Speckle pattern is the result of the scattering of a coherent wave. When using an imaging sensor it looks like grains that are the result of random constructive or desctructive interferences. The size of the grains is a constant: it depend of the aperture of the light coming from the illuminated scattering object or in other words the max angle of light incoming. To identify it, it is good to check if the grain size match more or less with your optical configuration. The contrast of the speckle grains depends on the scatterer: the more it randomize the phase of the wave, the more contrast you have. Maybe you should give more clues about your experiment configuration: the source, the sensor , the geometry as a start.

Dear Laurent, thank for your answer

Dear Ramiro, the speed of the boiling effect should be also considered. Since, if you have a sensor with a limited speed some of the grains might be blurred. The contrast test should be the best approach to test this issue. See LASCA technique. Otherwise, I suggest the adoption of MHI technique if you want an online monitoring.

Dear Roberto, thank for your answer. I had began the studies recently, your answer is a good explanation for it.

Thousand thank

Ramiro

Dear Ramiro. Effectively, Prof. Braga got the point. A test with a laser speckle contrast imager should be performed to discriminate possible blurring effects.