CLASSE Safety Handbook

The FDA Radiation Control for Health and Safety Act of 1968, 1040.10 separates lasers into the following hazard classes, which are based on power and/or potential to cause biological damage to eyes and/or skin. In the definitions below, "acute" means immediate (for visible light, faster than the human blink response of 0.25 sec, and potentially shorter times for other wavelengths) and "chronic" refers to exposure over a period of time of seconds to minutes. Distinctions are made between hazard level when viewing with or without optical equipment (the latter having the potential to concentrate power and therefore inflict more damage), and for visible or invisible wavelengths (i.e. whether the human blink response can be helpful in preventing eye damage). The designations do not cover hazard levels that exist when the device is disassembled during "service" (repair), when even more precautions should be taken.

• Class I: Not hazardous with or without optical instruments, because of low power and/or an interlocked enclosure; however, there are wavelength-dependent power limits for this class
• Class IIa: Chronic hazard only if viewed for more than 1000 seconds; applies to visible wavelengths only and power below 4 mW; emissions must be "not intended for viewing" (note that this class is sometimes erroneously referred to as "Class Ia", which does not exist)
• Class II: Not an acute hazard; applies to visible wavelengths only and continuous wave (CW) power below 1 mW
• Class IIIa: Either an acute viewing hazard (when viewed using optical instruments) or a chronic viewing hazard (when viewed without optical instruments for more than two minutes); CW power range is 1 to 5 mW and power density below 2.5 mW per cm^2
• Class IIIb: Acute hazard to the skin and eyes from direct radiation; CW power from 5-500 mW
• Class IV: Acute hazard to the skin and eyes from direct and scattered radiation, very likely to cause permanent damage; CW power above 500 mW

To make classification slightly more confusing, there is a newer, international classification developed by the IEC (International Electrotechnical Commission) that differs somewhat from the older FDA standard above; since 2007, either standard may be observed in the US. There are still levels numbered by increasing hazard, but arabic numbering (1-4) instead of roman numerals (I-IV), and instead of subclasses "a" and "b" there are subclasses "M", "R", and "B".

• Class 1: Not hazardous with or without optical instruments
• Class 1M: Hazardous only if viewing with optical instruments
• Class 2: Not an acute hazard; applies to visible wavelengths only and CW power below 1 mW
• Class 2M: Acute hazard only when using optical instruments; possible chronic hazard; applies to visible only
• Class 3R: Not an acute hazard; applies to visible and invisible, CW power range is 1-5 mW
• Class 3B: Acute hazard for direct or reflective viewing; not a hazard for diffuse reflection or to skin; applies to visible and invisible and medium power; CW power from 5-500 mW
• Class 4: Acute hazard for direct or indirect viewing; applies to visible and invisible and high power; CW power above 500 mW

A comparison of the FDA and IEC classification schemes can be found here.

General comments on laser use

Class 2 (or class II in the old classification system) lasers should be used whenever possible. The output of these lasers is limited to 1 mW, which is sufficient for many purposes (such as alignment), but they are considered safe due to the blink reflex, which limits exposure to about 0.25 s. Users of class 2 lasers should be aware, however, that intentional suppression of the blink reflex can lead to eye injury. Users should not stare into the beam. Alignment lasers may be required for alignment of an x-ray flightpath at CHESS, which is generally at about eye level; in such cases, special care should be taken to prevent exposure to unaware persons outside of the hutch.

Although lasers producing 5 mW output (class 3R, or class IIIa in the old classification system) are commonly marketed as hand-held laser pointers in the United States, they are only considered safe when handled carefully and with restricted beam viewing. These lasers should only be used when class 2 lasers are inadequate. Due to the fact that these lasers are generally mounted in a fixed position, possibly at eye level, the risk of injury is much greater than with a hand-held laser. Use of class 3R and IIIa lasers in a fixed position should be treated as hazardous. A standard operating procedure will not be required, but the Laser Safety Officer (LSO) or his/her delegate should approve their installation. The laser output must be contained within a well-defined nominal hazard zone (NHZ), for example, an x-ray hutch or other room, and a sign must be posted outside the NHZ indicating the nature of the hazard. It will not be necessary to enclose the NHZ with a laser safety curtain for a class 3R or class IIIa laser, but it is the user’s responsibility to restrict laser radiation exposure to within the NHZ.

Class 3B (IIIb) and class 4 (IV) lasers present significant hazards, and require special operating procedures and LSO approval before use.

Lasers that are not labeled with a classification or output power cannot be used without approval from the LSO.

Lasers at CHESS

See the page on Lasers at CHESS.

Lasers for ERL

Lasers for ERL Phase 1 are located in Wilson Lab, Rooms 141A and 143, adjacent to the ERL control room. The laser beam, which undergoes several stages of amplification, shaping, and wavelength shifting on laser tables in the two rooms, is transported through the wall between 141A and 143 and later through the wall to L0 to another laser table, and from there to the electron gun nearby. The lasers range in classification up to class IV (4), and protective measures in both rooms assume the maximum hazard in either. Rooms 141A and 143 both have access restricted to authorized personnel and are signed accordingly. These rooms are protected by keypad entry locks; for unauthorized emergency access (e.g. by fire investigators), the doors can be forced open, and this action shuts down the lasers. A multi-point interlock system is in place, which, depending on where a trip originates, can shut down front-end laser components, close the beam stop between 141A and 143, and/or close the shutter between 143 and L0. Several crash buttons are located in both rooms.

A laser system is also set up in the rear of Room 128 for the gun/photocathode studies to be conducted there. The laser room is located within the radiation exclusion area and is therefore interlocked to the light beam marking this boundary; in addition the door itself is interlocked to the laser system power. Equipment and required precautions are similar to those in Rooms 141A/143.

In routine operation, light beams and optical elements not on laser tables are completely enclosed in sealed, opaque rigid tubing. On laser tables, beams are frequently transported inside clad optical fibers, and the two L0 laser tables are covered. However, frequent changes on the L0 laser tables occur; hence the table covers are interlocked with microswitches so as to close the shutters from room 143.

Increased hazard exists during modifications to laser setups and alignment, for which a special bypass interlock mode and associated procedures exists. Authorized personnel have the generic EHS and ERL-specific laser training, and when necessary wear laser safety goggles appropriate for the wavelength and intensity.