In the last decade, since the publication of EN 62471:2008 “Photobiological Safety of Lamps and Lamp Systems” in tandem with the low voltage directive 2014/35/EU, the debate surrounding optical radiation safety has shifted from being focused on UV or laser lamp manufacturers. With the aim of overcoming the challenges faced in applying EN 62471, as well as reducing the measurement trouble of luminaire manufacturers, an innovative strategy has been devised for assessing the photobiological safety of luminaires.
The latest edition of the luminaire standard EN 60598-1: 2015 reports that in the evaluation of the photobiological safety of luminaires, two hazards are considered based on the type of source: the retinal blue light hazard and the actinic UV hazard to the eye and skin. The latter has been part of the luminaire standard since 2002, assuring appropriate shielding of discharge sources to avoid UV leakage. The new factor is the consideration of retinal blue light hazard, evaluated in the application of IEC TR62778, “Application of IEC 62471 for the assessment of blue light hazard to light sources and luminaires.”
An Overview of Blue Light Hazard
IEC TR 62778 offers directions for evaluating the retinal blue light hazard of light sources intended for lighting applications and principally emitting in the visible region of 380–780 nm. This evaluation is based on establishing — by measuring spatially averaged spectral radiance—whether or not a source leads to a retinal blue light hazard in excess of risk group one (RG1) at a distance of 200 mm.
A source categorized as RG1 does not present a hazard as a result of normal behavioral restrictions on exposure, such as not actively staring at the source and aversion response. If a luminaire exceeds the RG1 limit, dthr, which is the distance at which RG1 is presumed to be found, it should be ascertained and reported on a warning label. This strategy, which involves using the robust concept of hazard distance, offers a complete analysis. Although the likelihood of ocular exposure at very close quarters, such as 200 mm, to a luminaire might be low, it depends more on the location and type of luminaire, and the viewer considered, consumer, service engineer, child, and so on.
It should be noted that RG1 is not the lowest risk group of EN 62471. RG0, which is usually termed “exempt” to prevent mention of the word “risk,” is presently specified as a low-risk group in EN 62471. However, it will be re-specified as an extremely low-risk group in a forthcoming edition of the standard. The application of RG0 is pointlessly prohibitive; however, from the time EN 62471 was published, it is the one that is often found in commercial specifications, government regulations, and product standards. It would be challenging to unravel the misconception of the importance of the risk groups.
One Technical Report, Two Points of View
A significant encouragement in composing IEC TR 62778 was to reduce the measurement trouble for luminaire manufacturers. This is accomplished by two ways: first, by furnishing conditions under which the risk group categorization of a primary light source (lamp, LED chip, or module) might be transferred to a luminaire; and second, by proposing a choice of evaluation methodologies, two of which are dependent on commonly available data. Hence, this article should be viewed from two different points of view: that of the luminaire and that of the primary light source.
Table 1. Possible IEC TR 62778 assessment results of primary light sources and luminaires.
|Primary Light Source
||Does not exceed limit of the blue light hazard RG1 in any case†
||Does not exceed limit of the blue light hazard RG1
||Illuminance at which upper limit of RG1 found
||Distance from luminaire at which Ethr found
† Provided operating current in the luminaire is not higher than that at which assessment is performed.
One Technical Report, Three Assessment Methods
Three techniques have been suggested for evaluating the blue light hazard, a summary of which is detailed below, in order of needed inputs.
Table 2. Overview of IEC TR 62778 assessment techniques
||Spectral radiance/ irradiance
(300- 780 nm)
A table of illuminance values, as a function of CCT (≤ 8000 K), is given, below which RG1 results. By analyzing the table for a source of known CCT, the reported illuminance value can be adopted as Ethr. This value might be given in the datasheet of a primary light source and transformed to dthr for a luminaire. If the latter process results in dthr ≤ 200 mm, less than the evaluation distance, then, it is mandatory to report RG1. The safety factor of this process is two. Hence, it cannot yield a transferable risk group classification.
A table of illuminance values, as a function of CCT (≤ 8000 K), is given, below which RG1 results. Apart from knowing the CCT of the source, a measurement of luminance (cd⋅m−2) is needed. The field of view (FOV) of measurement used for determining the luminance should not be more than the luminous area of the source. If the measured luminance of a primary light source is less than the value reported in the table, “RG1 unlimited” is applicable; in contrast, for luminaires, “RG1” is applicable. The safety factor of this process is two. If the measured luminance is more than the tabulated values, methods A or C should be considered.
