Schlerien optic images allow you to observe (among other things) changes in the density of air that you would not otherwise be able to see clearly. The heat plume from a torch changes the refractive index of the air air it is heated and using this technique you can actually image the air currents as they flow up in the space above the flame. I've seen images of several different types of flames (torches included)and IMHO the no-hood approach often fails to capture the entire torch plume. The bigger the torch, the more heat and turbulence there are to deal with. There are some of these systems that work OK, but these are usually in a high end studio or industrial setting. Home built systems often tend to be underpowered and under sized. My personal opinion is that a properly powered and vented hood is the better option for a small studio. You should also smoke test any ventilation system (with a real tester) with the torch running and you in working position to be sure that you really know where your torch plume is going.
Robert
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