Flame speed is extremely important as it affects the performances of many industrial systems. More- over, its significance makes it a major target for the validation of kinetic mechanisms, which explains the necessity to provide ever more accurate data. Flame speed dependence on pressure and temperature conditions is interestingly assessed using, among others, spherically expanding flame in constant volume chambers. In these conditions, the flame speed derivation, based solely on the pressure evolution in the chamber, requires empirical models. The current study describes a perfectly spherical chamber with full optical access allowing simultaneous recording of the pressure inside the chamber and, fully innovative, of the flame radius evolution until the flame vanishes at wall. A careful description of the new set-up and of the accuracy of the measurements, in particular of the flame radius, is presented here. In parallel with experiments, one-dimensional transient simulations were carried out to identify the limits of the proposed new method. Then, the simultaneous use of pressure and flame radius information is compared to the traditional constant volume method based on empirical models. A first advantage relies in the direct detection of the development of instabilities during the flame propagation. In addition, although the flame speed is extremely sensitive to the flame radius determination, the actual experimental accuracy allows significant improvements in terms of accuracy, notably as initial pressure and temperature are elevated. This new set-up will allow major advances in the measurement of laminar flame velocity under extreme thermodynamic conditions.