In general, a light beam is a slender stream of light. Light is an electromagnetic wave. Electromagnetic waves can be characterized by wave fronts. A wave front is a surface where the electromagnetic field of light is oscillating in the same phase. In the geometric optics approximation, light can be presented as a family of rays. These rays are always perpendicular to the wave front. Often the user needs a collimated beam—a beam of light in which all rays are parallel to each other and the wave front is a plane. Such a beam, in some cases, can be provided automatically by using an appropriate laser or can be formed by special optics (with possible significant loss of light energy) using conventional light sources such as lamps.
A laser beam is a group of nearly parallel rays generated by a laser; a light beam with a Gaussian shape for the transverse intensity profile: if the intensity at the center of the beam is I0, then the formula for a Gaussian beam is I = I0 exp(-2r2/w02), where r is the radial distance from the axis and w0 is the beam “waist” (the narrowest part of a Gaussian beam). The intensity profile of such a beam is said to be bell-shaped. Besides laser beam, a single-mode fiber with a core diameter of several microns also creates a Gaussian beam at its output.
The “spreading” of a light beam in general, and in particular of a laser beam as it moves away from the laser or light source is called beam divergence. The initial beam divergence of the light source is important for a light beam focusing on the target and for controlling the light spot diameter on the target surface. Typically, in order to control light-treatment effects, the light beam is focused onto the target surface by a lens, and the distance between output lens and target surface is varied to provide the needed light spot size and power density within the area of treatment. The radius of the beam in the focal plane of the lens with a focal length /is given by: w=f-9, where в is the beam divergence. Single-mode lasers or single-mode fibers have a minimal beam divergence and can provide minimal light spot size. Minimal light spot size in the focal plane of the aberration free optical system can be close to the wavelength l.