Powerful LEDs challenge optical safety standards

02 Jun 2003

John O'Hagan of the NRPB describes how optical safety standards are being amended to stop high-brightness LEDs from slipping through loopholes in the legislation.

From Opto & Laser Europe June 2003

As light-emitting diode (LED) technology continues to develop, bringing progressively more powerful devices to the marketplace, questions are beginning to be asked about the safety implications of their use.

Clusters of high-brightness visible and white LEDs are now being used in everything from traffic lights and cat's eye reflectors to portable torches and toys. Their long life, compact size and high efficiency looks set to maintain the popularity of LEDs with equipment makers. Many believe that it will only be a few more years before the semiconductor light emitters are fitted as standard in car headlamps.

Although few LED sources are capable of causing direct eye damage - unlike intense, highly directional laser beams - there are concerns that the brightness of some could be capable of dazzling those exposed to them. Drivers travelling at night may have noticed that LED traffic lights can be uncomfortably bright to look at, and there have been reports that some LED torches can cause unpleasant dazzling and after-images.

Unfortunately, addressing the safety issues surrounding the increasing use of LEDs will not be easy, and the problem has been somewhat swept under the carpet. No safety guidelines dedicated to LEDs have yet been drawn up and there have been arguments over whether they should be classified as lasers or incoherent sources. What's more, there are conflicting opinions as to how their output should be measured.

So how far do existing safety standards go to deal with LEDs? Here's a round-up of the steps taken by the most relevant standards organizations so far:

* ICNIRP The International Commission on Non-Ionizing Radiation Protection (ICNIRP) makes recommendations for optical exposure limits based on a review of biophysical data. It has published guidelines on the safe use of both incoherent optical radiation and laser beams.

As far as LEDs are concerned, ICNIRP has published a statement recommending that safety evaluations and related measurements of LEDs should follow the guidelines for incoherent sources. Although the statements are not in themselves legally binding, they offer a good starting point.

* CIE Because LEDs are already used in indicator lamps and in the near future will be employed in general lighting, their safety was addressed by the International Commission on Illumination (CIE) in the recently issued CIE S 009/E:2002 standard. The standard describes exposure limits and offers a reference measurement technique. It also provides a classification scheme for the evaluation and control of photobiological hazards from all electrically powered incoherent broadband sources of optical radiation. The exposure limits given are based on the ICNIRP guidelines.

The classification scheme includes an Exempt Group for lamps that pose no photobiological hazard. Many low-power LED products are likely to fall into this category.

* IEC The International Electrotechnical Commission (IEC) publishes manufacturing standards, including IEC 60825 which deals with the safety of laser products. Part 1 of this standard covers manufacturing requirements and has been adopted in Europe as EN 60825-1. The 1993 edition of the laser standard (which was adopted in Europe in 1994) was extended to include LEDs in an attempt to address concerns over the increasing output powers of the devices.

The problem is that the measurement conditions for assessing laser beams are not necessarily realistic in terms of the common applications of LEDs. Attempts have been made to address this by amending the measurement procedures, but unfortunately the amendments for the International and European standards, as well as some of the national standards, are not the same. The latest version of EN 60825-1 incorporates amendments and suggested corrections up to July 2002. The measurement conditions are now more realistic, but discussions on improving them further are still under way.

Exemptions EN 60825-1 does also allow for exemptions for LEDs that are considered completely safe. Laser and LED products that under all conditions of operation, maintenance and failure do not exceed the Class 1 accessible emission limit (AEL) are exempt. A new addition to the standard suggests that LED makers should provide more detailed data to their customers to help them decide whether their product is exempt or not.

Although many low-power LED products will fall into the "exempt" category, product makers are faced with the problem of how to demonstrate this without having to resort to complicated measurements.

I propose that the assessment of whether an LED product is exempt should start with the manufacturer of the LED. The company could provide the product maker with the LED's radiant optical power as a function of voltage, and also a maximum rated operating voltage. If the total optical power from an LED operated at its maximum voltage is significantly below the Class 1 AEL, it would be reasonable for the product maker to conclude that the product is exempt.

The analysis of the actual worst-case scenario of eye damage by exposure to an LED product can be complex and in some circumstances the most hazardous viewing position may be some distance from the source. This is the case with traffic-light LED arrays. When viewed close up, the radiation from only one LED is imaged in the eye, whereas from further away the radiation from all of the LEDs is imaged (see figure 1).

Another complication is the chance that product makers might increase the potential hazard posed by LEDs by operating them above the manufacturer's specified maximum conditions. The output from a novelty product intended to be operated at 12V DC is shown in figure 2 as a function of applied voltage. The measured radiant power at 24V is 2.4 times that at 12V. Although using it this way may shorten the life of the LED, it may still be operational for many hours.

Legislation What is apparent is that a series of clear safety standards that deals with LEDs needs to be completed as soon as possible, so that potentially hazardous products do not find their way onto shop shelves or roads.

The Commission of European Communities may ultimately play an important role in bringing about legislation. In 1993, it published a proposal for a European Directive concerning protection against physical agents in the workplace, including optical radiation. The initial drive has been to push through the sections on noise and vibration. A new draft on electromagnetic fields has recently been published and the optical version is expected to follow. The NRPB has reviewed the implications of such a directive and the report is available from the NRPB website.

If it is passed, this legislation could place a specific duty on the integrators of optical radiation sources to confirm that these sources do not present an unacceptable risk to health. To work well, it will require access to data from LED manufacturers to minimize the measurements required.