Movies for TV - Early Television Foundation

PRINCIPLES OF MOVIES p
are the crystals which form the image on exposure. If the mixture
is cooked for a long time at a high temperature and the nitrate is
added quickly, a fast, coarse-grain emulsion is formed. If the nitrate
is added slowly and cooked for a shorter time at a lower tempera-
ture, a slower, fine-grain film is produced.
It is necessary to wash the gelatinous mass formed from the
above operation to remove the excess salts which would otherwise
crystallize. Then a further amount of gelatin is added and the batch
recooked in what is sometimes called the second ripening. This in-
creases sensitivity due to the formation of sulphides of silver on the
halide crystals. This is a very important operation, for fogging and
sensitivity are affected by it. The mixture is then refrigerated until
required.
A complicated process of dyeing is involved prior to coating the
base with the emulsion, and the various types of emulsion, such as
panchromatic and blue-sensitive, positive films emulsion, are produced
at this stage. The layer is about 1/1000 inch thick (after
drying). The film is then perforated and slit into the required
widths.
This is a very exacting process, since it is important that the
film does not vary in width or it might stick in the camera or pro-
jector. After slitting, it is perforated and cut into suitable lengths
which may vary from as short as 50 feet for amateur camera use
to as much as 2000 feet, although 1200 feet is a more common
length than 2000 feet. Each batch of film is numbered along the
edge at frequently recurring intervals with a code number so that
its manufacture can be checked for quality control and for identification
purposes during processing. The effects of heat in the manu-
facturing process have already been pointed out; they are just as
important during storage, for if stored in an overheated place fog-
ging is liable to occur. For that reason,
film offered for sale in
tropical places should be regarded with suspicion unless the conditions
under which it was stored are known for certain to have been
properly maintained.
On being exposed to light, a chemical change occurs in the
emulsion forming a latent image of the scene in front of the lens.