Optical Eye Safety
Electromagnetic radiation from lasers can cause eye and skin injuries. The greatest danger is to the eye due to its inherent sensitivity to light. Injuries to the eye can be extremely painful and can result in permanent loss of vision. The hazards are spectrally dependent and require appropriate control measures. The optical sources used in this laboratory include lasers with pose a moderate risk of injury. They emit infrared (invisible) light. In addition to the general safety rules for a laboratory environment, the following safety rules must be observed.
- The lasers must have a warning label and the laser area must have a DANGER sign similar to that shown in Figure 2.
- The laser beam and its specular reflection must not be viewed directly or with any type of collecting optics. (This practice is recommended for all lasers regardless of classification.)
- The beam paths must be kept below eye level for both sitting and standing positions at all times. Beam blocks should be used so that the beam does not leave the tablThe beam should only be monitored indirectly by viewing it from a diffuse surface and by using a photodetector.
- Block the beam or turn off the laser when inserting optical components, e.g. lens and photodiodes, which may produce unpredictable specular reflections.
Note that laser light exiting the end of an optical fiber may be as hazardous as the original beam. Light sources in this laboratory do not represent a skin exposure risk.
Figure 2 — DANGER sign for Class 3 lasers
The eye responds differently to radiation in different parts of the optical spectrum. The eye consists several components including the cornea, the lens, the vitreous humor, and the retina as shown in Figure 3. The cornea is the transparent outer layer that has protective functions and provides part of the focusing power of the eye. The lens is located behind the iris and provides variable focusing power, i.e. accommodation. The vitreous humor is the fluid within the eye. The retina is located on the back interior surface of the eye and contains the light sensitive elements which are known as rods and cones.
Figure 3 — Structure of the Human Eye
Optical radiation or light is subdivided into ultraviolet, visible, and infrared portions of the electromagnetic spectrum (see Figure 4). It has smaller wavelengths, or larger frequencies, than radio waves and longer wavelengths, or larger energies, than X-rays. The wavelength l is associated with color in the visible portion of the spectrum and is conventionally the preferred designation rather than frequency f or energy. Note that wavelength is typically defined as the wavelength in vacuum and that wavelength l is related to frequency f by lf =c where c is the speed of light.
Figure 4 — Optical Spectrum
Visible radiation with wavelengths between 0.4 µm and 0.7 µm and near-infrared (invisible) radiation with wavelengths between 0.7 µm and 1.4 µm pass through the cornea and lens and are focused on the retina. This light is concentrated as much as 10,000 times by the eye! Retinal burns and other damage caused by excessive exposure cannot regenerate and may result permanent vision loss or blindness.
Mid- to far-infrared radiation with wavelengths greater than 1.4 µm and ultraviolet radiation with wavelengths less than 0.4 µm are absorbed in the cornea and the lens. Excessive exposure can lead to cataract formation in the lens and to burns and inflammation in the cornea. Permanent vision impairment is possible.
GENERAL SAFETY MEASURES
Lasers are powerful sources of electromagnetic radiation. They are particularly dangerous for the eye since their high collimation and coherence allow high concentrations of energy on the retina. Safe operation depends on wavelength, output power or energy, and continuous or pulsed operation. According to the American National Standards Institute’s (ANSI) Z136.1 Safe Use of Lasers (1993) standard, lasers are grouped according to hazard classifications. The higher the classification number, the more safety measures are required. The risk associated with each class are given below.
- "Class 1 denotes lasers or laser systems that do not, under normal operating conditions, pose a hazard. Direct viewing of the beam or its specular (mirror-like) reflections is included which is known as intrabeam viewing.
- Class 2a denotes low-power visible lasers or laser systems that are not intended for prolonged viewing, and under normal operating conditions will not produce a hazard if the beam is viewed directly for periods not exceeding 1000 seconds.
- Class 2b denotes low-power visible or laser systems which, because of the normal human aversion response (i.e. blinking, eye movement, etc.), do not normally present a hazard, but may present some potential for hazard if viewed directly for extended periods of time (like many conventional light sources). The aversion response time is conservatively estimated as 0.25 seconds.
- Class 3a denotes two groupings of lasers and laser systems. The first grouping includes lasers that normally would not injure the eye if viewed for only momentary periods (within the aversion response period) with the unaided eye, but may present a greater hazard if viewed using collecting optics (lens systems). A CAUTION label is required for this grouping. Another group of Class 3a lasers are capable of exceeding permissible exposure levels for the eye in 0.25 s and still pose a low risk of injury. DANGER labels are required for these lasers.
- Class 3b denotes lasers or laser systems that can produce a hazard if viewed directly. This includes intrabeam viewing of specular reflections. Normally, Class 3b lasers will not produce a hazardous diffuse reflection.
- Class 4 denotes lasers or laser systems that produce a hazard not only from direct or specular reflections, but may also produce hazardous diffuse reflections. Such lasers may produce significant skin hazards as well as fire hazards."
Laser Institute of America, Laser Safety Guide, ninth edition, (Laser Institute of America, Orlando, FL, 1993) pp. 13-14.
The recommended safety requirements differ for lasers within each of these classifications and are based on maximum permissible exposure (MPE) levels for various viewing conditions. For Class 1 lasers, there no safety requirements, but the laser should be labeled as Class 1 and the beam should not be viewed directly as a precautionary habit. For Class 4 lasers, eye protection for diffuse reflections are required and extensive access and control measures must be used.