Lasers are categorized for safety considerations depending on their capacity to cause harm to the eyes or skin of humans, and/or ignite a fire. The classes to which the lasers fall into are based on the power, energy, and wavelength of the emitted radiation. These classes are denoted by either Arabic or Roman numerals and may include a letter suffix (M, R, or B) indicating additional characteristics. 

This system was established in the early 1970s and updated by the Food and Drug Administration (FDA) in 2007. Within the United States, the FDA stands as a pivotal regulatory authority to establish guidelines and standards for laser safety. Specifically, the FDA's Center for Devices and Radiological Health (CDRH) oversees the regulation of medical devices, encompassing laser products among its responsibilities. In addition to the FDA, various national and international organizations play a role in shaping laser safety standards. The International Electrotechnical Commission (IEC) is an example of an international entity that sets global standards for laser safety, as evidenced by documents like IEC 60825. Regulatory bodies in different countries frequently adopt or tailor these standards to align with their specific safety requirements. In the United States, Americal National Standard Institute, ANSI Z136.1 (an old system) is still used and is very similar to IEC.

The classification of lasers are determined based on the accessible emission limits (AEL) and maximum permissible exposure (MPE) defined for each laser class. AEL typically represents the maximum power (in W) or energy (in J) that can be emitted within a specified wavelength range and exposure time, passing through a designated aperture stop at a specified distance. The MPE, on the other hand, is the highest power or energy density (in W/cm² or J/cm²) of a light source being considered safe, with a negligible probability of causing damage. It is generally set at about 10% of the dose that carries a 50% chance of causing damage under worst-case conditions. The MPE is measured either at the cornea of the human eye or on the skin, considering a specific wavelength and exposure time.

The following are the major laser safety classes:

• Class 1: A Class I laser remains safe for regular use in all conditions. This implies that the MPE should not be exceeded when viewing the laser with the naked eye or through typical magnifying optics such as a telescope or microscope. The wavelength range for Class 1 lasers can encompass ultraviolet (UV), visible, and infrared (IR) regions. This range typically spans from 100 nanometers (nm) in the UV region to around 1 millimeter (mm) in the far-infrared region. Some examples of Class 1 laser use are: laser printers, CD players, DVD devices, geological survey equipment, and laboratory analytical equipment. 
• Class 1M: This class is deemed safe for direct viewing with the naked eye but may pose risks when observed through optical instruments. The wavelength range for Class 1M lasers typically falls within the limits of 302.5 nm to 4000 nm, covering both the ultraviolet, visible, and infrared portions of the electromagnetic spectrum. Class 1M lasers emit beams with large diameters or significant divergence. The MPE for a Class 1M laser is typically within safe limits unless focusing or imaging optics are employed to narrow down the beam. However, if the beam is refocused, the hazards associated with Class 1M lasers may increase, potentially prompting a change in the product class. Some examples of Class 1M laser use are: barcode scanners, surveying instruments, and educational demonstrations.

• Class 2: These are low-power (< 1mW), visible light lasers that could possibly cause damage to a person's eyes. This class includes any laser product that falls within the wavelength range of 400 nm - 700 nm and allows human access to laser radiation not exceeding the AEL of Class 2. These laser products emit visible radiation and are considered safe for brief exposures. However, intentional and prolonged staring into the laser beam may pose a hazard. Some examples of Class 2 laser use are: classroom demonstrations, laser pointers, aiming devices, and range-finding equipment.
• Class 2M: These are visible lasers that emit radiation in the wavelength range of 400 nm - 700 nm. Accidental viewing with the naked eye is considered safe within this class, however, caution is advised even for accidental viewing when observed through optical instruments, similar to the risks associated with Class 1M. The MPE can be exceeded and eye injury may occur following exposure with telescopic optics such as binoculars for a collimated beam with a larger diameter. Some unique examples of Class 2M laser use are: retail scanners and consumer electronics.
• Class 3R: This class includes lasers that emit continuous wave radiation within the power range of 1 mW - 5 mW. Direct viewing of the Class 3R laser beam could be hazardous to the eyes. This class includes any laser product that allows human access to laser radiation exceeding the AEL of Class 1 and Class 2. However, it ensures that human access to laser radiation does not exceed the AEL of Class 3R for any emission duration and wavelength. The wavelength range for Class 3R lasers is typically between 302.5 nanometers (nm) and 1064 nm. This range covers a significant portion of the spectrum, including violet, blue, green, yellow, orange, and red wavelengths. Laser products in this category may emit radiation capable of surpassing MPE under direct intrabeam viewing (collimated beam making direct contact with the human eye), but the risk of injury in most cases remains relatively low. The AEL for Class 3R is constrained to five times the AEL of Class 2 for visible laser radiation or five times the AEL of Class 1 for non-visible laser radiation. The risk of injury escalates with exposure duration, and ocular exposure may be hazardous under worst-case conditions or intentional direct intrabeam viewing. Some examples of Class 3R laser uses are the same as Class 2 lasers with the most popular uses being laser pointers and laser scanners.
• Class 3B: This class of lasers have intermediate power ranging from 5 mW to 500 mW for continuous wave operation and 10 J/cm2 for pulsed operation. Direct viewing and diffuse reflections of beam of the Class 3B laser are hazardous to the eye. This class includes any laser product that allows human access to laser radiation exceeding the AEL of Class 1, Class 2, and Class 3R. However, it ensures that human access to laser radiation does not surpass the AEL of Class 3B for any emission duration and wavelength. The specific wavelength range for Class 3B lasers depends on the design and purpose of the laser device. These laser products are typically hazardous in cases of intrabeam ocular exposure (collimated beam making direct contact with the human eye), even with accidental short-time exposure. Class 3B lasers approaching the AEL for Class 3B may result in minor skin injuries or pose a risk of igniting flammable materials, although this is more likely with a small diameter or focused beam. Some examples of Class 3B laser uses are spectrometry, stereolithography, and entertainment light shows.
• Class 4: This class of lasers have high power ranging above 500 mW for continuous wave operation and above 10 J/cm2 for pulsed operation. The direct beam, as well as specular and diffuse reflections from Class 4 lasers, pose hazards to both the eyes and skin. Additionally, Class 4 laser devices may present a fire hazard depending on the target's reaction upon impact. The wavelength range for Class 4 lasers covers ultraviolet (UV), visible, and infrared (IR) regions. Most laser-related eye injuries occur from reflected beams of Class 4 laser light. Therefore, it is crucial to keep all reflective materials away from the laser beam. The reflectivity of a surface is determined by its roughness relative to the wavelength. When within a laser-controlled area, it is imperative to wear eye and skin protection that is suitable for the wavelength and power of the laser in operation. The safe operation of Class 4 laser devices necessitates stricter controls to mitigate potential risks. Examples of Class 4 laser applications include surgery, research, drilling, cutting, welding, and micromachining.

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