From: Breukel, C. (HKG) (C.Breukel@lumc.nl)
Date: 08/13/02-02:41:21 AM Z
Hi all,
I came across below message, might be of some interest...
Sorry for the scrambling, the source is at:
http://www.nature.com/nsu/020805/020805-10.html#1
Best,
Cor
Photography reinvented
Principles of photography overturned by a new one-step process.
9 August 2002
PHILIP BALL
Scientists have reinvented the 200-year-old process
of photography. A
new one-step method could make high-resolution
colour prints from
digital images.
Since its invention in the early 1800s, photography
has involved
basically the same process - using light to convert
silver salts into dark
particles of silver metal on film.
John Marshall and colleagues at the Polaroid
Corporation in Waltham,
Massachusetts, have effectively replaced silver by
acid. Exposure of the
photographic film to light produces acid, which
then converts colourless
dye molecules into coloured forms1.
The whole process, called acid-amplified imaging
(AAI), takes place in
a single sheet of film. It requires none of the
'wet' processing
conventionally used to develop and fix photographic
images.
At this stage, AAI film isn't sensitive enough for
snaps in ordinary
daylight, say the team. But it can be triggered by
strong light sources
such as lasers and light-emitting diodes. It would
be ideal for
applications such as digital printing, producing a
colour image directly
from an electronically controlled light source.
One of AAI's biggest potential advantages is that
it can generate
extremely sharp, high-resolution colour images. In
conventional
photography, the resolution limit is set by the size of the silver grains.
Reprinting history
For years, researchers have been trying to eliminate the messy wet
chemistry of conventional darkroom
processing. In the early years of the science, photographers had to carry
with them an entire miniature
chemical laboratory.
But despite impressive advances in photographic technology, today's
photographers still use roughly the
same technique as at its conception - when Frenchman Joseph Nicéphore
Niépce captured the view
from his window near Chalon-sur-Saône in 1816.
Light falls on a film impregnated with a silver salt, transforming the
silver ions into neutral silver atoms.
These then aggregate into tiny particles of silver. The developing process
'amplifies' the exposed silver
particles, growing them into grains big enough to appear dark.
In colour film there are three layers of light-sensitive material, each
containing a coloured 'sensitizing' dye
that absorbs light in the red, blue or green region of the spectrum, which
make up nearly all colours.
A colour negative is then made by depositing coloured dyes within each
layer, which either stick to the
silver particles or lodge in the unexposed regions. The silver is removed,
leaving the dyes behind. These
various stages involve wet chemical treatment of the film - even in instant
Polaroid cameras, where it all
happens in situ.
The Polaroid team's new process uses the same three-layer principle - but
the dyes that form the colour
image are switched on by acid.
First, light exposure splits apart a compound called an iodonium salt to
make a 'primary acid' in the AAI
film. Sensitizing dyes ensure that the salt is split only by red light in
the red-sensitized layer, and so on.
Next, the small amount of primary acid is increased as it catalyses the
formation of a 'secondary acid'.
Once amplified, the acid converts colourless dyes dispersed throughout the
film into coloured forms, a
different colour for each of the three layers. This all happens
automatically once the film is exposed.
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