CLASS II & III LASERS:
LLLT uses phototherapy to the use of photons at a non-thermal irradiance to alter biological activity. The photons are absorbed by mitochondrial chromophores in skin cells. Consequently, electron transport, adenosine triphosphate (ATP) nitric oxide release, blood flow, reactive oxygen species increase, and diverse signaling pathways get activated.
The most significant difference between these laser classes is not penetration depth. That is simply a marketing ploy. The contrast of the laser classes solely comes down to class II & III lasers use phototherapy
(Non-thermal), and class IV lasers use Photophysical (Thermal).
CLASS IV:
Class IV lasers are labeled as "High-intensity Lasers." The power output is anything above 500mW (0.5W). They are FDA cleared under the ILY product code along with Lamps, Infrared, and Therapeutic heating devices. The ILY category started from a heating pad and has expanded to thermal lasers, intended to provide topical heating by elevating tissue temperature to relieve muscle and joint pain temporarily. To classify a device under the ILY product code, the FDA requires heat performance testing that verifies the device will reach an idle temperature of 40⁰C and maintain that temperature throughout the treatment duration. This heat production leads to a photophysical effect that dilates blood vessels and increases blood flow. While this type of therapy may have some clinical benefits, it should not be interpreted or described as LLLT.
Class IV lasers claim to have a deeper depth of penetration to create biostimulation faster than class III lasers. However, this theory of depth of penetration has been debunked by several published journals and research articles. As stated above, Class IV lasers create a photophysical effect, not photochemistry.
The FDA code ILY is 510(k) exempt, meaning no clinical data is required to submit for clearance. This allows Class IV laser to file a substantial equivalent to the heating lamp to gain FDA market clearance, and then market depth of penetration.