|MadSci Network: Chemistry|
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 . EXPOSE 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 firstname.lastname@example.org Cheers!
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