The Scattering Contribution to Photoacoustic Signal

Insensitivity to scattering has generally been accepted as an inherent characteristic of photoacoustic detection. The physical reasoning underlying this assumption has been that the photoacoustic signal arises from the heat generated following optical absorption, and thereby is a measure of the cross section of such absorption. However, the thermal energy generated by the absorption of light is a function of the intensity distribution within the sample, which depends on its scattering characteristics. Consequently, deviation from Beer's Law should, in principle, affect the thermal signal generated. We will show that for highly scattering media, the photoacoustic signal is strongly dependent on the scattering parameters. On the other hand, for optically thin solids and liquids the photoacoustic signal is insensitive to light scattering up to a scattering coefficient (λα) of ~1/ℓ where ℓ is the sample thickness, λ is the fraction of light scattered, and α is the total attenuation coefficient of a coherent beam.