laser safety

Judy DonnellyThree Rivers Community College 1

Laser Safety


CHARACTERISTICS of LASER LIGHT

Monochromatic- “single color”

Coherent- waves are “in phase”

Highly Directional- resulting in very concentrated light energy (high “irradiance”)


COMPARE LIGHT FROM A LASER AND A FLASHLIGHT

Monochromatic

laser is single color; flashlight has rainbow spectrum

Coherent

Laser Speckle- due to wave interference

Highly Directional

Flashlight beam spreads much more than laser’s

Define Irradiance = Power/area; laser is lower power but much smaller area


What is Irradiance?

Irradiance depends on both laser power and on the area being irradiated. It is a concept of central importance in laser safety. The symbol for irradiance is “E”, and the units are usually mW/cm2.

E =P

A


EXAMPLE: CALCULATION of IRRADIANCE

A 5 mWatt laser makes a 2 mm by 3 mm spot on a wall.

Find the irradiance.

Power = 5 mWatt

Area = 0.2 cm x 0.3 cm = 0.06 cm2

Irradiance = Power/Area = 5 mWatt / 0.06 cm2

= 83 mWatt / cm2


Laser Safety Standards

Several organizations oversee standards for laser safety:

ANSI American National Standards Institute

Reference for laser users

CDRH Center for Devices and Radiological Health

Product safety standards for laser manufacturers

OSHA Occupational Safety and Health Administration

Enforces regulations in the workplace

IEC International Electrotechnical Commission

International standards organization, with 60 member countries


Laser Eye Hazards

The eye is the part of the body most vulnerable to laser hazards. Changes to the eye can occur at much lower laser power levels than changes to the skin. And, eye injuries are generally far more serious (life altering) than injuries to the skin.

iris

lens

cornea

pupil

retina

optic nerve

fovea


Laser Wavelength Region

IR-C = 1 mm to 1400 nm

IR-B = 3000 nm to 1400 nm

IR-A = 1400 nm to 700 nm

Visible light = 700 nm to 400 nm

UV-A = 400 nm to 315 nm

UV-B = 315 nm to 280 nm

UV-C = 280 nm to 100 nm

Absorption of Light by the EyeLens

Cornea RetinaMid and Far IR(1400 nm-1 mm)

Mid UV (180 nm-315 nm)

Near UV(315 nm-400 nm)

Visible and Near IR(400 nm-1400 nm)


Irradiance at the RetinaExampleA laser pointer produces a 2-mW beam. The beam enters the eye and is focused by the cornea and lens to a spot on the retina 16 um in diameter.

Find:The irradiance on the retina, assuming that all of the 2 mW of power is focused on the retina.

Solution:Area of spot

A = d2/4

= (1.6 x 10-3cm)2/4

= 2 x 10-6 cm2

Irradiance:

E = P/A

= 2 mW/[2 x 10-6 cm2]

= 1000 W/cm2

lens

cornea

pupil

retina

Rule of thumb: The optics of the eye increase irradiance by a factor of 100,000!


Example of retinal damage due to laser exposure


Not all viewing conditions are the same

Specular reflection Convex reflector Concave reflector

Diffuse reflection

Whether a reflection is specular or diffuse for a given surface depends on the laser wavelength. “Smooth” is relative to the laser wavelength.


LASER SKIN HAZARDS

•Thermal hazards (skin burns) from high level of optical radiation

•Photochemical hazards (accelerated aging and risk of skin cancer) due to ultraviolet radiation


NONBEAM HAZARDS

There are several nonbeam potential hazards associated with the use of lasers and laser systems.

1. Fire hazard2. Explosion hazard3. Electrical hazard4. Chemical hazard5. Laser generated air contaminants (LGAC)6. Other hazards

Although loss of sight may be life altering, electrocution is the hazard most likely to end life!


NONBEAM HAZARDS

FIRE HAZARD

Class 4 laser systems represent a fire hazard.

Irradiances exceeding 10 W/cm2 or beam powers exceeding 0.5 W.

The use of flame-retardant materials is advisable and necessary.

Fires have occurred in medical facilities where oxygen provides an explosive environment.


NONBEAM HAZARDS

EXPLOSION HAZARD

High-pressure arc lamps, filament lamps, and capacitor banks in laser equipment shall be enclosed resulting from component disintegration.

The laser target and elements of the optical train that may shatter during laser operation shall also be enclosed or equivalently protected to prevent injury to operators and observers.

Explosive reactions of chemical laser reactants or other laser gases may be a concern in some cases.


NONBEAM HAZARDS

ELECTRICAL HAZARD

This may occur from contact with exposed utility power use, device control, and power-supply conductors operating at potentials of 50 volts and above.

These exposures can occur during laser setup or installation, maintenance, and service


NONBEAM HAZARDS

ELECTRICAL HAZARD

The following potential problems have frequently been identified during laser facility audits.

