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---1 TRADITIONAL LASER SAFETY
The traditional way that laser safety has been addressed for decades is laid out in the American National Institute Standard for the Safe Use of Laser Z136.1.
This can be found in the Hazard Evaluation and Classification section of the standard.
Several aspects of the application of a laser or laser system influence the total hazard evaluation and thereby influence the application of control measures:
1. The laser or laser system's capability of injuring personnel or interfering with task performance
2. The environment in which the laser is issued
3. The personnel who may use or be exposed to laser radiation
Preceding this hazard evaluation protocol is the appointment of a laser safety officer or advisor, who will be referred to in this text as the LSO. It’s the LSO's responsibility to see that laser safety is adequately addressed at a facility or institution.
---2 LIFE CYCLE
A more comprehensive evaluation approach is to expand this evaluation from three components to five and then add two life cycle phases: design and disposal.
The five components are:
1. The laser or laser system's capability of injuring personnel
2. The beam path of the laser system
3. The interaction of the laser beam with its intended target
4. The environment in which the laser is used
5. The personnel who may use or be exposed to the laser radiation
---2.1 THE LASER OR LASER SYSTEM'S CAPABILITY OF INJURING PERSONNEL
Here are the factors of the laser source itself:
1. What type of laser? Here one is interested in details such as whether it uses a pulsed or continuous wave (CW) and the nature of the wavelengths being generated, that is, ultraviolet, visible, near infrared, mid- or far infrared? Each of these will have some effect on the nature and level of control measures and hazard the laser posses to an individual.
2. What is the output of the laser? Are we talking about milliwatts, nanojoules, megawatts, or joules of output? These answers and the wave length will have a dramatic effect on possible laser protective eyewear requirements.
3. What is the classification of the laser? In the research and development (R&D) environment almost all lasers are class 3R(3A), 3B, or 4.
---2.2 THE BEAM PATH OF THE LASER SYSTEM
You need to consider what happens to the laser beam once it leaves the laser source. In a very similar way you consider your commute from leaving the security of your garage to your work destination. Are you one of the lucky ones who has a minor commute of several minutes, or do you have a long arduous commute of highways, tunnels, and bridges? In laser terms the beam path could be open, contained in fiber optics, or enclosed. In addition, just like the driver going down a steep grade, the beam could be amplified or go through nonlinear optics and therefore produce a change of lanes in our driving example, but for photons it’s a change of wavelength.
This could occur several times along with possible chirp stretching or compression. Any of these steps or a combination of them will affect the safety requirements one might apply to a system.
---2.3 THE INTERACTION OF THE LASER BEAM WITH ITS INTENDED TARGET
Once the laser radiation reaches its destination, just like our driver reaching work, many options lie ahead, from that great day at work to violent meetings. A percentage of the beam may be reflected off a target, or beam interaction may generate gases as a result of products requiring ventilation. An intense pulse laser beam may generate ionizing radiation in the form of neutrons; gamma or x-rays even cause activation of products, hence generating additional ionizing radiation.
Maybe the end of the beam path is delivered through a robotic arm, which introduces new concerns for evaluation.
---2.4 THE ENVIRONMENT IN WHICH THE LASER IS USED
Now we have to consider factors from the workplace and how they contribute to our hazard evaluation. Do they make our job easier or harder? Places such as a clean room may do both. While adding to access control and thereby helping keep unauthorized persons out, cleanness requirements may make it harder to implement other controls. Other common laser use environments are the operating room, manufacturing floor, fabrication area, and our chief interest, the research laboratory.
---2.5 THE PERSONNEL WHO MAY USE OR BE EXPOSED TO LASER RADIATION
When we think of these people, authorized laser users, ancillary staff, visitors, and in some case consumers come to mind. As each of these groups is evaluated, items such as training requirements, personnel protective equipment, and even ergonomic factors require consideration.
---3 ADDITIONAL LIFE CYCLE ELEMENTS
The designing of safety into a laser product is a clear legal responsibility of the laser manufacturer, as called out in national product safety codes and regulations.
Items such as protective housings, interlocks, labels, and electrical safety controls are all rather standard and expected by the purchaser of such products. The reader should check the Web site of the Center of Radiological Devices and Radiological Health for a listing of requirements and guidance documents. The design of a research setup is more a by-product of environmental or experimental need to reduce air turbulence or keep out unwanted light pollution than part of an overall safety plan. In some settings the laser beam path may travel across several optical tables and even across walkways or through a wall. Time given to providing a safe work environment will pay dividends to the user and those visiting the laser use area. Designers also need to think about what goes into the laser and related equipment and how to dispose of it.
Few laser safety professionals think about disposal of laser equipment until the issue is brought before them. Unlike radioactive material or radiation-generating products, there is little control over who buys laser products or how they are disposed of. This may be changing. Laser products can contain hazardous materials whose disposal can require special care. This is highlighted by the international effort to rid electronic and other products of hazardous materials, that is, the "greening of products." Regulations that require this are Waste from Electrical and Electronic Equipment (WEEE) and Restriction of Hazardous Sub stances (RoHS). Both of these European norms and others push the concept of corporate social responsibility, which fits very well into the design and disposal phases of our approach.
Some of the common ways to dispose of laser equipment rather than sending it to landfills are:
1. Donating the equipment to educational institutions
2. Contacting resale firms
3. Looking for a home for the equipment within one's own organization
4. Auctioning the equipment
5. Returning it to the manufacturer (Some manufacturers do have limited return for disposal programs).
Above: Fig. 2 Risk vs. hazard classification.
TABLE 1 Laser Hazard Classification Class Basis for Classification Class 1:
Safe--Visible and nonvisible Lasers that are safe under reasonably foreseeable conditions of operation; generally a product that contains a higher-class laser system but access to the beam is controlled by engineering means.
Low power--Visible only For CW lasers, protection of the eyes is normally provided by the natural aversion response, including the blink reflex, which takes approximately 0.25 sec. (These lasers are not intrinsically safe.) AEL = 1 mW for a CW laser.
Safe without viewing aids 302.5 to 4000 nm Safe under reasonably foreseeable conditions of operation. Beams are either highly divergent or collimated but with a large diameter. May be hazardous if user employs optics within the beam.
Safe without viewing aids Visible only Protection of the eyes is normally provided by the natural aversion response, including the blink reflex, which takes approximately 0.25 sec. Beams are either highly divergent or collimated but with a large diameter. May be hazardous if user employs optics within the beam.
Low and medium power 302.5 nm to 1 mm Risk of injury is greater than for the lower classes but not as high as for class 3B. Up to 5 times the AEL for class 1 or class 2.
Medium and high power Visible and nonvisible--Direct intrabeam viewing of these devices is always hazardous.
Viewing diffuse reflections is normally safe provided the eye is no closer than 13 cm from the diffusing surface and the exposure duration is less than 10 sec.
AEL = 500 mW for a CW laser Class 4:
High power Visible and nonvisible Direct intrabeam viewing is hazardous. Specular and diffuse reflections are hazardous. Eye, skin and fire hazard.
Treat class 4 lasers with caution.
Laser hazard classification gives the user or laser safety officer an initial sense of the hazard the laser system or product presents to the user and others in the area.
Rather than using colors to alert one to the fire hazard level in laser safety, a numerical code is used. The higher the number, the greater the hazard potential.
Potential is the key word, for any laser system can be made safe. The hazard levels range from class 1, no hazard, to class 4, maximum potential hazard (Table 1.1).
ANSI, the International Electrotechnical Commission (IEC), and the Center for Devices and Radiological Health (CDRH) each had slightly different classification systems until 2005, when they all adopted a uniform approach.
A key component of laser safety is the hazard classification scheme ( Fgr. 2), which is an indication of the laser's capability of injuring personnel. All laser or laser systems are classified according to their accessible radiation during operation, which in a research setting can be different from the classification of the laser source. Thus, a class 3B laser beam can be amplified on an optical table to class 4. Likewise, a class 3B or 4 laser beam can be attenuated to a lower classification as part of an optical set up. Laser products sold in the United States are usually classified in accordance with the Federal Laser Product Performance Standard, which falls under the Food and Drug Administration (FDA), CDRH.
Those sold in Europe are classified to meet IEC 60825-1. Be aware that under CDRH Laser Notice 50, laser products sold in the United States can also be labeled with a certification as meeting IEC 60825-1. If the laser has been modified subsequent to classification by the manufacturer, its falls upon the LSO to classify the new laser system or product.
