F-Number

The focal length of lens divided by its usable diameter. In the case of a laser the usable diameter is the diameter of the laser beam or a smaller aperture which restricts a laser beam.

Failsafe Interlock

An interlock where the failure of a single mechanical or electrical component of the interlock will cause the system to go into, or remain in, a safe mode.

Femtoseconds

10^-15 seconds. 1 fs = 0.000,000,000,000,001 secs

Fiber Optics

A system of flexible quartz or glass fibers that use total internal reflection (TIR) to pass light through thousands of glancing (total internal) reflections.

Flashlamp

A tube typically filled with krypton or xenon. Produces a high intensity white light in short duration pulses.

Fluorescence

The emission of light of a particular wavelength resulting from absorption of energy typically from light of shorter wavelengths.

Flux

The radiant, or luminous, power of a light beam; the time rate of the flow of radiant energy across a given surface.

Focal Length

Distance between the center of a lens and the point on the optical axis to which parallel rays of light are converged by the laser.

Focal Point

That distance from the focusing lens where the laser beam has the smallest diameter.

Focus

As a noun, the point where rays of light meet which have been reflected by a mirror or refracted by a lens, giving rise to an image of the source. As a verb, to adjust focal length for the clearest image and smallest spot size.

Folded Resonator

Construction in which the interior optical path is bent by mirrors; permit compact packaging of a long laser cavity.

Frequency

The number of light waves passing a fixed point in a given unit of time, or the number of complete vibrations in that period of time.