Crime or accident scene photographers capture images commonly in colour, but also in black and white. The photograph of the skid mark was made during reconstruction at the accident scene to show how and why the ladder had slipped and caused a serious injury to the user. Colour pictures are generally preferred because colour may be an important aspect of the trace evidence, for example. Thus traces of paint or dye on a piece of evidence may be crucial to linking the evidence with a crime or accident.
Various forces and different countries have different policies in regards to 35 mm film or digital photography. There are advantages and disadvantages to both. Conventional photography (even using disposable cameras) have a high resolution, enabling great enlargement should details in a picture need closer examination. Pictures from surveillance cameras are a growing source of evidence for courts, as are pictures taken by bystanders on mobile phones. The former are being used increasingly at accident blackspots, and bystanders may take pictures of events when no policeman or investigator is present, but yet may be critical to a case. Digital photographs usually have an automatic date and time marker on each image, so that authenticity can be verified. Conventional photographs without such marks must be authenticated by the photographer, usually in a witness statement. Pictures of the relative position of objects (as in a Palimpsest) can establish a sequence of events at a crime or accident scene. Due to continued advances in digital technology and software, digital single-lens reflex (DSLR) cameras are increasingly being used by most Police forces.
The images must be clear and usually have scales. They serve to not only remind CSIs and investigators of the scene but also to provide a tangible image for the court to better enable them to understand what happened. The use of several views taken from different angles helps to minimise the problem of parallax.
Overall images do not have scales and serve to show the general layout (e.g. the house where the murder is thought to have occurred). Context images show evidence in context (e.g. how the knife was next to the sofa). Close up images show fine detail of an artefact (e.g. a bloody fingerprint on the knife).
Road Traffic Incident (RTI) photographs show the overall layout at the scene taken from many different angles, with close-ups of significant damage, or trace evidence such as tire marks at a car crash scene. As with crime scene photography, it is essential that the site is pristine and untouched as far as is possible. Some essential intervention (rescuing a trapped victim, for example) must be recorded in the notes made at the time by the photographer, so that the authenticity of the photographs can be verified.
Like all evidence a chain of custody must be maintained for crime scene photographs. Sometimes a CSI (Forensic Photographer) will process his/her own film or there is a specific lab for it. Regardless of how it is done any person who handles the evidence must be recorded. Accident scene pictures should also be identified and sourced, police photographs taken at the scene often being used in civil cases.
Crime or accident scene photographs can often be re-analysed in cold cases or when the images need to be enlarged to show critical details. Photographs made by film exposure usually contain much information which may be crucial long after the photograph was taken. They can readily be digitised by scanning, and then enlarged to show the detail needed for new analysis. For example, controversy has raged for a number of years over the cause of the Tay Bridge disaster of 1879 when a half-mile section of the new bridge collapsed in a storm, taking an express train down into the estuary of the river Tay. At least 75 passengers and crew were killed in the disaster.
The set of photographs taken a few days after the accident have been re-analysed in 1999-2000 by digitising them and enlarging the files to show critical details. The originals were of very high resolution since a large plate camera was used with a small aperture, plus a small grain film. The re-analysed pictures shed new light on why the bridge fell, suggesting that design flaws and defects in the cast iron columns which supported the centre section led directly to the catastrophic failure. An alternative explanation that the bridge was blown down by the wind during the storm that night is thus unlikely. The re-analysis supports the original court of inquiry conclusions, which stated that the bridge was "badly designed, badly built and badly maintained".
- Introduction to Forensic Engineering (The Forensic Library) by Randall K. Noon, CRC Press (1992).
- Forensic Engineering Investigation by Randall K. Noon, CRC Press (2000).
- Forensic Materials Engineering: Case Studies by Peter Rhys Lewis, Colin Gagg, Ken Reynolds, CRC Press (2004).
- Peter R Lewis and Sarah Hainsworth, Fuel Line Failure from stress corrosion cracking, Engineering Failure Analysis,13 (2006) 946-962.
- Peter R. Lewis, Beautiful Railway Bridge of the Silvery Tay: Reinvestigating the Tay Bridge Disaster of 1879, Tempus, 2004,