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 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.
Scope of IEC62471:2006
The IEC62471:2006 standard “Photobiological Safety of Lamps and Lamp Systems” provides guidance for the evaluation of the photobiological safety of all electrically-powered, non-laser sources of optical radiation emitting in the spectral range 200-3000 nm, whether or not the emission of light is the primary purpose of the product. The inclusion of LEDs in the scope of this standard is specifically mentioned to highlight the removal of LEDs from the scope of the laser standard, IEC60825.
The potential hazards of exposure to the skin, the front surfaces of the eye (cornea, conjunctiva and lens) and the retina are evaluated through consideration of six specific hazards with respect to exposure limits (ELs) provided for an exposure duration of eight hours, taken as a working day. The standard does not consider the potential effects of long-term exposure, nor of abnormal photosensitivity.
In the case of hazards to the skin and front surfaces of the eye, it is sufficient to take into account the amount of light incident on the surface in question. However, to consider hazards to the retina, one must take account of the imaging properties of the eye. It follows that two distinct measurements are required: irradiance and radiance.
The standard provides specific guidance on the geometrical conditions under which these measurements should be made to take into account biophysical phenomena, such as the effect of eye movements on retinal irradiation. The spectral range over which radiance should be considered is reduced to 300-1400 nm, since the retina is essentially protected outside this range due to the transmission characteristics of the lens. Table 1 indicates the required measurement (radiance or irradiance) for different hazards.
Measurement of irradiance
Irradiance permits the evaluation of hazards to the skin and front surfaces of the eye. Irradiance is defined as the ratio of radiant power (dF) incident on an element of a surface, to the area (dA) of that element (Fig. 1). Its symbol is E and its units are W/m2.
Irradiance accounts for light arriving at a surface from the entire hemisphere above. However, due to its position with respect to the bridge and nose, the eye is shielded from wide-angle radiation. Within the scope of this standard, the measurement of irradiance in all but the case of the thermal skin hazard is performed over a 1.4-radian acceptance angle. Light emitted from a source outside this acceptance angle need not be measured.
In measuring irradiance, the measurement optic, typically a diffuser or an integrating sphere, should have a cosine angular response to correctly account for off-axis contributions. At a given angle from the surface normal, the projected area on the surface is increased by the cosine of the said angle, resulting in reduced irradiance.
Knowledge of source irradiance does not however give any information about the quantity of light coupled by the eye and imaged on the retina, for which a measurement of radiance is required.
Measurement of radiance
Radiance permits the evaluation of hazards to the retina. Radiance is defined as the ratio of radiant power (dF) emitted by area dA into solid angle dΩ at angle q to the source normal, to the product of solid angle dΩ and the projected area dA∙cos q (Fig. 2). The symbol is L and the units are W/m2sr.
In viewing a source, the eye collects light within a given solid angle set by the diameter of the pupil, and projects an image of the source onto the retina. As the pupil dilates (or contracts) according to the level of visual stimulus, or luminance, of the source, the retinal irradiance of the image increases (or decreases).
The law of conservation of radiance states that radiance cannot be increased by passive optical systems such as the lens of the eye. The retinal irradiance is therefore determined from the source radiance and the solid angle subtended by the pupil (2-7-mm diameter) at the retina (17-mm distant) in the reverse of the determination of radiance from irradiance, given below.
Radiance may be measured by two manners, either using an imaging technique or indirectly through an irradiance measurement. In both cases, the measurement is performed in a specific field of view (FOV) or solid angle of acceptance (often described by a planar angle, q) that defines the area of the source measured.
The imaging technique (Fig. 3) replicates the imaging of the eye. A telescope images the source under test onto a plane at which may be placed apertures of varying diameter to select the required FOV of measurement.
Alternatively, a measurement of irradiance with a cosine-corrected input optic may be performed (Fig. 4). An aperture is placed directly at the source to define the measurement FOV. The radiance is computed from the ratio of irradiance to the solid angle of the FOV in steradians.
For momentary viewing, the retinal image of a source subtends the same angle as does the source. The smallest image formed on the retina, according to IEC62471, has an angular extent of 1.7 mrad, given the imperfect imaging performance of the eye.
