Driven by the desire to evade issues encountered in applying IEC/EN 62471, and to decrease the measurement burden of luminaire manufacturers, a new approach to the evaluation of the photobiological safety of luminaires is now in place.
According to IEC/EN 62471, the photobiological safety of luminaires and lamps intended for general lighting service (GLS) applications is assessed by implementing the general lighting service (GLS) classification criterion, namely by reporting at a distance at which the source generates an illuminance of 500 lux, not less than 200 mm. Over the years, a number of issues in implementing this approach were encountered including:-
- Differences in interpretation over which luminaires should be considered in the GLS category
- Questions on whether the GLS criterion represents a realistic exposure scenario
- Realization that for most of the GLS sources, the result is “Exempt”
This resulted in the formation of a photobiological safety panel (now WG6) within IEC TC 34 and the introduction of a new approach, the most recent publication of which is IEC/TR 62778 Edition 2: “Application of IEC 62471 for the valuation of blue light hazard to light sources and luminaires”, and the amendment of a range of luminaire and lamp standards, published and updated under the Low Voltage Directive.
Photobiological Safety Hazards
Whilst IEC /EN 62471 provides consideration to six hazards to the skin and eye over the range 200-3000 nm (table 1), the optical radiation emitted by GLS products may not cover the whole spectral range, nor be of a level to present cause for concern. A consideration of photobiological safety relies therefore on lamp type, and is treated by technology-specific standards.
||Example source types
||2 mW•klm-1 effective UV
||Discharge lamps (excluding fluorescent lamps)
||Application of IEC TR 62778
||LED and discharge lamps
||Tungsten halogen lamps (non-vehicle)
Actinic UV Hazard
The treatment of the actinic UV hazard essentially continues to be unchanged, having been implemented in the luminaire standard from 2002 with the introduction of CIE S009 upon which IEC 62471 is based. The actinic UV hazard considers UV related damage to the skin and anterior surfaces of the eye, the wavelength dependence of which is described by the actinic UV spectral weighting function.
Actinic UV Spectral weighting function
The limit is based on an actinic UV hazard exempt risk ground classification (emission limit value 1 mW•m-2) or a GLS source (evaluated at 500 lux). Diving 1 mW•m-2 by 500 lux expressed in klx, and noting that lx.m-2 is by definition lumens, yields the 2 mW•klm-1 limit. Whilst the overall luminous flux units might lead one to think that this measurement should be effected by an integrating sphere-based measurement, because of the optical performance of integrating spheres in the UV, it is essential to adopt an approach based on irradiance.
IEC TR 62778
IEC TR 62778 has been written to provide assistance in the assessment of the retinal blue light hazard of all lighting products, emitting principally in the visible region, 380–780 nm, and on the transfer of data from LED/lamp to finished products. Using IEC/EN 62471 as a basis, assessment carried out for determining whether or not the luminaire under test exceeds the limits of risk group 1 (RG1) at a distance of 200 mm. A source classified as RG1 is one which does not pose a hazard due to usual behavioral limitations on exposure, including not actively staring at the source and aversion response.
Retinal blue light hazard is the name given to acute (type II) photochemically induced retinal damage brought about by absorption of light by the retinal pigmented epithelium and the choroid, with peak sensitivity around 440 nm for the phakic eye, hence the “blue” appelation. The spectral dependence is accounted for by the blue light hazard weighting function.
Blue light hazard is evaluated through a spatially averaged spectral radiance over a spectral range 300-780 nm, encompassing the range of the blue light hazard (300-700 nm) and the CIE 1924 luminous efficacy function (380-780 nm). Where a source subtends an angle less than 11 mrad, according to IEC 62471, the blue light small source approach applies through a measurement of spectral irradiance.
|Component Lamps or LEDs
|RG0 unlimited (very rare)
||RG0 (very rare)
|Ethr, threshold illuminance (lx) at which RG1 found
||dthr, threshold distance (m) at which Ethr found
There are two assessment methodologies offered, one based on available data (including a safety factor of two) and a second on direct measurement. The price to pay for the simple approach can be extremely significant indeed.
