Beam diameter is a measurement of the size of a beam of light or other electromagnetic radiation, such as a laser. It is the diameter on any line that is perpendicular to and intersects the beam axis, and is double the length of the beam radius. For a circular beam, its length is defined as the length of a line segment that passes through the center of the beam and has its endpoints on the beam's opposite edges. If the beam is elliptical, its diameter can be specified as the length of either the major or minor axis of the ellipse. If the beam does not have circular symmetry, the beam width is often referred to instead.
Most electromagnetic beams do not have sharply defined edges, as solid objects do, and beam divergence means that their width is not constant along the entire length of the beam. Thus, there are a number of ways to define the beam's diameter. Beam diameter measurement is done with a device called a laser beam profiler. The point on the beam where the beam diameter is narrowest is called the beam waist.
Beam diameter is an important attribute of lasers. Beams with a larger diameter suffer less beam divergence, which is a measurement of how rapidly the light of the beam spreads from the beam waist. Beams with low divergence thus have higher beam quality, a measurement of how tightly focused a laser beam remains as it travels. A beam's optical intensity is the amount of optical power the beam delivers per unit of area at the target, so a laser with low beam divergence will have greater optical intensity than a beam with the same optical power but higher beam divergence. This is important for many laser applications, such as cutting, drilling, and remote welding in industry and laser microscopy in biological science.
There is a trade-off between laser beam quality and the size of the laser, as a laser with a smaller lens has a smaller beam diameter and will suffer greater beam divergence, all other things being equal. Making a laser smaller, which is often desirable for reasons of convenience and cost, while maintaining high beam quality requires improvements in other parts of the design. This can be done by using higher quality optical components, optimized resonator design and alignment, and the use of laser gain media that is less prone to distorting thermal effects such as thermal lensing.