A uniform linear monochromatic soft X-ray source is created by combining a X-ray pinhole transmission grating spectrometer with a uniform line-fooused laser produced plasma. As an application of such X-ray source, a new method is described for calibration of X-ray photographic film that the response of film density versus relative exposure is provided by a step wedge filter foil of known thickness and transmissivity at a given wavelength. Based on this method, the 5F X-ray film is calibrated for the first time in the soft X-ray region.