International projectionist (Jan 1963-June 1965)

Of prime importance to the projectionist: Characteristics of film Of prime importance to the theatre projectionist are the nature and working qualities of the film he projects. And by the term film we refer to perforated and processed motionpicture positive stock which consists of a plastic base coated with one or more thin layers of gelatine emulsion. Now, the trems used here are strictly projection terminology inasmuch as "film" and "emulsion" have somewhat different meanings to the photographic engineer. The word "film" in photographic jargon refers only to the hardened gelatine coating which carries a photographic image in the form of dispersed silver or salt grains, or as colored organic dyes. This "film" may be coated upon glass, paper, or transparent plastic; and to facilitate the formation of a coating of uniform thickness, it is applied not directly to the supporting material, but to a "substrate," or thin bonding layer, of clear pre-coated gelatine. The substrate and gelatine film, taken together, are what the propectionist calls "emulsion" ( as when he speaks of scratches in the emulsion, or of scraping off the emulsion preparatory to splicing a print ) . To chemists, however, an emulsion is a uniform dispersion of a very finely dividen liquid or sold in a liquid — a milky suspension of microscopic silver bromide grains in a solution of gelatine for example. Such a solution-like emulsion of light-sensitive silver salts hardens to a pellicle — or photographic film — when spread out upon a smooth support material and allowed to "set" in the dark. An actual photographic emulsion is made by adding solutions of silver nitrate and sodium bromide to a warm solution of gelatine. A chemical reaction occurs in which these PROPERTIES OF DIFFERENT TYPES OF MOTION-PICTURE FILM PROPERTY OF BASE OR FILM 5-mil CELLULOSE NITRATE 5i-mil CELLULOSE DIACETATE 5i-mil CELLULOSE ACET0PR0PI0NATE 5i-mil CELLULOSE TRIACETATE ifmil CR0NAR Tensile strength (lbs/in2) 15,000 10,000 1 1 ,000 13,000 16,000 Elongation at break {%) 10 20 10 5 70 Tear strength (grams) 65 ^5 55 60 180 Rigid ity (105 lbs/in2) 6.5 lf.0 k.2 5-3 5-5 Flexibility (folds) 16 7 16 15 20,000 Swelling, 30 min water 0 17 0.52 0A7 0.37 0.07 Per cent shrinkage: 2 yrs. storage 2 yrs projection use 0.30 0 60 0.!*0 1.5 0.35 1 . 0.25 o.to 0.05 0.05 Decomposition temp F ( ) 300-380 375 400 h-50 700 Focus drift Moderate Great Great Moderate Slight Buckling effects Slight Great Great Moderate Slight Frame embossing Slight Moderate Moderate Moderate Slight Brittleness tendency Slight V. great Great Moderate None Rel projection life 1200 t+oo 600 1000 10,000 Solubility in solvents: Alcohol & ether mixt sol si. sol. insol. insol. insol. Acetone sol. sol. sol. si. sol. insol. Methyl acetate sol. si. sol. sol. si sol. insol. Isoamyl acetate sol. insol. insol. insol. insol. Acetic acid (glacial) sol. sol. sol. sol. insol. Dloxane sol. sol. sol. sol. insol. Chloroform insol. si. sol. si. sol. si. sol. insol. Methylene chloride si. sol. sol. sol. sol. insol. two salts are converted to the desired insoluble silver bromide and to soluble sodium nitrate, a by-product. Smal lamounts of chlorides and iodides may be introduced into the gelatine solution to make emulsions of special photographic properties, while panchromatic and infrarad negative emulsions require the addition of sensitizing dyes. Without these special dyes, the film would be sensitive only to blue, violet, and ultraviolet rays, and all yellow, orange, and red objects would photograph as black! To make negative emulsions "faster," or more light-sensitive, they are ripened for prescribed periods of time at high temperatures before being coated upon the base material. Because the ripening process also increases the size of the silver-salt grains, the fastest films are inclined to give "grainy" images. The positive emulsions employed for releaseprint films are neither sensitized to the longened wave lengths of light nor ripened for increased photographic speed. They are thus "slow" and red-blind, but also remarkably fine-grained. Gelatine, so necessary to the manufacturer of photographic films, is an unusual substance. It can absorb (FIG 1) large amounts of water without dissolving. The gelatine merely swells. Up to a limit, it can be repeatedly melted by warming and "set" by cooling. Contrary to popular opinion, gelatine does not occur in nature. It is a derived protein made by chemical treatment of collagen, the principal protein of the connective tissue found in muscle, hide, and the ossein of bones. Gelatine contributes to the photographic sensitivity of the silver bromide crystals, and it allows developers and other processing solutions to enter and leave the emulsion without dissolving it. It would indeed be difficult to find a satisfactory substitute for gelatine in photography. The substrate, or thin bonding layer of gelatine directly coated upon the transparent base material is necessary because the emulsion, itself, is water-attracting, while the base is water-repelling. In order to obtain a coating of emulsion of uni 12 International Projectionist June, 1964