[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

RE: Green Light



Christopher Bacon wrote in a reply to a message to Craig dated 98-03-15
16:21:25 EST:

............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").

Negative film on a telecine has already captured the broad spectrum of
light on the negative by recording the image in its yellow, cyan, and
magenta images. there are only three color dyes used for this image Yellow,
Cyan and Magenta (YCM). This is the negative of Red, Green, and Blue (RGB)
as we all know. 

Film is subtractive color and TV is additive color. Therefore you only need
a vary narrow bandpass for the red light, the green light and the blue
light to pass through the film and be detected by the RGB sensors no matter
what they might be (PMT or CCD). 

By using narrow band red, green and blue light the orange mask, which is
primarily for contact printing on film with white light, is not even a
factor because the telecine is adjusted for equal RGB output from the three
sensors. In other words the orange mask is transparent on an RGB light
source. 

A telecine with an adjustable RGB light source can give outstanding results
because you can optimize the telecine on the worst channel for best signal
to noise and reduce the lights on the other two channels for a balance.
This means you don't have to turn up the gain (and noise) to acheive a
balance. Hollywood has been doing this in the film labs for decades. The
Peterson/Bell & Howell RGB lamp house used on most optical printers is a
classical example of this. This technique I have used many of times when
doing matte shots from TV cameras and film matte transfers. I think Bob
Kertesz, of Blue Screen LLC, does this also to his sessions He does a lot
of matte work. The green light on the Cintel is okay for this but it
already is being pushed hard for responce in the blue region and red
regions. That is why those channels are noisey but they must balance to
give you true white (minimum subcarrieron NTSC). White light that is
filtered so as not to contain infrared or ultra violt is reffered to "cold
light" and is not to be confused with cool light (blueish) or warm light
(Redish). It can be split via dichroics and trim filters into narrow band
RGB light. The new Sony Multi-standard telecine to be shown at the NAB does
use an RGB lamphouse.

I am hopeful that this tends to make the green light issue better
understood by all.

Note: I am not an employee of Sony and do not recieve any compensation from
them.

Jim Mendrala
Real Image Technology, Inc.
805-294-1049 Fax:805-294-0705
Mailto:J_Mendrala at compuserve.com

---
Thanks to Martin Banks for supporting the TIG in 1998..
No product marketing allowed on the main TIG.  Contact rob at alegria.com
957 subscribers in 36 countries on Sat Mar 21 21:22:26 PST 1998
complete information on the TIG website http://www.alegria.com/tig3/