The magic 4 J/cm²

By Jan Tunér

From early publications on the subject of PBM and wound healing, the dose of 4 J/cm² appeared to be close to an optimum. While still rather valid, this belief is also an intellectual trap for many persons. Because what is a J/cm² - really?

We have to understand that these early papers were performed on superficial structures, meaning that 4 J/cm² for deeper structures is something quite different. In a superficial wound, a lot of the applied energy is absorbed by the tissue. Only a few mm below the surface, there are no 4 J/cm² any longer. Maybe only 0.4 J/cm².

But possibly the most important misconception is the importance of time. In the early days of PBM, the output of the HeNe lasers was very low, rarely as high as 10 mW. In order to reach the level of 4 J/cm², these lasers had to be applied for a long time. To apply 4 J/cm² to a wound of 1 cm² and using 10 mW you had to irradiate the area for 400 seconds. More than 6 minutes! Not very practical but probably effective, irradiation time in itself has been proven to be an important factor in PBM (Castano et al. 2007, downloadable at

Another problem with the dose is its correlation with the energy. And here the size of the probe enters. With a thin probe, a high dose can easily be achieved, even if a very modest energy has been applied. Dose as well as energy have to fall within the “therapeutic window”. In the picture below, an orange aperture (laser eye) of 0.25 cm² is applied in contact in the middle of the imaginary green area of 1 cm². If 1 J is applied, the calculation of the dose (J/cm²) is: 1 J divided by 0.25 cm² = 4 J/cm²! So the 1 J (energy) resulted in a dose of 4 J/cm².

Figure 1

Now, the same laser probe is kept at a distance and the divergent light from the laser will cover a larger area, in this case 1 cm² (Figure 2). Then the calculation becomes 1 J divided by 1 cm² = 1 J/cm². This means: same energy, different dose. Then, if only the dose is reported, and not the energy, what do we know? Sometimes the spot size and the dose are reported, leaving the reader to calculate the energy for himself. Several studies have reported a reasonable dose (J/cm²) but used a very thin probe. The dose is then quickly reached, but insufficient energy is applied to the target.

Figure 2

With the advent of the indium red lasers, output power of PBM lasers increased considerably and quite possibly shorter time with higher power could result in similar or even better results. And certainly it made the use of PBM more attractive for clinicians. But still, is more always better? Saving time is fine, if possible. But research shows (Castano) that time in itself is an important factor. The cells need have time to adapt. Imagine waking up a child by tenderly tapping her over the cheek several times over a minute. Works well for both parties. But imagine adding the force in all those tender taps into one skin-tingling slap! The kid will wake up for sure, but how?

Figure 3

So much for the dose = J/cm². But what about the power density – mW/cm²? In Figure 2, it is obvious that the energy is spread over a larger area. This means fewer density of photons in the irradiated area. It makes sense that the cells will react differently. But if a laser 10 times stronger is used, things change. In Figure 3 we can see that the single joule (J) can be achieved in a very short or a very long time. The biological response will be different.

Now, if you found this difficult, stop here and try to digest the central message: dose as well as energy always need to be considered and duly reported. If you want to go deeper into the complicated subject of light parameters, you are recommended to download this paper: