[en] Linear absorption coefficient of materials for penetrating light depend on its energy. γ-ray which is emitted by radioactive materials, the linear absorption coefficient for penetrating material can be found from its table. In the case of x-ray, it cannot be decided easily, because x-ray is not monoenergitic. In this investigation radiographic method is applied to determine the practical value of the linear absorption coefficient for an x-ray instrument, by using film detector. Type of this film is Structurix Industrex-C, which is manufactured by KODAK. The results showed that the values of practical linear absorption coefficient depend on the voltage difference of its anode and cathode, i.e. the smaller of the voltage difference, the bigger is the value of the practical linear absorption coefficient. The connection between the voltage difference (KV) and the value of the practical linear absorption coefficient (μ) of an x-ray instrument for certain materials, represent the characteristics of the x-ray instrument. (author)