Dichroism measurements in forensic fibre examination. Part 2 - dyed polyamide, wool and silk fibres.

A number of dyed polyamide, wool and silk samples were examined with plane polarized light on their dichroic behavior by optical light microscopy (OLM) and microspectrophotometry with plane polarized light (MSP-PPL). It was found that most of these acid dyed peptidic fibres possess dichroism, but these are weaker than the effects previously described for polyester fibres. The small effects may be not observed, especially for wool, but these can be measured using MSP-PPL. In the three peptidic fibre classes, for the first time, a so called "inverse dichroism" is observed which appears in the absorption spectra as a hyperchromic effect.