Copper Sulphate B&W Reversal Bleach

dcy said:

Out of curiosity, any idea why papers use AgCl + AgBr but film uses AgBr + AgI ?

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Speed?
Native spectral sensitivity?

MattKing said:

Speed?
Native spectral sensitivity?

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I tried to look into this but I'm out of my depth. I did learn a couple of things though:

(1) These lecture notes from a college in India say:

Silver bromide (AgBr) is the most widely used of the silver salts for paper and film products...
Silver iodide (AgI) ... [its speed] is approximately one-third that of silver bromide
Silver chloride (AgCl) ... Its speed is approximately one-eighth that of silver bromide.

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(2) This paper goes into the details of the photochemistry of AgCl, AgBr, AgI. This paper is way over my head, but perhaps others can glean something of value from it.

Having said that, the paper did say one thing that I understood:

... Accordingly, the ionic relaxation time in AgCl grains was two or three orders of magnitude longer than that in AgBr grains. This result indicates that, on exposure of AgCl grains for a short time, trapped electrons are inclined, not to react with interstitial silver ions, but to recombine with positive holes, and that photographic performance on exposure for a short time is therefore deteriorated more intensely in an AgCl emulsion than in an AgBr emulsion.

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Ok. So the latent image from AgCl will vanish sooner. That makes it unsuitable for film. While AgI is the opposite.