MadSci Network: Physics

Re: What happens to your film when light hits it to make the negative?

Date: Sat Feb 12 11:46:28 2000
Posted By: Harry Adam, Research Associate, Research Division, Kodak Limited
Area of science: Physics
ID: 949412219.Ph

Hello Dustin – first you need to know what the light sensitive part of 
film is. Film consists of layers of coated and dried-down gelatine in 
which is dispersed colloidal materials, to capture and store the scene, 
then later to allow the information to be amplified and converted into a 
permanent, usable image. In colour film, some of this colloidal material 
is solid crystals of silver halide, (AgX where X = Cl-, Br- or I-) some of 
it is oil droplets containing chemicals waiting to be converted into dyes. 
Your question only asks about the action of light on film, so I’ll 
restrict this answer to explaining the action of light on silver halide 
crystals in negative films.

First, silver halides are naturally sensitive to light – blue and shorter 
wavelengths – e.g. UV. To make a practical film – whether colour or black 
and white, we need to sensitize the film to the colours we humans see – 
red and green, as well as blue. (Look at your PC screen with a magnifier, 
and you’ll see it is made up of red, blue and green dots only.) So with no 
more detail, you can imagine film containing some crystals made to respond 
specifically to these colours. They are in separate layers in colour films 
and often in a single layer in B & W films.

When the camera shutter focuses the scene on the film, grains which are 
exposed to light to which they are sensitive, decompose by absorbing the 
light which converts the Ag+ ion of the silver halide lattice into a 
silver atom (Ag0), in the process releasing the associated halide ion as a 
radical – X.  The halide has been “persuaded”  by the light to hand back 
the electron to the silver – which, being a noble metal, is happy to 
accept and get back to the form it really wants to be in, but at the same 
time the halide radical is very unhappy (energetic) and is desperate to 
react with anything to get its electron back. IF it grabs it back from the 
silver – then nothing has happened – no image. On the other hand if it can 
find something else to react with the silver atom survives. Years and 
years of R & D have gone into improving this process to yield films of 
ever higher speeds with small crystals and low graininess.
So – after exposure we are left with a film which if you could see it 
would look no different to what was there before exposure (if you could 
have seen that. Of course you can’t see either the before or the after 
without totally exposing the film. Some of the grains have small specks of 
silver as a result of exposure while the unexposed grains have none.

That difference is enough so that the developer can tell the difference 
and essentially turns the exposed grains completely into silver without 
doing anything to the unexposed ones. In negatives then you end up with 
black silver in the exposed (light areas of the scene) if it’s a B & W 
film or in colour you have dyes corresponding to exposure where no 
exposure forms no dye. It’s as “simple” as that!

I’m sure that won’t have satisfied your curiosity fully – so if not, and 
you can’t find any good reference books or web-sites, I’ll be happy to 
answer more questions for you. E-mail me at

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