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Phosphor-converted LEDs with low circadian
action for outdoor lighting
AkvilėZabiliūtė,1,* Rimantas Vaicekauskas,2Pranciškus Vitta,1,2 and Artūras Žukauskas1
1Institute of Applied Research, Vilnius University, Saulėtekio al. 9, bldg. III, LT-10222 Vilnius, Lithuania
2Department of Computer Science, Vilnius University, Didlaukio g. 47, Vilnius LT-08303 Lithuania
*Corresponding author: akvile.zabiliute@tmi.vu.lt
Received October 25, 2013; revised December 17, 2013; accepted December 17, 2013;
posted December 18, 2013 (Doc. ID 200048); published January 24, 2014
Dichromatic phosphor-converted (pc) light-emitting diodes (LEDs) with low circadian action are proposed for low-
luminance photobiologically safe outdoor illumination. The LEDs feature the partial conversion of blue radiation in
an orange phosphor with the resulting correlated color temperature in the “firelight”range of 1700–2500 K. The
circadian action factor, which is the ratio of the biological efficacy of radiation due to the excitation of intrinsically
photosensitive retinal ganglion cells to the mesopic luminous efficacy of radiation, is considerably lower than that of
commercial white pc LEDs. The equivalent general color-rendering index estimated with regard to the reduced
color-discrimination ability of human vision at low luminances has appropriate values in between those of common
white pc LEDs and high-pressure sodium lamp. © 2014 Optical Society of America
OCIS codes: (160.2540) Fluorescent and luminescent materials; (160.5690) Rare-earth-doped materials; (230.3670)
Light-emitting diodes; (330.5380) Physiology; (350.4600) Optical engineering.
http://dx.doi.org/10.1364/OL.39.000563
Solid-state lighting technology is rapidly penetrating all
applications of artificial light. In particular, improved
efficacy, longevity, directionality, and controllability of
light-emitting diodes (LEDs) offer substantial savings for
outdoor lighting [1–3]. Additional benefits are foreseen
due to the easiness of the instantaneous dimming of
LEDs and dynamic patterning of illuminance, which fol-
lows the traffic participants in outdoor environments
[4–6].
The dominating approach to the use of solid-state tech-
nology in general lighting is based on white phosphor-
converted (pc) LEDs, which operate due to the partial
conversion of short-wavelength electroluminescence
from semiconductor chips to photoluminescence in phos-
phors [7–9]. Typically, such LEDs have blue-enriched
spectral power distribution (SPD), which is energy benefi-
cial in mesopic conditions peculiar to outdoor lighting due
to the Purkinje effect, which is the shift of the spectral sen-
sitivity of the human eye to shorter wavelengths with
reducing ambient (adaptation) luminance, L. Recently,
however, blue light was found to initiate the physiological
pathway of melatonin-production suppression through
the excitation of intrinsically photosensitive retinal
ganglion cells (ipRGCs) containing melanopsin photopig-
ment [10–12]. When continuously experienced in the
evening hours and in the first half of the night, such sup-
pression, as well as other possible effects of exposure to
blue light, disrupt circadian rhythms and may pose serious
health issues [13,14]. Also common white LEDs have cor-
related color temperatures (CCTs) in the range above
2700 K, which are believed to be inappropriate for well-
being at low luminances typical of street and pedestrian
area lighting standards (0.1–2cd∕m2). This belief origi-
nates from Kruithof’s hypothesis [15], which has been
partially validated in terms of increased feelings of pleas-
antness, comfort, and relaxation at lower CCTs [16].
The above considerations point out the need for the
development of pc LEDs with low circadian action and
extra-low CCTs for outdoor lighting applications. In this
Letter, we meet this need by proposing pc LEDs with
SPDs composed of components due to semiconductor
electroluminescence and appropriate phosphor photolu-
minescence, respectively.
