Lighting and its Photobiological Safety Applications

In the last decade, since the publication of EN 62471:2008 “Photobiological Safety of Lamps and Lamp Systems” harmonized to the low voltage directive 2014/35/EU, the talk regarding optical radiation safety has shifted from being the preserve of laser or UV lamp manufacturers to be a general discussion point in the lighting sector, mainly with regards to retinal blue light hazard. Motivated by the wish to circumvent issues faced in applying EN 62471, and to decrease the measurement load of luminaire manufacturers, a new method for evaluating the photobiological safety of luminaires is currently in place.

According to the most recent edition of the luminaire standard EN 60598-1: 2015, the photobiological safety assessment of luminaires takes into consideration two hazards, based on source type: the retinal blue light hazard and the actinic UV hazard to the eye and skin. The latter has been included in the luminaire standard since 2002, guaranteeing ideal shielding of discharge sources to stop UV leakage. What is new is the consideration of retinal blue light hazard, evaluated in application of IEC TR 62778, “Application of IEC 62471 for the assessment of blue light hazard to light sources and luminaires”.

An Overview of Blue Light Hazard

The IEC TR 62778 gives guidance on the assessment of the retinal blue light hazard of sources of light projected for lighting applications and emitting mostly in the visible region, 380-780 nm. This evaluation is based on determining, through a measurement of spatially averaged spectral radiance, whether or not a source signifies a retinal blue light hazard in excess of risk group one (RG1) at a distance of 200 mm.

A source classified as RG1 is one which does not cause a risk because of normal behavioral restrictions on exposure (including aversion response and not vigorously staring at the source). Where a luminaire does surpass the RG1 limit, the distance at which RG1 is estimated to be found, dthr, should be defined and printed on a warning label. This method, using the robust concept of hazard distance, provides a complete analysis. Although the likelihood of ocular exposure at such close quarters as 200 mm to a luminaire may be low, it does rely a lot on the type and location of luminaire, and the viewer considered, consumer, service engineer, child etc.

It is important to note that RG1 is not the lowest risk group of EN 62471. RG0, usually called “exempt” to avoid mention of the word “risk” is presently referred to in EN 62471 as a low risk group, but will be restated as very low risk group in an upcoming edition of the standard. The application of RG0 is needlessly restrictive, but since the publication of EN 62471, is one that can repeatedly be found in government regulations, product standards, and commercial specifications. It can be hard to unravel the misinterpretation of the implication of the risk groups.

One Technical Report, Two Points of View

A key motivation in the writing of IEC TR 62778 was the reduction of the measurement load for luminaire manufacturers. This is accomplished in two manners, primarily by providing conditions under which the risk group classification of a chief light source (LED chip, lamp, or module) may be shifted to a luminaire and secondly in providing a choice of assessment methodologies, two of which are based on universally available data. This article should thus be considered from two different points of view: that of the main light source and that of the luminaire.

Table 1. Possible IEC TR 62778 assessment results of primary light sources and luminaires

Primary Light Source Luminaire
Assessment Result Definition Assessment Result Definition
RG1 unlimited Does not exceed limit of the blue light hazard RG1 in any case† RG1 Does not exceed limit of the blue light hazard RG1
Ethr Illuminance at which upper limit of RG1 found dthr Distance from luminaire at which Ethr found

† provided operating current in the luminaire is not higher than that at which assessment performed

One Technical Report, Three Assessment Methods

Three approaches are proposed for the evaluation of blue light hazard, an overview of which is available in the following paragraphs, in order of required inputs, below.

Table 2. Overview of IEC TR 62778 Assessment Techniques

Method A Method A Method B
Input(s) CCT CCT Spectral radiance/ irradiance
(300- 780 nm)
Result(s) Ethr RG1 (unlimited)
Ethr
RG1 (unlimited)
Ethr

Method A

A table provides illuminance values, as a function of CCT (≤ 8000 K), below which RG1 will take place. Referring the table for a source of identified CCT, one can adopt the reported illuminance value as Ethr. This value may be provided in the data sheet of the main light source and converted to dthr for a luminaire. Where the latter process produces dthr ≤200 mm, below the assessment distance, then RG1 should be stated. This technique contains a safety factor of two and cannot create a transferable risk group classification.

Method B

A table provides luminance values, as a function of CCT (≤ 8000K), below which RG1 will occur. Besides knowledge of the CCT of the source, a measurement of luminance (cd.m-2) is needed. The field of view (FOV) of measurement used in defining luminance should not go beyond the luminous area of the source. Where the measured luminance of the main light source is below than stated in the table, “RG1 unlimited” applies for luminaires, “RG1”. This technique comprises of a safety factor of two. Where the measured luminance surpasses the tabulated values, one should take into account methods A or C.

