Drug wraps and Schlieren lines

By Anna Sandiford 03/06/2011


Life is spooky: it was interesting to see the blog post about Schlieren imaging by Physic Stop‘s Marcus Wilson because it reminded about this simple yet effective technique that I have not seen in use in New Zealand (yet) and related directly to a conversation I had yesterday with my crime scene consultant.

We were talking about drugs. More specifically about how cross polarised light can be used to visualise patterns across foodbags. In England & Wales, it is routine in forensic science laboratories (or it was when I was still working there three years ago) for drug wrappings to be put on a light box with cross polarised light in order to see the otherwise invisible patterns:


In this image you can see three semicircular pieces of material – these are sections of kitchen food bags that were taken from a Defendant by the Police. Each of them is a drug ‘wrap’ – they are made by cutting off the corners of food bags and wrapping small quantities of drugs in them.

The Defendant denied dealing drugs and said that he had just bought these wraps from a dealer (they contained either heroin or cocaine). The Police went to the Defendant’s home address and took a roll of food bags from his kitchen drawer.

The wraps and the food bags were sent to the forensic science laboratory. The sections of wraps were cut open and laid under the light on the light box. The food bag was also cut open and laid next to the wraps. Instantly, you can see the way the patterns are extremely similar across both the bag and the sections of drug wraps. These patterns change very rapidly as a result of the manufacturing process so to have such similar patterns across allegedly unrelated bags is extremely significant from an evidential viewpoint: had these bags originated from different boxes then I would expect the patterns to be totally different from one another. However, it isn’t possible to say that the bags originated from the same box, only that they probably originated from the same batch of bags.

Added to this can be the use of Schlieren patterns (or what I have always been led to believe were called this). This is a way of photographing drug wraps (foodwrap/clingfilm/Gladwrap, whatever you want to call it, as well as food bags) so as to highlight the different densities in the polymer sheets. This method shows lines of varying shades of grey through to black that also vary rapidly during manufacturing due to the the way bags/film is made (and I know this to be true because I have been to a clingfilm & bag factory to see how it’s done – very interesting, and I don’t care if this makes me sound like a geek).

Visualising otherwise invisible patterns on bags and cling wrap is very useful for cases where the ends of bags or clingfilm haven’t been cut with scissors but where the ends might have been torn/ripped and therefore distorted, thus removing the possibility of being able to physically ‘fit’ a bag to a given roll or a section of clingfilm with the end of a given roll.

This information can be added to the results of chemical analyses of the drugs contained within the wraps – in some cases it all adds up to hefty evidence against a Defendant. On the other hand, it can also add to a Defendant’s account of events, depending on the results.