What is a collimated beam?
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A collimated beam is a beam of light in which the light rays travel approximately parallel to one another. This means that the beam stays roughly the same width as it travels, rather than spreading out rapidly.
Collimated beams are most commonly produced using lasers. The light coming directly from a semiconductor laser diode is actually divergent: it spreads out as it leaves the diode. A lens or optical system is therefore used to collect the light and make the rays approximately parallel, producing a collimated beam.
No real beam remains perfectly parallel forever. Even a well-collimated laser beam gradually expands because of diffraction, but over the relatively short distances used in most medical and therapeutic applications, this expansion can be very small.
A familiar example is a laser pointer. The beam remains narrow enough to produce a small spot on a screen several metres away. At the other extreme, extremely well-collimated lasers can be directed towards reflectors on the Moon, although even these beams spread considerably by the time they get there.
LED light can also be made more directional using lenses and other optics, and can be approximately collimated. However, LEDs naturally emit light over a much wider range of angles than lasers, so achieving a narrow, highly collimated beam is considerably more difficult.
In practical terms, collimation means that the beam travels in a narrow, pencil-like path with relatively little divergence. If nine collimated laser beams are arranged in a 3 × 3 array, for example, they remain as nine separate beams and produce nine distinct spots when they reach the skin.