Edwin Land and Instant Photography - American Chemical Society

On June 17, 1970, Edwin Land patents the SX-70 camera. From the article:
"press the camera button and watch as the image developed before their eyes.

Until this point, Polaroid films required a step that interfered with Land’s vision of absolute one-step photography: After being ejected from the camera, the user had to peel back the negative sheet to reveal the final photograph. Some early films required additional steps by the user, such as swabbing the developed image with a coating to stabilize it or adhering the image to a hard backing to prevent curling.

The development of the SX-70 and its film required a complete reformulation of the Polaroid system. Above all, the film was integral, meaning that the negative, positive, and developers were all contained within a film unit and would remain there after developing. To accomplish this, the positive layers had to be transparent to allow light to penetrate them and expose the negative, below. Polaroid’s solution was to develop a chemical opacifier—a chemical screen that was clear when undeveloped but opaque immediately when ejected from the camera and able to become clear again to reveal the developed image.

Minimizing the complexity of the undertaking, Land described the project to The Photographic Journal in 1974:

“It is an interesting experience to see how all of Absolute One-Step Photography can happen very simply if it happens sequentially, involving both the camera and film in some two hundred to five hundred steps…

“When the film is ejected potassium hydroxide in a few drops of water is spread in a layer 26/10,000 inch thick and ‘all hell breaks loose,’ but in a much more orderly way than that phrase implies. For several minutes chemical reactions occur rapidly one step after another in that thin sandwich and then this progression slowly stops. There is peace again and the picture is complete.”

The simplicity of the SX-70 system for photographers belied its technical complexity. Within the 2 millimeter thick film unit was a sandwich of thin polymer sheets, a positive image-receiving sheet, reagent, timing and light reflecting layers, and the tri-color negative—17 layers in total. The camera itself was a remarkably sleek design. When it was first sold in 1972, the product represented the culmination of Land’s 1943 dream of absolute instant photography."