ABSTRACT
The lowest order transverse electromagnetic mode output of a laser is a beam with a spherical gaussian wavefront. The characteristics of this beam are typically radially symmetric about its axis, the principal direction of propagation. This chapter reviews the relevant Commission Internationale de l'Eclairage terms that are most widely accepted internationally and provides them appropriately to discuss, clarify, and define the diameter of a laser beam. There are several terms in vogue used to express the diameter of a laser beam. The half-power beam diameter is that diameter of the beam core which contains 50% of the total radiant beam power, k = 1.1744. Radiometry is the technology surrounding the measurement of electromagnetic radiant power and energy across the entire spectrum of wavelengths. Photometry, which involves the sensitivity of the eye and has its own units, is that part of radiometry that deals only with the visible portion of the electromagnetic spectrum ranging approximately 400-720 nm.