Lasers are classified according to their potential to cause biological damage.
The pertinent parameters are laser output energy or power, radiation wavelengths, exposure duration, and cross-sectional area of the laser beam at the point of interest. In addition to these general parameters, lasers are classified in accordance with the accessible emission limit (AEL), which is the maximum accessible level of laser radiation permitted within a particular laser class.
In laser safety standards and regulations, laser hazard classifications are used to signify the level of hazard inherent in a laser system and the extent of safety controls required. These range from class 1 lasers (which are inherently safe for direct beam viewing under most conditions) to class 4 lasers (which require the most strict controls). The laser classifications are described as follows:
---4.1 TIME BASE FOR CLASSIFICATION
Different time bases are used for the different classes and wavelength ranges as follows:
1. 0.25 s for class 2, class 2M, and class 3R in the wavelength range from 400 to 700 nm
2. 100 s for laser radiation of all wavelengths above 100 nm, excepted for cases listed in items 1 and 3
3. 30,000 s for laser radiation of all wavelengths less than or equal to 400 nm and for laser radiation greater than 400 nm where intentional long term viewing is inherent in the design or function of the laser product 126.96.36.199 Class 1 Any laser or laser system that cannot cause eye or skin injury during normal operation qualifies as class 1. They cannot emit laser radiation at known hazard levels (typically CWs of 0.4 µW at visible wavelengths). Users of class 1 laser products are generally exempt from radiation hazard controls during operation and maintenance (but not necessarily during service). The maximum exposure duration is assumed to be no more than 30,000 sec, except that for infrared systems not intended to be viewed (>0.7 µm), 100 sec must be used. The exemption strictly applies to emitted laser radiation hazards and not to other potential hazards.
---4.1.2 Class 1M They are safe under reasonably foreseeable conditions of operation but may be hazardous if observed using viewing optics. Examples of this class are Light Emitting Diodes (LED) and fiber communication systems.
Two hazardous conditions apply:
1. If the beam is diverging and someone uses ( For example) a lens within 100 mm of the aperture to collimate or otherwise concentrate the beam into the eye
2. If the beam is of a large diameter and collimated and someone uses a lens to increase the proportion of the beam that can enter the eye
---4.1.3 Class 2--These are low-power visible lasers that emit above class 1 levels but emit a radiant power not above 1 mW. The concept is that the human aversion reaction to bright light will protect a person. Aversion or blink response is less than 0.25 sec. A CW HeNe laser above class 1 but not exceeding 1 mW radiant power is an example of a class 2 laser. The laser safety professional should be aware that counter to common belief, studies have indicated the 0.25 sec blink reflex may be present in less than 25% of the general population.
---4.1.4 Class 2M Class 2M lasers (safe if not using viewing aids) are restricted to the wavelength range of 400 to 700 nm. Protection of the eyes is normally provided by the aversion response. The difference between a class 2 and a class 2M laser is that the total power in the beam of a class 2M laser can be much higher. However, the beam will either be highly divergent or collimated and have a large diameter so that the proportion of the beam that can normally enter the eye is small. Class 2M lasers are generally not safe if viewing optics are used.
Two hazardous conditions apply:
1. If the beam is diverging and someone uses ( For example) a lens within 100 mm of the aperture to collimate or otherwise concentrate the beam into the eye
2. If the beam is of a large diameter and collimated and someone uses a lens to increase the proportion of the beam that can enter the eye 188.8.131.52 Class 3R This class consists of lasers and laser systems that have an accessible output between 1 and 5 times the class 1 AEL for wavelengths shorter than 0.4 µm or longer than 0.7 µm, or less than 5 times the class 2 AEL for wavelengths between 0.4 and 0.7 µm, meaning 1 to 5 mW. The "R" stands for reduced requirements. As with class 3A below, which is only hazardous for intrabeam viewing, class 3R can be considered safe for momentary viewing except by optics.
---4.1.6 Class 3A--This is the predecessor of class 3R: intermediate-power lasers (CW: 1 to 5 mW).
Such lasers are only hazardous for intrabeam viewing and can be considered safe for momentary viewing except by optics. Some limited controls are usually recommended.
