regulations | SAFE T Y
tion of CCT (≤8000K), below which RG1 will
result. In addition to knowledge of the CCT
of the source, a measurement of luminance
(cd/m2) is required. The field of view (FOV) of
measurement employed in determining luminance should not extend beyond the luminous
area of the source, as discussed in the following section. Where the measured luminance of
a primary light source is below that reported
in the table, RG1 unlimited applies while for
luminaires RG1 applies. This method includes
a safety factor of two. Where the measured
luminance exceeds the tabulated values, one
should consider methods A or C.
Method C: The implementation of this
technique was discussed in detail in the
December 2013 article. Representing the
direct spectroradiometric assessment of
blue light hazard, this route will in all cases
yield the most accurate result.
A fourth option
In the future, a fourth method will be proposed, based on the calculation of the luminance in an 11-mrad FOV from luminance
distribution data of a source. A spectral
measurement of the same source allows
determination of KB,v, a parameter defined
in the TR as the ratio of the blue-light hazard quantity to the photometric quantity.
The combination of these two will result in
a blue-light radiance value to be compared
with the RG1 limit.
Since the assessment method is not
reported alongside the result, the impact
of transferring data from a primary light
source, obtained through methods A or B,
to a luminaire, should be considered. The
safety factor of two can be quite a high price
to pay for simplicity.
Finally, given that CCT and KB,v are typically computed from a total spectral radiant-flux measurement of the source, one
should consider whether or not the resulting spectral distribution be representative
of that obtained in the blue light radiance
measurement. In some instances, such as
that highlighted in Fig. 1, the spectral distribution obtained in the radiance measurement would have a significantly higher blue
peak and therefore different values of CCT
and KB,v than that obtained in the total flux
measurement, encompassing emission from
the entire source. The safety factor of methods A and B ought to cover this eventuality
but the determination of KB,v does not.
Conditions for the transfer of
risk group classification
A prerequisite for the transfer of a blue-light
hazard classification, relying upon the law
of conservation of radiance, is that the measurement be performed over a circular measurement FOV that does not extend beyond
the luminous area of the source. This measurement is sometimes called “true” radiance to distinguish from the measurement
of “physiological” radiance considered in
the photobiological safety assessment. Fig.
2 depicts the two measurement conditions.
According to Method C, where a source
has a diameter < 2. 2 mm, spectral irradi-
ance should be measured to report only Ethr,
whereas for sources with a diameter > 2. 2
mm, spectral radiance should be measured,
for which RG0 unlimited, RG1 unlimited,
or Ethr can result. This need comes from the
fact that at 200 mm, the required 11-mrad
FOV encompasses a circular area of 2.2-mm
diameter. To avoid confusion in determin-
ing whether a measurement of radiance or
irradiance should be performed, it would be
more accurate to consider if the source fully
extends beyond a circle of 2.2-mm diameter
or not. If a primary light source under-fills
this area then a risk group classification can-
not be transferred. The FOV could in this case
be reduced to ensure the measurement of true
radiance, but this may give rise to an overes-
timation of the hazard.
Taking this idea further, in measuring a primary light source, and adopting the result as a
measure of true radiance, one makes assumptions about the radiance uniformity across the
11-mrad FOV. Such assumptions may not be
valid if the radiance profile included regions
significantly higher than the average. This
case may be considered in the future.
FIG. 2. Conditions differ for measurement
of “true” (upper) and “physiological”
FIG. 3. The sequence depicts an example
of the variation of blue light radiance
in an 11-mrad FOV with distance. The
average radiance over the FOV falls
below the RG1 limit only where the
source falls entirely within the FOV.