Optical wireless users should pay more attention to eye safety ratings of systems

Issues of eye safety are set to become a topic of contention among optical wireless equipment vendors in the months ahead as equipment based on the emerging technology gets deployed. The Free-Space Optics Alliance, which will hold its inaugural meeting at WCA 2001 in Boston later this month, will surely address the subject. The problem as it stands is that most vendors claim their products to be eye safe when it’s not necessarily the case. David Kahn, VP product development at Plaintree Systems, an optical wireless equipment vendor, sets the record straight. Optical wireless (OW) or Free Space Optics (FSO) is the newest contender in the race to provide affordable last mile connections that can bring the multimedia Internet into the home and office. It uniquely combines the power of fibre with the convenience of wireless while avoiding leasing, licensing or spectrum planning hassles. Telecom carriers, ISPs and institutions are all eyeing the prospects and possibilities of optical wireless but are facing a battery of conflicting claims about the hazards to the eyes of inadvertently staring at the invisible beams. Predictably, all vendors claim that their systems are eye-safe, but a little research reveals that there is more to the issue than meets the eye! A review of the market will show that products carry a variety of designations that are all showcased as being synonymous with eye-safe. These designations are typically "Class 3A", "Class 3B" or "Class 1M". What is invariably unstated is the fact that all these designations imply that the products are potentially dangerous to the eyes. Only one designation, Class 1 tells that the product is safe under all foreseeable conditions. All the other classes are hazardous to varying extents. The danger is that powerful invisible beams could enter the eye and concentrate within it, causing heating and possible lesions resulting in both temporary and permanent sight impairments. For operators who plan to use systems designated other than Class 1, the challenge is to quantify the level of risk. What are the chances that anyone can get into the path of the beam? What are the chances that they will then stare into the beam? What are the chances that they will be using binoculars, increasing the hazard level 50 times? There will be situations where the optical path is entirely under the control of the operator and the risk management is then much easier. But in other situations, this will not be the case and the fact that the chances of injury are very low will not serve as an effective defence in a court of law. Operators will also rightly be concerned to avoid putting installation and maintenance personnel at risk especially where binoculars or gunsights are required for alignment. There are two prime types of optical wireless systems, those employing lasers and those employing LEDs (Light Emitting Diodes). They differ considerably in several respects including eye hazards. Laser light is far more dangerous to the eyes than LED light of the same power because the eye is able to focus laser light to a point resulting in a very high power concentration. In contrast, LED light being from an extended source cannot be focussed down so much. The International Electro-technical Commission standard IEC 60825-1 is the authoritative reference for eye safety and addresses both lasers and LEDs. It defines the various designations and hazard levels associated with products and specifies what steps must be taken to identify them and protect users and the public. The standard implies that the maximum safe power level entering the eye directly from a LED transmitter can be up to 10 times greater than that from a similar laser transmitter. In January this year, the standard was updated, introducing the Class 1M rating that by definition applies to systems dangerous to view when using optical aids such as binoculars. Such systems require a warning notice or label. Most laser wireless products fall under a Class 3 category with a few in the new Class 1M category. They are therefore hazardous to varying degrees. LED wireless products generally fall under the Class 1 category and are therefore safe under all foreseeable conditions.

Optical wireless users should pay more attention to eye safety ratings of systems

News | 06/11/2001 4:00 am EDT

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