Micro-spectrophotometry (MSP) is an essential tool in the forensic analysis of many kinds of trace evidence . It uses either visible or infrared light to determine the light transmission, absorption, or reflectance properties of a material. MSP is particularly valuable in the investigation of hair, textile fibers , and paint.
The chemical bonds within the molecular components of trace evidence interact with light in a characteristic way. They will absorb, transmit, or reflect specific frequencies of visible and infrared light. When we see a piece of cloth as red, for example, this means that although white light falls upon the material, all the color frequencies making it up except red are absorbed by the dye molecules in the material. It is therefore the red frequencies of light that are reflected back. A blue cloth contains different dye molecules, which reflect back only blue frequencies. MSP is a more sophisticated and highly accurate way of recording exactly what color an object is.
When an opaque or translucent specimen is inserted into an MSP instrument, it is exposed to a range of visible or infrared 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 spectra are held in databases. It is also possible to compare a piece of trace evidence with a control sample. A fiber found at the scene of the crime can be compared with one found on a suspect's clothing, for instance. If the two fibers' MSP spectra are identical, then they come from the same source. The same is true of hairs and paint flakes. Indeed, MSP can reveal whether someone's hair has been dyed, bleached, or treated in some way, as well as when the person last visited the hairdresser. This could be useful, for example, in linking hair found on a suspect to that taken from the victim and so place the suspect at the scene of the crime, or eliminate them as a suspect.
see also Micro-fourier transform infrared spectrometry; Spectroscopy.