Photoacoustic spectroscopy Photoacoustic spectroscopy Photoacoustic spectroscopy is the measurement of the effect of absorbed electromagnetic energy (particularly of light) on matter by means of acoustic detection. The discovery of the photoacoustic effect dates to 1880 when Alexander Graham Bell showed that thin discs emitted sound when exposed to a beam of sunlight that was rapidly interrupted with a rotating slotted disk. The absorbed energy from the light causes local heating, generating a thermal expansion which creates a pressure wave or sound. Later Bell showed that materials exposed to the non-visible portions of the solar spectrum (i.e., the infrared and the ultraviolet) can
Photoacoustic effect added steady light does not produce any photoacoustic effect by itself, but changes the photoacoustic response due to the modulated/pulsed probing light. The resulting signal serves as a reference to all other measurements in absence of the background light. The photothermal part of the reference signal is maximal, since at photosynthetic saturation no energy is stored. At the same time the contribution of the other mechanisms tends to zero at saturation. Thus the reference signal is proportional to the total absorbed light energy. In order to separate and define the photobaric and photothermal contributions in spongy samples (leaves, lichens) one