3. Visualization, qualitative or quantitative methods?

In general, physics is not satisfied with a qualitative approach. The question of the "nobility" of visualization methods therefore arose very early on. Quite apart from any controversy, the real question is: what physical quantities can be measured using a visualization image? An initial answer can be found in the work of Reynolds (see historical box): of course, observing a flow of dye in a stream of water and highlighting eddies is eminently qualitative; carefully measuring the characteristic parameters of an observed regime change is already quite quantitative!

Indeed, this apparently insidious question leads to a different way of classifying visualization methods than the phenomenological approach of the previous paragraphs; four main families emerge.