The angular momentum of a particle can be distinguished in external (orbital) or intrinsic (spin) depending on whether it changes with a shift of the axis about which is calculated. After reviewing the origin of optical angular momentum, I will answer the question: Is the orbital angular momentum of a field intrinsic or external? A proper analysis of this issue involves the study of the angular momentum spectrum. An interferometric experiment is proposed to measure efficiently the exact angular momentum spectrum and variance for light beams with any arbitrary spatial distribution.