halide with silver ions that can modify image-forming dyes. The solubility and thus diffusivity of the dyes changes locally according to the local levels of these silver ions and in turn controls the amounts of dye that can diffuse to the image-receiving layer, and in which places. The dyes then diffuse through the stack driven by electrostatic and chemical gradients, and deposit into the image-receiving layer. The reagent allows for the dyes to move up the stack via diffusion. Dye release fees dye molecules to move up the stack via diffusion into the image-receiving layer. Additive color film Additive film (such as Polavision and Polachrome slide film) uses a color mask of microscopically thin transparent red, green, and blue lines (3000 lines per inch) and a black and white emulsion layer to reproduce color images in transparency film. The resulting dye developers (unexposed emulsion) block the colors not needed and project the color or combination of colors which form in the resulting image. Since the lines are so close to each other, the human eye easily blended the primary colors together to form the correct color, much like an LCD display or television. For instance, a photo of a yellow flower would expose the emulsion beneath the red and green masks and not the blue mask. The developing process removed the exposed emulsion (under the red and green masks) and diffused the unexposed dye developer (under the blue mask) to its receiving layer, blocking light from coming through. This resulted in the projected light shining through the red and green masks but not the blue mask, creating the color yellow. Because of the film density, film speeds were necessarily slow. High precision was required for the production of this film. Film brands Polaroid Polaroid Corporation invented and produced the widest range of instant film. Roll film was distributed in two separate negative and positive rolls and developed inside the camera. It was introduced in 1948 and was manufactured until 1992. Sheet film was introduced in 1958 for 4x5" film holder #500. Each sheet contains a reagent pod, negative and receiving positive, and was loaded separately and developed outside the film holder. In 1973 Polaroid introduced 8x10" Instant film. Pack film was distributed in a film pack which contained both negative and positive sheets and was developed outside the camera. It was introduced in 1963. Integral film is also distributed in