The examination of samples obtained from an accident or crime scene can be a sophisticated process. Some of these examinations can determine the presence of certain compounds or materials. Various microscopic methods can be used to visually examine samples. The choice of the microscopy technique can be determined in part by the size of the target. For example, gunshot residue may be too small to be seen using a visible light microscope. Rather, the increased magnification available through the use of electrons or laser light is needed to resolve the residue.
Another sophisticated form of microscopy that is useful in forensic analyses is called confocal microscopy. As one example, the technique has been used to visualize the marks on bullets and cartridge cases that were otherwise not easily seen using conventional light microscopy
In confocal microscopy, the source of illumination is laser light. A laser light wave can be focused to a very small area on a specimen, which permits very detailed examinations on a sample to be done.
As well, the wavelength of light used can be specifically selected. This is advantageous because some molecules and stains that can be applied to a sample will fluoresce when exposed to the particular wavelength. Fluorescence occurs when the sample molecules acquire more energy when they absorb the laser light. This energy increase is transient, and some energy is subsequently emitted. The emitted energy is the fluorescent light.
When a solid sample is examined, the confocal microscope can be equipped with detectors to capture the light that is reflected back off of the surface and the fluorescent light that is emitted. This information can be analyzed using a computer that is connected to the microscope and a very detailed image of the specimen can be produced.
If a specimen is transparent to the laser light, then the light beam can be progressively focused at different depths through the thickness of the specimen. The information collected from the reflected light at each of these so-called optical planes can be stored in the computer. Subsequently, each image can be analyzed separately to assess the composition and structure of the specimen throughout its depth. As well, the collected images can be merged together to produce a three-dimensional image of the specimen.
When confocal microscopes first appeared in the 1980s, they were expensive and beyond the range of many labs. However, now they are quite affordable and have become a popular addition to a forensic laboratory.
see also Analytical instrumentation; Crime scene investigation; Fluorescence; Imaging; Scanning electron microscopy.