The Laser Diode (LD) delivers a divergent beam of true laser energy, with all photons are exactly the same color and in phase in time and space. With the addition of a set of collimating optics, a parallel beam is achieved, as in a laser pointer. This means the beam can be focused to an extremely small point with a lens, such as the ocular lens, showing how LDs can pose a real ocular hazard.
The LED also delivers a divergent beam, with most of the photons at the rated wavelength, but totally out of phase, giving non-coherent, yet quasi-monochromatic light. It is impossible to force collimation on LED energy, and therefore precise focusing is also impossible.
Semicollimation can, however, be achieved, giving a less divergent beam and hence increasing the photon intensity to give a better phototherapeutic effect.
Laser light flows in phase – it does not spread appreciably over distance. LEDs are non-coherent (spread) and do not travel as far due to signal degeneration. Summary – laser light is much more concentrated (coherent) than LED which makes it ideal for medical therapeutic use.
Lasers have been proven to help manage pain and have been cleared by the Food and Drug Administration (FDA) for that purpose. Devices that use LEDs may fall short of providing the power output (and tissue penetration of energy) required for pain management and inflammation.