In this article we present a novel method for absolute quantification of the flux and angular distribution of a beam source for atomic hydrogen or atomic deuterium. It is based on quantitative determination of the erosion of an amorphous, hydrogenated carbon (a-C:H) film. A heated tungsten capillary serves as the radical source. Atomic hydrogen is produced in this capillary by thermal dissociation of hydrogen molecules. A large-area a-C:H film is exposed to the source at a substrate temperature of 650 K. Interaction of atomic hydrogen with the a-C:H film causes erosion. From the spatial variation of the erosion rate one can deduce the angular distribution of the impinging hydrogen atoms. This angular distribution was also measured by mass spectrometry and showed excellent agreement with the erosion profile. The absolute flux of atomic hydrogen was also determined by mass spectrometry. With the absolute flux of atomic hydrogen known from mass spectrometry, measurement of the lateral variation of the erosion rate can be directly used as a probe for absolute quantification of the angular distribution of the impinging H (D) flux. The erosion yield is (2+/-0.7)X10(-2), which is consistent with the microscopic erosion mechanisms of a-C:H by atomic hydrogen known from the literature.