Light safety4
Ophthalmic instrumentation
1.1 INTRODUCTION
e goal of this chapter is to discuss the hazard, in particular
laser light. We will cover a number of topics that the reader may
think they are familiar with, but in the case of safety, repetition is
not a bad thing. In addition, there is coverage of a number of laser
incidents. e author of this chapter hopes that by the end the
reader will have an appreciation for the potential harm they may
be exposed to. A number of resource materials will be referenced,
and the author wishes to thank those individuals who allow open
access to their material.
An awareness of laser safety is extremely important, not
only for the benet of users and ancillary/support sta but also
to answer patient questions and concerns. With the increase
in an aging population comes an increase in eye diseases.
Ophthalmology has responded with technology such as mydriatic
and nonmydriatic fundus cameras, phacoemulsication devices,
femtosecond lasers, supercontinuum light sources, and optical
coherence tomography. ese advances support the need for
understanding an appreciation of laser safety.
1.2 WHY ALL THE CONCERN OVER
LASERS?
With all the hazards one faces during their day, why do we
feel laser safety deserves special attention? While laser light or
radiation has properties that distinguish it from natural light,
the answer is rather simple. Visible (400–700 nm) and near-
infrared wavelengths (700–1400nm) are focused by the lens
of the eye to a spot size an order of magnitude smaller than
that of natural or incoherent visible light, a 300–200µm spot
size to one of 10–20µm. us producing an irradiance (power
per square centimeter) much higher than incoherent light. e
item that gets lost sometimes is while no one expects to look
directly into a laser beam, with this magnication of 100,000
to the macula even a small reection of a laser beam between
400 and 1400nm has the potential to cause some level of
retinal injury.
See Figures 1.1 and 1.2 for a graphic display of this point.
A1mW/cm
2
beam in is 100 W/cm
2
at ones macula.
1.3 CONTINUOUS WAVE
Laser systems that can produce uninterrupted laser energy are
called continuous wave (CW) lasers. As long as they are turned
on, a beam of laser light is emitted. By convention, any laser that
emits light for longer than 0.25 s is called a CW laser. External
shutters can be used to “chop” the beam so it has a strobe
appearance, but the output power remains constant even in the
chopped beam. e seemly pulse of the bar code scanner is an
example of a chopped CW laser. e power output of CW lasers
is measured in units of watts.
1.4 PULSED LASERS
Pulsed laser systems emit a beam that is less than 0.25 s in
duration. e 0.25 s demarcation between CW and pulsed
lasers is based on the approximate time for a human reex to
very intense light to work (aversion response). e pulsing in a
pulsed laser system is achieved internal to the laser. Some employ
an electrical “Q-switch” or are “mode locked” to achieve the
shorter pulse widths. e pulse is dened by the pulse width
(oremission duration) and the number of pulses per second
known as pulse repetition frequency. Unlike the CW laser,
all of the energy is emitted in short bursts. e total energy is
compacted into a shorter time interval so the peak energy output
can be very high. is makes shorter pulses more hazardous.
Many pulsed lasers have pulse widths measured in nanoseconds
(1 billionth of a second). Pulse rates to attosecond (10
18
s) pulse
widths have been routinely produced (but only in vacuum).
A laser must output pulses at a rate faster than 1Hz to be
considered a repetitive pulsed laser. e output is measured in
terms of Joules (Watts × seconds). Turnkey units are available for
the generation of femtosecond laser pulses, 10
15
s.
With the development and renement of the laser diode,
a highly reliable and convenient laser source became available
for medical applications. A source that is robust and does not
require an in-house service person to keep it running has few
maintenance needs. Many laser systems combine the diode or a
diode array with a ber optic system. e ber optics is used as a
means to deliver the laser radiation without the use of open beam
paths or beam enclosures.
1.10.5 Report 5: MDR Report Key 4105971 ophthalmic femtosecond laser, lensex laser system 12
1.10.6 Report 6: Report Key 4076806 wavelight EX500 excimer laser 12
1.10.7 Report 7: MDR Reporting Key 3735743 Wavelight FS200 femtosecond laser 13
1.10.8 Report 8: MDR Report Key 4124364 Catalys precision laser system ophthalmic femtosecond laser 13
1.10.9 Report 9: Laser re 13
1.10.10 Report 10: Model No. Purepoint LIO nature of problem, malfunction 13
1.10.11 Report 11: Event, negligence 13
1.10.12 Report 12: Device problem, device inoperable 13
1.10.13 Report 13: LASIK eye surgery 13
1.10.14 Report 14: Medlite C6 13
1.10.14.1 Manufacturer response 13
1.10.15 Report 15: Electrical shock 13
1.10.15.1 Manufactures response 14
Bibliography 14
Worth reading references 14