Thermal Properties of Melanin Determined from Photoacoustic Techniques

摘要

Photoacoustic signals from synthetic melanin powders and films were detected and analyzed; their signification to vision research is discussed. Consistency between results from our experimental arrangement and Rosencwaig-Gersho theory was confirmed. The light beam from a continuous-wave, krypton laser was chopped at frequencies ranging from 200 to 1200 Hz, and the corresponding photoacoustic signals from the melanin were recorded . Experiments were carried out on melanin powders and thin films. Identical measurements were carried out for carbon black powder used as a standard. As expected, the acoustic amplitude varied omega to the -1 power for the carbon black standard. The same dependence was observed for melanin powders obtained by evaporating water at neutral ph. For melanin thin films, however, an omega to the 3/2 power dependence for the acoustic signal was observed. For melanin powders obtained by evaporation of water at low ph values or by washing with acetone, the variation of the acoustic signal with chopping frequency displayed an omega to the -1 power behavior from 200 to 650 Hz and an omega to the 3/2 power behavior from 650 to 800 Hz. The switch in slopes plotted on log-log scale permitted an estimate of the thermal diffusivity of acetone melanin. The central role played by water in these results is under scored. Acoustic signals were also measured while the laser intensity was varied; the dependence was found to be linear in accordance with the Rosencwaig-Gersho theory. (aw)