Light (heat) is easily dispersed when travelling through air, so any little bit of air between the applicator and the skin will affect your results. Especially with dry skin which has lots of tiny air pockets. It also serves to reduce internal reflectance thus increasing the transmittance of your applicator’s light (heat). When applied to the skin it serves as a coupling medium that creates a tight bond between the skin and the applicator letting light (heat) transmit directly to the skin. It also serves to protect both your light guide and most importantly your patient. The “barrier” created by the coupling gel prevents the skin, hair and other particles that may be on the skin to make direct contact with the light guide. The viscosity of your coupling gel is extremely important since it must be able to withstand the heat generated by your applicator and still perform its primary purpose. The weaker the viscosity the quicker it will start to melt away thinning the bond between your applicator and the patient’s skin. This allows air in which increases reflectance ultimately weakening the application strength resulting in poor results; not to mention the risks of burning your patient or damaging your light guide. Minimum application thickness should always be maintained at 3 millimetres (depending on the viscosity) but the more coupling gel you use the better.