Abstract: A method and apparatus for the radiometric calibration of an X-ray detector, such as a spectrometer, for wavelengths in the range of 2 to 200 angstroms. A hot filament X-ray diode for generating a point source of X-rays is disposed within a vacuum manifold and is surrounded by a housing having two ports equally spaced from the point source. A center line of each port passing through the point source forms substantially the same angle with respect to the line defined by the stream of electrons passing from the cathode to the anode of the X-ray diode so that the apparent intensity of the point source through each port is substantially identical. A reference detector is placed opposite one port to intercept a known portion of an X-ray beam emitted therefrom, while the detector to be calibrated is placed opposite the other port for interception of a known portion of an X-ray beam passing through that port.