Page 16 - 2015-03

The Exciter Lamp

- A look at the technical aspects of this simple device

Mike Trickett
e all know what they are used for, but It becomes obvious that we should use a DC
Whow do they work? source to supply the lamp, and that we can't
tolerate the filament of the exciter being oper-
Efficient operation of the exciter lamp, the ated from AC, or can we?
associated optical system and the optical sen-
sor is necessary to obtain quality sound from a Over the years, various methods of smoothing
sound-on-film optical track. the exciter voltage have been used and that is
the basis of this article - a look at the different
The starting point in the chain is the exciter methods used to obtain an acceptably smooth
lamp - be that a traditional incandescent lamp light output from the exciter.
or the more modern LED (Light Emitting Di-
ode), the requirements are much the same. Obtaining a smooth DC voltage would seem to
be a fairly simple thing to achieve - but it was
The first is obvious - it must emit light - be that not all that easy until modern times.
white light in older systems, red
light in the case of later projec- The need for a fine, compact fila-
tors. ment in the exciter lamp, meant
that it had to be a low voltage lamp.
We all know that the slight vari- The usual voltage being 4 or 6
ations in light, caused by the film volts. In the case of Bell & Howell,
sound track passing between the 4 volts at .75 amps was their
light source and the optical sen- choice, others opted for 6 volts,
sor, causes the sensor to produce while in 35 mm cinema projectors,
an output voltage which corre- 6 volts at 8 amps was common
sponds to the audio of the film (more on that later).
sound track. This audio voltage
is then amplified and eventually In electronics, the common method
applied to a speaker/s which con- of obtaining DC is to rectify an AC
veys the sound to the audience. source using diodes or valves and
then smooth the resultant output to remove the
Because slight variations in light level are the ripple voltage; this is commonly done using
basis of the operation of the system. What capacitors and resistors (or inductors).
happens if there is a slight variation in the light
output from the exciter. [Rectifying an AC voltage will produce pulsat-
ing DC; that is Direct Current with a large AC
Feeding the exciter with AC, derived from a component (ripple) impressed on it. It then
transformer, would under normal circumstanc- requires some form of filtering to remove the
es cause the light level from the exciter lamp ripple to produce pure DC.]
to vary 100 times* per second, which of course
would be picked up by the sensor and repro- I the days of valve (tube for those using
duced as a 100 Hz hum in the speaker. American terminology) technology (pre about
1960), it was not so simple. Valves are high
impedance devices and don’t take too kindly

16 REEL DEALS March 2015