With increasing exposure time, due to eye movement (saccades) and task-determined movement, the retinal image is smeared over a larger area of the retina, resulting in a corresponding reduction in retinal irradiance. A time-dependent function is defined to represent the spread of the retinal image in the range from 1.7 to 100 mrad. This covers the range from 0.25s (aversion response time) to 10,000s exposure.
In the context of photobiological safety, the measurement of radiance is performed in a manner that reflects this phenomenon i.e. the FOV of measurement is chosen to account for the light falling within a given area of the retina. The measurement FOV follows therefore the same time dependence, from 1.7 to 100 mrad, regardless of the size of the source measured.
The measured quantity is more accurately termed physiological radiance as opposed to true radiance, which by definition samples only the emitting area of the source (Fig. 5). Where the physiological radiance is measured in a FOV greater than the angle subtended by the source, the resultant radiance is an average of the true source radiance and the dark background. Furthermore, since the angular subtense of a source varies with distance, physiological radiance, unlike true radiance, is a function of measurement distance.
In the above, reference was made to irradiance and physiological radiance with no consideration for the spectrum of the source, which is clearly very important within the context of this standard. These quantities should, in practice, be evaluated at each wavelength with a monochromator. This yields spectral irradiance and spectral physiological radiance. The resultant spectra should be weighted, where required, against hazard weighting functions to take account of the strong wavelength dependence of three of the hazards considered (Fig. 6). The result should be integrated over the required wavelength range prior to comparison with ELs.
The distance at which a source should be evaluated depends upon its intended application. Two exposure scenarios are considered; general lighting service (GLS) and all other applications (non-GLS).
The present definition of GLS is ambiguous, but relates to finished products that emit white light and are intended for illuminating spaces. Evaluation should be reported, not necessarily measured, at a distance at which the source produces an illuminance of 500 lux. This distance may be less than one meter for household luminaires, but many meters for street lighting, for example.
Irradiance measurements may be performed at a convenient distance and scaled to 500 lux. However, physiological radiance, which depends on the source subtense with respect to the applicable FOV, should be performed at the correct distance.
The rationale for the 500-lux condition is arbitrary and a bone of contention within the lighting industry since this in many cases does not represent a realistic exposure scenario. In the next part of this article, we shall consider how this issue is currently being addressed.
Non-GLS sources should be measured at a distance of 200 mm from the (apparent) source. This distance represents the near point of the human eye. At shorter distances than 200 mm, the retinal image is out of focus, resulting in lower retinal irradiance.
Here, the concept of apparent source is important. Where a lens is used to collimate the output of an LED, a magnified virtual image is produced behind the chip. The 200-mm measurement distance should be taken with respect to this apparent source, since it is this which the eye images.
The measurement at 200 mm may represent a worst-case exposure condition for the retina. However, this is not the case for the skin and front surfaces of the eye where the exposure distance may be closer. This latter eventuality has not yet been taken into account in this standard, for which the primary concern is acute retinal damage.
Comparison with ELs
ELs are provided in terms of radiant flux for thermal hazards or energy (radiant flux multiplied by time) for photochemical hazards: a measured irradiance result can be directly compared with the former, and an exposure time obtained for the latter. This procedure does not apply to the measurement of radiance, for which the FOV of measurement is time dependent.
A pass/fail test is therefore applied to the retinal hazards based on measurements at FOVs corresponding to the minimum exposure times of the classification system in turn, starting from the exempt risk group. Where the resultant radiance exceeds the maximum-permissible radiance for a given risk group, the next risk group is tested. The detailed evaluation of retinal hazards is rather more convoluted since source size and level of visual stimulus should be taken into account in determining which ELs to apply.
As outlined in part 1 of this article series, a classification system, based on the minimum exposure time before the EL is exceeded, is defined ranging from exempt (no risk) to risk group 3 (RG3; high risk). The limit irradiance (radiance) of each risk group can then be determined, and the measured irradiance (radiance) may be compared against these limits.
IEC62471 is intended as a horizontal standard, and as such does not include manufacturing or user-safety requirements that may be required as a result of a product being assigned to a particular risk group. Such safety requirements vary according to application, and should be dealt with in vertical, product-based standards. However, IEC TR 62471-2 does provide some further guidance on the measurement and provides a recommendation of labeling for each hazard and risk group (Fig. 7).
