Irradiance is the power received by a surface per unit area, W/cm2. Radiant exposure (fluence) is the energy received by a surface per unit area, J/cm2. We already know that energy is power multiplied with a unit time. Fluence is the most common adjustable treatment parameter you will encounter in photothermal laser therapy.
The question now is how much fluence do I need to create a desired result? There is no short answer to this. It is not only the amount of energy that you put into the skin over a given area that is important. More important is what happens with the energy the moment it hits the skin. The fat and water content of the skin determine scattering for a large part. This means that the beam gets broken up in many smaller beams and various factors may cause it to increase or dilute the energy density in the area you need it. The same treatment will have a different result depending on the hydration level of the skin and the quantity of fat present in the treatment area. Apart from this, the density of target chromophores in the path of the laser beam also effects the fluence needed.
The spot size has a great influence on penetration depth and it has been shown in various studies that the larger the spot size is, the deeper penetrations the beam has. This is because of the phenomenon that scatter away from the target area mainly appears on the outer edge of the beam. Thus, the larger the spot size the more energy remains within the target area and it leads to deeper penetration depth.
Traditionally, high fluence laser for hair removal range up to 120 J/cm2. However, this has been challenged in various ways lately. Low fluence, high frequency diode lasers have become quite popular in the name of “pain-free” hair removal. This is where 10 J/cm2 to 20 J/cm2 are rapidly pulsed while the therapist constantly glides the hand piece over a larger area of 10 cm2 to 15cm2. It is believed that ‘staging’ of the heat will lead to destruction of the dermal papilla. From studies, it seems that the low fluence, high frequency motion technique has an advantage on areas with thick dark hair due to the reduction in pain. However, this seems not to be the same all over the body and in particular not the bikini area.
The larger the area over which you spread the energy, the lower the energy will be at any specific point within that area. If you have thick dark hair, which is usually the case for at least the first two treatments, the higher chromophore content requires less energy to create sufficient heat to kill the dermal papilla. Therefore, it seems that it may be beneficial to start a hair removal treatment series with a low fluence, high frequency technique and switch midway over to more traditional high fluence technique.
The depth of the target chromophore also determines the fluence require. If you treat a pigmented lesion on the epidermis, much less energy is required than treating a vein in the dermis. The deeper the target chromophore the more important a higher fluence become. The penetration depth of a laser beam is greatly determined on the machine side by the fluence in combination with the spot size.
Capability to generate a high energy level over a large enough area, thus high fluence is one of the factors that is greatly reflected in the purchase price of a machine. The luxury of a large spot size with sufficient energy for hair removal comes at a high price. Most manufacturers of low-end equipment tries to disguise this by eliminating fluence (J/cm2) as a parameter from the user interface and instead goes with energy (J) only.
Energy density (radiant exposure) and power density (irradiance).
