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Green Light



A short while back there was some debate about "Green light"  on the TIG
and how it is used by some to describe CRT based film scanning.

It is worth remembering that today's CRTs put out a broad spectrum of light
that happens to peak in the green and this is why it looks biased towards
green when viewed directly.
Conversely a Xenon lamp source biases towards the blue.
 
What is important in film scanning is not the color of the light, or indeed
its intensity at any single point in the optical path.


The only requirement that matters is that the complete path is accurately
matched, and that the detectors have the dynamic range and sensitivity to
capture accurately minute changes in light level,(corresponding to the
subtle density changes in negative and intermediate film images and the
extreme dynamic ranges of print stocks).


We are in danger of being brainwashed into the belief that 'blue-rich'
Xenon light is a pre-requisite for color negative. I know it's several
years since I installed and serviced both contact and optical film
printers, but my recollection is that they all used Q.I. incandescent
"yellow biased" light. In film printing it is the print stock, that has the
"gain" in it to compensate for the color negative or inter-positive orange
mask. I guess you could consider in this instance the print stock as the
"detector". 

Incidentally have you ever seen the effect of exposing a frame of negative
with a concentrated slit of unfiltered incandescent light from a 150 watt
lamp, let alone Xenon light. It can take a worryingly short time for the
magenta layer to start to fade, permanently and irreversibly.  Negative
film does not take kindly to extreme light levels.


Today's film scanning CRTs are optimised for the best illumination while
protecting the film's dye layers.

The light source is just one component in a film scanning system, so beware
of judging it in isolation.


Peter Swinson



---
Thanks to Howard Sisko for support in 1998..
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