MadSci Network: Chemistry

Re: How does instant film work

Date: Sun Feb 28 13:02:01 1999
Posted By: Harry Adam, Staff, Research Division, Research Division, Kodak Limited
Area of science: Chemistry
ID: 919616767.Ch

Hi, Ed – thanks for your question on instant photography.  Well, you are 
right that the rollers through which the print emerges from the camera is 
responsible for breaking a capsule of chemicals – but it is only one 
capsule. The rollers also serve to spread the chemical evenly between 
layers within the pack. Once this is done, the water in the material which 
has been spread diffuses into the imaging and non-imaging layers either 
above, below or both above and below the “plane of spread”. Water is the 
first and most important chemical, as up to that point, all the layers 
have been dry, and no processing can occur while the layers are dry. Apart 
from water, the capsule contains a thickener, to help even and smooth 
spreading, and base – to raise the pH and allow the necessary chemical 
reactions to take place.
The following diagram illustrates how, and this example is taken from the 
Kodak Instant System, no longer sold, so no secrets are being divulged! 
Imagine you are looking down on this – i.e the viewing side is the top – 
that is where the image gets formed. The exposure takes place in the 
camera from below.

.                                      VIEW                             .
.                            Backing Layer                              .
.                  Estar support (clear plastic)                        .
.                                Mordant                                .
.                 Opaque reflective layer (TiO2)                        .
.                           Cyan dye-releaser                           .
.            Red sensitized reversal emulsion                           .
.                 Oxidised developer scavenger                          .
.                         Magenta dye-releaser                          .
.         Green sensitized reversal emulsion                            .
.                   Oxidised developer scavenger                        .
.                       Yellow dye-releaser                             .
.           Blue sensitized reversal emulsion                           .
.                  Ultra-violet absorbing layer                         .
.     ********** Separation layer – for spreading   ********            .
.                              Timing layer                             .
.                                 Acid layer                            .
.                             Estar support                             .
.                            Backing layer                              .

Imagine you took a picture of the sky – i.e. only blue light hits the film 
on the exposure side. Latent image is formed in the blue sensitized layer, 
and because this is a reversal emulsion, the effect is to render that 
emulsion undevelopable. Only those emulsions not exposed will develop. OK, 
the print emerges from the camera, the pod or capsule at the front of the 
print is burst by the exit rollers and the liquid is spread through the 
spreading layer. Not only does this contain water and base, but also 
carbon black to stop light penetrating to the emulsion layers as the print 
emerges from the camera. It also contains a reversible developer – called 
an electron transfer agent (ETA). The water, base and ETA diffuse into the 
pack above and in the layers not exposed cause development. When the ETA 
develops the silver halide into silver it becomes oxidised, and then it 
reacts with the dye releaser in the adjacent layer. This releases dye, and 
re-reduces the ETA which can then engage in more development. The cycle 
continue with the ETA transferring electrons from the dye releaser to the 
silver halide, until development is complete. Meanwhile, the released dye 
diffuses through the opaque white layer and is trapped on the mordant, 
below clear plastic and forming the image. In our example, only magenta 
and cyan dyes diffuse onto the mordant. When you view, the white light you 
are viewing with, is a mixture of red, green and blue – the primary 
colours. The magenta dye absorbs all the green, and the cyan all the red, 
so the blue light is the only one reflected back to the eye. So, you see a 
picture of the blue sky you pointed the camera at in the first place. 
Finally, the base slowly causes the timing layer at the bottom to open up, 
and acid is released from the layer below that. This neutralises the base 
and all the reactions stop. The silver halides and developed silver remain 
in the pack, hidden from view – on one side by the opaque TiO2, and on the 
other by the carbon black that was contained in the “goo” the was spread 
from the capsule. If you are still with me, I hope this explains how 
instant photography works. There are various other ways of doing it, but 
the general principles of dye diffusion from one layer to another obtain 
in them all. Again, a single pod or capsule is burst to spread a uniform 
layer. In some older systems (e.g. the Polaroid peel-apart system),  you 
peeled the picture apart to separate the image from the emulsion layers, 
which were discarded. Any questions remaining? E-mail me at		Cheers!

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