NOTE: There are different labeling requirements for class 3a lasers with a beam irradiance that does not exceed 2.5 mW/cm2 (caution logotype) and those where the beam irradiance does exceed 2.5 mW/cm2 (danger logotype).
---4.1.7 Class 3B--These are moderate-power lasers, invisible wavelengths: cannot generate radiant energy greater than 125 mJ, CW: 5-500 mW, pulsed: in less than 0.25 sec., visible wavelengths: CW 5-500mW, pulsed cannot produce radiant energy greater than 30 mJ per pulse. In general, class 3B lasers won’t be a fire hazard nor are not generally capable of producing hazardous diffuse reflections except for conditions of intentional staring done at distances close to the diffuser. Specific controls are recommended. The application of these controls should be graded since a direct exposure from a 10 mW laser does not present the hazard a direct exposure would from a 400 mW laser. The diffuse reflection from both might not present any hazard at a distance greater than 0.1 meters.
---4.1.8 Class 4
High-power lasers (CW: 500 mW, pulsed capable of generating over 125 mJ in less than 0.25 sec) are hazardous to view under any conditions (directly or diffusely scattered) and are a potential fire hazard and a skin hazard. Significant controls are required of class 4 laser facilities. A graded approach may be justified.
Class 1 Product: A laser system or product that contains a completely enclosed laser or laser system of a high classification.
The international system is defined in the IEC laser safety standard IEC 60825-1.
This has been adopted by a number of countries within their national standards.
The United States and ANSI z136 has recently adopted this system.
While it’s common to think of the responsibility for laser safety as the sole responsibility of the LSO, the responsibility really falls among many levels.
The employer will provide employment and a safe and healthful place of employment for his employees. This statement comes from the regulation for the California Occupational Safety and Health Administration. Similar wording and expectations can be found in many regulatory sources. Part of management's responsibility for providing a safe work place, from a laser safety perspective, is the appointment of an LSO. This appointment does not have to be, and generally is not, a full-time position. Once the appointment is made, the employer through its management chain takes on certain responsibilities. Quoting the ANSI Z136.1 2000 version Section 184.108.40.206, "the management shall provide for training to the LSO on the potential hazards, control measures, applicable standards, and any other pertinent information pertaining to laser safety and applicable standards or provide the LSO adequate consultative services. The training shall be commensurate to at least the highest-class laser under the jurisdiction of the LSO; Training also includes consideration for non-beam hazards."
---5.2 LASER SAFETY OFFICER OR ADVISOR
The role of the LSO is critical to ensuring laser safety, particularly in the R&D setting. Without an appointed LSO, an organization cannot say it addresses laser safety. The LSO is commonly defined as an individual with the authority and responsibility to monitor and enforce the control of laser hazards. In addition, the LSO must be able to knowledgeably evaluate and control laser hazards. The LSO either performs the stated tasks or ensures that the tasks are performed.
Some of the LSO's responsibilities are listed below:
2. Hazard evaluation
3. Control measures
4. Procedure approval
5. Protective equipment
6. Signs and labels
7. Facility and equipment
8. Safety feature audits
10. Medical surveillance
11. Record keeping
12. Accident investigation
13. Review of laser system operations
1.5.3 RESEARCH GROUP LEADER OR SUPERVISOR
This individual, depending on the organization, is known by several names: principal investigator, responsibility individual, senior researcher, group leader, or supervisor. It’s common for these individuals to not see themselves as super visors or want the responsibilities that go along with the title. As in so many things, the attitude is set from the top down. If management or the group leader only present lip service to laser safety, but no time or resources, soon everyone will sense how unimportant laser safety is to management and their actions will reflect this. Several kinds of laser accidents can be traced back to management's lack of belief in or support of laser safety.
No matter how detailed or creative a laser safety program may be, if the employees don’t see a reason for laser safety, the laser safety system will fail. The employees must understand that they play the most critical role in laser safety, particularly in the R&D setting. Some items they must feel free to do are:
1. Bring real-life issues to the attention of group leadership or the LSO.
2. Have the flexibility to get corrections or modifications made to work procedures.
3. Be aware that complaining to oneself or to peers generally does not produce any positive changes. There must be a mechanism to raise concerns to a higher level.
