Don Larsson writes:

>A correspondent off this list posed a question that the more
>technologically-inclined here might be able to answer: exactly (or
>approximately) how much time of each 1/24th of a second during normal
>sound projection is "dark"?

This depends on the size of the shutter blades and the time taken by the
intermittent mechanism to advance the film.  Both vary between projector
designs.  The smaller the shutter and/or quicker the movement, the more
light-efficient your xenon lamp is (i.e. you get more foot-lamberts per
watt of light), but the more critical your shutter timing is.  Larger
blades and/or smaller movements make shutter timing easier but are less
light-efficient.

A small blade and/or quick movement are highly desirable if showing
silents, because they will tolerate a lower speed before a flicker becomes
visible.  The most efficient projector design I know of in this regard is
the Philips/Kinoton FP20, which will happily go down to 18fps without a
significant flicker and without fitting a special shutter.  However, the
tiny margins for error this system creates work only because there are no
belts in the mechanism and the shutter is single-blade, double-geared (i.e.
it goes through two complete rotations for every pulldown).  Belt-driven
machines such as the Cinemecannica Victoria 5 have larger blades and thus a
longer 'dark time', but this is needed because the variance in transmission
as the belt wares through its lifetime would eventually cause ghosting if
the blades were any smaller.

Leo

Dr. Leo Enticknap
Director, Northern Region Film and Television Archive
School of Arts and Media
University of Teesside
Middlesbrough  TS1 3BA
United Kingdom
Tel. +44-(0)1642 384022
Fax. +44-(0)1642 384099
Brainfryer: +44-(0)7710 417383

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Online resources for film/TV studies may be found at ScreenSite
http://www.tcf.ua.edu/ScreenSite