So far, a method that could quantify circadian response
to a light source in all conditions is not completely estab-
lished, although the common opinion is that the circadian
action increases with increasing retinal irradiance and
shifting the light source spectrum to blue and blue/green
regions [17]. Despite some limitations, several approaches
to the modeling of spectral sensitivity of human circadian
system have already been developed. An approach intro-
duced by Gall [18] is based on a single circadian spectral
function Cλ, which approximates the measured action
spectra of melatonin suppression by narrowband light
[11,12]. A nonlinear approach, which accounts for the cir-
cadian action due a spectral opponent input to the ipRGC
from the blue-yellow visual channel (S-cone excitation),
has been proposed by Rea et al. for wideband (polychro-
matic) and narrowband light [19]. Further improving the
model of circadian spectral sensitivity might require ac-
counting for the long-wavelength enhancement of the
blue-light effect due to melanopsin bi-stability [20]. For
a particular application in low-circadian-action outdoor
lighting, here we use a simple model of circadian effi-
ciency function proposed by Gall. (For low CCTs, the cir-
cadian action of light sources assessed within this model
does not differ from that obtained within the model of
Rea et al.)
Our approach is based on the solution of the optimiza-
tion problem for the SPD of model pc LEDs through the
minimization of mesopic circadian action factor (CAF)
[21]. The mesopic CAF is defined as the ratio of circadian
efficacy of radiation and mesopic luminous efficacy of
radiation (LER):
ac;mes Kc0R780 nm
380 nm CλSλdλ
Kmes0R780 nm
380 nm VmesλSλdλ;(1)
where Sλis the SPD of a light source, Vmesλis the
mesopic spectral luminous efficiency function defined
February 1, 2014 / Vol. 39, No. 3 / OPTICS LETTERS 563
0146-9592/14/030563-04$15.00/0 © 2014 Optical Society of America
within visual-performance based MES-2 photometric sys-
tem [22], Kmes0683∕Vmes555 nmlm∕W is the maxi-
mal value of spectral luminous efficacy for mesopic
vision, and Kc0is the maximal value of the spectral cir-
cadian efficacy. For convenience, we set the value of Kc0
to 1817 biolm∕W, which is such that acis dimensionless
and equals unity for the CIE standard illuminant A
(2856 K blackbody radiation) at photopic conditions.
The thin lines in Fig. 1display the spectral luminous
efficacy functions in the photopic and scotopic limits
(solid line and dashed line, respectively) and for two mes-
opic adaptation luminance values that correspond to the
illumination requirements for a low-class and high-class
road (0.3cd∕m2and 2cd∕m2; dotted line and dash-
dotted line, respectively). The bold line in Fig. 1shows
the Gall’s circadian efficacy function normalized to a
maximal value of 1817 biolm∕W. The peaks of the spec-
tral luminous efficacy functions are within the range of
505 to 555 nm, depending on the adaptation luminance,
whereas the circadian efficacy function has the maxi-
mum at a wavelength of 450 nm [18]. With decreasing
adaptation luminance, the overlap of the circadian effi-
cacy function and luminous efficacy function is seen
to increase, which indicates that the content of blue ra-
diation in low-CAF SPDs is to be traded off between the
unwanted photobiological effect and desirable energy
efficiency of outdoor lighting.
Following the results of optimization presented in [21],
the main guidelines for the development of solid-state
sources of light with low circadian action for outdoor
lighting applications are as follows: (1) the SPD must have
as low as possible CCT and must contain only two com-
ponents; (2) the short-wavelength component is to be
peaked in the range around 440 nm, and the complemen-
tary long-wavelength component is to be peaked in the
orange range of the spectrum (such a selection of compo-
nent peak wavelengths ensures the lowest mesopic CAF
due to a low partial power of the short-wavelength com-
ponent and a high mesopic LER of the compound SPD);
and (3) for outdoor lighting, the reduced photopic general
color-rendering index (CRI) of such two-component SPDs
can be tolerated due to the reduced color-discrimination
ability of human vision at mesopic adaptation luminances.
The above guidelines ground the feasibility of low-CAF
pc LEDs based on the matured technology of partial
conversion of deep-blue emission efficiently generated
in InGaN semiconductor chips within appropriate
phosphors. Below we demonstrate and assess two-
component blue–orange SPDs of low CAF pc LEDs
having CCTs in the “firelight”range (1700–2500 K), which
is below the common “white”region (>2500 K). The “fire-
light”range also covers typical CCTs of high-pressure
sodium (HPS) lamp (∼1900 K), which is an efficacious
light source still widely used in outdoor lighting.
