Photomedicine Parameters

Device power (watts)

The term "low level laser therapy" refers to the use of light at much lower levels than those used for tissue ablation or photocoagulation. Newer, high-powered therapy devices that deliver power similar to surgical lasers (but with a less concentrated beam) no longer constitute low level laser therapy. Class IV devices, for example, routinely heat tissue, rendering the term "cold laser" inaccurate.

Photomedicine units that produce more intense beams reduce the time required to deliver treatment. That said, ideal "dosing" of light for specific clinical conditions (described below) remains controversial, considering that a wide range of photomedicine treatment applications have demonstrated benefit. In general, the advantage of more intense beam strength relates to stronger analgesia, whereas less intense light with a longer period of application offers better tissue healing results.

Spot size (cm²)

A laser's power, combined with spot size, determines power density (see below). The spot size amounts to the area to which the photons affect the target. A small spot size cones down the light to a concentrated area, whereas a larger spot size spreads it out, reducing the intensity per cm² As such, two devices or applicator heads that emit light with dramatically different spot sizes can change treatment characteristics and temperature elevation even if the power is identical.

Typically, the spot size of an applicator head does not change, meaning that the user must be cognizant of the clinical significance of a small or large spot size in terms of how it affects the delivery of light. That is, if treating a broad expanse of tissue, a device with a larger spot size (eg, 1 cm²) will accomplish the desired outcome more efficiently than a device with a much smaller spot size. Furthermore, delivering a large amount of light to a tiny area increases the risk of excessive heating and tissue burn.

Power density (W/cm²), or "dose" (J/cm²)

Power density describes the amount of photons directed to a site, which in turn provides information about the energy and heat delivered to a treatment target.

Power densities usually range from 10–100 mW/cm², and energy densities range from 4–50 J/cm². Some practitioners use higher power and energy densities to attempt to treat deeper joint, spinal, or brain problems.

The specific dose(s) of laser required to heal tissue and treat pain remains unclear. Calculating actual joules of energy delivered is considerably complex and, ultimately, uncertain. Fortunately, a wide range of doses has shown benefit for human patients and experimental animals, despite the wide variety in size, color, and hair coat.

Wavelength (nm)

Therapeutic photomedicine devices typically use red (630–680 nm) or near infrared (700–1100 nm) light, although lasers and LED units that emit visible purple, blue, and green light, with shorter wavelengths, have also become available.

Some laser therapy units emit 2 or more beams to target multiple tissues and varying depths.

Laser beams differ from other types of light therapy, including LEDs, as they are monochromatic (existing within a narrow band of wavelengths), coherent (tightly aligned), and collimated (photons travel in parallel). The more light scatters within tissue, the less intense the effect, which may or may not be the desired outcome. However, debate continues about the relative value and differences between laser light and LEDs.

The types and depth of tissue that respond to light therapy depend on the wavelength delivered. Certain molecules, such as melanin and hemoglobin, preferentially absorb light in the 630—670 nm interval. To reach deeper tissues, wavelengths (810 nm, 980 nm) in which photons proceed through superficial layers unabsorbed, reach deeper sites such as bone, nervous tissue, and internal organs. The ideal wavelength for photobiomodulation of nervous system tissue reportedly ranges from 810 nm to 830 nm.

Pulsation (Hz) and pulse width

Photomedicine devices have frequency settings that can provide a "pulsed" treatment versus continuous wave. Pulsation frequency provides information about the on/off sequencing of light, whereas pulse width indicates how long the light is on versus off. Pulsing reduces the number of photons delivered per unit time and allows tissue to cool during the "off" periods.

The clinical benefits of both regular "pulsed" light delivery and of "super-pulsed" remain unclear. "Super-pulsed" photomedicine devices deliver high-powered light (25 W or greater) in nanosecond bursts. This may allow the number of photons to reach a clinically significant level, deep in the tissue, while offsetting the risk of thermal injury with the brief exposure, allowing cells to cool between bursts. Laser device manufacturers are increasingly introducing units with super-pulsing options.