Two-scale factor universality in O2: experiments under density gradient

In this study, we show that light transmission measurements directly on the image
of a critical fluid sample under magnetic compensation of gravity as provided by a
solenoid can give turbidity data with the same precision as the classical
techniques using a laser beam. The novelty of this technique is noticeable as it
also authorizes both a detailed observation of the sample and the local
measurement. When density gradients exist, as it is often the case in space
experiments, different image zones can be chosen and compared, the more turbid zone
corresponding to a density closest to the critical density. It was shown that light
turbidity data, a quantity due to density -induced refractive index fluctuations,
could be directly obtained from image analysis. As an example, images of oxygen
taken near their critical temperatures of 155K are analyzed and values of
isothermal compressibility and fluctuation correlation lengths are compared with
literature values.