Sudden Color Film

It is 14 years since the first Polaroid cameras began developing and printing their own black-and-white snapshots in a matter of seconds. Though photographers have been yearning ever since for someone to produce an equally swift, self-processing color film, most chemists agreed that the job was incredibly difficult. It seemed improbable that it would ever be accomplished.

But the very complexity of the problem was what appealed most to Dr. Edwin H. Land and his colleagues at the Polaroid Corp. in Cambridge, Mass. This week they began to market the improbable. Polacolor, a self-processing color film. Now, just 50 seconds after the snap of a shutter, a surgeon can record a sharp color shot of a delicate operation; an alert military reconnaissance pilot can produce a revealing picture of an enemy operation; a doting parent can turn out a portrait of his child in remarkably accurate tints.

Linked Molecules. The new color film can be used in most Polaroid cameras, but it depends on new chemicals, designed to work with the precision of molecular machines. There are three layers of emulsion containing fine, light-sensitive grains of silver halide. The grains in the top layer are sensitive to blue light; those in the middle are sensitive to green; those in the bottom layer are sensitive to red. When a many-colored picture is focused on the film, the blue, green and red components of the light that has entered the camera form three latent (undeveloped) images on the three layers of silver halide.

Conventional color films work in much the same way. But just below each layer of Polacolor's silver halide is a layer containing strange double molecules synthesized by Polaroid's chemists. The molecules are shaped roughly like dumbbells. Each of them has at one end a submolecule of photographic developer. At the other end is a submolecule of brilliantly colored dye. Connecting the dye and developer is a strong chain of carbon atoms.

While the film is dry, the linked molecules remain quiescent, but after the picture is snapped, a pair of rollers in the camera breaks a pod of thick, alkaline liquid and spreads it evenly over the film. The liquid penetrates quickly through the layers, waking the linked molecules to active chemical life. They start moving, and most of them eventually touch a grain of silver halide in the nearest light-sensitive layer. If that grain has been exposed to light, it is ready for action. It grabs the developer end of the molecule, holds it tight, and uses it to turn the silver halide into metallic silver. This develops the images in the three light-sensitive layers, and it also immobilizes the linked molecules that have taken part in the developing process. Only the molecules that have not been captured by exposed grains of silver halide can continue to move through the wetted film.

This is the secret of Polacolor. The three superimposed images--blue, green and red--capture developer molecules with dyes of appropriate color attached to them. In spots on the film that have been exposed to blue light the silver halide grains in the top layer capture and hold all the yellow dye, which lies in the layer just below. Since no red or green light has reached this part of the film, the magenta and cyan dyes in the deeper layers are free to move to the surface. Acting together, they make a spot of blue.*

The same molecular machinery produces the other colors. When green light from foliage forms a latent image on the green-sensitive layer, the magenta dye, which is nearest that layer, is captured. The other dyes, yellow and cyan, are free to go to the surface and become the green leaves in the finished picture. Similarly, yellow and magenta make red. Intermediate colors form at places where the images overlap weakly, allowing fractional amounts of dye to escape. White light in the picture (such as a cottony summer cloud) makes exposed spots on all three layers, capturing all the dyes and leaving the finished picture white. When all three dyes reach the surface, they form spots of black corresponding to parts of the film that have received no light at all.

Tough Picture. When the released dyes reach the surface, they hit a sheet of white paper coated with large, stationary molecules of an acid material. These clutch the dyes as they arrive and form them into a tough, many-colored surface that reproduces the colored image focused by the camera's lens. The picture needs no further treatment. Its blues are sometimes slightly greenish at first, but after a few moments the excess green tint disappears permanently.

Polacolor is not entirely foolproof. For one thing, the user must take some account of temperature, both when snapping a picture and developing it. This is presumably why the new film is first being introduced in Florida; it will not be sold in the north until the weather warms up. With elementary care, though, any amateur should be able to take good pictures with Polacolor.

Among the big users of Polacolor will be industrial and scientific laboratories, which often need to take quick color shots of a fleeting stage in a process or experiment. But of all Polacolor's potential users, it is the military from whom Chemist Land may get his largest orders. The ability to photograph the enemy in color and see the picture almost immediately will be of enormous advantage in many dangerous situations. No enemy of the U.S. is likely to enjoy this advantage for years; in spite of frantic efforts, says Land, the Russians have not yet succeeded in copying even black-and-white Polaroid film.

* The dyes used are subtractive colors, each of which transmits about two-thirds of white light. The yellow transmits the green and red components, blocking blue; magenta transmits red and blue; cyan transmits blue and green. When two of the colors overlap equally, they produce the color that is common to both. Cyan and magenta give blue; cyan and yellow give green; magenta and yellow give red.

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