Everything You Need To Know About Class 3 vs. Class 4

Many clinicians ask us: What is the difference between Class 3 and Class 4 Lasers?

There is a slew of false information in the public domain, perpetuated mainly by the manufacturers of these devices, regarding the effectiveness and cellular mechanisms activated during class 4 laser light irradiation. Many class 4 laser manufacturers are intentionally or unintentionally misleading healthcare practitioners into believing that higher power and longer NIR wavelengths equate to deeper tissue penetration and better clinical efficacy. Nothing could be further from the truth.

The truth is…

Unfortunately, all of these claims turn out to be fancy sales gimmicks, as they have not the standing in the clinical or scientific journals to support their claims. The clinical and scientific facts are clear that because of the very high absorption of NIR laser light by water at wavelengths greater than 950 nm, 99% of the energy produced at this wavelength or above is absorbed before penetrating the dermis of the skin, leading to a high risk of thermal damage and a low depth of penetration. Promoting that a laser is a class 4 laser states absolutely no information about the wavelength of the device, but simply informs the purchaser about the risk of thermal tissue damage.


A CO2 laser (wavelength = 10,600 nm), for example, is a common class 4 laser that is absorbed in the first 10 microns (0.0004 inches) of tissue; thus, primarily in the epidermis. The same holds for the excimer (XeCl, wavelength = 308 nm) laser which is also absorbed in the epidermis. At 970 and 980 nm, the depth of penetration is less than 300 microns (< 0.01 inches); thus, total absorption is achieved within the dermis of the skin. For any given wavelength, the tissue properties are determined by the reduced scattering (µs’) and absorption (µa) coefficients of the specific tissue structures resident in the tissue.

These scattering and absorption coefficients determine the penetration depths and ultimately govern the overall depth of penetration of a laser beam. A Class 4 laser typically has higher incident power at the tissue surface and if the beam is defocalized to prevent charring tissue, then a larger treatment area; however, the depth of penetration is superficial and is restricted to a few hundred microns at best (i.e.: the top layer of the dermis). Even with higher incident powers and large treatment areas there is no biochemical effect due to lack of photobiochemical targets at the wavelengths used and hence cellular mechanism activation; therefore, the thermal effects of a class 4 laser are the only mechanism of action remaining. Once the thermal effects of tissue have been exceeded, tissue damage is guaranteed.

Certain manufacturers use the limited knowledge of their customers to claim that a Class 4 laser has greater efficacy than a class 3B laser. This is unsubstantiated rubbish. Laser classification (Class 3b versus Class 4) is only used according to IEC-60825-1: 2014 guidelines to determine the possible risk for eye and skin damage and has nothing to do with the efficiency or efficacy for that matter in treatment of conditions. Laser classification is determined by not just a question of optical output power, but also wavelength, divergence of the beam, emission area, pulsing parameters, exposure rates, et cetera.

Regarding Class 4 high power lasers, it has not been proven in the scientific and clinical literature that high power is better than low power, in fact, the opposite has been proven to be true. As mentioned above, there is a therapeutic “optical” response window between 600 and 950 nm and a biphasic dose response curve governed by the Arndt-Schulz law, within which the positive bioregulatory effects occur.

The use of CLT in animals and humans almost exclusively involves light in the range above 600 nm and below 950 nm with the maximum effective “optical window” ranging from 650 nm to 930 nm.11

As an example, a class 4 laser emitting 880 and 970 nm laser light at 10 W average power with a beam surface area of 10 cm² producing a radiant exposure of 1000 mW/cm²; thus, dramatically exceeding the safe exposure limits known as the Maximum Permissible Exposure (“MPE”) limits, which range from 200 mW to 500 mW/cm² depending on wavelength (typically 200 mW/cm2 in the visible range from 400 to 700 nm and increasing up to approximately 500 mW/cm2 in the NIR range); therefore, these devices need to be treated as thermal invasive devices.

The use of class 4 lasers have a significant potential of delivering non-optimal treatment doses of laser energy due to their lack of penetration and excessive MPE; thus presenting a greater risk of burning patients, particularly with dark hair follicles.

Let’s say that you wish to deliver energy to a tissue surface of 1 cm² with a dose of 10 joules/cm² of energy. With a 10 Watt laser, this would take one second of treatment time. If however, you wished to deliver 2 to 4 joules of energy to the same surface area, which is a more common therapeutic dose, this would take 0.2 to 0.4 seconds.

Most Class 4 manufacturers treat up to 5 minutes with their technology, thus they have exceeded the therapeutic dose of tissue not only in wavelength by being outside the optical window, but also in power by exceeding the MPE by 20 times and the therapeutic dose by 500 times. This logic suggests that too much power and the wrong wavelength simply equates to the expense of more money without the requisite return in better clinical effects. I therefore regard lasers with output powers exceeding 500 mW as unnecessarily strong and downright dangerous to conduct CLT treatments.

Class 4 lasers for phototherapy is not new and not innovative, as such lasers have been on the market for years, but have been approved strictly for surgical applications; such as: general surgery and tissue ablation for port wine stains, spider veins, et cetera. Just advertising the advantage that a laser is class 4 and hence, is a better instrument than a class 3B laser is akin to claiming that the Chrysler 600 is a better vehicle than the Mercedes Benz 500, just because the number is higher.

The above criticism is directed towards the gross generalizations and false claims of vendors of Class 4 lasers who purport their use for therapeutic purposes, not against the use of class 4 lasers for their eligible claims in laser surgery and tissue ablation.

One thing remains certain, current scientific and clinical research proves that class 3B lasers are best suited for therapeutic applications and class 4 lasers are best suited for tissue destruction.


1) In vivo effects of low level laser therapy on inducible nitric oxide synthase. Moriyama Y,   Nguyen J, Akens M, Moriyama EH, Lilge L. 3, March 2009, Lasers Surg Med, Vol. 41, pp. 227 -231)

2) Primary and secondary mechanisms of action of visible to near-IR radiation on cells. T, Karu.  1, Mar 1999, Photochem Photobiol, Vol. 49, pp. 1-17

3) Mechanisms of Low Level Light Therapy, T.N, Hamblin M.R and Demidova. [ed.] SPIE. 2006. Vol. 614001

4) Nitric oxide and mitochondria. GC. Brown. 12, Jan 2007, Front Biosci, Vol. 1, pp. 1024-1033

5) Novel effects of nitric oxide. Davis KL, Martin E, Turko IV, Murad F. 2001, Annu Rev Pharmacol Toxicol, Vol. 41, pp. 203-236