|A consideration of the potential hazards to the human body posed by exposure to optical radiation has, in the past, been limited to lasers and sources of UV, with a minimalist approach being adopted for LEDs. This latter treatment may have been acceptable in the past, where LED performance had not reached current levels. However, a brief glimpse of many of the LEDs of today attests to the significantly-improved optical performance, and that a consideration of the photobiological safety of LEDs within an appropriate framework is now very much required.
This article is the first in a three-part series that takes a wide-ranging view of the place of LEDs in photobiological safety standards, from the underlying photobiological concerns to the implementation of current product-safety standards.
Overview of photobiology
Photobiology is the study of the interaction of optical radiation with living organisms. Optical radiation is defined as electromagnetic radiation having wavelengths between 100 nm in the deep ultraviolet (UV) to 1 mm in the far infrared (IR). However, this range is often restricted for practical purposes to 200-3000 nm due to atmospheric absorption below 200 nm, and the negligible effect of low-energy photons in the far IR.
Since optical radiation is strongly absorbed in tissue, with penetration depths of a few microns for UV to millimeters for IR, it follows that it is the skin and eyes of the human body that are most at risk of exposure. The biological response to exposure results from a variety of energy-transformation processes, broadly categorized as either photochemical or thermal interactions. While photochemical interactions dominate in the short-wavelength range, where photon energies are greatest, thermal effects tend to dominate at the long-wavelength end of the spectrum.
In a photochemical interaction, light of a specific wavelength (and therefore energy) excites electrons in cellular molecules, leading to the breaking or reorganization of chemical bonds therein. This may have direct consequences to DNA, whereby base pairs are bound together, creating a disruption in the DNA strand. Indirectly, an excess of highly-reactive free radicals may be produced. These can interact with DNA to cause structural reorganization, and with other cells such as retinal photoreceptors to cause deterioration of cellular function and cell death. Importantly, damage to DNA, if not repaired, has the potential to give rise to cancer.
The mechanisms underpinning thermal interactions are related to the absorption of light giving rise to an increase in temperature at the exposure site, leading to protein denaturation and thermally-induced cellular damage.
While thermal interactions pose the same hazard over all wavelengths, the strong wavelength dependence of photochemical interactions is characterized by hazard-weighting functions (Fig. 2). Such functions are the reciprocal of dose (or energy) required at each wavelength to elicit a given level of response and normalized to unity: a low response requires a high dose, and vice versa.
Furthermore, while the effects of low-level thermal exposure may be mitigated by thermal conduction from the exposure site, photochemical interactions generally follow the Bunson-Roscoe law of reciprocity. This states that photochemical processes are dose dependant, meaning that low-level, long-term exposure gives rise to the same damage as high-level, short-term exposure.
Photobiological hazards posed to skin and eye
In consideration of the hazards posed to skin and eye, three exposure scenarios should be taken into account: exposure of the skin, of the front surface of the eye (cornea, conjunctiva and lens), and of the retina.
On exposure of skin, a proportion of incident light is reflected, the remainder being transmitted through the epidermis and dermis. The principle concern for the skin resides in UV exposure, which presents a photochemical hazard due to direct damage of DNA, giving rise to the familiar inflammatory response producing erythema (sunburn). Another hazard is the production of reactive free-radicals which may attack DNA and other skin cells, such as collagen. This structural protein gives skin its elasticity, and collagen damage gives rise to elastosis, resulting in wrinkles and aged skin. The risk of thermal burn is also present, yet is of less concern since exposure is generally limited due to the associated feeling of pain. Skin may develop a protection mechanism upon repeated exposure to UV: this results in the thickening of the upper skin layers to reduce UV transmission and the production of UV-absorbing melanin, the pigmentation of tanned skin.
Exposure of the superficial structures of the eye demonstrates a response analogous to that of skin. The dominant concern is in the UV region, where photokeratitis (arc eye/snow blindness) may result: this is an inflammatory photochemical response, akin to sunburn, that occurs in the cornea and conjunctiva. Another possible result is a UV cataract (clouding) of the lens. In the IR, a thermal response to chronic high-level exposure may cause an infrared cataract.