4. Report accidents or problems without fear of their careers being negatively impacted.
---5.5 FIELD SERVICE STAFF AND VENDORS
These people need to realize the vital role they play in laser safety. They are looked upon by many to be the system experts and therefore the source of laser safety controls and guidance. A casual word or outright disregard for laser safety can have disastrous effects.
---6 PROBABILISTIC RISK ASSESSMENT
For some technologies, such as nuclear power generation and chemical plants, people want to know they are going to be safe. The problem is that ultimate safety cannot be achieved, so generally techniques are used to determine if certain events or actions are likely, the results from these events or actions are determined, and numbers are assigned. How do these numbers evolve? Do we need to go to these lengths when using our laser product? Of course, it depends on what can go wrong and the consequences.
We could use the same formal techniques developed for other technologies, and there are many - For example, probabilistic risk assessment, fault-tree analysis, HAZOP, and so on. These often make use of data from real incidents and accidents. The media like to quote the probability of us being killed per mile driven, riding in a train, and flying in an airplane. Of course, behind these figures are, tragically, real cases of people who have been killed using these modes of transport. Does information exist for people who have been injured or killed using your particular laser product? It may, but probably not. What if your laser product is unique and part of a research project? How can you evaluate it? In general, it turns out that we can go a long way toward assessing safety issues without even switching the laser on.
As an illustration, we can carry out a thought experiment - so favored by Albert Einstein, but we will be a bit simpler. Banana skins have provided a source of amusement in slapstick comedy for at least a century. Can we find out the probability of someone being killed by slipping on a banana skin? We can define our probability here as the ratio of the number of people killed to the number of people who could have slipped on the banana skin or the ratio of the number of people killed to the number of people who actually slipped, which shows us the first difficulty. We need to be careful to understand what numbers mean.
We could do the actual experiment. We could take the population of a small town, say of 1,000 people, and ask them one by one to walk along somewhere where we have left a banana skin. We would have to make sure that the volunteers could not communicate with each other and could not see what was happening; otherwise they could bias our experiment (by learning from other people's experience - for once something we don’t want to happen). At the end of the experiment, we could count up the number of people who:
1. Stepped over or to the side of the banana skin
2. Slipped on the banana skin but suffered no fall or injury
3. Slipped on the banana skin, fell, and either:
a. Were not injured
b. Received minor injuries
c. Received major injuries
d. Received fatal injuries
There are a number of problems with trying to do this experiment, not least of which is the potential litigation you could face. It would also be extremely difficult to do the experiment with a single community without your volunteers finding out about the fate of earlier participants. However, within national data bases of accidents and emergencies you could almost certainly find some real data.
So, how do we get a gut feeling? You probably already have it. Without doing the experiment, you could probably think through - from your experience of everyday life - the probability of being killed by slipping on a banana skin.
Your estimate is probably as good as any official statistics. You could also try a comparison of estimates with colleagues or friends over a coffee or beer.
Is there a correct answer to how many people will be killed? Not until you do the actual experiment. Even then, the answer is only correct for that experiment. For 1,000 people, the estimated answer is 0 or 1. If you thought 5, you would not be wrong. Of course it’s possible that all 1000 people will be killed.
There may be reasons why the likelihood of slipping and death is higher. The participants could be blindfolded and, through careful design of the route they are instructed to take, the likelihood of slipping could be practically certain. The banana skin could also be positioned at the edge of a 100-m drop off a cliff, increasing the probability of death.
We can use exactly the same thought experiment technique to assess our laser application. However, we also need to think about risk assessment. We all need to be very good at real, practical risk assessment in our everyday lives. A natural selection process exists for those who are not good at risk assessment. As an example, we can take the process we go through when crossing a road, generally subconsciously.
As you approach the edge of the roadway, you start the process. You may use all of the senses available to you, but you will primarily use your eyes and ears. Essentially, you start looking at the traffic on the road - but what are you looking for? It might be the volume of traffic, its speed, and the type of traffic.
You may take into account other factors to do with the environment, including how far along the road notice and whether the traffic is all moving in the same direction. Let us concentrate on the traffic. What does this consist of? There could be cars, vans, trucks of various sizes, and perhaps bicycles.
