lighting geekery: photoacoustic spectroscopy

I’m still amazed that after about 20 years specializing in lighting, I can still learn new things that lighting does.  Today’s crazy new thing: Photoacoustic spectroscopy.  Basically, light has enough energy in it to cause material to vibrate, generating acoustic signals that can be measured.

I find this interesting because I’ve felt the affect first hand – holding my hand in the beam of a super powerful, crazy-narrow beam LED fixture (which doesn’t have any IR or UV energy in the beam), you can feel the power of the photons.  Mostly what I felt is probably thermal absorption in my skin, but still — I “felt” the light.  This is a scientific application of that principle.

According to Wikipedia:

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 and through thermal expansion 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 also produce sounds.

A photoacoustic spectrum of a sample can be recorded by measuring the sound at different wavelengths of the light. This spectrum can be used to identify the absorbing components of the sample. The photoacoustic effect can be used to study solids, liquids and gases. [1]