During excitation of the laser medium, this is the point where lasing begins.

This term is applied to laser protective eyewear (LPE) filters, protective windows and barriers. The TL is an expression of the “resistance factor” for beam penetration of a laser protective device expressed in W·cm^-2 or J·cm^-2. It is taken to be the maximum average irradiance or radiant exposure at a given beam diameter for which a laser protective device (e.g., barriers, LPE, curtains, windows, etc.) provides adequate beam resistance. Thus, laser exposures delivered on the protective device(s) at or below the TL will limit beam penetration to levels at or below the applicable MPE.