Method C

The direct spectroradiometric measurement is called for here, producing the most accurate assessment result. On the level of the main light source, where the luminous area of the source completely overfills the 2.2 mm diameter circle establishing the 11 mrad FOV at 200 mm and the measured radiance is below the limit of RG1, the product can be evaluated as RG1 unlimited, otherwise Ethr should be mentioned. Where the main light source under-fills this measurement FOV, a measurement of spectral irradiance is necessary to report Ethr. The requirement for the diverse measurement type to establish Ethr is not essential and shall be taken off in the future. In the case of luminaires, an assessment is directly carried out in the 11 mrad FOV at 200 mm. If the measured radiance is below the limit of RG1, the product can be considered as RG1, if not dthr, should be stated.

What is Spatially-Averaged Radiance?

Computation of Ethr

The threshold illuminance, Ethr, at which RG1 is found, may be calculated by taking into account the irradiance-based emission limit for this RG1 measurement in an 11 mrad FOV. The product of the blue light hazard RG1 emission limit radiance (10000 W.m-2.sr-1) and the solid angle corresponding to an 11 mrad FOV (9.50332.10-5 sr) produces the blue light irradiance emission limit of 1 W.m-2. The ratio of luminance (cd.m-2) to blue light radiance (W.m-2.sr-1) being equivalent to that of threshold illuminance, Ethr, to blue light irradiance emission limit (1 W.m-2), Ethr (lux) can be easily achieved from the spectral radiance measurement, 300-780 nm.

Determination of dthr

With the threshold illuminance, the low measurement burden path suggested by IEC TR 62778 is to use current goniophotometric data on the luminaire to establish the peak luminous intensity and use the inverse square law to calculate dthr. Without the goniophotometric data, an illuminance meter can be used directly to establish the position at which Ethr is found.

This method although simple, does not take into consideration the fact that the measurement must be evaluated in an 11 mrad FOV. If the luminaire under test subtends >11 mrad at dthr it follows that emission from the source outside the 11 mrad FOV contributed to the measurement of illuminance used to discover the location of Ethr, resulting in an over-estimate of dthr. Although in some circumstances this may pose no application-related issues, it can have an impact in the promotion of the product since one will naturally be inclined to those sources having a shorter dthr, perceived as being “safer”.

From a measurement at 200 mm, Ethr was determined from the ratio of luminance to blue light radiance and dthr determined using a luxmeter. Evaluation of the 11 mrad FOV at dthr (depicted in yellow) shows that the source extends beyond the FOV. The assessment is overly conservative.

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 luxmeter. 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

In IEC TR 62778, guidance is offered to address the case where a source subtends >11 mrad at the primary estimate of dthr. This binary method considers the threshold distance of a single emitter and establishes whether or not other emitters come within the 11 mrad FOV at that distance. Essentially, this method requires that all other emitters in the luminaire be switched off or covered, which in a number of instances is neither easily achievable nor practical.

(Upper) Source extending beyond 11 mrad at 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.

Figure 2. (Upper) Source extending beyond 11 mrad at 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.

Given that the current LED technology produces a maximum blue light radiance between four and eight times the RG1 limit, one can estimate the amount of area over which the 11 mrad FOV should increase to produce a spatially averaged blue light radiance equal to the RG1 limit. In the case of omni-directional sources, it is assessed that the true dthr will not surpass 600 mm though for directional sources dthr may be great, but then the greater dthr, the more probable the source falls completely within the 11 mrad FOV. A measurement-based technique is suggested, starting with the calculation of dthr according to the technical report, and assessment of whether or not the source spreads beyond 11 mrad. If the source extends further than 11 mrad, the measurement should be done again at greater distances, in steps of 200 mm until such point that the measured blue light radiance is lower than the RG1 limit.

Reporting Assessment Results

The assessment result for the main light source should be reported in the datasheet of the main light source together with the current at which the calculation was performed while that of the luminaire should be stated in the product literature of the luminaire. Where dthr occurs, a label warning not to stare at the source within dthr should be posted on the luminaire and in the installation manual.

Conclusion

The execution of IEC TR 62778 and the new method to the assessment of the photobiological safety of sources proposed for lighting applications will in numerous instances result in a simpler assessment. In others, where a refinement of dthr is required, extra interpretation will be needed, and yet interpretation in standardization can be challenging. This emphasizes the need for a sounder metrological method to the determination of this parameter. Revision of IEC TR 62778 is happening as a new international standard, IEC 63109. In TC 34, work is underway to confirm that a simple and strong approach to photobiological safety testing is offered to the lighting sector.

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.

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