## Power Density

### Power

Power and energy are closely related. Power is the rate at which energy is delivered, not an amount of energy itself.

__Formula: __

Power = Energy / Time 1 Watt = 1 Joule / Second

Therapeutic Energy = Power (Watts) or Joules/sec x Time (sec).

### Power Density

Power density or Irradiance refers to the __amount of power delivered per unit area__. Power density indicates the __degree of concentration__ of the laser output. It is expressed in __Watts per square centimeter____ (____W/cm2__), or miliWatts per square centimeter (mW/cm2). Some studies have concluded that the power density may be of even greater significance than the dose.

__Example:__ A laser’s output is 4 Watts, and it is illuminating a circle of 3 centimeter diameter.

First find the area of the circle, 3.14 x 1.5 x 1.5 = 7 cm2.

Then divide the power by the area, 4W / 7cm2 = 0.6 W/cm2.

__Formula:__ Irradiance (PD) = W/cm2

### Energy Density(ED) / (Fluence)

The energy density expresses the __total amount of energy delivered per unit area__, in __Joules per square centimeter (J/cm2__). The energy is measured in __Joules__, and is calculated by multiplying the power output of the laser times the amount of time elapsed during the laser treatment.

__Example:__ A 4 Watt continuous wave laser would deliver 240 Joules in one minute.

(4 Watts x 60 seconds = 240 Joules)

Then simply divide the total energy by the area to arrive at the energy density in Joules per centimeter squared.

__Formula__: Fluence (ED) = Power x Exposure Time, measured in Joules/cm2 (Watts x Seconds)

Key: __The amount of energy delivered, determines the magnitude of the laser interaction within the tissues and the individual cells.__

Effect of Laser Spot Size on Tissue Distribution of Light Energy

A beam of light incident on tissue may be:

- Reflected
- Absorbed
- Scattered

Scattering in tissue broadens the incident beam, decreasing the effective fluence in the intended target area.

__For effective penetration, light needs to avoid scattering and surface absorption__.

Doubling the spot size will increase the effective volume by a factor of eight. A larger spot size usually enables faster and more effective treatment in dermatologic applications such as treatment of vascular lesions, laser hair removal, etc. However, more photons must be supplied by more complex and expensive power supplies, components, and delivery devices. As a general rule, doubling the spot size and halving the fluence will yield an equivalent effective fluence at a given depth. This effect becomes more pronounced with increasing depth.