Digital images contain their maker's mark

IF YOU thought your digital photos could not be traced back to you, think again. It turns out that digital cameras leave a telltale fingerprint buried in the pixels of every image they capture. Now forensic scientists can use this fingerprint to tell what camera model was used to take a shot.

To capture an image, digital cameras use a light-sensitive microchip called a charge-coupled device, or CCD, made up of millions of bucket-like wells filled by electrons. The total charge of each well depends on the amount of light that hits it. Each well is topped by a lens and a colour filter - either red, green or blue - so that a mosaic of three of them provides the information needed to generate one pixel.

To translate this into a usable colour and brightness signal, every camera has built-in "demosaicing" software. This software has to be tailored to a particular camera type to cater for the many peculiarities of each model, including colour filter arrays, CCD chips and lenses. One of the algorithm's tasks is to work out the colour a screen pixel should adopt without jarring with the colours of neighbouring pixels.

Nasir Memon and his team at the Polytechnic University in Brooklyn, New York, have discovered how to work backwards from neighbouring pixel values in a photo to identify the model-specific demosaicing algorithm that created it. Early tests have shown the technique can identify cameras with 90 per cent accuracy (Digital Investigation, DOI: 10.1016/j.diin.2008.06.004).

If it proves viable in further tests, the idea could make a big difference to detective work, says Mark Pollitt, a former FBI crime lab scientist who is now at the University of Central Florida in Orlando. He gives the example of a photo of a kidnap victim being emailed to a press agency: "If we can identify the camera, then there is a possibility that we can identify who bought it and where."
If we can identify the camera, we might be able to identify a kidnapper 

While many people own the same camera models, Pollitt believes that this technique can still be used forensically. He says that because digital cameras have a shelf life of only 18 months, this can help to narrow down when and where it was sold.