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 which 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 l0(-34) Joule-sec).
PHOTOSENSITIZERS Chemical substances or medications which 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.
PIGMENT EPITHELIUM A layer of cells at the back of the retina containing pigment
granules.
PLASMA SHIELD The ability of plasma to shop transmission of laser light.
POCKEL'S CELL An electro-optical crystal used as a Q-switch.
POINT 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 to
a single plane, rather that the innumerable planes rotating about the vector axis.
Various forms of polarization include random, linear, vertical, horizontal, elliptical
and circular.
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 necessary prerequisite for laser action.
POWER The rate of energy delivery expressed in watts (joules per second). Thus:
1 Watt = 1 Joule x 1 Sec.
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 protective housing is 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, it 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.
PULSE MODE Operation of a laser when the beam is intermittently on in fractions
of a second.
PULSED LASER Laser which delivers energy in the form of a single or train of
pulses.
PUMP To excite the lasing medium. See Optical Pumping or 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.