Implementation of IEC62471 in Europe
In the European Union, CE marking demonstrates product safety by compliance with the relevant applicable EU directive, such as the low-voltage directive (LVD), through application of European Norme (EN) standards harmonized to the directive under consideration. While compliance with these EN standards is not mandatory, it does provide presumption of compliance with the essential health and safety requirements of the directive considered.
Optical radiation is specifically considered under the terms of the LVD. This is applicable to electrical products operating at voltages of 50-1000V AC. The European adoption of IEC62471, namely EN62471:2008, is harmonized to the LVD.
From September 1, 2011, evaluation of LEDs against the laser standard (IEC60825) no longer allows presumption of conformity with the essential health and safety requirements of the LVD.
From April 2010, the EU artificial optical radiation directive (AORD), 2006/25/EC, came into force. This adopts exposure limits slightly different to those of IEC62471. For consistency, EN62471 adopts the exposure limits of the AORD and is the standard to be used to evaluate worker exposure to non-laser sources of optical radiation.
Also relevant to LEDs is the EU Toy Safety directive, to which is harmonized EN62115 “Safety of electric toys.” This standard has in the past referenced the laser standard (EN60825) for the classification of LEDs. It is currently under review, but it is expected that reference will be made to EN 62471 where measurements are required.
Finally, where products are not covered by the LVD or toy directives, consideration should also be made of the general product-safety directive to which few standards are specifically harmonized, yet for the evaluation of non-laser sources of light, EN62471 is the relevant EN standard.
Implementation of IEC62471 in ROW
While many standardization bodies around the world are considering the adoption of IEC62471, few have yet issued national standards let alone a legal framework to render testing mandatory. Of the activity seen, much is related to the lighting industry, for which a well-defined standardization framework is in place and under active development to accommodate solid-state lighting.
To the knowledge of the author, China is presently alone in having formally implemented a voluntary standard – GB/T 20145-2006 – with Japan expected to publish JIS C 7550 in November 2011.
Some countries, such as Australia and New Zealand, are currently working on the adoption of IEC62471 as a voluntary standard. Another group (e.g. Hong Kong, Republic of Korea) are presently content to reference IEC62471 on a voluntary basis, while others (e.g. Canada) are at the stage of considering implementation and potential regulations.
Finally, in the US, where ANSI RP27.1 exists as a voluntary standard, there is currently no mandatory requirement for the evaluation of non-laser sources. However, following a meeting in August 2011 of the standards technical panel of UL/ANSI 8750 “Light Emitting Diode (LED) Equipment for Use in Lighting Products,” a task group has been formed to consider the implementation of photobiological safety standards for those lighting products covered by this UL standard.
It is clearly not possible to measure every LED in use, and indeed in many cases there is no need to do so. For example, the low visual response elicited from low-power white or colored LEDs leads one to reason that no photochemical safety concerns exist. However, as one considers LEDs of increasing optical power, the point at which one can no longer make such assumptions may not be obvious.
When are measurements required?
In the first instance, IEC62471 recommends that detailed measurements are not required for sources having a luminance less than 104cd/m2. This level is considered as one visually comfortable to view. The guidance is based on the expectation that, at this level, exposure limits will not be exceeded. However, it only applies to white or broadband sources emitting over the visible region.
Luminance does not fully take into account the emission of colored LEDs, nor does it take into account UV or IR emission. The luminance of a UV source may be below this level, yet one cannot use this information to base a conclusion on a potential UV hazard. In practice, this threshold luminance is particularly low, and is exceeded by many, even low-power, white LEDs.
Where the luminance of a white-light source exceeds this level, and for all other sources, one should proceed with the evaluation of photobiological safety, at the appropriate distance – 500 lx or 200 mm – depending on the intended application of the finished product.
General lighting service (GLS) sources are defined as white-light sources used to illuminate spaces. Within the context of LEDs, consideration is made of two technologies: phosphor-converted (PC) and color-mixed LEDs. Due to the narrow-band emission of LED chips, and the limited emission range of LED phosphors, one can restrict consideration to the visible region: no risks are posed in the UV or the IR.
