An optical diagnostic is reported which measures the parameters of an intense hydrogen or deuterium neutral beam by observation of the Doppler shifted Balmer alpha or beta lines emitted by fast atoms in the beam at several locations in the neutralizer. The spectrum of blue shifted and red shifted light emitted along optic axes which lie in planes perpendicular and parallel to the grid rails of the ion source is recorded by an optical multichannel analyzer which is interfaced to a computer. The light is Doppler shifted sufficiently to resolve the three energy components of the beam resulting from the H+, H+2, and H+3ions accelerated by the ion source. From the Doppler broadening of each peak the beam divergence parallel and perpendicular to the accelerator slots can be measured. A model is presented whereby the energy distribution can be determined from the intensity of each peak. The spectroscopic results for energy distribution are in agreement to within 5% with momentum analysis by a magnetic mass spectrometer.