Photobiological Safety In Lighting Applications

Driven by the desire to circumvent issues encountered in applying IEC & EN 62471, and to reduce 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 lamps and luminaires intended for general lighting service (GLS) applications is evaluated by implementing the general lighting service (GLS) classification criterion, namely by reporting at a distance at which the source produces an illuminance of 500 lux, not less than 200mm. 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,
• realisation that for the majority of GLS sources, the result is “Exempt”.

See our applications page on Photobiological Safety of Lamps and Lamps Systems

This led to 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 assessment of blue light hazard to light sources and luminaires”, and the amendment of a range of lamp and luminaire standards, published and updated under the Low Voltage Directive.

Photobiological Safety Hazards

Whilst IEC /EN 62471 gives consideration to six hazards to the skin and eye over the range 200-3000nm (table 1), the optical radiation emitted by GLS products may not cover the entire spectral range, nor be of a level to present cause for concern. A consideration of photobiological safety depends therefore on lamp type, and is treated by technology-specific standards

Actinic UV Hazard

The treatment of the actinic UV hazard essentially remains 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 takes account of 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.

The limit is based on an actinic UV hazard exempt risk ground classification (emission limit value 1mW.m^-2) or a GLS source (evaluated at 500 lux). Dividing 1 mW.m^-2 by 500 lux expressed in klx, and noting that lx.m^-2 is by definition lumens, yields the 2mW.klm^-1 limit. Whilst the total luminous flux units might lead one to think that this measurement should be effected by an integrating sphere-based measurement, due to the optical performance of integrating spheres in the UV, an approach based on irradiance should be adopted.

IEC TR 62778

IEC TR 62778 has been written to provide guidance 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 is made to determine whether or not the luminaire under test exceeds the limits of risk group 1 (RG1) at a distance of 200mm. A source classified as RG1 is one which does not pose a hazard due to normal behavioural limitations on exposure, including aversion response and not actively staring at the source.  Retinal blue light hazard is the name given to acute (type II) photochemically induced retinal damage caused by absorption of light by the retinal pigmented epithelium and the choroid, with peak sensitivity around 440nm for the phakic eye, hence the “blue” appellation. 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 300-780nm, encompassing the range of the blue light hazard (300-700nm) and the CIE 1924 luminous efficiacy function (380-780nm). Where a source subtends an angle less than 11mrad, according to IEC 62471, the blue light small source approach applies through a measurement of spectral irradiance.

Classification

There are two assessment metholodologies provided, 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 very significant indeed.

Computation of E_thr

The threshold illuminance, E_thr, 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, E_thr (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 E_thr only. Since E_thr 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

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 E_thr only. Since E_thr depends only on the spectral distribution of the source under test, there is no need to evaluate this parameter through a separate irradiance measurement.

In the case of luminaires in excess of the limit of RG1, the distance from the luminaire, d_thr, at which the threshold illuminance, E_thr, is to be found, should be determined and reported on marking on the luminaire.

The recommended method to determine d_thr is to use goniophotometric data of the product under test, where available. Knowledge of the maximum luminous intensity, I_max (cd), and the inverse square law allows calculation from d_thr = √(I_max /E_thr). This procedure ignores the fact that for many sources, the inverse square law will not be applicable, particularly for directional sources. In the absence of such data the location of E_thr can be found directly using an illuminance meter.

In the determination of the threshold distance, the use of goniophotometric data or a direct illiminance meter measurement encompasses light from the entire luminaire. Where the source subtends an angle greater than 11mrad at d_thr, too large an area of the source was included in the determination and this value of d_thr can be said to be overly conservative. To what extent d_thr is over-estimated depends on the size and the directionality of the luminaire under test, a factor of ten (and more!) has been seen. Over reporting is the correct way to err in a safety assessment, but the impact of an over-estimated d_thr 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 provided in annex D to address the case where a source subtends >11mrad at the initial estimate of d_thr, here labelled d_N for clarity. Having determined d_N, one should evaluate whether or not the luminous area of the source extends beyond the circular area described by an 11mrad FOV at d_N, of diameter 0.011.d_N. Where the source falls entirely with this area, then d_N =d_thr, otherwise (as is the case in figure 1, upper) d_N is overly conservative and can be refined using the following process. 

The distance, d₁, at which the E_thr of a single LED package occurs, should be determined then the area described by an 11mrad FOV at d₁ of diameter 0.011.d₁ should then be considered. If only one emitter falls within this area as is the case in figure 4 (lower) then d₁ = d_thr. Where more than one emitter falls within this area, and where E_thr was taken from the LED emitter datasheet, as opposed to resulting from the measurement of the luminaire, it does not necessarily follow that RG1 be exceeded at this distance. It is in this case recommended to perform a spectral radiance measurement at d₁ in an 11mrad FOV, if the result is below the RG1 limit, d₁=d_thr. In all other cases the true value of d_thr lies between these extremes, so the default position is to adopt the worst case, d_N =d_thr.

The refinement of d_thr in this manner is in the majority of cases not practically feasible (how does one turn off or block emission from all but one emitter in a luminaire?) and therefore can be considered largely redundant.