Visualisation of convective flow patterns in liquid helium

The advantages of using liquid helium as the investigative fluid in Rayleigh-Benard experiments are reviewed. A low temperature shadowgraphy apparatus is described that permits convective flow pattern visualization in liquid helium, thus overcoming the main disadvantage until now of using quantum fluids. The factors involved in maximizing the optical resolution for both the shadowgraph and schlieren methods are examined for several fluids and this discussion is applied to the low temperature apparatus in assessing its performance. Some preliminary results on pattern formation in liquid helium are presented.