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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

Spectral-Band
Wavelength
Designation
Laser
Wavelength (nm)
Vacuum-Ultraviolet
10-200nm
Near-Ultraviolet
100-280 nm
UV-C

Argon-Flouride

193

Neodymium: Yag (quadrupled)
255
280-315nm
UV-B
Xenon-Cloride
308
315-400nm
UV-A

Helium-Cadmium

325
Ruby (doubled)
347.1
Krypton
350.7,365.4
Nd:YAG (tripled)
355
Nitrogen
337
Argon
351.1,363.8
Visible
400-700nm

Helium-Cadmium

441.6

Argon
497.9,467.5,etc.
Helium-Selenium
460.4-1260

Neodymium: YAG
(doubled)

532
Helium-Neon
632.8
Krypton
647.1,530.9,etc
Ruby

694.3

Rhodamine 6G
(dye laser)
450-650
Near-Infrarer
700-1400nm
IR-A
Gallium-Arsenide
905
ND:YAG
1064
Helium-Neon
1080,1152
Mid-Infrarer
1400nm-3μm
IR-B
Erbium:Glass
1540
Far-Infrared
3μm-1mm
IR-C
Carbon Monoxide
3390
Helium-Neon
4000-6000
Hydrogen-Flouride
5000-5500
Carbon Dioxide
10,600
Water Vapor
118,000

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.

eye components

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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