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Re: Green Light

In a message dated 98-03-15 16:21:25 EST, Craig asked:

<< 1.  What is the difference in light output between a CRT at 300 ma and a
Xenon lamp at 300 watts ?   [...] >>

There is no way to directly relate the two.  A 300-watt xenon lamp makes a
fine light source for a film projector in a screening room, while a CRT at 300
ma might make a good night lite.  But that proves nothing, since you also have
to consider the optical path and sensors used in a telecine.

<< 2.  In the case of negative specifically, What is the difference between
"imprinting" an orange based image on a green biased light (CRT) , and
"imprinting" it on a relatively nuetral, (slight blue or green Xenon).  [...]
In both scenarios isn't filtration required to remove any color bias derived
from light source or film base color ? Is the arguement here really who
provides a better filtration system? >>

First, the orange base of color negative film is a filter.  Only orange light
passes through it with minimum attenuation.  "Cancellation" in optical filters
is a little different than mixing paint: an optical filter absorbs light of
all colors except those around a narrow range, so pure green or blue light
should not even go through a good orange filter.  Fortunately, CRTs, xenons,
and film base are broad-banded enough that this presents no serious
difficulties.  Of course there is a lot less light on the other side from a
CRT than a xenon bulb, but that doesn't prove anything since some types of
sensors need very little light to operate, while other types need more.

As for the second point, if you look at television from the receiver
backwards, the three primary colors are represented by voltages which have
specific values.  These  must be respected all the way back to the telecine.
Since light sensors are sensitive to more than one color of light, it is
necessary to split and filter the light, so the green sensor "sees" image
information only around green, the red sensor "sees" red information, and so
forth.  It's a thorny problem; if the filters are too "narrow," some colors
won't be reproduced by the system; too broad and color rendition suffers.  If
so much light is lost through the film, optics, beam splitter, and filters
that there isn't enough to get a decent signal out of the sensors, the result
is electronic noise in the image.  So good telecines have to be designed as
complete, end-to-end systems, not bits and pieces.  The advantage of having
more light to begin with in the Spirit might easily be nullified by more
sensitive sensors in the C-Reality (notice I say "might").

<< With regard to source brightness again, aren't all film scanning systems
calibrated around a light source that will sufficiently penetrate Dmax? I
believe in the Kodak 2k and 4k scanners used with Cineon [...] Does it not
then make sense that variable light output is crucial to making good pictures
in a scanning environment moreso than what part of the spectrum that light
exists in? >>

At maximum density, the minimum amount of light gets through the film and the
minimum signal comes from the image sensors.  The lower useful limit is
reached when noise from the sensors and subsequent electronics outweighs the
image signal.  In properly functioning equipment, there's enough light so Dmax
of normally exposed and processed film can be transferred without an
unsatisfactory level of noise.   (Exactly what constitues "unsatisfactory" is
a topic of great controversey.)  If your image sensors have enough dynamic
range that you can satisfactorily handle the whole range of exposed film from
under to over exposed (by way of normal) right in the machine's electronics,
then you don't need variable light sources!  

Best regards,
Christopher Bacon

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