Divided into two equal beams for special applications.


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

Threshold Limit

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.

Transformation Hardening

A process well-suited to lasers which involves hardening metal by heating it to the critical temperature for transformation, quenching it, and solidifying it with uniform distribution of its carbon content.

Transverse Electromagnetic Mode

Abbreviated TEM. The cross-sectional shape of the working laser beam. An infinite number of shapes can be produced, but only a relatively small number are needed for industrial applications. In general, the higher the TEM, the coarser the focusing.