The angular width of a radiation beam. With respect to a conical beam of light, it is the vertex angle of the cone. The beamwidth is a measure of the rate of divergence or convergence of a light beam. Laser divergence is often specified as that circular cone which contains a percentage (usually 90) of the total beam energy. The least beamwidth of a laser or other optical system is determined by its aperture. In radians, the least beamwidth is approximately l
/A where l
is the wavelength and A the aperture. No optical instrument can have a beamwidth less than this.