Nearly every task we undertake as forensic photographers will require us at some stage to replace or increase the existing ambient lighting with a supplementary one. This can be for numerous reasons, overall low light levels within a scene, areas of shadow detail, or to increase our depth of field. The use of electronic flash offers a solution to many of these issues, providing a portable bright, daylight balanced light that can be manipulated and controlled as required.
3.1 How does it work?
I am sure we are all familiar with either using flash, or seeing flash being used, but do we understand the physics of the modern flash unit?
The requirement for an artificial source of light started back in 1839 with Ibbetson, who used Limelight, which was a ball of calcium carbonate heated in an oxygen flame until it became incandescent. This was followed in the 1860s by magnesium wire and then in the 1880s by flash powder. All created clouds of smoke and white dust making them unsuitable for studio work. It wasn't until 1929 that the first flash bulb was produced, called the ‘Vacublitz’, swiftly followed by the first mass production flash the ‘Sachalite’ made by GEC in the USA (General Electric Company).
In 1931 the first electronic flash tube was developed by Harold Edgerton and, unlike its flash bulb predecessors, it could be recharged and used time after time with repeatable results.
Most modern flash units (Figure 3.1) have three main parts:
- A power supply, ...