The potential hazards of exposure to optical radiation have long been understood – sun burn and arc eye being common examples. With increasing optical output, can the LED sources that we use to light our lives cause us harm?
In the European Union, CE marking demonstrates the safety of goods. The mark indicates that they comply with the essential health and safety requirements of the relevant directives, established by applying European (EN) standards relevant to the directive.
Optical radiation is dealt with under the Low Voltage Directive (LVD). Before the era of LEDs, lighting standards did not consider photobiological safety in any depth, and simply minimised ultraviolet radiation with a 2mW per 1,000 lumens UV limit.
In 1993, Nichia introduces commercially viable blue GaN LEDs, and they were followed by white phosphor conversion (PC-) LEDs. Clearly, optical radiation exposure had to be considered again.
LEDs may be considered as a technology between lasers and conventional lamps. For this reason, LED safety was in the past evaluated with reference to the laser standard. As a result, hazards were often overestimated. This situation became untenable as LEDs were used in more applications.
Set the standard
In 2007, the International Electrotechnical Commission (IEC) decided to remove most LEDs from the scope of the laser standard, adopting guidelines from the Commission Internationale de l´Eclairage (CIE) to publish IEC 62471: 2006 Photobiological Safety of Lamps and Lamp Systems.
In Europe, this was published as EN 62471: 2008 and harmonised to the LVD. From 1 September 2011, evaluating LEDs against the laser standard will no longer demonstrate conformance with the LVD.
IEC/EN 62471 contains guidance for the evaluation of the photobiological safety of lamps and lamps systems, excluding lasers, that emit light in the spectral region 200-3,000nm. It covers six hazards to the skin and eyes for exposure up to eight hours. It defines a four-tier classification structure, from Exempt (no risk) to Risk Group 3 (hazardous even with momentary exposure).
The damage mechanisms considered are photochemical, when light of specific wavelengths causes chemical changes in tissue, or thermal, resulting in local tissue heating.
Although the effects of low level thermal exposure canbe mitigated by thermal conduction away from the exposure site, photochemical hazards are cumulative – the same damage may arise from short duration, high level exposure as long duration, low level exposure.
In practice, the evaluation consists of a series of measurements of spectral irradiance (200-3,000nm), to evaluate exposure to skin and the front surfaces of the eye, and spectral radiance (300-1,400nm), to evaluate exposure to the retina.
All measurements are performed in conditions designed to account for biophysical phenomena, such as eye movements and pupillary constriction.
According to IEC/EN 62471, evaluation of general lighting service sources should be performed at a distance at which the source produces an illuminance of 500 lux. All other sources are measured at a worst case condition of 200mm, the near point of the human eye.
The IEC is still discussing the implementation of this standard in the lighting industry because the application of the 500 lux condition is not always realistic. For example many home and office lighting products may have a 500 lux distance of several metres, but the sources are rarely that far from the occupants of the room, particularly when they are standing.
Do LEDs pose a photobiological hazard?
Since the lighting industry is concerned with the visible spectrum, and given the limited spectral distribution of LEDs, there are no concerns about UV and IR radiation. What remains is the potential of photochemical damage to the retina from blue light. The strong wavelength dependence of blue light is described by the blue light hazard function.
Two properties of LEDs contribute to the hazard. Their small size causes high retinal irradiance (glare) and the wavelength which has the highest potential to cause damage coincides with the typical peak wavelength of the blue LEDs that pump the phosphor in white PC-LEDs.
The blue peak of cool white LEDs is and significantly greater than that of warm white LEDs. It is the popular cool white LEDs that have a greater potential for damage.
At present there are no concerns about acute exposure – today’s highest power LEDs are classified at the lower end of RG2. It would be necessary to look at the source from a distance of 200mm for 40-50 seconds before reaching exposure limit values.
However, cumulative exposure over eight hours should be considered, and further research should be done into the reported effects of long-term, low-level exposure on age-related macula degeneration.
As the trend is clearly for LED devices with higher outputs, there is no question that the risk will increase in the future.
- Leslie Lyons is technical support manager at Bentham Instruments and member of BSI and IEC optical safety committees TC76.