Literature

to refer to – all about laser protection

It is surely less embarrassing to ask a stupid question, but to be not able to answer it. Here you find answers on
practical questions of the laser protection which we are frequently asked.

The European standards according to which laser safety products are tested comprise also an annex B each with rules for the proper selection. These rules must be supplemented by expertise. Due to a myriad of varieties in the parameters for the beam source and the beam guidance system no more generalized no rules can be given. Here are some examples ...

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In Europe the safety of laser eyewear is measured not only by the optical density, but also by the resistance against laser radiation. This might be a big difference. If for example laser safety filters fail after a too short time of irradiation, they do not protect the eyes sufficiently, even if they have a sufficient optical density.

The laser protection level may be given separately for different laser operation modes which is part of the marking as per EN 207, e.g.

1064 D LB7 + I LB8
> 1064-1200 DIR LB6 OB

This marking means that the laser spectacle reaches protection level LB7 for the wavelength 1064 nm at continuous mode operation, level LB8 at impulse mode and LB6 in the range larger than 1064 nm up to 1200 nm at the operation modes continuous mode, impulse and giant impulse. Further the marking as per EN 207 shows the manufacturer's shortcut.

The subject laser pointer is now significantly discussed emotionally. How big is the real hazard?

Lasers can be designed so that their radiation is emitted as a very sharply defined beam with extremely small divergence. This is why laser pointers are used. With relatively little power you can generate a good perceptible light point, replacing the pointing stick also at greater distances from the screen.

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Every chain is as strong as it's weakest part. Not only the filters, but also the frame of laser safety eyewear contributes to eye safety. Especially the following items are important:

  • side shield and general protection against oblique incidence of laser rays.
  • low transmittance for laser radiation. Most materials show a transmittance increasing with the wavelength.
  • high resistance against laser radiation. High softening temperature and heat conduction of the frame material are advantages.

For laser eyewear frames comfort, safety design and costs have to be optimised.

Side shield type and goggle frames are distinguished. Goggles allow also to wear a normal correction spectacle and have very high protection levels in most cases. Side shield type frames show less fogging.

Lasers are distinguished by their mode of operation:

  • continuous wave operation D means continuous release of laser radiation.
  • impulse operation I means release of single or periodically repeated laser pulses.
  • giant impulse operation R is basically the same as impulse operation, but at a very short pulse time.
  • mode coupled operation M means the release of all energy stored in the laser medium within the shortest possible time.

Some laser types can be operated in different modes of operation.

Filters of laser safety eyewear must meet special requirements. They must protect sufficiently against laser radiation and have a high luminous transmittance. This is relatively easy to achieve with filters for UV or IR lasers. In the visible range this becomes much more difficult, especially at about 550 nm where the sensitivity of the human eye is highest. Filtering of laser radiation in this range may lead to considerable losses of luminous transmittance. Also the visibility of neighboured wavelengths might be impaired. This might be important for medical laser systems, if e.g. therapy and pilot beam both are in the red wavelength range.

Laser safety filters are distinguished by absorption and coating. Absorption filters protect by damping along the optical path. Coatings are thin layers and protect by mirroring depending on the wavelength. Laser protection by mirroring effects may cause problems, because the reflected laser radiation may lead to damages elsewhere.

There are also distinguished filters made of glass and plastic. Plastic filters are light, but less scratch resistant and have a low softening temperature or start to burn easily. This prevents to achieve high protection levels. Mineral filters are heavier, but are more scratch resistant and may achieve very high protection levels.

Cabin safety windows allow to observe safely processes inside sealed-off laser systems. The same applies as for laser safety filters. Large area cabin safety windows made of glass are safer than those made of plastic, but they are difficult to manufacture and therefore expensive in most cases.

The user of a laser system is responsible for the safety. You may try to evaluate the required protection level yourselves by our online service Protection level, but we are also ready to support you in selecting an appropriate eyewear, because this has to be done according to different norms which are not easy to understand even for more experienced laser specialists. For this we normally need the following data:

  • laser power (continuous wave operation) and/or pulse energy (impulse or giant impulse operation)
  • pulse duration and pulse repetition frequency (impulse or giant impulse operation); please note that frequently the pulse repetition frequency and pulse energy are not independent from one another.
  • diameter of the laser beam which may hit the eye
  • wavelength of the laser

We can support you best, if you send us a copy of the technical data of your laser system which in most cases do not comprise more than one page of the user's manual.

At mode-locked operation lasers may emit extremely high power over a very short time which may change the optical properties of laser safety filters (Q-switch effect and emission of secondary radiation).

The subject laser pointer is now significantly discussed emotionally. How big is the real hazard?

Lasers can be designed so that their radiation is emitted as a very sharply defined beam with extremely small divergence. This is why laser pointers are used. With relatively little power you can generate a good perceptible light point, replacing the pointing stick also at greater distances from the screen.

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Lasers are an indispensable part of daily life. They are used diagnostically and therapeutically in medicine, they are used in industrial micro and macro materials processing, and they do their job almost unnoticed in CD drives and printers. Although lasers from the physical point of view are only one method to produce electromagnetic radiation they are still surrounded by an aura of mystery and danger. When are lasers really dangerous?

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This document explains laser safety in an easy and plain way.

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There is frequently warned against the danger of laser radiation when viewed through magnifying optical instruments. But is the exposure when viewed through microscopes or telescopes actually more dangerous than the direct exposure? Part of the answer is simply the magnification. Magnifying optical instruments enlarge everything, including the laser beam. This reduces at least the irradiance on the retina, if the radiation passes through there. This is the case in the wavelength range 400-1400 nm and the hazard of eye injuries is actually less than at direct exposure. But there are exceptions. If the radiation doesn’t pass through to the retina, the answer may be difficult. It is therefore necessary to consider each case separately. Here some scenarios are considered which frequently occur in industrial laser applications.

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Laser radiation is directed electromagnetic radiation which may be highly ordered in space and time. The spectrum ranges from the ultraviolet through the visible to the far infrared.

Laser radiation in medical applications has some advantages over conventional sources:

  • It can be focused on much smaller areas than for example sunlight and allows to process much finer structures.
  • Special effects at interacting with tissue can be achieved due to the sharply limited wavelength.
  • The high temporal and spatial order allows non-contact measurements at high precision.

The applications make use of different types of interaction between laser radiation and matter:

  • The thermal effect is based on the conversion of the absorbed laser energy into heat. In medicine, this allows, for example, coagulation and vaporization of tissue.
  • Chemical effects are achieved based on laser induced reactions in the tissue. This is utilized, for example, in photodynamic therapy.
  • A diagnostic effect is achieved by scattering of laser radiation in the tissue depending on the wavelength.

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Laser eye protectors are covered by the European Directive 89/686/EEC (personal protective equipment) and must be CE marked before placed on the EC market as usually known. However, many users do not know how complex this type of market access is in order to prevent damages from the users. Additionally, the proper selection of laser eye protection is not easy.

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The resistance of laser safety filters to laser radiation doesn’t only depend on the power or energy density but also on the beam diameter. The present study illuminates these findings.

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