This initial process is hazard spotting. Hazards are defined as the things that can cause harm. We’re interested in the harm to people, essentially us at this stage. We may take into account who may be harmed by the hazard and how they could be harmed. If it’s us crossing the road then obviously we are at risk of something happening to us. However, we may have other people with us. A group of fit young adults will primarily look after themselves but will (generally) still keep a lookout for others in the group. However, an adult with young children will do the risk assessment for the whole group. We will define risk as the result of combining the probability that a hazard will do us harm and the outcome from that interaction. Interacting with a high-speed heavy truck can have drastic con sequences for us; we could be spread over a large part of the roadway. However, we may expect to survive an interaction with a bicycle, and cars are somewhere in between.
Our perception of the risk will depend on a number of factors, not least on our past experience. If we have crossed this particular road many times before with no near-misses, we may become complacent. If something has changed - for instance, the traffic is moving much faster than usual - we may exercise more caution.
We also need to consider our tolerance of the risk. Let us assume that you are crossing the road to catch a bus. You have been offered a new job where your salary is to be doubled, but you have to get there by a certain time. The bus has just arrived at the stop on the other side of the road. The traffic is very busy.
What do you do? Do you wait for a reasonable opportunity to cross and risk missing the bus, or do you make a dash between the traffic? Under these circum stances you may well expose yourself to a much greater risk because you perceive that a benefit could result. Crossing under the same traffic conditions at other times would be an intolerable risk to you.
Going back to standing at the side of the road, there are a number of things we could do. We could decide not to cross the road. That way, we have removed the risk because we won’t be exposed to the hazard. We could wait for a suitable gap in the traffic and cross. We may or may not know how long this will take.
We could look for something to help us cross the road. Perhaps the first is a pedestrian crossing. This is only effective if the drivers actually stop. We could call such a control measure an administrative control measure because it’s procedural. You are told when learning to drive that you should stop at such crossings, but nothing in the vehicle control system forces you to do so. If the crossing has lights, then this may be a bit better. The driver is now being guided by a control system (rather than necessarily observing you), but still there is nothing to physically prevent the driver from running you over. This is an improvement on the administrative control measure, but it’s by no means perfect.
How else could we get to the other side of the road? There may be a pedestrian bridge. This may mean walking farther and the additional effort of climbing steps or walking up a ramp. There is obviously the potential risk of a driver hitting any supports for the bridge, or even a vehicle approaching that is too tall to fit under the bridge. Generally, the probability of these events is small, but they are possible.
Another approach is to use an underpass or tunnel. The probability of a vehicle dropping down through the roof is very unlikely, but is there other potential problems. In some neighborhoods and at some times of the day (or more accurately, night) you may consider that the risk of death by mugging is much greater than the risk of being run over by a truck.
Both the bridge and the tunnel are examples of engineering control measures.
They both separate us from the hazard so that the risk of harm from the hazards is very small. However, the control measure may introduce other risks, as is the case with the tunnel.
We build up our experience of crossing various roads throughout life and we call on that experience all of the time. If you see an incident involving someone crossing a road at a particular location, that event may come to mind whenever you cross at that location.
Let us assume we are standing at the side of a relatively quiet road. A cyclist is approaching and he is wearing a bright green helmet. He is not particularly close and you judge that since the traffic is otherwise clear, you will now cross the road. Just as you step onto the road surface the cyclist speeds up and appears to try to run you down. However, you make it to the other side of the road and the cyclist speeds off into the distance. The following day you come to the same place and are ready to cross the road. You see a cyclist approaching with a bright green helmet. What goes through your mind? You may stand back and let him past or you may take some other action; it depends on your personality. It may not even be the same cyclist as yesterday. What you have done is learned from the experience and modified your assessment.
When you cross the road for real, this whole assessment process takes place very quickly and without much conscious thought. We can summarize the process as follows:
1. Spot the hazards.
2. Decide who may be harmed by the hazards and how they could be harmed.
3. Identify existing control measures and assess the residual risk.
4. Record the findings from the assessment.
5. Periodically review the assessment.
These are the so-called five steps to risk assessment, which are commonly applied to laser safety in the United Kingdom and are slowly gaining a foothold in the United States. The above example shows that risk assessment can be based on common sense. All of this feeds back into the five steps of the comprehensive hazard evaluation that we started the SECTION with.
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