Safe light sources control photobiological hazards

Have you ever worried about damage to your eyes due to a light source? Lamps became commonplace long before industry-wide safety standards came into force.

Indeed, lamps have been used and produced in large quantities for more than 100 years. Although they became ordinary and daily life depends on them much more than we are generally aware of, their safety has not really been the centre of concern for the general public. To observe natural events like solar eclipses, people are strongly advised to protect their eyes with special glasses while laser pointers, which are used to indicate items on a presentation screen, have made people aware of the dangers of bundled lights. Similarly, large scale laser displays must be directed away from the naked eye.

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Most light sources are safe to view. We all have an innate aversion response to very bright light and this normally protects our vision since we do not continuously stare into a potentially hazardous light source. Just momentary viewing of the sun produces this response. If someone stares directly into the sun, he or she can permanently damage their retina after about 90 seconds, but since this would require overcoming the aversion response, this is extremely rare. When directly exposed to a laser or LED pointer the aversion response limits the exposure to a fraction of a second, or, in any case, to less than a few seconds. Since lasers were introduced in the 1960s it has been possible to experience a retinal injury within a fraction of a second, too fast for protection by the aversion response. Hence, laser safety standards became necessary.

The IEC has had safety requirements for laser products since the 1980s (for instance, the IEC 60825 series). Some lamp standards have included special requirements to limit ultraviolet emissions of certain lamp types for a number of years. However, with the advances in light emitting diode (LED) technology, people have become increasingly concerned with the potential hazards to the retina from exposure to these bright, solid-state lamps. Obviously, lengthy staring at many conventional lamps may damage the eye.

The photobiological effect of light can be beneficial, for example in treating depression, but it can also be harmful to organs such as the skin or the retina. So there is a need to be able to evaluate the safety of lamps and lamp systems (including luminaires) so as to specify emission limits, reference measurement techniques and a classification scheme for photobiological hazards from electrically powered sources, such as LEDs. This guidance helps to evaluate the photobiological effects, their dose relationships and measurements.

The science of photobiology is the study of the effects of visible light and other optical radiation (ultraviolet and infrared) upon biological systems, both plants and animals. For example, sunburn is a photobiological response resulting from excessive exposure to the ultraviolet radiation in sunlight. Most photobiological effects depend upon both the wavelength and the dose. That dose depends upon the exposure level (the “irradiance”) and the exposure duration. As it turns out, LEDs are generally quite safe to view. However, blue and other short-wavelength LEDs may pose a theoretical eye hazard if someone were to force themselves to overcome their natural aversion response to bright light and stare at an array of them for several minutes, long enough to be exposed to a potentially hazardous dose.

Optical radiation hazards from lamps and lamp systems
Since 2002, the International Commission on Illumination (the CIE) has had an international standard (CIE S-009) on the photobiological safety of lamps and lamp systems. This was recently issued by the IEC as a dual-logo standard. Lamps may be hazardous from several aspects and therefore a classification scheme of four risk groups enables the user of a lamp or lamp system to know the degree of risk from exposure to the ultraviolet, visible and infrared radiant energy emitted by a LED or another lamp type.

Most lamps used in the home are in the “exempt” group (no reasonably foreseeable risk), therefore safety measurements are not needed. In Risk Group 1 the lamp may pose a risk from very lengthy exposures at close range. Risk Group 2 includes lamps that may pose photobiological hazards to the eye or skin from even a moderate exposure duration, and Risk Group 3 comprises lamps that may be harmful even from momentary viewing (for example, high-intensity, short-arc xenon high-pressure lamps). Aware of the danger, the user can determine if any safety measures are needed, such as the use of a protective fixture, limiting exposure durations, etc.

To evaluate the risk groups the manufacturer of the LED or other light source must perform radiometric measurements for each lamp. This can be quite complex – even more so than the measurements required for the laser hazard classification in IEC 60825-1. Laser manufacturers normally measure the radiant power through one or two apertures, at a specified distance. However, the lamp manufacturers must measure the entire spectral emission from the ultraviolet to the infrared at a specified distance, and then in some cases also determine the source size.

Most lamp manufacturers are set up to measure the lamp spectrum, but some special geometrical details must be applied in this instance. Normally, a LED emits only over a limited wavelength band and the spectral measurements of a LED may actually be simpler than those required for a conventional lamp. Furthermore, single LEDs will not require a source-size determination. Indeed, it was the challenge of determining the source size of a LED in terms of the laser standard that led many to recognize that most LED sources really should be treated along with other lamps in a separate lamp safety standard.

Safety of people, animals and plants
IEC lamp safety standards will improve the safe use and design of lighting systems and limit potentially hazardous exposure conditions. As noted above, most domestically-used lamps are safe. Some high-intensity discharge lamps and tungsten-halogen lamps require either a built-in glass filter to reduce ultraviolet radiation to safe levels or need to be used with a fixture that blocks the ultraviolet radiation. The joint, double-logo standard does not provide manufacturer requirements. Instead, IEC Technical Committee 76 (Optical radiation safety and laser equipment) is preparing a technical report on possible manufacturer requirements depending upon the lamp (LED) application. Except for the application of warning labels, it is not practical to establish a single set of control measures for each risk group. Instead, the safety requirements, if needed, depend upon the application area.

Adopting an existing standard
TC76 decided not to prepare a completely new standard for lamp safety since the CIE standard already existed. The CIE is highly regarded for its technical reports and standards for light measurement conditions. For this reason, and to offer a truly International Standard in a very short time, the TC decided to create a dual-logo standard where measurement requirements and detailed definitions for risk group classification exists within the CIE standard. Labelling and performing requirements, if applicable, will be defined in other IEC standards. Maintenance of the dual-logo standard will be a joint task of CIE TC 6-47 and IEC TC 76/Working group 9 (Non coherent sources), along with representatives from IEC TC34 (Lamps and related equipment) and Subcommittee 34A (Lamps). The joint task group met in Cleveland, USA, in July 2006 to draft revisions to make the standard more useful to manufacturers.

Interdisciplinary approach to safety
Issues in laser safety have become more clearly defined and have been largely resolved over the past decade. However, the refinement of lamp safety standards will require effective co-ordination across a range of technical committees. TC76 plans to complete a technical report on the implementation of lamp risk groups and recommend for consideration some performance requirements for safety that could be included in other standards. After a couple of years of experience, the technical report could evolve into an International Standard.

IEC 62471 Photobiological safety of lamps and lamp systems, is an example of the close relationship and collaboration between different groups in the standardization business. Furthermore, it shows that the safety of light sources have to be treated as an interdisciplinary matter.

IEC 60825 (all parts), Safety of laser products