LEDsmagazine.com NOVEMBER/DECEMBER 2015 53
regulations | SAFETY
Although LEDs bring many benefits to the world of general illumination, solid-state lighting (SSL) manufacturers still need to ensure that products present no photobiological hazards to humans.
In the December 2013 edition of LEDs
Magazine, an overview was given of a new
approach to the evaluation of the photobiological safety of light sources intended for
lighting applications ( http://bit.ly/1NW
zvGK). In this article, we will take a wider
view of the IEC TR 62778 document and discuss in depth the challenge of the correct
treatment of extended sources such as LED
arrays and linear LED light engines.
IEC TR 62778, “Application of IEC 62471
for the assessment of blue light hazard to
light sources and luminaires” provides guidance on the evaluation of the retinal blue-light hazard of sources of light intended
for lighting applications. This assessment
is based on determining whether or not a
source presents, at a distance of 200 mm,
a retinal blue-light hazard in excess of risk
group one, as defined by the horizontal photobiological safety standard, IEC 62471. Currently presented as a Technical Report (TR),
it is implemented through new editions of
normative IEC lamp and luminaire safety
standards. At the time of writing, the majority of these updates have been adopted in
Europe by CENELEC as EN standards, and
harmonized to the EU low voltage directive,
While IEC TR 62778 was first published
in 2012, the move away from implementing
IEC 62471 for lighting applications has been
slow, but appears to be gaining momentum.
The complex procedure of evaluating sources
to IEC 62471 will be replaced by a rather simpler approach but with significant complications presented in the treatment of extended
sources. This latter case is, to a certain extent,
addressed by IEC TR 62778 Edition 2: 2014.
A significant motivation in the writing of
this TR document was the reduction of the
measurement burden for luminaire manufacturers. This is achieved in two manners: first, by providing conditions under
which the risk group (RG) classification of
a primary light source may be transferred
to a luminaire; and second, in presenting
a choice of assessment methodologies, two
of which are based on commonly available
data. This TR should therefore be considered
from two distinct points of view: that of the
primary light source (Table 1) and that of the
luminaire (Table 2).
One TR, three assessment techniques
Three techniques are proposed for the
assessment of the blue light hazard, an
overview of which is presented, in order of
required inputs as defined in Table 3.
Method A: A manufacturer can report a
table of illuminance values, as a function of
CCT (≤8000K), below which RG1 will result.
Consulting the table for a source of known
CCT, one can adopt the reported illuminance
value as Ethr (threshold illuminance). This
value may be reported in the datasheet of a
primary light source and converted to dthr
(distance threshold) for a luminaire. Where
the latter process yields dthr ≤200 mm, below
the assessment distance, then RG1 should be
reported. This method includes a safety fac-
tor of t wo and cannot produce a transferable
risk group classification.
Method B: A manufacturer can again
report a table of luminance values, as a func-
Risk group determination characterizes
photobiological safety in LED lighting
Expanding on a prior series of articles focused on the photobiological safety of LED-based lighting,
LESLIE LYONS further details risk group classifications and methodologies that come into play with
extended sources such as LED arrays.
LESLIE LYONS is the technical support
manager of Bentham Instruments Limited, UK
( bentham.co.uk), and a member of BSI and IEC
committees TC 76, Optical Radiation Safety
and Laser Equipment, and the Photobiological
Safety Panel of IEC TC 34/SC 34A - Lamps.
FIG. 1. A white phosphor-converted
LED chip is composed of a single,
small blue LED emitter behind a larger
phosphor layer. The spectral radiance
measurement in an 11-mrad FOV,
centered on the blue chip, will yield
different values of CCT and KB,v to the
total spectral radiant-flux measurement
of the same source.