From: Monnoyer Philippe (monnoyer@imec.be)
Date: 06/07/02-08:59:45 AM Z
Bill,
Bare silver halides absord high energy wavelenghts only. Therefore,
their natural spectral sensitivity is limoted to UV and blue.
A sensitizing dye absorbs lower energy wavelenghts, due to their
molecular structure. These wavelenghts go from blue to infrared,
depending on the dye molecule(cyanine length).
Some dye absorb light up to the green region and give orthochromatic
emulsions. Others absorb up to the red and give panchromatic emulsions.
But for a dye to be effective, there are more conditions. For example,
the dye molecule has to have the right structure, composition and
aggregation properties, to correctly adsorb (and not absorb) on the
surface of the grains.
When the conditions are met with an appropriate dye, the low energy
photon (red for example) absorbed by the adsorbed (on the grains) dye
molecule, will be transmitted to the silver halide grain. That way, a
latent image can be obtained with a red photon. Without dye, this
wouldn't happen.
Some natural dyes meet somehow the photographic requirements. A lot more
other synthetic dyes have been developped to be very efficient.
Voila,
Philippe
Belgium
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