The SPDs of firelight pc LEDs were composed using a
deep-blue component provided by a commercially avail-
able InGaN electroluminescent emitter (444 nm peak
wavelength) and an orange component provided by an
inorganic phosphor selected from those described in
the literature. Five phosphors of different chemical com-
position groups having different peak wavelengths and
FWHMs have been considered as follows: a nitridosilicate
phosphor Ba;Sr2Si5N8∶Eu2(599 nm peak wavelength,
81 nm FWHM) [23], an yttrium magnesium alumosilicate
garnet phosphor Y3Mg2AlSi2O12 ∶Ce3(602 nm peak
wavelength, 145 nm FWHM) [24], a chalcogenide pho-
sphor Ca;SrSe∶Eu2(590 nm peak wavelength, 54 nm
FWHM) [25], an oxonitridosilicate phosphor Ca −α−
SiAlON∶Eu2(591 nm peak wavelength, 95 nm FWHM)
[26], and a orthosilicate phosphor Ba;Sr2SiO4∶Eu2
(587 nm peak wavelength, 102 nm FWHM) [27].
Figure 2shows the SPDs of the five proposed “fire-
light”pc LEDs with the CCTs in the range of 1700–
2540 K. The orange component due to phosphor emission
is seen to dominate in all SPDs. The pc LED with the
Eu2-doped chalcogenide phosphor has the most struc-
tured SPD, which is almost void of spectral power in the
range around 500 nm, whereas the LED with the Ce3-
doped garnet phosphor has the broadest spectrum
extending to the deep red region.
Figure 3displays a portion of the CIE 1931 color-
mixing diagram with the chromaticity coordinates of the
InGaN emitter and the five phosphors. The chromaticities
of the resulting mixtures are obtained at the intersection
of the thin lines connecting the InGaN chromaticity with
the chromaticities of the phosphors and the Planckian
locus (bold line). The proximity of the chromaticities of
the phosphors to the Planckian locus explains a low
400 500 600 700
0
500
1000
1500
Kph0V(λ)
Ksc0V'(λ)
Kmes0Vmes(λ) (L=2 cd/m2)
Kmes0Vmes(λ) (L=0.3 cd/m2)
Kc0C(λ)
Spectral efficacy (lm/W)
Wavelength (nm)
Fig. 1. Solid, dashed, dotted, and dash-dotted thin lines: pho-
topic, scotopic, and mesopic (for two mesopic adaptation lumi-
nances of 0.3, and 2cd∕m2) luminous efficacy functions,
respectively [19]; solid bold line, Gall’s circadian action efficacy
function normalized to 1817 biolm∕W[
18].
400 500 600 700
Spectral power (arb. units)
Wavelength (nm)
(Ba,Sr)2Si5N8:Eu2+
Y3Mg2AlSi2O12:Ce3+
(Ca,Sr)Se:Eu2+
Ca-α-SiAlON:Eu2+
(Ba,Sr)2SiO4:Eu2+
Fig. 2. SPDs of the proposed “firelight”LEDs.
564 OPTICS LETTERS / Vol. 39, No. 3 / February 1, 2014
partial power of the blue component in the SPDs of the pc
LEDs (Fig. 2).
Table 1presents the parameters of the SPDs of the pro-
posed pc LEDs as well as commercially available warm
white and cool white pc LEDs (Philips Lumileds, Rebel
brand), an HPS lamp, and the CIE standard illuminant A.
Columns 2–5 of Table 1display the CCT, the photopic
general CRI (Ra), the relative partial radiant flux of the
blue component, and the limiting radiant efficiency, η0,
which accounts for the Stokes shift of phosphor photo-
luminescence in respect to the primary blue emission
[28]. Also are shown the CAF (normalized to that of
the CIE standard illuminant A at photopic conditions
as indicated above), mesopic LER, and the mesopic
equivalent of the general CRI (Ra;mes) for two adaptation
luminances (0.3 and 2cd∕m2). The CRI equivalent was
estimated in accordance with [21] with regard to the re-
duction of the color discrimination ability at decreased
mesopic adaptation luminance:
Ra;mes 100 −γL100 −Ra;(2)
where γLis the color shift rescaling factor, which is the
ratio of the mean size of the MacAdam ellipses at
photopic conditions to the mean size of the ellipses at
a particular mesopic luminance [29]. This factor equals
0.26 and 0.51 for adaptation luminances of 0.3cd∕m2
and 2cd∕m2, respectively.
