Metallic dust explosion is a challenging research topic. One key point of investigation is the flame propagation velocity, which can be determined experimentally by direct visualization of the explosions. This paper presents the results of flame propagation during an aluminum dust explosion inside a vertical prototype of 700 mm height and 150x150 mm square cross section, with glass walls. The study considers direct visualization: the light emitted by the aluminum flame is recorded with a fast camera. Aluminum flames look white and highly luminous. A special attention is therefore requested to collect images without saturation. Indeed, aluminum flame images from literature are mostly saturated; therefore comparison of images obtained with and without saturation is of high interest. The flame propagation velocity, which corresponds to the flame speed in the laboratory referential, is determined from saturated and non-saturated images. The burning velocity, i.e. the consumption rate of the reactants by the flame front, is then presented. With saturated images, the flame surface area is under-estimated, around 10-20 %, yielding to over-estimated burning velocity.