Chapter 2

Terms and Definitions

Although a form of electromagnetic radiation, because of its characteristics, lasers
present us with a new set of terms and definitions. Some of these pertain to laser
systems and some pertain to the eye, the organ of primary concern for laser injury.
Each is important for understanding the hazard that a particular laser system may
pose.

2.1 Radiation Characteristics

The pulse duration is the duration (usually msec, μsec, or nsec) of a pulsed laser
flash, usually measured as the time interval between the half-peak-power points on
the leading and trailing edges of the pulse. If the energy is delivered over a shorter
period of time, say nanoseconds, instead of milliseconds, the potential for tissue
damage is greater because the tissue does not have sufficient time to dissipate the
deposited energy.

The pulse repetition rate describes how often during a time period (i.e., Hz,
kHz) the laser is allowed to emit light. If the pulse repetition rate is low, tissue may
be able to recover from some of the absorbed energy effects. If the repetition rate is
high, there are additive effects from several pulses (rather than from a single pulse)
over a period of time.

The wavelength, is the distance between two peaks of a periodic wave. It
is inversely related to the frequency, f, the number of waves per second, and is also
inversely related to the energy (i.e., the shorter the wavelength, the greater the energy;
E = h = hc/). Table 2.1 lists the various optical band designations along
with some of the common laser systems. Tissue penetration of electromagnetic energy
depends upon wavelength. Some wavelengths in the infrared region penetrate
deeper into the tissue than certain wavelengths in the UV region. Theoretically, every wavelength has its own penetration characteristics. Other considerations pertaining
to penetration include percentage of water in an organ, the reflectivity or
focusing characteristics at the surface of the tissue, etc.

Table 2.1: Optical Spectral-Band Designation

Lasers are characterized by their output. The output of a continuous-wave
laser is normally expressed in watts, W, of power and the output of a pulsed laser
is expressed as energy in joules, J, per pulse. For pulsed systems, multiplying the
output by the number of pulses per second (repetition frequency) yields the average
power in watts (W = J/s). The peak power for a pulsed laser depends upon the
pulse duration. The shorter the duration, the higher the peak power. Peak powers
for very short duration pulsed lasers can be in the terawatt (TW or 1012 W) range.

Pulsed laser output is normally characterized by the radiant exposure or energy
density which is the magnitude of the energy flux and describes the quantity of
energy across the face of the beam that is arriving at a tissue surface at any one point
in time, expressed in joules/cm2. The greater the energy, the greater the potential
for damage. CW laser beams are characterized by the irradiance or power density,
the rate of energy flow per unit area in the direction of wave propagation, typically
measured in units of mW/cm2 or W/m2. This is a factor of both the output and
beam diameter (usually expressed in mm).

2.2 Components of the Eye

From the laser effects viewpoint, the eye is composed of several subsystems: light
transmission and focusing, light absorption and transduction, and maintenance and
support systems.

Figure 2.1: Eye Components

Transmission and Focusing System

The cornea is the transparent membrane which forms part of the front of the eye and
separates it from the air. It covers the colored portion (iris) and the pupil of the eye.
The cornea is continuous with the sclera (white of the eye). The greatest amount
of refraction of the laser beam takes place in the cornea. The cornea transmits
most laser wavelengths except ultraviolet and far-infrared irradiation which, at high
energies, may burn it.

The sclera or the “white of the eye” is the white membrane which forms the
outer envelope of the eye, except its anterior (front) sixth which is occupied by the cornea. The iris and pupil are the colored diaphragm with an aperture (pupil) in
its center. The iris is composed in large part of muscular tissue which controls the
amount of light entering the eye by widening (dilating) the pupil at twilight, night,
and dawn; narrowing (constricting) the pupil at daylight. Therefore, eye-hazard
lasers are much more dangerous under low light conditions; more wavelengths enter
the eye through the wide pupil hitting the retina.

