LASER BEAM SAFETY
Eyes can suffer severe damage when they are not protected adequately when working with laser beams. Choosing the correct protection depends on lens density and color which is based on the wavelength and power of the specific laser being used., A good example is that workers who might be exposed to direct laser beams or reflections, should not use plastic goggles Instead for adequate protection, these workers should wear filter safety-glass goggles. A severe retinal burn from a laser beam was the subject of an Office of Environment, Safety and Health (ES&H) in January 1986. Study is found in Serious Accident bulletin (Issue No. 11, January 1986).
Even though the worker was using a laboratory-approved plastic laser safety goggles, the laser beam melted a hole in the goggles. According to the Investigators. their findings were that this injury would not have occurred had the worker been wearing filter-glass eye protection. Testing indicated that safety glasses made of KG-3 filter glass provided better protection for those working in laboratories where there are high-peak power and high-average power (repetitively pulsed or continuous wave) lasers.
In determining laser eye protection it should be selected based on how well it will protect the eye against the maximum exposure anticipated, while allowing the greatest amount of light possible to enter the eye to ensure proper sight.
The booklet also provides step-by-step guidance for selecting the correct eye protection for various laser operations and is available from the Laser Institute of America, 12424 Research Parkway, Suite 130, Orlando, FL 32826 (Phone: 407-380-1553).
ANSI Standard Z136.1-1986, "Safe Use of Lasers," also provides guidance on choosing the correct protective eyewear when working with lasers. All laser users should comply with the guidance provided in ANSI Z-136.1, and questions centered on this standard are used during radiation protection appraisals.
An essential element of on-the-job eye safety, is wearing the correct type of eye protection. The following recommendations provide guidance for minimal eye injuries.
- Worksite hazard analyses is important in determining what eye hazards exist and what type of eye safety equipment will provide the right protection.
- Safety procedures that clearly identify the appropriate eye protection should be posted in work areas where such protection is required.
- Providing instruction before hiring those workers whose tasks require eye protection, and also scheduling hands-on training in the selection and usage of appropriate eye protectors on a regular basis.
- Ensure that no exemption from wearing eye protection in areas where it is required, is given to anyone, including top-level managers, public officials, or other visitors to the work site.
- In addition to using machine guards, engineering controls, etc, use eye protection as well and not as a replacement.
- Always make sure that eye protection fits properly (i.e., firmly and comfortably, without restricting vision).
- If prescription eyewear is worn by a worker, they should wear eye protection as well. Although most plastic lenses of eyeglasses are impact-resistant (by Federal regulation), they are not meant for use in hazardous situations. (Contact lens wearers should use the same type of eye protection as those who do not wear prescription eyeglasses.)
- Eye protectors should be selected on the basis of the exact narrow waveband being transmitted by the laser, choosing the correct lens color and density for that specific waveband.
- To protect the eye against the maximum exposure anticipated for a particular wavelength and viewing duration, select eye protectors that best afford the protection required.
- Use administrative controls to ensure that eye protectors are worn and that any other hazards (e. g., reduced visual capacity of the wearer) are controlled.
What are the effects of laser energy on the eye?
The site of damage depends on the wavelength of the incident or reflected laser beam:
- Laser light in the visible to near infrared spectrum (i.e., 400 - 1400 nm) can cause damage to the retina resulting in scotoma (blind spot in the fovea). This wave band is also know as the "retinal hazard region".
- Laser light in the ultraviolet (290 - 400 nm) or far infrared (1400 - 10,600 nm) spectrum can cause damage to the cornea and/or to the lens.
Are there any specific symptoms of laser eye injuries?
- Exposure to the invisible carbon dioxide laser beam (10,600 nm) can be detected by a burning pain at the site of exposure on the cornea or sclera.
- Exposure to a visible laser beam can be detected by a bright color flash of the emitted wavelength and an after-image of its complementary color (e.g., a green 532 nm laser light would produce a green flash followed by a red after-image).
- When the retina is affected, there may be difficulty in detecting blue or green colors secondary to cone damage, and pigmentation of the retina may be detected.
- Exposure to the Q-switched Nd:YAG laser beam (1064 nm) is especially hazardous and may initially go undetected because the beam is invisible and the retina lacks pain sensory nerves. Photoacoustic retinal damage may be associated with an audible "pop" at the time of exposure. Visual disorientation due to retinal damage may not be apparent to the operator until considerable thermal damage has occurred.
What types of laser safety eyewear are available?
Â Â Â Goggles: Fit tightly on the face, they are typically worn over vision-correcting prescription eye glasses and are usually constructed with frame vents to minimize lens fogging. Larger, heavier than spectacles or wraps.
Â Â Â Spectacles: A frame that usually has two separate lenses with side shields that can be made with vision-correcting prescription eye glasses.
Â Â Â Wraps: A frame with a single lens that covers both eyes and is usually lighter than spectacles/goggles
What are the technical considerations for eye safety?
Â Â There are two important concepts:
- (MPE), or Maximum Permissible Exposure is the level of laser radiation to which a person may be exposed without hazardous effects or biological changes in the eye. MPE levels are determined by exposure time and pulse repetition, as a function of laser wavelength. The MPE is usually expressed either in terms of radiant exposure in J/cm2 or as irradiance in W/cm2 for a given wavelength and exposure duration.
- Exposure to laser energy above the MPE can result in tissue damage.
- The ANSI 136.1 standard defines MPE levels for specific laser wavelengths and exposure durations. Generally, the longer the wavelength, the higher the MPE; the longer the exposure time, the lower the MPE.
- (NHZ) or The Nominal Hazard Zone is the physical space in which direct, reflected or scattered laser radiation exceeds the MPE. LSE must be worn within the NHZ.
- In practical terms, when using dermatologic lasers the entire laser procedure room should be considered to be within the NHZ because the laser fiber or hand piece can be directed anywhere in the room.
What factors should be considered when selecting specific eyewear?
- Laser wavelength at which protection is afforded.
- Optical density (OD) of the LSE for the wavelength being used. OD refers to the ability of a material to reduce laser energy of a specific wavelength to a safe level below the MPE. It can be expressed by the following formula:
OD = log10(Ei /Et)
Ei = incident beam irradiance (W/cm2) for a "worse case exposure"
Et = transmitted beam irradiance (MPE limit in W/cm2)
Example: OD of 4.0 allows 1/10,000 of the laser light energy to be transmitted.
The required OD for any given laser can be determined by:
The OD of the LSE will decrease if the LSE is damaged. The damage threshold refers to the maximum protection that the LSE will provide for at least 5 - 10 seconds following noticeable melting or flame.
- consulting nomograms or tables (e.g., ANSI 136.1 guidelines), or
- consulting the laser manufacturer.
- Comfort of the design to enhance compliance.
- Field of view provided by the design of the eyewear.
- Absence of irreversible bleaching when the LSE filter is exposed to high peak irradiance.
- Effect on color vision: the colored filter material may reduce color vision and contrast, creating additional hazards. For example, certain LSE may interfere with visualizing monitoring equipment or detecting cyanosis during general anesthesia.
- Impact resistance. LSE must be resistant to dust, heat, etc., so that they will not loose their effectiveness.