The use of an intensity scale sensitometer with a white light source does not simulate the discrete spectral emission by rare earth intensifying screens and as such does not truly measure the characteristic curve of rare earth screen film combinations. More traditional methods such as inverse square sensitometry are impractical in the oridinary clinical setting. The sensitometry cassette described here uses an intensifying screen with a neutral density filter to expose the x-ray film. Direct x-ray exposure of the film is eliminated by insertion of a lead glass plate between the intensifying screen and film layers. Both front and back emulsions may be exposed equally or the unequal exposure normally resulting from the absorption of x-ray photons by the front screen may be simulated. Optical spectometry of a lead glass indicates this method is valid for emissions of wavelengths greater than 400 nm which includes the rare earth phosphors. Comparison of characteristic curves obtained by this method and other more traditional methods will be shown to agree well. The system is simple, accurate, and tests can be formed on a clinically functional x-ray source and processing combination.