26 Mar 2004
The class assigned to a laser is only part of the picture when deciding how it should be used safely, as Karl Schulmeister reports.
From Opto & Laser Europe April 2004
In Europe, as well as many countries elsewhere, every product that contains a laser or an LED needs to be assigned a safety class. This includes everything from products that contain low-power diodes, such as CD players, right up to the high-power lasers used in industry for materials processing.
This class is defined by the European standard EN 60825-1 (or the international standard IEC 60825-1) and indicates, in broad terms, the harm to the eyes or skin that could result from exposure to emitted radiation. The meanings of the various laser safety classes and their limitations can be quite involved, which inevitably means that they are easily misunderstood.
Such misunderstandings can arise because worst-case assumptions are built into the classification procedure. In other words, laser warning labels can sometimes indicate a level of hazard that does not apply in reality. For instance, many Class 3B and Class 4 products are safe in practice because the hazards associated with them only crop up in special situations. Examples include products with highly divergent beams such as line lasers, scanned lasers, or emissions from optical fibres, where the hazard only exists close to the source.
If a Class 3B line projector is mounted on the ceiling, for instance, people on the ground might be well below the maximum permitted exposure limits and have no need for eye protection. In this case, the hazard only becomes relevant for maintenance personnel, who would need to observe safety precautions if they were up a ladder cleaning the optics, for example. Other examples are Class 3B and 4 lasers that have a larger hazard zone but in which the radiation is contained by an enclosure. This is typically the situation for higher-power materials-processing installations.
Because the classification is not solely dependent on the laser's power it is possible in theory for devices that contain high-power lasers to be Class 1 products, as long as they are completely enclosed and a number of strict criteria are fulfilled. However, it is often unnecessary, impracticable and too expensive for manufacturers to meet these rigorous requirements.
For example, many materials processing lasers have walk-in access, meaning that in theory, the user could walk inside the housing, close the door and start the laser. And some laser systems do not have a roof, so there is the possibility of hazards resulting from reflections or single fault conditions. For an embedded product to really be Class 1 it needs to be fully enclosed; walk-in access has to be detected; and the housing has to be able to withstand the radiation even under fault conditions. Without these additional measures, a higher class must be assigned because classification is based on the design of the product and cannot rely on appropriate behaviour from the user.
Analysing the risks In practice, as long as the user treats the laser with respect, it is not really important how the product is classified. However, a Class 3B or 4 classification usually means that a risk analysis should be performed to evaluate the safety of the product in a specific application.
Risk analysis is usually performed by a qualified laser safety officer, but it might also be acceptable to rely on the analysis that needs to be done on a general basis by the manufacturer in the process of CE marking. A CE mark denotes that the manufacturer deems the machine safe, even if it is labelled as a Class 4 laser product. The manufacturer also has to inform the user of the specific hazards presented by the laser product, and procedures for safe use. The result of the risk analysis might well be that no further control measures are necessary, except to provide some information (in the manual) or training regarding the special situations in which the hazard can be present. However, it is important that the user still analyses the specific risks on a case-by-case basis.
It must also be recognized that correctly classifying the product according to the laser classification standard and following the hardware requirements for the respective class does not necessarily make the product safe. Even if a Class 4 laser system with a collimated open (i.e. not enclosed) beam meets all the applicable requirements of its class, such as warning labels, emission indicator, beam attenuator and key control, it is obviously not a safe product. It is necessary for the user to be trained, to adopt safe working procedures and to use personal protective equipment to reduce the risk.
Private uses General workplace safety legislation that calls for user training and controls already applies to Class 3B and 4 systems, and this is well established in practice. However, as more powerful lasers become affordable for private use, there is an urgent need in most countries to define the legal and practical implications of private laser use. For instance, 50 mW continuous-wave green lasers are already marketed for private laser shows, in which the patterns tend to involve the beam resting for a few seconds. These products feature all of the requirements of the laser safety standard and the CE mark based on the low voltage directive, but are certainly not appropriate to be sold to the general public. Users would have to install the system at a height at which maximum exposure limits are not exceeded - this is usually 6 m above the ground.
It follows that such a product, even if it meets all the requirements of the laser safety standard and the manual tells the user to observe national safety regulations, is not appropriate for the intended use according to general product safety legislation.
Likewise, the hazards of lasers can be increased by misapplication of the standard. For instance, a green laser pointer with a Q-switched Nd:YAG laser might be designated as a Class 2 source, based on its average power of 1 mW. However, the peak power of such devices is much higher, meaning that they should actually be classified as Class 3R. They are also not really appropriate for use by members of the general public.
When the private use of lasers is regulated, however, it is possible for both Class 3B and Class 4 lasers to be safe in practice and to be used without special training and, in many cases, without eye protection. Examples include laser lawnmowers and laser hair removal systems, of which prototypes have emerged recently. These are designed for safe use and include safety features such as automatic detection of whether an appropriate "target" is in place. It would unnecessarily hamper laser technology to restrict the private use of lasers to Class 2 and below.
Considering the whole picture These examples show that the safety classification of a laser product is only one of many factors that contribute to the safe use of such devices. It is important for the manufacturer and, in many cases, the user to perform a risk analysis and minimize the risk by modifying the design of the product or adding other safety controls. While the ambiguity in the classification system is not entirely satisfactory, no system can realistically account for the specific risk presented by every product.
The standards committee recognizes that the classification system is relatively complicated and that the information provided on the labels does not always reflect the meaning of the classes appropriately. To improve the situation, a standards project was set up last year with the aim of reducing the complexity of laser safety labels and moving more detailed information into the manual.
The long-term challenge for the laser safety community is to provide standardized tests which are prescriptive in nature but not over-restrictive. It is also important to provide a classification scheme that is not too complex but still fully characterizes the hazard. The tendency in standardization is to decrease the level of worst-case assumptions. However, this would also increase the number of potential situations in which a given product of a certain class might be more hazardous than its class suggests.
There is no ideal solution to this challenge, but manufacturers can help by recognizing that, in addition to ensuring correct classification of their product, it is important to carry out a risk analysis. It is also crucial that they give the user complete and detailed information on the risks associated with their product in the different modes of use - and even in situations of misuse.