Visible microspectrophotometry is a very useful tool in the forensic analysis of many kinds of trace evidence . It combines a microscope with a spectrophotometer so that the light absorption properties of a very small sample can be recorded. The technique is particularly valuable in the investigation of hair, textile fibers , and paint, which are typically of microscopic dimensions. A fiber, for instance, may have a diameter of only around 20 micrometers.
The chemical bonds within the molecular components of trace evidence interact with light in a characteristic manner. They will absorb, transmit, or reflect specific frequencies of visible light. When human eyes see a piece of cloth as blue, for example, this means that although white light falls upon the material, all the color frequencies making it up except blue are absorbed by the dye molecules in the material. It is therefore the blue frequencies of light that are reflected back. A yellow fiber contains different dye molecules, which reflect back only yellow frequencies. Visible spectrophotometry is a more sophisticated and highly accurate way of recording exactly what color an object is.
When an opaque or translucent item of trace evidence is inserted into the visible microspectrophotometer, it is exposed to a range of visible frequencies. The frequencies where it reflects, absorbs, or transmits, depending on the mode of the instrument, are recorded at a detector as a spectrum, or fingerprint, of that material. Comparisons can be made with materials whose visible spectra are held in reference databases. It is also possible to compare a piece of trace evidence with a control sample. A textile fiber found at the scene of the crime can be compared with one found on a suspect's clothing, for instance. If their visible spectra are identical, then they likely come from the same source. The same is true of hairs and paint flakes. Visible microspectrophotometry is also a useful and non-destructive way of analyzing colored inks in the investigation of questioned documents .
see also Micro-fourier transform infrared spectrometry; Spectroscopy.