5.11.2 Polyester Base (Motion Picture Presentation - theatrical projection)
Description
This article is from the rec.arts.movies.tech FAQ, by Scott E. Norwood snorwood@nyx.nyx.net with numerous contributions by others.
5.11.2 Polyester Base (Motion Picture Presentation - theatrical projection)
Polyester stock (`ESTAR (tm) ' is a trademark for polyester stock
manu- factured by the Eastman Kodak (tm) company) is a fairly new
development for print film. Like triacetate stock, it is nonflammable.
The primary differences between it and the older nitrate/triacetate
stocks are strength and thickness. Unlike other films, polyester stock
does not break. If stressed, it simply stretches. This can be either
good or bad, depending upon the degree to which it is stressed; for
example, a jammed platter feed mechanism can cause the still-running
projector to pull an essentially immovable piece of film through it,
causing great damage to the projector itself, and, of course, damaging
several feet of the film. If this circumstance occurred with
triacetate film stock, the film would have simply broken, and no
damage would have occurred.
The severity of this and other problems varies substantially among
films manufactured by different companies. Further, the resistance to
breakage is the primary reason why polyester is not used on camera
films, as the risk of damage is much greater when the film is run
through expensive camera equipment. (Polyester camera film is
manufactured and used for high-speed cameras used to capture
slow-motion images for scientific and engineering work, as the
mechanisms of these cameras run so quickly that they would be severely
damaged if the film were to break while the camera was running).
Polyester stock is also thinner and lighter than acetate stock (one
can identify it as polyester by holding a reel up to a light source in
a sideways position (such that it appears round from the viewer's
point of view); if one can see light through it, then it is
polyester). This can reduce the number of shipping reels, and the
shipping cost, but may require adjustment of gate pressure in the
projector in order for the film to run properly. Also, the stock is
more sensitive to low humidity than triacetate, as it tends to pick up
static electrical charge, sometimes preventing it from running
smoothly on a platter system. The most often recommended solution to
this ailment is to ensure that the platters are properly grounded, and
that a humidifier is present in the projection booth. This will also
help to avert unnecessary dust accumulation on the print.
The texture of polyester stock is substantially different from that of
triacetate stock, and cement splices are not useful on polyester films
(either tape or ultrasonic splices must be used). Thus, projectionists
usually use the more-visible tape splices to join film together.
The static and strength problems were particularly acute with many
prints of American President, one of the first major features to have
35mm prints distributed on polyester stock. Commonly, when run on
platters, the film layers would `stick' together, jamming the feed
mechanism, and, usually, causing the whole projector to stop (by means
of `failsafe' assemblies which stop the motor when there is excessive
tension on the guide rollers).
It should also be noted that the IMAX (tm) format requires that
polyester-based film be used, due to the relatively high linear speed
at which the film moves through the projector (about three times that
of 35mm), and the potential damage to the projector should there be a
film break in the middle of a show. However, IMAX (tm) equipment was
designed for polyester film, and has several safeguards not present in
most 35mm projection equipment in order to avert potential disasters
in the projection booth.
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