The lens is a transparent structure located immediately behind the iris and pupil
which focuses light on the retina. It thus forms one of the refractive media of the
eye. Visible and near-infrared light pass through the lens, but near-ultraviolet light
is absorbed by it. The aqueous humor is the water-like liquid between the cornea
and the iris. The vitreous humor, the jelly-like substance filling the eye between the
lens and the retina, is transparent to both visible and near-infrared radiation. The
vitreous humor also serves as a structural support for the retina.

Absorption and Transduction System

The retina lines the inside of the eyeball and consists primarily of photoreceptors
and nerve cells. The nerve cell layer lies on top of the photoreceptor cells but is
transparent, so light entering through the pupil actually passes through the nerve
cell layer before reaching the photoreceptor cells. Beneath the nerve cells is the
pigmented epithelium of the eye, it is a layer of cells in which pigment able to
absorb scattered light and stop light reflection is formed. Light is focused by the
cornea, lens, and various fluids of the eye onto the layer of rods and cones of the
retina. These photoreceptor cells convert the energy of absorbed light into nerve
impulses. These impulses are received by the nerve cells which transmit them along
nerve fibers from layer to layer through the retina to a nerve complex, the optic
nerve, that leads to the brain through the back of the eye. The retina is particularly
sensitive to laser irradiation since the laser beam is well focused on it. This is true
for visible and near-infrared laser beams. For example, all the light entering a 5
mm pupil is converted to an image 0.05 mm or smaller in diameter on the retina,
multiplying the energy density 10, 000-times or more. If the beam enters the eye
through binoculars or other magnifying optics, it is more dangerous since the energy
concentration may increase up to a million times. The retina is composed of the
macula, fovea, and retinal periphery.

The macula lutea or macula, is the area in the retina that is in direct line with
the visual axis. The eyes are fixed in such a manner that the image of any object
looked at is always focused on the macula. In the macular region, the inner layers of
the retina are pushed apart, forming a small central pit, the fovea centralis, or fovea.

The fovea is the central 1.5 mm area at the back of the eye. The fovea is the only
part of the eye in which precise vision takes place, enabling location of small and
distant targets and detection of colors. If an object is looked at directly, imaging
takes place at the fovea inside of the macula. If the object happens to be a laser
beam sufficiently strong to cause tissue damage, sharp vision is lost and the person
may be blinded; barely able to see the top letters on the eye chart and unable see
colors. The fovea and fine visual function can also be affected by retinal injuries
occurring at some distance from the fovea. Many injuries, especially those caused
by lasers, are surrounded by a zone of inflammation and swelling which, when it
extends into the region of the fovea, can reduce foveal function. The actual degree of
visual impairment will depend upon the location and extent of both injury and the
inflammatory response. Generally, the closer the injury is to the fovea, the greater
the chance of severe dysfunction.

The retinal periphery, all of the retinal area surrounding the fovea, is involved
in a variety of functions. Because it has a high concentration of photoreceptor
cells which operate during dim or dark conditions, night vision is one of its primary
functions. During bright conditions the peripheral retina detects motion (peripheral
vision). Unlike the fovea, however, the peripheral retina is unable to detect small
or distant objects or to distinguish between fine shades of color. A laser injury
restricted to this portion of the retina will have a minimal effect on normal visual
function. Workers with isolated laser injuries in the retinal periphery may report
having been dazzled at the time of exposure and may detect a dark spot (blind
spot) in their peripheral vision; they should be able to perform all fine visual tasks
normally. After a time, a worker will adapt to the presence of small- to mediumsized
blind spots. Even though the retina may be permanently damaged, the worker
will eventually become unaware of it. Laser injuries which involve large portions of
the peripheral retina may cause large defects in the individual’s peripheral vision.
These will always be a noticeable impairment and the worker will always be aware
of these.

Maintenance and Support Systems

The maintenance system consists primarily of the choroid, a rich network of blood
vessels on or behind the retina. If this network is injured by a laser beam, it bleeds
and may lead to partial or complete, temporary or permanent blindness. The eyelids
are the most relevant parts of the support system; they may be able to limit a laser
exposure to 0.25 seconds, the duration of the blink reflex. The eyelids themselves
may be burned by high energy infrared laser irradiation together with surrounding skin and the cornea.

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