Fig. 2.

Schematics of the experimental setups used to characterise gel wax properties. (a) Setup to measure the speed of sound and acoustic attenuation of gel wax. An ultrasonic pulser/receiver was used to a drive an ultrasound (US) transducer and receive reflections from a metal reflector. Gel wax-based slabs of different thicknesses were positioned between the US transducer and the reflector. The received signals were digitized by an analog-to-digital converter (ADC) and then transferred to a personal computer (PC) for processing. (b) Setup to acquire photoacoustic signals from gel wax slabs for spectroscopic and photostability measurements. Each slab was mounted on top of a rectangular acrylic frame with a thin plastic membrane. Excitation light was delivered through a fibre-coupled Nd:YAG pumped optical parametric oscillator (OPO), and a thin polyvinylidene fluoride (PVDF) ultrasound transducer was used to receive the photoacoustically-generated ultrasound waves. Signals from the PVDF transducer and the photodiode were captured simultaneously with an oscilloscope and transferred to a PC for processing.