Computation of Ethr
The threshold illuminance, Ethr, at which RG1 is found, may be computed by considering the irradiance-based emission limit for this RG1 measurement in an 11mrad FOV. From the blue light hazard RG1 emission limit radiance (10000 W•m-2•sr-1) and the solid angle corresponding to an 11mrad FOV (9.50332.10-5 sr), one obtains the blue light irradiance emission limit of 1W.m-2. Since the ratio of blue light radiance (W•m-2•sr-1) to luminance (cd•m-2) will be equal to the ratio of the blue light irradiance emission limit (1 W•m-2) to the illuminance at which this threshold is obtained, Ethr (lux) can be easily obtained. In an effort to reduce the measurement burden of luminaire manufacturers, guidance is provided on the transfer of data from primary light source to luminaire. Where the primary light source under test over-fills the 11mrad FOV at 200mm (or a 2.2mm diameter circular area), the measured result is radiance as opposed to a spatially averaged value. To the former can be applied the law of conservation of radiance: provided that the light source be operated at the same current, the classification can be transferred. In this case, a RG1 unlimited classification can be attributed. In those cases where the source under-fills the 2.2mm diameter circular area, IEC TR 62778 requires the measurement of spectral irradiance and the reporting of Ethr only. Since Ethr depends only on the spectral distribution of the source under test, there is no need to evaluate this parameter through a separate irradiance measurement.
Primary Light Sources
As part of an effort to reduce the measurement burden of luminaire manufacturers, guidance is offered on the transfer of data from primary light source to luminaire. Where the primary light source under test over-fills the 11 mrad FOV at 200 mm (or a 2.2 mm diameter circular area), the measured result is radiance as opposed to a spatially averaged value. The former can be applied to the law of conservation of radiance: provided that the light source be operated at the same current, the classification can be transmitted.
In this case, a RG1 unlimited classification can be attributed. In those cases where the source under-fills the 2.2 mm diameter circular area, IEC TR 62778 needs the measurement of spectral irradiance and the reporting of Ethr only. Since Ethr depends only on the spectral distribution of the source under test, there is no need to examine this parameter through a separate irradiance measurement.
|Blue Light Radiance in 11 mrad FOV (W•m-2•sr-1)
||RG0 unlimited (very rare)
||Ethr, threshold illuminance (lx) at which RG1 found
The blue light radiance of a luminaire is measured and then compared with the limits of RG1.
|Blue Light Radiance
|in 11 mrad FOV (W•m-2•sr-1)
In the case of luminaires in excess of the limit of RG1, the distance from the luminaire, dthr, at which the threshold illuminance, Ethr, is to be discovered, should be determined and then reported on marking on the luminaire.
The recommended method to determine dthr is to employ goniophotometric data of the product under test, where available. Knowledge of the maximum luminous intensity, Imax (cd), and the inverse square law permits calculation from dthr = √(Imax /Ethr). This procedure overlooks the fact that for a number of sources, the inverse square law will not be applicable, mainly for directional sources. In the absence of such data, the location of Ethr can be directly found using an illuminance meter.
In the determination of the threshold distance, the use of a direct illuminance meter measurement or goniophotometric data encompasses light from the entire luminaire. Where the source subtends an angle greater than 11 mrad at dthr, too large an area of the source was included in the determination and this value of dthr can be said to be excessively conservative. To what extent dthr is over-estimated relies on the directionality and the size of the luminaire under test, a factor of ten (and more!) has been seen. Over reporting is considered to be the correct way to err in a safety assessment, but the impact of an over-estimated dthr on the marketing of a product may prove problematic, and may have a negative impact on the perception of hazard associated with luminaires.
In the 2014 edition of IEC TR 62778, guidance is offered in annex D in order to address the case where a source subtends >11 mrad at the initial estimate of dthr, here labeled dN for clarity. Having determined dN, one should assess whether or not the luminous area of the source extends beyond the circular area described by an 11 mrad FOV at dN, of diameter 0.011.dN. Where the source falls wholly with this area, then dN = dthr, otherwise (as is the case in Figure 1, upper) dN is overly conservative and can be advanced using the following process.
Source extending beyond 11 mrad FOV at dN: over-estimation of threshold distance. A refined, but not practically realizable assessment shows that at d1 (lower), only one emitter falls in 11 mrad FOV. d1 = dthr.
The distance, d1, at which the Ethr of a single LED package takes place, should be determined and then the area described by an 11 mrad FOV at d1 of diameter 0.011.d1 should be considered. If only one emitter falls within this area as seen in the case in figure 4 (lower) then d1 = dthr. Where more than one emitter falls within this area, and where Ethr was taken from the LED emitter datasheet, as opposed to resulting from the measurement of the luminaire, it does not essentially follow that RG1 be exceeded at this distance. This case is recommended to perform a spectral radiance measurement at d1 in an 11 mrad FOV, if the result is less than the RG1 limit, d1 = dthr. In all other cases, the true value of dthr exists between these extremes, so the default position is to adopt the worst case, dN = dthr.
The modification of dthr in this manner is in the majority of cases not practically feasible (how does one block or turn off emission from all but one emitter in a luminaire?) and thus can be considered majorly redundant.
This information has been sourced, reviewed and adapted from materials provided by Bentham Instruments Limited.
For more information on this source, please visit Bentham Instruments Limited.