Direct spectroradiometric measurement is necessitated here, which produces the most accurate evaluation result. On the primary light source level, in which the 2.2-mm diameter circle that defines the 11 mrad FOV at 200 mm completely overfills the luminous area of the source and the measured radiance is lesser than the limit of RG1, the product can be evaluated to be RG1 unlimited; else, Ethr should be reported. If this measurement FOV is under-filled by a primary light source, a spectral irradiance measurement is necessitated to report Ethr. There is no need for the distinctive measurement type to establish Ethr and it can be removed in future. Considering luminaires, an evaluation is directly carried out in the 11 mrad FOV at 200 mm. In case the measured radiance is lesser than the RG1 limit, the product can be evaluated as RG1; else, dthr should be reported.
What is Spatially Averaged Radiance?
Computation of Ethr
Ethr, which is the threshold illuminance at which RG1 is found, can be calculated by taking into account the irradiance-based emission limit for the RG1 measurement in an 11 mrad FOV. The blue light irradiance emission limit of 1 W⋅m−2 is produced as a product of the solid angle corresponding to an 11 mrad FOV (9.50332⋅10−5 sr) and the blue light hazard RG1 emission limit radiance (10,000 W⋅m−2⋅sr−1). A ratio of luminance (cd⋅m−2) to blue light radiance (W⋅m−2⋅sr−1) that is equal to the ratio of blue light irradiance emission limit (1 W⋅m−2), Ethr (lux), to the threshold illuminance (Ethr) can be easily obtained with the help of the spectral radiance measurement, 300–780 nm.
Determination of dthr
With the given threshold illuminance, IEC TR 62778 recommends the use of prevalent goniophotometric data on the luminaire as the low measurement burden path to determine the peak luminous intensity and the use of the inverse square law to calculate dthr. When there is a lack of goniophotometric data, an illuminance meter can be employed for directly determining the position at which Ethr is found.
Although this method is simple, it does not consider the fact that the measurement should be assessed in an 11 mrad FOV. In case the luminaire being tested subtends more than 11 mrad at dthr, it can be observed that emission from the source outward of the 11 mrad FOV contributed toward the measurement of illuminance used to find the location of Ethr, resulting in overestimation of dthr. Although this may not lead to application-related problems in particular instances, it could affect the marketing of the product since it is natural for a user to choose those sources that have a shorter dthr, and regarded as “safer.”
Figure 1. From a measurement at 200 mm, Ethr was determined from the ratio of luminance to blue light radiance and dthr determined using a lux meter. Evaluation of the 11 mrad FOV at dthr (depicted in yellow) shows that the source extends beyond the FOV. The assessment is overly conservative.
Refined Determination of dthr
IEC TR 62778 offers guidance for addressing the circumstances in which a source subtends more than 11 mrad at the initial estimate of dthr. This binary method takes into account the threshold distance of a single emitter and ascertains whether or not other emitters come into the 11 mrad FOV at that distance. In practice, this method mandates all the other emitters in the luminaire to be covered or extinguished, which cannot be achieved easily or impractical in majority of the cases.
Figure 2. (Upper) Source extending beyond 11 mrad at an initial estimate of dthr. At dthr of single emitter (lower), only one emitter falls in 11 mrad FOV. The latter distance is reported as dthr. To perform this analysis is often not easy, the lower image being obtained thanks to Photoshop.
Since the existing LED technology produces a maximum blue light radiance that is four to eight times the RG1 limit, the required increase in the area over the 11 mrad FOV to produce a spatially averaged blue light radiance equal to the RG1 limit can be estimated. Considering omnidirectional sources, it is proposed that the actual dthr is not more than 600 mm; in contrast, for directional sources, dthr might be more. However, if dthr is greater, chances are high that the source may completely fall within the 11 mrad FOV. A measurement-based method, which starts by computing dthr based on the technical report, and assesses whether or not the source exceeds 11 mrad, is recommended. If the source exceeds 11 mrad, it is necessary to repeat the measurement at greater distances, in steps of 200 mm, until a point at which the measured blue light radiance is lower than the RG1 limit.
Reporting Assessment Results
In the datasheet of the primary light source, the evaluation result for the primary light source should be reported beside the current at which the evaluation was carried out. In contrast, evaluation result of the luminaire should be reported in the product literature of the luminaire. In case the result is dthr, a label that warns the users not to stare at the source within the dthr should be put on the luminaire as well as in the installation instructions.
The execution of IEC TR 62778 and the new method for assessing the photobiological safety of sources intended for lighting applications will, under various circumstances, result in a simpler evaluation. In other cases where refining of dthr is desired, further interpretation will be needed; yet interpretation in standardization could be challenging. This emphasizes the need for a more robust metrological strategy for determining this parameter. Amendment of IEC TR 62778 is ongoing as a new international standard, IEC 63109. In TC 34, researchers are striving hard to make sure that a robust and easy method for photobiological safety testing is offered for the lighting industry.
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.