Laser Safety Guide

Control Measures

Engineering Control Measures for the Four Laser Classes

• Laser Barriers and Protective Curtains:
  
  Area control can be effected in some cases using special barriers which have been specifically designed to withstand either direct and/or diffusely scattered beams. In this case, the barrier will exhibit a Barrier Threshold Limit (BTL) for beam penetration through the barrier during some specified exposure time (typically 60 seconds). The barrier is located at a distance from the laser source so that the BTL is not exceeded in the "worst case" exposure scenario.
  
  Currently available laser barriers exhibit BTL's ranging from 10 W/cm(2) to 350 W/cm(2) for different laser wavelengths and power levels. An analysis is usually required (done in a manner similar to the NHZ evaluations described previously) that establishes the recommended barrier type and installation distances for a given laser.
  
  Important in the design is the factor of flammability of the barrier. It is essential that the barrier not support combustion and be consumed by flames following an exposure.
  
  
• Protective Housing:
  
  A Laser shall have an enclosure around the laser which limits access laser radiation at or below the applicable MPE level. A protective housing is required for all classes of lasers, except of course, at the beam aperture.
  
  In some cases, the walls of a properly enclosed room area can be considered as the protective housing for an open beam laser. Such a "walk in" enclosure can also be a CDRH Class I provided that controls preclude operation with personnel within the room (vis: pressure sensitive floor mat switches, IR sensors, door interlocks..etc.).
  
  
• Master Switch Control:
  
  All Class IV lasers and laser systems require a master switch control. The switch can be operated by a key or computer code. When disabled (key or code removed), the laser is not capable of operation. Only authorized system operators are to be permitted access to the key or code. Inclusion of the master switch control on Class IIIB lasers and laser systems is also recommended but not required.
  
  
• Optical Viewing System Safety:
  
  Interlocks, filters or attenuators are to be incorporated in conjunction with beam shutters when optical viewing systems such as telescopes, microscopes, viewing ports or screens are used to view the beam or beam reflection area. For example, an electrical interlock could prevent laser system operation when a beam shutter is removed from the optical system viewing path. Such optical filter interlocks are required for all but Class I lasers.
  
  
• Beam Stop or Attenuator:
  
  Class IV lasers require a permanently attached beam stop or attenuator which can reduce the output emission to a level at or below the appropriate MPE level when the laser system is on "standby." Such an beam stop or attenuator is also recommended for Class IIIA and Class IIIB lasers.
  
  
• Laser Activation Warning System:
  
  An audible tone or bell and/or visual warning (such as a flashing light) is recommended as an area control for Class IIIB laser operation. Such a warning system is mandatory for Class IV lasers. Such warning devices are to be activated upon system start up and are to be uniquely identified with the laser operation. Verbal "countdown" commands are an acceptable audible warning and should be a part of the standard operating procedures (SOP).
  
  
• Service Access Panels:
  
  The ANSI Z-136.1 standard requires that any portion of the protective housing that is intended to be removed only by service personnel and permit direct access to an embedded Class IIIB or Class IV laser will have either an interlock or require that a tool is used in the removal process. If an interlock is used and is defeatable, a warning label indicating this fact is required on the housing near the interlock. The design shall not allow replacement of a removed panel with the interlock in the defeated condition.
  
  
• Protective Housing Interlock Requirements:
  
  Interlocks which cause beam termination or reduction of the beam to MPE levels must be provided on all panels intended to be opened during operation and maintenance of all Class IIIA, Class IIIB and Class IV lasers. The interlocks are typically electrically connected to a beam shutter and, upon removal or displacement of the panel, closes the shutter and eliminates the possibility of hazardous exposures.
  
  For embedded Class IIIB and Class IV lasers only, the interlocks are to be "failsafe". This usually means dual redundant electrical series connected interlocks are associated with each removable panel.
  
  Adjustments or procedures during service on the laser shall not cause the safety interlocks to become inoperative or the laser radiation outside a Class I laser protective housing to exceed the MPE limits, unless a temporary laser controlled area is established.
  
  
• Remote Interlock Connector:
  
  All Class IV lasers or laser systems are to be provided with a remote interlock connector to allow electrical connections to an emergency master disconnect ("Button panic button") interlock or to room, door or fixture interlocks. When open circuited, the interlock shall cause the accessible laser radiation to be maintained below the appropriate MPE level. The remote interlock connector is also recommended for Class IIIB lasers