Practically, the sole hazard in consideration is the blue-light retinal hazard, which dominates over the retinal thermal hazard for exposure times greater than ten seconds. It follows that it is the blue LED of both PC and color-mixed LEDs which gives the main cause for concern.
Consideration of the blue-light hazard of GLS sources is most conveniently demonstrated in evaluation of radiance through a measurement of irradiance, comparing the blue-light-weighted irradiance with the illuminance of the source (Fig. 1).
For a given illuminance, the higher the emission in the region of the blue-light hazard function, the greater the blue-light hazard posed. An increasingly prominent blue-emission peak lends a source a blue appearance, characterized by an increasing correlated color temperature (CCT). It can be demonstrated that at 500 lx, only LEDs having very high CCT (greater than approximately 10,000K) exceed the limits of the blue-light exempt risk group (RG), and that no sources will exceed blue-light RG1 (risk groups are discussed in Part 1). Since such high-color-temperature sources are seldom used in SSL applications, one can conclude that few GLS sources will pose any hazards at the 500-lx evaluation distance.
On the subject of GLS, two other points should be made. Firstly, with regard to certain sources, such as desk lamps and household spot lights (for which the determined 500 lx distance may be significantly greater than a likely exposure distance), the lack of clarity in the definition of GLS in IEC62471 has led to disagreement between laboratories of whether GLS or non-GLS measurement conditions should apply.
Secondly, and counter-intuitively, consider two GLS products, differing only in number of component LEDs or drive current. If the spectral output of both sources is the same, then the IEC62471 hazard evaluation is also the same, albeit performed at different 500-lx distances. Such a result may make sense where the two sources are used in distinct applications. However, in the not-uncommon case that they are marketed as alternatives for the same application, this further demonstrates that the evaluation at 500 lx is not a satisfactory point of reference.
The non-GLS category takes into account all types of LED, through the spectrum from the UV to the IR, including white LEDs used in non-GLS applications. Depending on the application, the optical output of such LEDs can vary significantly from very low-level indication to high-power LEDs used for example in industrial and signaling applications.
The analysis here is rather more detailed than is the case for GLS sources: at the close proximity of 200 mm, elevated risk-group classifications may indeed result, and there may be cause to consider multiple hazards for a single product. Table 1 provides an overview of the maximum reported RG of LED-based non-GLS products for each hazard considered by IEC62471. This excludes the thermal skin hazard, not part of the classification system.
In terms of the irradiance-based hazards, RG3 is certainly attainable, if not from a single LED, then by an array of LEDs. On the other hand, in the case of radiance-based hazards, since the measurement field-of-view (FOV) generally encompasses one, or a small number of, component LEDs, the maximum classification depends less on the collective effect of multiple LEDs in an array than on the output of individual LEDs.
While current blue-LED technology exceeds blue-light hazard RG1 by up to an order of magnitude, the RG2 limit is a further two orders of magnitude away. Furthermore, the often-cited fact that even the sun is an RG2 source would suggest that blue-light RG3 sources do not exist. Also, LED radiance is not sufficient to cause thermal damage to the retina; such damage can generally only be elicited by directly viewing certain lasers or arc lamps.
Analysis based on LED maker’s data
In order to avoid the cost and effort of evaluating the photobiological safety of finished products, pressure has in the past been brought to bear on LED manufacturers to provide photobiological safety information which may be transferred to the finished product. It is clear that an IEC62471 evaluation of a bare LED is not directly transferable to a finished product, which may include multiple emitters and beam-shaping optics, so another strategy should be employed.
The irradiance of the finished product cannot in any way be predicted. However, in the case of radiance-based hazards, a measurement of the true radiance, coupled with the law of conservation of radiance, may be used to determine the maximum possible radiance of any finished product using a given LED.
IEC TR 62471-2 introduces this principle for the evaluation of the blue-light hazard (the dominant concern for retinal injury) through a measurement of true radiance of the component LED at 200-mm distance and 1.7-mrad FOV. The resulting value is adopted as the blue-light radiance of the final product, to be compared with the exposure limit values of each risk group in turn. It is important to note that care should be taken to ensure that the data provided by the manufacturer provides a correct analysis for the operating conditions of the finished product.