1. Uncovered electrical terminals

2. Improperly insulated electrical terminals

3. Hidden “power-up” warning lights

4. Non-earth-grounded or improperly grounded laser equipment


NONBEAM HAZARDS

CHEMICAL HAZARDS

Certain dyes are highly toxic or carcinogenic.

These dyes frequently have to be changed, special care must be

taken when handling, preparing solutions, and operating dye lasers.


NONBEAM HAZARDS

LASER GENERATED AIR CONTAMINANTS

LGAC result from the interaction of high-energy laser radiation,

assist gases used in material processing, and the material itself.

When lasers are used in a medical setting, particles of biological

origin such as bacteria may be released into the air. Air filters and/or

ventilation systems are usually required.


NONBEAM HAZARDS

OTHER HAZARDS

•Compressed gases•Cryogenic liquids


Laser Hazard Classifications

Class 1: Cannot, under normal operating conditions, emit a hazardous level of optical radiation.

Included in this category is laboratory equipment using lasers with all beam paths and reflections enclosed. These are called “embedded lasers.”

Examples:• very low powered lasers (< 0.4 microwatts)• CD players, laser printers

Class 1M (NEW!) Eye safe unless focused by a optics



Class 2: low-power visible laser of more than 0.4 microwatts but less than1 milliwatt. The eye is protected by the “blink reflex.” That is, the laser does not have enough output power to injure a person accidentally, but may injure the eye when stared at for a long period.

A “caution” label is required.

Examples:• Many low power HeNe lasers, especially in school labs• Lasers used for alignment procedures • Bar Code scanners

Class 2M (NEW!) Visible output, less than 1 mW, eye safe unless focused by a optics



Class 3a lasers—rated in power from 1 milliwatt to 5 milliwatts Will not normally injure a person when viewed briefly with the unaided eye but may cause injury when viewed with a focusing device such as a lens or telescope.

A danger or caution sign must label the device, depending on its irradiance.

Examples:• many red laser pointers• some HeNe lab lasers

IEC Class 3R is similar.


Laser Hazard Classifications (continued)

Class 3b lasers from 5 milliwatts to 500 milliwatts can produce eyeinjury when viewed without eye protection. This class of laser requires a danger label and could have dangerous specular reflections.

Eye protection is required.

EXAMPLES :•12 mW HeNe•50 mW HeCd


Laser Hazard Classifications (continued)

Class 4 lasers above 500 milliwatts in power can injure you if viewed directly or by viewing either the specular and diffuse reflections of the beam. These lasers can also present a fire hazard. A danger sign will label this laser.Eye and skin protection are required.


MAXIMUM PERMISSIBLE EXPOSURE (MPE)

Maximum permissible exposure (MPE) limits indicate the greatest

exposure that most individuals can tolerate without sustaining injury.

MPE depends on:

• Wavelength

• Output Energy and Power

• Size of the Irradiated Area

• Duration of Exposure

• Pulse Repetition Rate

MPE is usually expressed in terms of the allowable exposure time (in

seconds) for a given irradiance (in watts/cm2) at a particular wavelength.

MPE’s are useful for determining optical densities for eyewear, filters or

windows.


Nominal Hazard Zone (NHZ)This zone describes the region within which the level of direct, reflected, or scattered (diffuse) laser radiation is above the allowable MPE. The distance depends on whether or not the beam is direct, focused, or diffused, as well as the power andMPE.

LASER

df0 NHZ

NHZ illustrated for a focused laser beam


Choosing laser eyewear: Optical Density

The ability of a material to absorb light is sometimesexpressed in terms of optical density.

Optical density is a logarithmic quantity.

In terms of optical density (OD), transmittance is:

T 10 OD


Optical Density

Example:

Laser goggles with OD = 2 at a particular wavelengthhave a transmittance of

T 10 2 0.01

The goggles transmit 1% of the incident light at thespecific rated wavelength.


Optical Density

T = 10-OD

OD Transmission %Transmission

0 1 1.0 100%

1 10–1 0.1 10%

2 10–2 0.01 1%

3 10–3 0.001 0.1%

4 10–4 0.0001 0.01%

5 10–5 0.00001 0.001%

6 10–6 0.000001 0.0001%

7 10–7 0.0000001 0.00001%


Choosing the Optical Density for laser glasses

ODLog10(T)Log10EoMPE

To determine the required OD for safety glasses, comparethe irradiation incident on the eye(Eo) to the MPE (what is allowed to be transmitted to the eye) and take the log of the ratio.


SAFETY RULES FOR LAB LASERS1. Avoid looking directly into any laser beam or at its reflection.

Be aware of the beam’s location.

1. Only trained qualified personnel should work with lasersDon’t let friends and visitors to the lab play with the lasers

1. Keep room lights on whenever possible

2. Remove all watches, jewel and unnecessary specular (shiny) reflecting surfaces from the work area.

3. Don’t bend down below beam height

4. Use beam blocks

5. Wear laser safety eyewear

6. Report accidents immediately.In the case of eye exposure consult an opthalmologist.

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