P

Phase

Waves are in phase with each other when all the troughs and peaks coincide and are “locked” together. The result is a reinforced wave in increased amplitude (brightness).

Photocoagulation

Use of the laser beam to heat tissue below vaporization temperatures with the principal objective being to stop bleeding and coagulate tissue.

Photometer

An instrument that measures luminous intensity.

Photon

In quantum theory, the elemental unit of light, having both wave and particle behavior. It has motion but no mass or charge. The photon energy (E) is proportional to the EM wave frequency (v) by the relationship: E=hv, where h is Planck’s constant (6.63 x 10^-34 Joule-sec).

Photosensitizers

Chemical substances or medications that increase the sensitivity of the skin or eye to irradiation by optical radiation, usually to UV.

Picosecond

A period of time equal to 10-12 seconds.

Planoconcave lens

Lens with one cancave surface (curves in) and one flat surface.

Planoconvex lens

Lens with one convex surface (curves out) and one flat surface.

Plasma shield

The ability of plasma to stop transmission of laser light.

Pockel’s cell

An electro-optical crystal used as a Q-switch.

Pointing source

Ideally, a source with infinitesimal dimensions. Practically, a source of radiation whose dimensions are small compared with the viewing distance.

Pointing errors

Beam movement and divergence, due to instability within the laser or other optical distortion.

Polarization

Restriction of the vibrations of the electromagnetic field vector to a single plane. Prevents optical losses at interfaces between the lasing medium and optical elements.

Population inversion

A state in which a substance has been energized, or excited, so that more atoms or molecules are in a higher excited state than in a lower resting state. This is a necessary prerequisite for laser action.

Power

The rate of energy delivery expressed in Watts (Jjoules per second). Thus, 1 Watt = 1 Joule x 1 second.>

Power meter

An accessory used to measure laser beam power.

PRF

Pulse Repetition Frequency. The number of pulses produced per second by a laser.

Protective housing

A device designed to prevent access to radiant power or energy.

Pulse

A discontinuous burst of laser light or energy, as opposed to a continuous beam. A true pulse achieves higher peak powers than that attainable in a CW output.

Pulse duration

The “on” time of a pulsed laser. May be measured in terms of milliseconds, microsecond, or nanosecond as defined by half-peak-power points on the leading and trailing edges of the pulse.

Pulsed lasers

Lasers that emit energy in a series of short bursts, or pulses, and are inactive between each pulse. Pulsed lasers—not to be confused with modulated lasers—typically deliver several Watts of peak power per pulse.

Pulse mode

Operation of a laser when the beam is on intermittently in fractions of a second.

Pump

To excite the lasing medium. See pumping or optical pumping.

Pumped medium

Energized laser medium.

Pumping

Addition of energy (thermal, electrical, or optical) into the atomic population of the laser medium. Necessary to produce a state of population inversion.