This procedure leads in many cases to an over-estimation of the hazard, since account is not taken of physiological radiance. This is demonstrated in Table 2, where a comparison is made between an IEC62471 analysis and a worst-case analysis of a particular product. In the former case, each RG is considered in turn, with measurements being performed in the correct FOV and compared with the RG exposure limit (resulting in an RG1 classification). In the latter case the worst-case radiance is assumed and compared with the limits of each risk group in turn (resulting in an RG2 classification).
A similar result is obtained in many instances, especially when considering high-power LEDs used in SSL applications. According to IEC TR 62471-2, blue-light RG2 requires the use of a warning label. This means that the lighting industry has been faced with the decision of either determining how to implement the recommendation of labeling, or not accepting such worst-case analysis evaluation, which clearly has no bearing on the true hazard posed by the source in the intended application. This procedure has generally been discontinued while awaiting a more acceptable solution, as will be seen below.
Analysis based on LED data-sheet values
Where no photobiological safety-evaluation information is available from an LED manufacturer, some have sought to make estimations based on data-sheet values, which typically report beam-emission angle and either total flux or intensity in photometric (lumen, candela) or radiometric (W, W/sr) quantities, depending on whether the LED emission wavelength is within or without the visible region.
Given the emission angle and the evaluation distance, the area illuminated by the LED may be determined and either total flux or intensity used to make an estimate of irradiance. To estimate physiological radiance, it is required to know both the intensity and the FOV area corresponding to the RG considered. Where intensity is not directly reported in the datasheet, it may be calculated from the total flux and beam-emission angle. In the case of white or colored LEDs, where photometric data is often provided, a conversion factor (lm/W) must be determined to convert to radiometric units.
In the case of hazards requiring the application of a hazard-weighting function, estimation without taking such into account represents an over-estimation. This errs on the correct side of caution, as befitting such an analysis. Again, care should be taken to ensure that the data provided by the manufacturer provides a correct analysis for the operating conditions of the finished product. Should such a calculation indicate the existence of a classification that is higher than exempt, correct measurements are recommended. It need not be stated that the uncertainty associated with such estimations are necessarily high.
IEC TR 62471-2 also introduces the concept of mapping out the photobiological hazards associated with a source by determining hazard distance information to cover all potential applications. This procedure consists of the evaluation of a source at the minimum accessible distance, no less than 200 mm for the retinal hazards, and the determination (should any hazard be in excess of the exempt RG) of the distance from the source at which exposure is decreased to the required level for each remaining RG.
For irradiance-based hazards, this procedure is relatively straightforward, although one may be hampered by the requirement of measurement in a 1.4-radian FOV for all but the thermal skin hazard. The inverse-square irradiance law may be used with caution, but such calculations should not be necessary since irradiance can readily be measured at other distances by a number of techniques, such as the use of a luxmeter to seek an illuminance corresponding to the given level of irradiance sought.
Radiance-based hazards are more difficult to handle since measurements should be made in a specific FOV. Where the source subtends an angle less than the field of view, the hazard distance can be predicted since it will reduce with the square of the measurement distance, as the area of dark covered by the FOV increases.
Where a single emitter subtends an angle greater than the FOV, as a first approximation, physiological radiance will be constant until the distance at which the source subtends an angle less than the FOV. In the case of arrays, the physiological radiance may not decrease sufficiently before more LEDs fall into the FOV in which case, as a first approximation, physiological radiance will be constant until the distance at which the entire array subtends an angle less than the FOV (Fig. 2).
Hazard distance of LED luminaires
In awaiting an update of luminaire standards, evaluation of the photobiological safety of LED luminaires is currently performed through implementation of IEC62471. This situation has provided little satisfaction due to issues with the evaluation at 500-lx distance and the implementation of worst-case analysis to permit the transfer of LED manufacturer’s data. IEC committee SC34A is currently working on this issue, in considering the implementation of a restricted version of the hazard-distance analysis relative to the sole concern of white LEDs in GLS applications, namely the retinal blue-light hazard.
Based upon the assumption that light sources classified as exempt or RG1 for blue-light hazard are suitable for GLS applications, both component LEDs and finished products should, in the first instance, be evaluated at 200 mm in an 11-mrad FOV, with the spectral range extended to 300-780 nm to cover both blue light and photopic regions. This measurement serves as both an analysis of blue light RG1, and as a worst-case analysis, assuming that true radiance is measured.
