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Collecting Fingerprints from Optical Media

December 1, 2011

You would think there isn’t anything complicated about this topic. There really isn’t, but we continually hear about people getting it wrong, so I thought it deserved an article addressing it.

I have to say up front that I am not a “fingerprint expert” in any way. I have some common knowledge about the subject but most of what I can bring to it is knowledge of optical media. So, if you are reading this and see something that I am incorrect about, please let me know.

There are many methods to collect latent fingerprints from all kinds of different surfaces. Some of these methods are quite compatible with optical media and will not harm it in any way. There are others, however, that will virtually destroy optical media making it impossible to read anything from the disc. I am going to identify these two categories of methods and while I may not be able to list every single technique available, you should be able to evaluate any technique that I have left out as to whether or not it is destructive to optical media.

First, it needs to be said that processing optical media for fingerprints must be done before the content of the disc is gathered forensically. Putting the disc into a drive will spin it at up to 5,000 RPM in a hot, dry environment. This will seriously degrade any fingerprints on the disc. It is also possible that the disc may not be able to be read without cleaning (or at least wiping it off), thus eliminating any fingerprints from the data side.

That means fingerprint processing MUST come first. Plenty of agencies have policies or procedures which instruct examiners to first try to get as much off the disc as possible because the collection of fingerprints has historically destroyed the disc, making collection of the data on it impossible. The point of this article is to make fingerprint collection non-destructive so it can be done first, and then the disc can be cleaned as needed so it can be read completely.

The most destructive technique for fingerprint collection is lift tape. On CD media it can peel the reflector off the disc and on DVD media it can delaminate it. You might get lucky a few times but more than 50% of time lift tape is going to destroy at least a portion of the disc.

We got a call one day from the US Secret Service where an examiner was nervously asking how a CD with a reflector peel might be repaired. They had the disc processed for fingerprints before trying to read it but the fingerprint process used involved lift tape. There were two discs affected, one with 75% of the reflector removed and the other with only about 25%. How could such discs be repaired?

The answer is that it is impossible. Write-once CDs and DVDs use an organic dye to hold the information and this dye is sandwiched between the reflector and the polycarbonate substrate of the disc. With CDs there is only lacquer over the reflector which makes it very susceptible to damage, whereas with DVDs there is another piece of polycarbonate bonded over the reflector. The problem is, the dye has no particular affinity for either the polycarbonate or the reflector so statistically speaking about 50% of the dye is going to stay on the polycarbonate and the other 50% goes with the reflector. Think of it as every other bit is taken away. Considering these “bits” are as small as a 700nm dot (that is nanometers or one billionth of a meter), you would be faced with trying to align the bits up and they are too small to see, even with a microscope.

So lift tape is just something to be completely avoided. Don’t even threaten the disc with holding it nearby.

Cyanoacrylate fuming is another technique that is used for degraded fingerprints and those on surfaces which cannot be processed by other means. The fact that it sometimes works with degraded fingerprints makes it a likely candidate if a disc isn’t properly handled. This technique can be used with success as long as care is taken to keep the data side of the disc shielded. There are a number of products available that can be used for this purpose, one of which that pops up is called d_skin – more information here. This is assuming the label side of the disc is the one being processed – cyanoacrylate is not compatible with the data side of a disc and will result in difficulty reading the disc, perhaps the entire disc.

So what is safe to use? There are two that I have known about for years and should be obvious: ordinary powder with photography and alternate light source with photography. With powder it is important to clean off the disc before reading it, but dunking it in ordinary water and blotting it dry should be fine.

A technique that I have heard about is using microsil (sold under various trade names) which is a silica compound. I’ve never seen this used and understand it is something that is used rarely and almost never on smooth, polished surfaces. It is compatible with optical media, to the extent that it is important to completely clean any residual material off of the disc. Failure to completely clean the disc can result in damage to a drive when the material flies off inside the drive with the disc spinning at 5,000 RPM.

So, from this we should be able to see some pretty general rules about developing latent prints from optical media. The first thing you want to avoid is sticking anything to the disc – like lift tape. Microsil is perhaps an exception to this in that it comes off very easily. The second rule is to avoid techniques and substances – like cyanoacrylate – which are going to leave a residue on the disc. Anything that can be easily cleaned off with ordinary water (like fingerprint powder) is perfectly safe.

Finally, while I haven’t discussed it in detail, avoid any technique that involves solvents. Any sort of volatile solvent, for example ammonia, benzene, naphtha, etc., can potentially attack the lacquer, the bonding of DVDs or the polycarbonate substrate itself.

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