Читать книгу Patty's Industrial Hygiene, Physical and Biological Agents - Группа авторов - Страница 77

Many types of lasers have been produced that vary in their wavelengths and temporal patterns of output. Hundreds if not thousands of different active laser media have been discovered, but only a few types have the characteristics and properties that favor widespread use and have properties suitable for industrial, scientific, or medical applications.

Depending upon how the excitation energy is applied and the characteristics of the internal laser cavity, the output beam of a laser will be either pulsed or continuous‐wave (CW). Some lasers, such as the solid‐state ruby laser or any glass laser, cannot normally be operated as a CW laser because of problems with overheating and resulting damage to the laser crystal and adjacent components.

The output of pulsed lasers can vary greatly in the duration and energy of individual pulses as well as their pulse repetition frequency (PRF). This performance depends upon many factors including among others, the nature of the stimulation process, the duration of the exciting energy, the optical configuration, and temperature of the laser cavity. A pulsed laser may be designed so that its output consists of pulses as short as a few femtoseconds (1 fs = 10⁻¹⁵ s) to many milliseconds. The pulses may be delivered individually, in groups or continuously over a broad range of frequencies. A laser with an output emission lasting more than 0.25 second is considered a CW laser for safety purposes (5).

The energy output of pulsed lasers is concentrated into very short pulse durations. This compression in time causes the laser radiation to be delivered much more rapidly than in a CW laser. The rate of energy delivery is termed power and is measured in watts (W). Power in watts is equal to the energy measured in joules (J) divided by the pulse duration in seconds (s), i.e. 1 W = 1 J s⁻¹. This means that short pulses can compress small energies into very high peak powers. A very small amount of energy delivered in extremely short pulse duration can achieve very high peak powers for brief periods of time. The absorption of such high peak powers can result in effects other than simple heating, i.e. optical “nonlinear” phenomena. These short pulses may be required for laser material surface ablation with minimal heat dispersion or for optical breakdown and plasma production as employed in some medical applications. The hazard can vary with exposure duration, since less and less energy is required to produce a threshold lesion in biological tissue (such as the retina) for shorter and shorter exposure durations (9).

Pulses may be produced so the laser energy is delivered over a wide variety of durations. A flash‐lamp driven (optically pumped) solid‐state laser will produce laser emission that follows the output of the flash‐lamp and will have a pulsed output of about 0.1–2 milliseconds (ms). Further compression or shortening of pulses can be achieved with special techniques. Two ways to achieve this are “Q‐switching” and “mode‐locking.” Q‐switching is an optical technique to compress the laser output into ∼4 to 20 nanoseconds (ns = 10⁻⁹ s, = billionths of a second, = thousandths of a microsecond). Mode‐locking is a technique that further compresses the emitted light into shorter pulses of 10 to 40 picoseconds (ps = 10⁻¹² s, = trillionths of a second, = millionth of a microsecond). Still shorter, ultra‐short pulses are measured in femtoseconds, and the shortest pulse for which we currently have guidelines for human exposure is 100 fs (2, 3, 5).

Ultra‐short laser pulses that produce so‐called “nonlinear effects” have safety implications. For example, the failure of laser eye protectors when irradiated by ultra‐short pulses has been an issue in recent years because of reduced attenuation during the pulse; this phenomenon has been termed “saturable absorption” or “reversible bleaching.” Wavelength broadening in fs lasers also can alter laser attenuation if the filter.

The very high radiance (“brightness”) of a laser (MW and GW cm⁻² sr⁻¹) is responsible for the laser's great value in material processing and laser surgery, but it also accounts for its significant hazard to the eye (Figure 4). When compared to a xenon arc or the sun, even a small He–Ne alignment laser is typically ten times brighter. A collimated beam entering the relaxed human eye will experience an increased irradiance of about 10⁵, 100 000 times. In other words, an incident laser beam with an irradiance of 1 W cm⁻² at the cornea creates 100 kW cm⁻² at the retina (9, 17). Of course, the retinal image size is only about 10–20 μm, considerably smaller than the diameter of a human hair. Thus, some workers may wonder: “So what if I have such a small lesion in my retina?” An individual does have millions of cone cells in his or her retina. But, the size of the retinal injury is always larger because of heat flow and acoustic transients following the initial absorption of energy, and even a small disturbance of a very small retinal area can affect vision. This is particularly important in the region of central vision referred to by eye specialists as the macula lutea (yellow spot), or simply the “macula.” The central region of the macula, the fovea centralis, is responsible for detailed “20/20” or “6/6” vision. Damage to this extremely small (about 150 μm diameter) central region can result in significant vision loss even though 99% of the total retinal area remains unscathed. The peripheral retina surrounding the macula is useful for movement detection and other tasks but possesses little acuity. After all, this is why the eye moves across a line of print for reading, because our detailed vision is limited to a very small angular subtense. Outside the 400–1400 nm retinal hazard region the cornea – and even the lens – can be damaged by laser beam exposure (9).

Under certain conditions, such as viewing by diffuse reflection, larger areas of the retina may be exposed to laser radiation. In this regard, the threshold energy for thermal injury increases with increasing image size. However, if the retinal image size doubles, with a resulting fourfold increase in retinal area the increased energy necessary to reach a damage threshold does not increase by four, but only by a factor of approximately two. This result is due to reduced efficiency of the retina to dissipate energy by radial heat flow for larger images. This inverse relationship of thermal injury threshold with increasing spot size is valid from approximately 25 μm to nearly 1.3–1.7 mm. These factors, as complex as they are, are all incorporated into the current laser safety maximum permissible exposures (MPEs), and their basis is presented in International Commission on Nonionizing Radiation Protection (ICNIRP) (3) and Lund and Sliney (18).