This study defines, then formalizes, in microdensimetric analyses of wood, the notion of optimum exposure to X-rays, taking account of technical restraints: characteristics of wood specimen, type of film, standard, source of radiation, exposition parameters (distance source-film , tension, intensity, time of exposure). One of the most important problems is the conversion of the film's optical density in wood specific gravity of the material analysed. The described method allows to avoid the use of the continued coin's radiography of the standard, commonly used instead of the optical glass coin of the microdensitometer. It formalizes a theoric model of exposure conditions. Besides, it shows with an example that it's easy, by experiments, to get this optimum in every particular case, by changing the peak voltage and the exposure time.