Where the resulting blue-light radiance is below the RG1 exposure limit, the component LED or finished product may be considered exempt/RG1 in all conditions. Where the RG1 exposure limit is exceeded, the RG1 hazard distance should be determined through the evaluation of radiance as an irradiance measurement, with respect to a corresponding RG1 exposure limit expressed in blue-light-weighted irradiance. Given the ratio of luminance to blue-light radiance, the illuminance level at which the RG1 blue-light irradiance should be obtained is determined.
In the case of finished products, the distance at which this illuminance is obtained should be reported by using a luxmeter: at distances closer to the source than this distance, RG2 applies, elsewhere RG1/exempt applies. In the case of component LEDs, the illuminance value is simply reported in the data sheet such that the finished-product manufacturer can apply the aforementioned procedure to determine the RG1 distance for the particular product under consideration. This procedure is alas not quite as simple as it looks since the measurement should be performed in an 11-mrad FOV: not doing so will over-estimate the RG1 distance.
How luminaire standards will in future implement the RG1 hazard distance is still a work in progress, but it is clear that one can tolerate greater RG1 distances for ceiling-mounted applications compared for example to portable luminaires.
IECEE CB scheme & product marks
It is solely in Europe that IEC62471 has been implemented within a legal framework. However, IEC62471 has worldwide renown, through the implementation of the IECEE CB scheme and a wide range of product marks.
The IECEE CB scheme was set up to facilitate international trade in electrical equipment and is based on IEC product standards and a principle of mutual recognition of test results. Put simply, a manufacturer in country A, wishing to market his product in country B, need only have the product tested by a CB (certification body) testing laboratory in his home country. The CB test report will be accepted by the national CB (NCB) in country B and used to grant any required certification marks. Since 2009, testing to IEC62471 under the IECEE CB scheme LITE category (which requires testing to a number of other standards) is mandatory for LED-based GLS products.
In China, the mandatory CCC mark scheme requires testing of luminaires, for which IECEE CB test reports are currently accepted: through this certification route, testing to IEC62471 can be said to be mandatory for LED-based GLS products for sale in the Chinese market.
Furthermore, in many countries throughout the world, voluntary product-mark schemes are in existence. These are used to enhance the status of a product, by providing the consumer with increased confidence in its quality. Examples of such schemes include the UK BS kite, the German GS, the ENEC mark and the Korean KS mark, which are increasingly taking account of photobiological safety through application of IEC62471. Future prospects
The revision of IEC62471/ CIE S009 by IEC TC-76 and CIE D6 is underway, yet given the proposed adjustment of certain exposure limits by ICNIRP (and the correction of the current retinal thermal hazard weighting spectrum) a publication date for the update has not yet been given. Two additional parts to the IEC62471 series are being drafted by IEC TC-76, including part 4, related to guidance on measurement methods. The CIE D6 has also formed working groups with terms of reference to consider the implementation of IEC62471.
In addition to this, the European Commission has asked the Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR) to look into the health effects of artificial light: it will be some time before this study reports its findings.
LEDs may not represent as great a hazard as lasers, yet given their widespread use and ever-increasing optical performance, it is correct that account should be taken of the potential hazards associated with these sources. One should also be aware that while IEC62471 is based on normal behavior, and the innate aversion response, consideration should be made of those overcoming the aversion response, particularly in the case of children who have a natural curiosity and no appreciation for potential hazards.
Reports of the potential effects of chronic blue-light exposure, resulting in age-related macular degeneration and loss of vision in the central visual field, are fairly well supported (but in need of much further research). However, there have only been a few contentious reports relating to acute exposure to LEDs, including ostensible retinal damage due to exposure to a violet LED and the suggestion that LEDs may be particularly dangerous for children, whose undeveloped lenses do not offer the retina sufficient protection in the UV.
As a last note to finished-product manufacturers, while little can be done to circumvent irradiance-based hazards (other than limiting access to the source), physiological radiance can be modified by design, by minimizing the optical power in a given FOV through appropriate spacing of the LEDs and using more low-power LEDs to do the job of a single high-power chip.
Article source: ledsmagazine.com