Emissivity determination using the photoacoustic effect

We show that emissivities in the near infrared can be determined relative to a reference surface employing the photoacoustic effect. The photoacoustic cell is equipped with two windows and a pair of synchronously moving chopping wheels so that the cell alternately views the test and the reference surface. The acoustic signals produced in the cell are detected with a microphone and that output is fed to a lock-in amplifier. The temperature of the test surface is varied to produce a null in the lock-in amplifier, which permits determination of a relative emissivity. Results of measurements for several plastic and metal surfaces are reported.