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Laser safety: eyewear is not the only option

20 Feb 2003

Making a laser facility safe often involves much more than going out and buying a pair of laser goggles. Jacqueline Hewett reports on the other essential items you will need.

From Opto & Laser Europe March 2003

In this month's Buyer's Guide we take a look at the range of equipment available to help you make your laser laboratory safer. Eyewear is often the first - and sometimes the only - item that many laser users think of buying to protect themselves, but safety experts agree that a pair of goggles should in fact be the last line of defence.

If you want to ensure that your laser installation presents no danger, the following equipment is highly recommended:

• signs and lamps to warn employees that a room contains a laser;

• enclosures and beam blocks that act as a physical barrier between the laser beam and anyone in its vicinity;

• failsafe devices that automatically shut down the laser or render its beam safe in the case of an unintended interruption;

• appropriate eyewear.

If you're not sure which of the above measures you need to take, help is at hand. Advice is available from specialist laser-safety consultancies, or you could subscribe to a training course on the topic. Bodies such as the UK's National Radiological Protection Board, the National Physical Laboratory and the Laser Institute of America regularly organize such courses. Visit the websites mentioned at the end of this article for more information.

If you decide that it is appropriate to increase your level of laser protection, the next step is to contact a laser-safety equipment vendor. Make sure you have the following key specifications to hand:

• what type of laser you are using;

• what class of laser you are using;

• what wavelength your laser is emitting;

• what power your laser is emitting at the given wavelength;

• whether your laser is running in continuous-wave mode or pulsed mode;

• if your laser is pulsed, its repetition rate and pulse length.

This seemingly trivial set of information will allow experts to assess the hazards of your laser system and provide a unique safety solution tailored to your needs.For an insight into the different kinds of safety equipment on the market and the role that each item plays, Opto & Laser Europe has prepared the following breakdown.

Warning signs and interlocks Laser safety starts outside the lab. Anyone about to enter a room that contains a laser needs to be warned. This means posting the appropriate warning signs on the door and installing an exterior lamp that lights up whenever the laser is on. Some lamps display different colours to indicate the operational status of the laser: in general, green means that it is safe to enter, amber indicates that the laser is enabled, and red that it is in operation.

Interlocks for doors are used to control entry into a laser laboratory, and come in various configurations. Some use a coded numeric keypad or magnetic badge-controlled entry system to prevent people from accidentally wandering into a room in which a laser is operational. Others have a mechanism for shutting down the laser or blocking its beam if the door to the lab is unexpectedly opened. Inside the lab, there should be a small safety alcove where you can pick up the appropriate eyewear before beginning work with the laser.

Safety barriers Whenever possible, enclose the laser and its beam within a series of metallic surrounds so that you can work freely without coming into contact with the laser beam. These sheets of metal need to stand up to the worst foreseeable exposure to the laser in use and give protection from stray direct and/or scattered beams. Beam terminators - small metal or graphite blocks - can also be placed on the table to directly block the path of the beam. These can be easily moved around or removed completely during operation of the laser. Also consider the use of a beam stop that bolts onto the optical table.

Laser safety curtains - layered protective fabrics that have the ability to withstand direct laser exposure - can be used to enclose a working area, divide a laboratory into cubicles or create safety alcoves. The exact composition of the curtain depends on the output parameters of your laser, including beam power, beam size, exposure time and separation distance between the laser and the curtain. Vendors offer many forms of laser safety curtain to suit any application. Ready-made ceiling- or wall-mounted curtains, roller blinds, window blocks or free-standing screens are all available.

Laser-safety windows should be considered if researchers are working alone in the lab. A filter can be placed over the glass that allows those outside to maintain line-of-sight visual contact with the lone worker.

Laser eye protection A vendor will want to know the details of the exposure conditions you are working in before selling you a pair of safety glasses. You will need to be able to tell the vendor whether you are interested in protection from momentary accidental direct exposure to a laser beam, or from diffuse reflections for an extended period of time.

Another consideration when selecting safety glasses is the transmittance of the eyewear for a particular wavelength, which is known as its optical density (OD). To offer adequate protection, eyewear must have the appropriate OD. Laser safety standards in the US and Europe allow vendors to calculate the OD that your eyewear requires based on the exposure limits of your specific application and taking into account the EN 60825-1 standard.

In terms of figures, the OD can be thought of as the logarithm of the reciprocal of the transmittance. For example, a filter with an OD of 4 blocks ten times the amount of light as a filter with an OD of 3.

In addition to having the right OD, eyewear must offer protection for long enough for the wearer to take evasive action. The duration of protection is known as the L number, and is calculated using a standard called EN 207. Vendors also offer a selection of glasses that allow you to see enough of the beam to align it. In some instances, though, it is better to use a low-power laser for alignment rather than relying on safety eyewear.

Crucially, your eyewear must provide a high transmission in the visible range so that you can see what you are doing. Be aware that if the eyewear has a high OD in the near-infrared it may also be very effective at blocking red light and hence any warning lights on your equipment.

The lenses in eyewear are made from either glass or polycarbonate filters. Glass filters are typically more expensive but give better protection (owing to their greater OD) and better colour recognition. Bear in mind that safety eyewear only filters a specific range of wavelengths. If you are using a tunable laser, such as an optical parametric oscillator, make sure that you mention all applicable wavelengths to the vendor. It may be necessary to buy several pairs of glasses to cover broad tuning ranges. Research is currently ongoing to develop broadband filters that can deal with multiwavelength sources.

If you know that you will be wearing the eyewear for an extended period of time, comfort is also an issue. A large choice of frames is available, but be careful to choose a style that protects against stray beams entering the eye. All eyewear comes with the wavelength range it blocks etched onto the lenses or the arm of the glasses.

Non-beam hazards The hazards associated with using a laser are not limited to potential damage to your eyes. Except for the visible and infrared between 400 and 1400 nm, the eyes and skin are at equal risk from laser radiation. This means that when you are working in the UV or the infrared B and C ranges, protecting yourself against eye damage only is inadequate.

A laser installation often uses water-cooling systems and large high-voltage power supplies. These are hazardous in their own right and their use means that electrocution is a common cause of many laser-related accidents. Another danger comes from the possibility of interaction between a laser beam and certain materials, such as plastic or wood. This can generate fine particulate matter, resulting in air contamination and potentially dangerous fumes.

It is important that you seek expert advice to safeguard your lab against such risks. It can often be a good idea to fit an emergency "off" switch just inside the lab door that switches off the electrical power to all of the apparatus in the room except for emergency lighting. Appropriate fume extraction or air monitoring equipment may also be required.

Other secondary non-beam risks include:

Plasma radiation Infrared systems such as CO2 or Nd:YAG lasers emit invisible radiation. But when these invisible beams interact with metals, a visible emission is generated as a bright spot of plasma that can be rich in ultraviolet and broadband visible.

Collateral radiation This is radiation generated by the laser system in its normal operation. It refers to by-products of the laser, rather than the beam itself. For example, a CO2 laser generates its beam in plasma tubes. If you open up the laser, the tubes will be glowing violet and this violet light is a source of collateral radiation.

Collateral radiation can also take the form of X-rays. This becomes an issue when using pulsed laser systems operating at high potential differences, when the rapid deceleration of the electric charge produces X-rays.

Opto & Laser Europe would like to thank the following companies for their help in compiling this article.

Laser Institute of America

Lasermet

Noir

Trinity Technologies

Wilson Industries

Synopsys, Optical Solutions GroupSPECTROGON ABEaling UGSchaefter und Kirchhoff GmbHart Photonics GmbHALIO IndustriesPhoton Engineering, LLC
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