Photoacoustic spectroscopy is a technique in which the absorption of periodically intensity-modulated light is detected by the sound that it produces at the (acoustic) modulation frequency in a closed volume of gas or liquid in thermal contact with the material absorbing the light. The spectroscopic aspect of the technique relies on the ability to scan the wavelength of the light that stimulates the sound. Thus one can determine the absorption as a function of wavelength through the conversion of absorbed energy to heat and thence to sound. The acoustic detection is carried out synchronously with respect to the imposed intensity-modulation frequency on the light, making it possible to use narrow-band noise reduction. The existence of this technique for detecting sound generated by the absorption of light led Wong and co-workers to investigate the possibility of using the same techniques for microscopy.