Plateau of residual
Light on Light off
Pupil dilation slope
shows that it is essentially dictated by the
color temperature. This reveals a seemingly
unescapable tradeoff, where low-blue can
only be obtained at the cost of a very yellow
light tint. In fact, this tradeoff stems from
a basic property of conventional LED technology: the reliance on blue-pump LEDs to
generate a white spectrum, which causes the
amount of blue radiation and the light chromaticity to be fundamentally correlated.
A blue-free solution
Having identified and understood the root
cause of this tradeoff, it is actually possible to
circumvent it and offer true white light with
minimal blue content. One example is the
BlueFree SSL technology developed by Soraa.
It hinges on replacing standard blue-pump
LEDs with violet-pump LEDs, and carefully
shaping the spectrum of the light source to
create a blue gap. By removing energy in a relatively narrow spectral range, it is possible to
retain the color temperature of an incandescent light while virtually removing any blue
radiation from the spectrum.
Although the use of blue-pump LEDs has
been unquestioned for two decades, the last
five years have seen the rise of full-spectrum
white LEDs based on violet-pump LEDs. This
may be counterintuitive at first, since conventional wisdom asserts that the lower
Stokes-loss of blue-pump LEDs is preferable.
However, Soraa has demonstrated bulk-gal-lium-nitride (GaN) violet LEDs with a record
efficiency, making full-spectrum LEDs competitive for lighting applications (http://bit.
Violet-based full-spectrum LEDs offer a
significant improvement in quality of light
( http://bit.ly/2qwYIC1), as was previously
discussed in an LEDs Magazine feature article
( http://bit.ly/VFt9D2). In these full-spectrum
products, the violet pump is complemented
with three phosphors (blue, green, and red)
to produce a full white spectrum with very
high color rendering. In the BlueFree case,
however, only green and red phosphors are
used. By carefully selecting these phosphors,
it is possible to retain very good color rendering despite the absence of blue radiation — in
particular, the crucial rendering of red tones,
which is most important to color preference,
Fig. 3 compares the spectra of various
white sources at the same color tempera-
ture of 2700K and illustrates the BlueFree
approach. Standard LEDs have a high spec-
tral content in the range 440–490 nm, where
the circadian sensitivity is maximal. In con-
trast, the BlueFree spectrum shows very little
radiation in this range.
Testing the theory
To verify the effectiveness of the BlueFree
approach, Soraa asked Jamie Zeitzer, professor at Stanford University and a global
expert in the field of sleep medicine, to
study its physiological impact. He devised an
experiment measuring the post-illuminance
pupil response (PIPR) of subjects after exposure to light ( http://bit.ly/2paI46v). This
response is of interest because, after temporary exposure to light (during which the
pupil goes from full dilation to full constriction), the pupil returns to a state of residual
constriction whose magnitude is controlled
by the same ipRGC receptors that govern the
circadian cycle. A higher dose of blue light
induces more residual constriction, which
suggests higher circadian stimulation.
Fig. 4 shows the pupil dilation speed after
participants are subjected to a light stimulus at an illuminance of 35 lx, induced by two
light sources: a standard LED and a BlueFree
LED, both at a color temperature of 2700K.
These two sources are designed to have the
same chromaticity, so participants can’t dif-
ferentiate them visually. The BlueFree LED
induces a significantly larger re-dilation
speed. This is because the pupil returns to
a lower level of residual constriction, as evi-
dence of its reduced circadian disruption.
This experiment confirms that the Blue-
Free spectrum is able to significantly reduce
circadian stimulation without resorting to
an unacceptably low color temperature.
BlueFree light can therefore be used seam-
lessly throughout a domestic environment,
producing white light and good color ren-
dering with less influence on our sleep cycle.
Due to the existence of blue-sensitive
receptors in our eyes that regulate our circadian cycle, light in a domestic setting
(especially blue-rich LED lighting) has the
potential to disrupt our sleep patterns in the
evening and cause long-term health issues. A
few commercial products currently attempt
to mitigate this effect by delivering light of
a very low color temperature, which suffers
from an unpleasant yellow tint. In contrast,
a newer technology can employ violet LEDs
to create a true white light with virtually
no blue radiation. A clinical experiment by
Stanford’s Professor Zeitzer indicates that
BlueFree bulbs are significantly less disruptive to our circadian system, when compared with standard LED sources.
FIG. 4. The graph on the left characterizes pupil response to different light sources. The
pupil constricts during the 30s light stimulus, then dilates back to a plateau of residual
constriction, whose amplitude is governed by the ipRGCs. More blue light leads to
more residual constriction in the plateau. The dilation speed is measured in the gray
zone. Faster dilation speed indicates a return to a lower constriction. The right graph
shows experimental results of dilation speed for seven participants induced by two light
sources having an illuminance of 35 lx and a color temperature of 2700K. The larger
slope for the BlueFree LED indicates a lower circadian stimulation.