|MadSci Network: Chemistry|
Thanks for your question Alex. I am sorry my reply is so late. I am still a little uncertain about what you want to do. You asked for “a substance which would react to intense light, such as that found in a scanner, to see if it is plausible to make a form of ink or paper which could help stop people simply scanning and re-printing photographs”.
I assume that you propose using an overcoat of some kind to apply to a photo you want to protect from copying. You mentioned silver nitrate as being too sensitive. I assume also that you meant silver halide not silver nitrate. Silver nitrate is one of the starting materials for making photographic emulsions based on silver halide salt crystals. It is very reactive and corrosive and not useful for your intended purpose. Silver halides on the other hand have been refined in their design and production methods over time and can be made extremely sensitive. Sometimes it only takes a few photons per crystal to make them developable into silver. It is possible, though, to make them much less responsive or “slower” by not using the techniques which are used to make them sensitive.
For example the crystals can be made very small. The smaller they are the more light intensity is needed for a crystal to capture enough photons to make it developable. Various other ways can be used including adding impurities to the crystals.
You suggested that there may be a way of using something which would respond to the bright scanner light but not to normal light levels. Silver halide crystals respond to the total amount of light they are exposed to, that is the intensity times the exposure time. Their response can be made independent of exposure time ( and therefore intensity ) over a wide range of exposure times but they can also be treated in a way which makes them less efficient at extremely long or alternatively at extremely short exposure times. There used to be a photographic paper product which was used for recording the rapid movement of an intense light beam. The paper was insensitive enough to be handled in fairly normal but dim light but when scanned by the light beam, would produce a visibly dark line. Unfortunately the exposure time needed to produce this difference in response was very much shorter than in a flatbed scanner so although your thinking is imaginative I can’t offer you much hope.
While I was working in photography I did investigate ways of protecting photographs and other prints from copying. I have to admit I didn’t find a method which would not spoil the viewing of the photograph. The difficulty is that if you can see a photograph clearly, it is possible to photograph it. A modern digital camera is easily good enough to copy an old photograph so there is no need to use a flat-bed scanner.
So, the bad news is that although in principle a silver halide layer could be made which darkened faster in a scanner that when exposed to normal light, it would degrade the quality of the image and most probably spoil the image by darkening by light or airborne sulphur-containing pollutants.
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