All students in advanced laser courses using the high-power laser lab (V15) are required to watch a safety video, attend a lab orientation session, and complete an assignment on laser safety.

MANDATORY ATTENDANCE
NO EXCEPTIONS

Students who miss a safety orientation session (e.g. through late admission to a course) will be required to view the video, review the following safety information, and successfully complete a quiz on laser safety prior to being allowed into the lab.

This refresher course is by no means a complete and thorough treatment of laser safety: it is designed solely as a reminder of safety concepts as well as a demonstration of specific hazards in the college laser lab. It is not a replacement to the health and safety course(s) at the college.

First, watch the short video on laser safety from Coherent, Inc. The video covers a range of general laser safety topics.

Next, we will review the following topics, all of which were covered in the video, but deserve special attention and with specific examples from our labs including:

1. Class I-IV laser definitions and safety requirements for each
• Class 1:
  No biological injury is possible and generally considered safe to view. Maximum power is limited to 40 microwatts at wavelengths below 450nm, 400 microwatts at wavelengths above 500nm (e.g. a small HeNe laser). A very powerful laser such as a carbon-dioxide laser used for cutting can be considered class-1 if in an enclosure which prohibits access to the beam or reflections (our laser cutters are all in class-1 enclosures however these limits do not apply when the system is open for servicing, at which point they must be treated as class-4).
• Class 2:
  Ocular hazards for chronic viewing. This class is for visible, CW, lasers only and limited to a maximum power of 1mW (e.g. a small lab HeNe). Safety relies on the natural aversion of the eye to bright light (the 'blink' response).
• Class 3A:
  Ocular hazards for chronic viewing. This class is for visible, CW, lasers only and limited to a maximum power of 5mW (e.g. a larger lab-type HeNe such as the external-optics type used in V12A).
• Class 3B:
  Ocular and skin hazards for direct exposure. Covers visible or invisible lasers limited to a (CW) maximum power of 500mW. Pulsed lasers covered as well. Safety glasses are required.
• Class 4:
  Ocular and skin hazards for direct or diffuse exposure. Covers high-power lasers. Safety glasses are required.


• For all lasers class 3B or above, 'controlled area' requirements apply in which the NHZ must be protected by barriers.
• Safety glasses are required for all lasers classed 3B and 4.
2. Exposure limits (MPE)
• Maximum Permissible Exposure (MPE) is set by ANSI, OSHA and other agencies
• For visible lasers, MPE can be calculated for a 0.25s exposure since the eye has a natural aversion to bright light
• For IR lasers a 10s exposure is assumed to be worst-case
• Pulsed lasers have a different set of criteria since damage can occur from a single pulse which occurs faster than the eye can possibly blink (e.g. Q-Switched lasers)
• Examples (taken from OSHA guidelines):
• Argon-ion, CW, 514nm, 2.5*10^-3 W/cm²
• HeNe, CW, 633nm, 2.5*10^-3 W/cm²
• Nd:YAG, CW, 1064nm, 5.1*10^-3 W/cm² (10s)
• Nd:YAG, Q-Switched, 1064nm, 17*10^-6 W/cm² (10s)
3. Defining Nominal hazard zones (NHZ) and use of barriers
• An NHZ is the space within which the exposure exceeds the MPE
• Since a beam expands, and so irradiance decreases, the NHZ does not extend forever
• NHZ's are much smaller for diffuse reflections than for intrabeam (direct viewing)
• Example: a 100W YAG has an NHZ of 793m for direct viewing (the length, after which, the beam will have diverged enough to be rendered safe), 0.79m for diffuse viewing
• Barriers are used to limit access to diffused reflections and limit the hazard zones
4. Calculating OD for safety glasses
• Safety glasses are rated by wavelength and OD (Optical Density)
• OD = -log(transmission) so an OD of 4.0 means a transmission of 0.0001 or 0.01%
• Normally, a wide-open pupil of 7mm diameter (0.4cm² area) is assumed
• Example: To compute the required OD of safety glasses for an argon laser ...
  1. Compute the irradiance of the laser (E₀) assuming the beam enters a fully-dilated pupil (7mm diameter or 0.4cm² area)
     Example: for a 10W argon laser E₀=10W/0.4cm² = 25W/cm²
  2. look-up the MPE for the laser in use
     Example: for an argon laser (514nm), MPE=2.5*10^-3W/cm²
  3. compute the OD required to attenuate the beam (E0) to the MPE
     Example: OD = log(E₀/MPE) = log(25/2.5*10^-3) = 4.0
• Obtain a set of glasses with an OD of 4.0 minimum at a wavelength of 514nm

Another Look at OD ... Another (simplified) way to look at OD is that the purpose of safety glasses is to reduce a dangerous beam intensity down to a class-2 beam intensity. Assume we have a 10W argon laser (above) passing through safety glasses rated at OD=4. An OD of 4 means the intensity is reduce by a factor of 1/104 or 1/10000 so that 1mW passes through the glasses into the observer's eye: that is under class-2 limits and considered "safe". The formula used is the same as that used for neutral-density filters and many other optical elements: Pout = Pin / 10OD This is a simplified view because it assumes that "class-2" is 1mW. This is true for a CW visible laser (e.g. HeNe or Argon) but safe-limits vary by type of laser and are considerably lower for invisible lasers and pulsed lasers such as a Q-switched laser ... hence why the MPE calculations shown above are used.

5. High voltage safety
• Interlocks are used for beam safety (e.g. to shutdown a laser when a cover if opened)
• Interlocks are also employed for electrical safety (remove power when a cover is opened)
• Do not defect electrical-safety interlocks (but don't trust them either, turn the supply OFF)
• Demonstration: typical interlock devices
• Capacitors used in many lasers store energy, filter DC, etc.
• Capacitors usually have 'bleeder resistors' on them to reduce charge gradually
• Without a 'bleeder' capacitors store energy indefinetely
• Demonstration: energy storage in a capacitor
• Do NOT trust your life to a 50-cent resistor, always short capacitors
6. Chemical and fire safety
• Dyes used in dye lasers are often carcinogenic (cancer causing)
• Solvents are often toxic and flammable
• Gloves are required when handling all dyes and solvents

Finally, complete the Laser Safety Quiz on Blackboard certifying that you have seen the video and understand the safety procedures required in our lab. The quiz must be successfully completed at least one day before the first lab in order to be admitted to the lab. Unsuccessful (i.e. with as mark of less than 17/20) or incomplete quizzes will be required to be submitted again before admission to the lab. If a student shows-up to the lab WITHOUT a successfully completed quiz, he/she will be DENIED entrance to the lab and will be marked absent (with a 50% penalty) for each lab session until the quiz is successfully completed.

Copyright (C) Niagara College, Canada, 2009-2011