American Cinematographer - May 1933 - 9

1C.

American Cinematographer

*

May 1933

tensity across width. With the sharp uniform line the fre¬
quency response was 25,000 cycles contrasted with a fre¬
quency response of only 9,000 cycles for the line whose
edges were not sharp and the intensity varying across its
width.
This difference is unquestionably due to the fact that
ordinarily when the line of light is not uniform across its
width the variation is a falling off of intensity from the
center to the edges which gives a one half sine wave con¬
dition to the line of light, this half wave subtracts from the
sine wave which is being recorded on the film to the amount
of its magnitude, which, of course, depends upon how fast
the intensity falls off toward the edges.

This graph shows a gamma of .45 obtained
by using a .003" and .0006" lines of light
in combination width.

This condition of falling off towards the edges increases
as the line of light is made finer, particularly as the mechan¬
ical stops are brought closer together increasing the ratio
of penumbra or edge shadow to the illumination area so
that in attempting to create an extremely narrow line of
light you run the risk of not only losing exposure, but also
of not gaining the expected high frequency. And, of course,
in the case of the two .0006" lines of light the one falling
off at 9,000 cycles can be duplicated by using a line of light
of good character that is .002" in width.

Opt ical Twin
Fidelity With

Therefore when greater exposure is desired for glow lamp
recording it would naturally follow that the line of light
should be as wide as possible, consistent with its character
as one means of increasing the exposure.

Gl ow Lamp
by
Hartley Harrison

N

OT since 1880, when Edgar Fritts patented a means
of recording and reproducing sound vibrations, has
there been anything new in the principle of the
light beams for the recording or reproducing of sound.
The accepted practice has been to create as fine a line of
light as possible not only for both recording and reproduc¬
ing, but also for both variable density and variable area
recording. The theory being that the narrower the width
of the beam the higher the frequency response, and of
course, the better the quality.
It should therefore be of considerable interest in the realm
of sound when some different principle, namely the use of
two lines of light, is employed in the recording of sound.
But, before going into a description of the two line principle
let us re-familiarize ourselves with some of the optical
conditions of sound recording.
Although theoretically the width of the line of light
determines the frequency response of the optical system,
in actual practice, the character of the width of the line,
such as sharpness of edge, uniformity of light across the
width is so powerful a factor in controlling the frequency
response that the width of the line is of secondary im¬
portance.
This has been demonstrated by
light whose edges were very sharp
light across the width very uniform
line of light whose edges were not

using a .0006" line of
and the intensity of the
compared with a .0006"
sharp and a varying in¬

Experiments with a .003" width line of light having
excellent character showed almost enough exposure for
straight line recording burning the glow lamp at a static
current of 5 millamps. connected from a noise reduction
amplifier, and the quality of the lower register was far
superior to the quality obtained with narrower lines of light.
Although the frequency response cut off at 6000 cycles the
quality of the lower register can be accounted for by the
fact that the increased width increased the film motion
tolerances by about three times over a normal line of light.
(The theory of film motion relative to aperture size is I
think too well understood to merit discussion in this article.)
Of course, for good recording it is necessary to record
much higher frequencies than 6000 cycles and further ex¬
periments were conducted by using a .0006" line of light
in combination with the .003" line of light. The result of
this test proved more than gratifying, the frequency range
was extended to 17,000 cycles although maintaining the
same quality in the lower register and the exposure as shown
in the accompanying graph gave a gamma of .45 with fifty
per cent transmission with the lamp current still at 5 millamps.
The performance of this dual optical system may be lik¬
ened to the dual horn combinations where one horn
responds from 25 to 5000 cycles and the second horn
responds from 4,000 to 16,000 cycles, except that the
narrow line of light which records the high frequencies is
recording all of the time, adding to the exposure of the wide
line of light, although not changing the quality of the lower
register.
While at this writing there has been only a few thousand
feet of commercial recording completed with this system,
the first picture was composed of solid string orchestration
and violin solos, the quality of which can only be appreciated
by actually hearing the sound.
American Cinematographer - May 1933

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