The data presented in Table 1show that the CAF of the
pc LEDs decreases with decreasing CCT. For the pc LED
with a CCT of 1704 K, the CAF is 4–4.6 times lower than
that of a common warm white LED depending on the
adaptation luminance. (The CAF of each “firelight”LED
slightly increases when the luminance decreases due to
the increased overlap of the blue component with the
mesopic luminous efficacy function.)
The LER of the pc LEDs has a tendency to decrease
with decreasing CCT. Also for pc LEDs with a low blue
content, a decrease of LER with decreasing adaptation
luminance can be traced, in contrast to white pc LEDs.
The pc LEDs with narrower phosphor bands (e.g., those
with nirtidosilicate and chacogenide phosphors) are
superior in LER in respect to LEDs with broader
phosphor bands. However, narrower phosphor bands re-
sult in the reduced color-rendering ability. The limiting
radiant efficiency of the proposed LEDs decreases with
decreasing CCT. However in the entire “firelight”range,
the limiting radiant efficiency is above 70%, which is only
slightly smaller than that of white pc LEDs.
Finally, all proposed pc LEDs have appropriate mes-
opic equivalents of the general CRI in excess of 75 at
a low adaptation luminance of 0.3cd∕m2. At a high adap-
tation luminance of 2cd∕m2, two of the five proposed pc
LEDs have the general CRI equivalent below 70, which
might affect the performance of some visual tasks,
however.
In conclusion, we have designed the SPDs of dichro-
matic blue–orange pc LEDs operating in the “firelight”
range of CCTs (1700–2500 K) and assessed the photomet-
ric, photobiological, and colorimetric properties of these
LEDs for mesopic conditions. The proposed pc LEDs
have low CAF and are advantageous in respect of
common white pc LEDs for the use in photobiologically
safe low-luminance outdoor lighting. Also, most of them
have the mesopic equivalent of the general CRI compa-
rable to the general CRI of common LEDs under photopic
conditions. The “firelight”pc LEDs can be used in many
outdoor environments (e.g., in pedestrian and residential
areas, old-town and fine architecture locations, etc.),
where a part of efficacy can be traded for photobiological
safety, reduced light pollution, visual comfort and illumi-
nation design diversity.
0,0 0,2 0,4 0,6 0,8
0,0
0,2
0,4
0,6
0,8
2000 K
3000 K
4500 K
6500 K
700 nm
620 nm
600 nm
580 nm
480 nm
400 nm
460 nm
y Chromaticity coordinate
444-nm InGaN
(Ba,Sr)2Si5N8:Eu2+
Y3Mg2AlSi2O12:Ce3+
(Ca,Sr)Se:Eu2+
Ca-α-SiAlON:Eu2+
(Ba,Sr)2SiO4:Eu2+
x Chromaticity coordinate
490 nm
Fig. 3. Portion of the CIE 1931 diagram with the chromaticity
coordinates of the InGaN emitter, five phosphors, and resulting
pc LEDs shown. Bold line, Planckian locus.
Table 1. Parameters of the Proposed pc LEDs and Common Light Sources
0.3cd∕m22cd∕m2
Phosphor/Source CCT
(K) Photopic
CRI Blue
Content η0CAF LER
(lm∕W) CRI
Equivalent CAF LER
(lm∕W) CRI
Equivalent
Ba;Sr2Si5N8∶Eu21704 39 0.0259 0.73 0.186 298 84 0.169 329 69
Y3Mg2AlSi2O12∶Ce32088 62 0.0443 0.71 0.363 250 90 0.346 263 81
Ca;SrSe∶Eu22101 16 0.0674 0.77 0.308 404 78 0.284 439 57
Ca −α−SiAlON∶Eu22425 53 0.0806 0.76 0.480 356 88 0.463 369 76
Ba;Sr2SiO4∶Eu22542 55 0.0895 0.75 0.532 343 88 0.517 353 77
Warm white LED 3652 62 0.1259 0.77 0.748 377 90 0.773 365 80
Cool white LED 5000 65 0.2130 0.82 1.173 394 91 1.280 361 82
HPS lamp 1886 12 - - 0.313 319 77 0.291 344 55
CIE A 2856 100 - - 0.940 170 100 0.912 160 100
February 1, 2014 / Vol. 39, No. 3 / OPTICS LETTERS 565
This work was supported by the Lithuanian Research
Council (grant no. ATE01/2012).
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