Photoacoustic spectroscopy

Photoacoustic spectroscopy ( PAS), also optoacoustic spectroscopy ( OAS) called, is a physical examination procedure, that is based on the photoacoustic effect. This was first described in 1880 by Alexander Graham Bell. In this case, a medium is fed in quick succession by flashes of light energy. The resulting continuous switching between heating and cooling results in an alternating expansion and contraction, and this vibration can be performed under appropriate circumstances, as sound.

The effect has been used since the 1970 years in various areas to investigate, among other gases, solids and tissues.

Solid surfaces

For material testing an absorbing surface is heated by modulated light. The time-dependent temperature generated by the thermal expansion of the sound (or ultrasound) in the solid state. This can be measured, for example, with a piezoelectric detector.

Via thermal conduction to the wide temperature fluctuations in the gas at the surface due to thermal expansion and produce pressure fluctuations or noise in the gas. For detection is used in this variant a normal microphone.

Performs the germ periodically at different modulation frequencies, can be distinguished under the surface within limits according to the depth. Similarly, PAS - analyzes can also be performed in solutions.

Investigation of gas components

For the investigation of gases made ​​of the fact that gas molecules absorb light only very specific wavelengths. Which is used is dependent on the chemical structure.

For the detection of a specific gas component in a mixture of a laser light pulses are used, the frequency of which can be absorbed only by one type of molecule. The measured in a container with the gas from a microphone sound waves are then progressively louder the higher its share of the gas mixture. Through acoustic resonance, the volume can be increased by up to a factor of 100 ( 40 dB).

The method can be used for example for emissions testing, or the detection of air pollutants. Here about concentrations of methane in nitrogen can be detected by only ten parts in a billion.

Use simpler instruments as a light source an infrared emitter in communication with a narrow-band filter. For them, the acoustic excitation between the examined sample and a known gas is compared. Both are doing in adjacent chambers that are separated by a thin membrane. Are there differences between the concentrations of the component to be examined in both gases, the membrane begins to vibrate.

Tissue studies

In medicine and biology of the photoacoustic effect is exploited as an imaging method. Photoacoustic tomography ( PAT) operates at very fast laser pulses (less than ten nanoseconds), which generate in the tissue to be investigated ultrasound. The difference to conventional ultrasound examination is that not only " echoes " are investigated on the surface of organs, but these produce the sound itself. Therefore the possible images are similar in detail of the computer tomography without causing dangerous radiation would be produced.

In laboratory experiments, the method was also used for measurement of the blood sugar level of diabetics.