Due to the transmission characteristics of the lens, exposure of the retina needs only to be considered over the wavelength range 300-1400 nm. The exception is in the specific case of the aphakic eye, in which the lens has either not yet developed or is removed during surgery. The dominant damage mechanism for exposure times greater than 10s is a photochemical blue-light hazard (photoretinitis), resulting in the production of free radicals which damage both photoreceptors and the retinal pigmented epithelium (RPE - a layer of cells on the outer surface of the retina, which supports the photoreceptors’ function). For shorter times, a thermal hazard dominates which causes the denaturation of proteins and key biological components of the retina.
The eye is afforded a number of protection mechanisms in response to visual stimuli (380-780 nm) only. These include an aversion response (blinking, head movement and constriction of the pupil to limit the amount of light reaching the retina) and continuous eye movement (saccades), ensuring that the same area of the retina is not continuously exposed.
Table 1 summarizes the six photobiological hazards to the skin and eye.
Evolution of safety standards for LEDs
In consideration of these photobiological concerns, the International Commission on Non-Ionising Radiation Protection (ICNIRP) publishes exposure-limit (EL) values for each hazard considered. These values are based on thresholds for damage obtained through reported effects of optical radiation and experiments on animal tissue. Whilst a safety factor is provided, account is not taken of abnormal photosensitivity or the presence of photosensitisers in the body or on the skin (including certain pharmaceutical compounds, cosmetics and plants).
In 1993, the year in which Nichia introduced commercially-viable blue GaN LEDs, the photobiological safety of LEDs was for the first time considered, as the International Electrotechnical Commission (IEC) took the decision to include LEDs within the scope of the existing laser standard, IEC60825. The rationale behind this decision was twofold; firstly that LEDs may be considered as a technology intermediate between lasers and conventional lamps, due their narrow spectral bandwidth, small source size and the potentially strongly-directional spatial distribution of the emitted light. The second reason was due to the use of IR-LEDs in optical-fiber communication systems for which laser diodes were also employed.
In 1996 and 2001, attempts were made to better accommodate LEDs within the laser standard, mainly through a revised safety philosophy, which had consequences for all lasers. However, difficulties were still encountered in that the hazards tended to be over-estimated, largely due to not taking into account the divergent nature of LED emission.
In parallel to the development of IEC60825, in 1996 the Illuminating Engineering Society of North America (IESNA) published ANSI/IESNA RP27.1, “Recommended Practice for Photobiological Safety for Lamps and Lamps Systems: General Requirements.” This heralded a series of standards concerned with non-laser sources. In 2002, the International Commission on Illumination (CIE) adopted the main body of ANSI/IESNA RP27.1 to publish the CIE Standard S009/E-2002, “Photobiological Safety of Lamps and Lamp Systems,” thereby disseminating this standard to the world.
Given that the application of laser limits to LEDs was considered by experts as being overly conservative, and given advances in LED performance and the attendant increase in application areas, the IEC took the decision to remove LEDs from consideration by the laser standard, updating IEC 60825 in 2007. The exception was for fiber-coupled and free-space-communications applications. This change required the provision of an alternative context in which to consider LEDs.
The introduction of IEC62471-2006
In 2006, the IEC adopted the existing CIE S009/E-2002 guidelines, to publish IEC62471:2006 “Photobiological Safety of Lamps and Lamp Systems” as a dual-logo standard with the CIE. The scope of this standard is to provide guidance for the evaluation of the photobiological safety of lamps and lamp systems, excluding lasers, emitting light in the spectral region 200-3000 nm.
A measurement methodology and exposure limit values (based on ICNIRP data) are given in the consideration of the six hazards (Table 1) to the skin and eye for an exposure duration of up to eight hours, taken as a working day. No consideration is taken of the potential effects of long-term exposure.
A four-tier classification structure, based on permissible exposure time before exceeding the EL of each hazard, is defined, ranging from “Exempt” to “Risk Group (RG) 3” (Table 2). In the case of retinal hazards, the aversion-response time of the eye is taken into account. It should be noted that this classification system is different from the class system used for lasers.
The evaluation consists of a complex series of measurements of spectral irradiance (200-3000 nm) in consideration of hazards to the skin and front surfaces of the eye, and spectral radiance (300-1400 nm) in consideration of hazards to the retina. Measurements are performed in specific geometrical conditions which replicate biophysical phenomenon, such as the effect of eye movements on retinal irradiation, and at a measurement distance dependant on the application of the source in consideration i.e. general lighting service (GLS) or non-GLS.
In the next part of this article, we shall adopt a more practical approach, considering the finer details of source measurement and the implementation of the standard in Europe and the rest of the world.