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Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | SubscribeqMqM | Next PageqqM qMMQmagsTHE WORLD’S NEWSSTAND ®standards | OPTICAL SAFETYIn the context of photobiological safety,the measurement of radiance is performedin a manner that reflects this phenomenoni.e. the FOV of measurement is chosen toaccount for the light falling within a givenarea of the retina. The measurement FOV followstherefore the same time dependence,from 1.7 to 100 mrad, regardless of the sizeof the source measured.The measured quantity is more accuratelytermed physiological radiance as opposed totrue radiance, which by definition samplesonly the emitting area of the source (Fig. 5).Where the physiological radiance is measuredin a FOV greater than the angle subtendedby the source, the resultant radianceis an average of the true source radianceand the dark background. Furthermore,since the angular subtense of a source varieswith distance, physiological radiance,unlike true radiance, is a function of measurementdistance.for household luminaires, but many metersfor street lighting, for example.Irradiance measurements may be performedat a convenient distance and scaledto 500 lux. However, physiological radiance,which depends on the source subtense withSpectral influenceIn the above, reference was made to irradianceand physiological radiance withno consideration for the spectrum of thesource, which is clearly very importantwithin the context of this standard. Thesequantities should, in practice, be evaluatedat each wavelength with a monochromator.This yields spectral irradiance and spectralphysiological radiance. The resultant spectrashould be weighted, where required,against hazard weighting functions to takeaccount of the strong wavelength dependenceof three of the hazards considered(Fig. 6). The result should be integrated overthe required wavelength range prior to comparisonwith ELs.Measurement distanceThe distance at which a source should beevaluated depends upon its intended application.Two exposure scenarios are considered;general lighting service (GLS) and allother applications (non-GLS).The present definition of GLS is ambiguous,but relates to finished products that emitwhite light and are intended for illuminatingspaces. Evaluation should be reported, notnecessarily measured, at a distance at whichthe source produces an illuminance of 500lux. This distance may be less than one meterFIG. 5. In each pair of images, the red circles show the measurement fi elds of view fortrue (left) and physiological (right) radiance. For true radiance measurements, thecircle encompasses only the light-emitting area, while the physiological radiance isan average of the true source radiance and the dark background.respect to the applicable FOV, should be performedat the correct distance.The rationale for the 500-lux condition isarbitrary and a bone of contention withinthe lighting industry since this in manycases does not represent a realistic exposurescenario. In the next part of this article,we shall consider how this issue is currentlybeing addressed.Non-GLS sources should be measuredat a distance of 200 mm from the (apparent)source. This distance represents thenear point of the human eye. At shorterdistances than 200 mm, the retinal imageis out of focus, resulting in lower retinalirradiance.Here, the concept of apparent source isimportant. Where a lens is used to collimatethe output of an LED, a magnified virtualimage is produced behind the chip. The200-mm measurement distance should betaken with respect to this apparent source,since it is this which the eye images.The measurement at 200 mm may representa worst-case exposure condition for theretina. However, this is not the case for theskin and front surfaces of the eye where theexposure distance may be closer. This lattereventuality has not yet been taken intoaccount in this standard, for which the primaryconcern is acute retinal damage.Comparison with ELsELs are provided in terms of radiant flux forthermal hazards or energy (radiant flux multipliedby time) for photochemical hazards:a measured irradiance result can be directlycompared with the former, and an exposuretime obtained for the latter. This proceduredoes not apply to the measurement of radiance,for which the FOV of measurement istime dependent.A pass/fail test is therefore applied to theretinal hazards based on measurements atFOVs corresponding to the minimum exposuretimes of the classification system inturn, starting from the exempt risk group.Where the resultant radiance exceeds themaximum-permissible radiance for a givenrisk group, the next risk group is tested.68 NOVEMBER/DECEMBER 2011 LEDsmagazine.comPrevious Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | SubscribeqMqM qMM MQmags| Next Page q qTHE WORLD’S NEWSSTAND ®
Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | SubscribeqMqM | Next PageqqM qMMQmagsTHE WORLD’S NEWSSTAND ®standards | OPTICAL SAFETYThe detailed evaluation of retinal hazards israther more convoluted since source size andlevel of visual stimulus should be taken intoaccount in determining which ELs to apply.ClassificationAs outlined in part 1 of this article series,a classification system, based on the minimumexposure time before the EL isexceeded, is defined ranging from exempt(no risk) to risk group 3 (RG3; high risk).The limit irradiance (radiance) of each riskgroup can then be determined, and the measuredirradiance (radiance) may be comparedagainst these limits.LabelingIEC62471 is intended as a horizontal standard,and as such does not include manufacturingor user-safety requirements thatmay be required as a result of a productbeing assigned to a particular risk group.Relative efficacy (a.u.)1010.10.010.0010.0001200Actinic UV400Blue light600Such safety requirements vary accordingto application, and should be dealt with invertical, product-based standards. However,IEC TR 62471-2 does provide some furtherguidance on the measurement and providesa recommendation of labeling for each hazardand risk group (Fig. 7).Implementation of IEC62471 in EuropeIn the European Union, CE marking demonstratesproduct safety by compliance withthe relevant applicable EU directive, suchas the low-voltage directive (LVD), throughapplication of European Norme (EN) standardsharmonized to the directive underconsideration. While compliance with theseEN standards is not mandatory, it does providepresumption of compliance with theessential health and safety requirements ofthe directive considered.Optical radiation is specifically consideredunder the terms of the LVD. This isapplicable to electrical products operatingat voltages of 50-1000V AC. The Europeanadoption of IEC62471, namely EN62471:2008,is harmonized to the LVD.From September 1, 2011, evaluation ofLEDs against the laser standard (IEC60825)no longer allows presumption of conformitywith the essential health and safety requirementsof the LVD.From April 2010, the EU artificial opticalradiation directive (AORD), 2006/25/EC,came into force. This adopts exposure limitsslightly different to those of IEC62471.For consistency, EN62471 adopts the exposurelimits of the AORD and is the standardto be used to evaluate worker exposure toRetinal thermal800Wavelength (nm)FIG. 6. Hazard weighting functions used by IEC62471.100012001400non-laser sources of optical radiation.Also relevant to LEDs is the EU Toy Safetydirective, to which is harmonized EN62115“Safety of electric toys.” This standard hasin the past referenced the laser standard(EN60825) for the classification of LEDs. Itis currently under review, but it is expectedthat reference will be made to EN 62471where measurements are required.Finally, where products are not coveredby the LVD or toy directives, considerationshould also be made of the general productsafetydirective to which few standards arespecifically harmonized, yet for the evaluationof non-laser sources of light, EN62471 isthe relevant EN standard.Implementation of IEC62471 in ROWWhile many standardization bodies aroundthe world are considering the adoption ofIEC62471, few have yet issued nationalstandards let alone a legal framework torender testing mandatory. Of the activityRisk Group 2CAUTION. Possibly hazardousoptical radiation emitted fromthis product. Do not stare atoperating lamp. May beharmful to the eye.FIG. 7. Example label according to IEC TR62471-2.seen, much is related to the lighting industry,for which a well-defined standardizationframework is in place and under activedevelopment to accommodate solid-statelighting.To the knowledge of the author, China ispresently alone in having formally implementeda voluntary standard – GB/T 20145-2006 – with Japan expected to publish JIS C7550 in November 2011.Some countries, such as Australia andNew Zealand, are currently working onthe adoption of IEC62471 as a voluntarystandard. Another group (e.g. Hong Kong,Republic of Korea) are presently content toreference IEC62471 on a voluntary basis,while others (e.g. Canada) are at the stageof considering implementation and potentialregulations.Finally, in the US, where ANSI RP27.1exists as a voluntary standard, there is currentlyno mandatory requirement for theevaluation of non-laser sources. However,following a meeting in August 2011 of thestandards technical panel of UL/ANSI 8750“Light Emitting Diode (LED) Equipment forUse in Lighting Products,” a task group hasbeen formed to consider the implementationof photobiological safety standardsfor those lighting products covered by thisUL standard.Part 3 of this series of articles will discussthe implementation of IEC62471 to the LEDdevices of today, and its potential futuredevelopment. Also, it will be shown thatIEC62471 does not remain unknown to theworld, principally through the implementationof international IECEE CB and numerousother certification schemes.LEDsmagazine.com NOVEMBER/DECEMBER 2011 69Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | SubscribeqMqM qMM MQmags| Next Page q qTHE WORLD’S NEWSSTAND ®
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