The simplest and most efficient of the gas lasers. An axial flow of gas is maintained through the tube to replace those gas molecules depleted by the electrical discharge used to excite the gas molecules to the lasing state. See Gas Discharge Laser.

Also Optical Axis.  The optical center-line for a lens system; the line passing through the centers of curvature of the optical surfaces of a lens.

Any device(s) employed to partially (e.g., laser attenuative translucent window materials) or fully (e.g., opaque curtains) diminish or restrict the transmission of incipient laser light.

Hardware assembly or optical device, such as a mirror, capable of changing laser beam direction; used to re-point the beam and in folded, compact delivery systems.

The diameter of that portion of the beam which contains 86% of the output power.