Salt hydrates are often used as heat storage materials according to their interesting thermal properties (high latent and specific heat). Their main drawback is their large supercooling, where the material may remain liquid below its melting point. However, this property should be turned into a benefit if a reliable way of triggering crystallization is developed, as heat could be released on demand. Sodium Acetate Trihydrate (SAT, CH3COONa.3H2O), which can exhibit 90K of supercooling, was used for this study. For a better understanding of how heat is released after crystallization triggering, caused by seeding, a fully new method was developed to measure the samples’ opacification, using a HeNe laser (633 nm) coupled with a photodetector, measuring the solid fraction thanks to the difference between liquid and solid specular transmittance. This technique was compared with temperature measurement and image processing, showing consistency between the different methods.