MEASUREMENT OF A 2-LAYER SAMPLE USING PSEUDO PULSE EXCITED PHOTOACOUSTIC SPECTROMETRY

To obtain surface layer thickness easily, a simple pseudo pulse excite d photoacoustic spectrometric method was proposed. An argon ion laser was chopped mechanically to generate a pseudo pulse (pulse width; 6.6 ms, duty 1.67 %), which was then led to a sample enclosed in photoacou stic cell. Two layer samples made of polymethyl methacrylate (PMMA) we re used as model samples. The photoacoustic signal waveform observed s howed a maximum from the negative edge of the pseudo pulse of the lase r. The delay of the signal increased concomitant with the sample surfa ce thickness. The delay time of the signal was calculated by a cross-c orrelation method. A linear relationship was obtained between the dela y time of the photoacoustic signal from the input pseudo pulse and the surface transparent layer thickness in the range of 0 - 90 mm. The re gression line between the film thickness x (cm) and the delay time was expressed with the thermal diffusivity of the film kappa, as follows; Delta tau (s) = 1.16 x 10(-1). K--1/2. x + 0.006. Using this result, the method proposed was successfully applied to the measurement of the thickness for laminated polyethylene film on papers. The method propo sed is simple and easy to perform without any modification of usual ph otoacoustic instrumentation.