intensity required to register an image is about 80 decibels. Similar experiments have been made successfully at much lower frequencies. For example, I have mapped the field intensity pattern inside an acoustic resonator that was generated by driving an ordinary loudspeaker with a signal of six watts at a frequency of 315 cycles per second.

"It is apparent that a pattern of wave interference could be recorded by superposing a reference field of sound waves on waves from a primary source. This method would be useful as a new means of visualizing acoustic fields as well as a possible method of making acoustic holograms. The relative insensitivity of the film to sound would seriously limit the usefulness of the technique, however, unless the experimenter had access to an acoustic source of high power.

"Numerous experiments can be undertaken to demonstrate the sensitivity of the pre-exposed films to heat. For example, the distribution of heat within the flame of a candle can be mapped by holding the sensitized pack vertically in the flame for a few seconds. The temperature distribution within the flame generates a corresponding distribution within the emulsion. To make a photograph of a candle flame I pre-exposed the film to magenta light. The bottom portion of the flame did not register in the image because it was in the margin of the film.

"A hot object, such as a soldering iron, can be photographed by focusing the heat rays on the film with a parabolic reflector [see illustration, above left]. The contrast in the resulting image can be improved by inserting a heat shield between the hot source and the film. The shield prevents unfocused rays from reaching the sensitized emulsion.

"The presensitized film will respond even to mechanical vibrations. This effect can be demonstrated by attaching the film to a Chladni plate with fast-curing cement and striking the plate with a hammer [see illustration at right]. The experiment works best with a plate of aluminum at least 1/4 inch thick. Regions in which the plate vibrates at maximum amplitude appear as dark bands in the resulting image."

Bibliography

X-BAND HOLOGRAPHS. R. P. Dooley in Proceedings of the IEEE, Vol. 53, No. 11, pages 1733-1735; November, 1965.

A METHOD FOR PHOTOGRAPHING MICROWAVE WITH A POLAROID FILM. Keigo Iizuka. Harvard University Technical Report No. 558. March, 1968.

OPTICAL FILM SENSORS FOR RF HOLOGRAPHY. H. E. Stockman and Berthold Zarwyn in Proceedings of the IEEE, Vol. 56, No. 4, page 763, April, 1968.

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