Light Pollution and Circadian Misalignment: A Healthy, Blue-Free, White Light-Emitting Diode to Avoid Chronodisruption - MDPI
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International Journal of
Environmental Research
and Public Health
Article
Light Pollution and Circadian Misalignment: A Healthy,
Blue-Free, White Light-Emitting Diode to
Avoid Chronodisruption
Amador Menéndez-Velázquez *, Dolores Morales and Ana Belén García-Delgado
Photoactive Materials Research Unit, IDONIAL Technology Center, 33417 Avilés, Spain;
dolores.morales@idonial.com (D.M.); ana.delgado@idonial.com (A.B.G.-D.)
* Correspondence: amador.menendez@idonial.com
Abstract: Sunlight has participated in the development of all life forms on Earth. The micro-world
and the daily rhythms of plants and animals are strongly regulated by the light–dark rhythm. Human
beings have followed this pattern for thousands of years. The discovery and development of artifi-
cial light sources eliminated the workings of this physiological clock. The world’s current external
environment is full of light pollution. In many electrical light bulbs used today and considered
“environmentally friendly,” such as LED devices, electrical energy is converted into short-wavelength
illumination that we have not experienced in the past. Such illumination effectively becomes “biolog-
ical light pollution” and disrupts our pineal melatonin production. The suppression of melatonin at
night alters our circadian rhythms (biological rhythms with a periodicity of 24 h). This alteration is
known as chronodisruption and is associated with numerous diseases. In this article, we present a
blue-free WLED (white light-emitting diode) that can avoid chronodisruption and preserve circadian
rhythms. This WLED also maintains the spectral quality of light measured through parameters such
Citation: Menéndez-Velázquez, A.; as CRI (color reproduction index).
Morales, D.; García-Delgado, A.B.
Light Pollution and Circadian Keywords: light pollution; artificial light at night (ALAN); circadian rhythms; chronodisruption;
Misalignment: A Healthy, Blue-Free, white light-emitting diode (WLED); blue-free WLED; color reproduction index (CRI); luminescent
White Light-Emitting Diode to Avoid organic materials; spectral converters
Chronodisruption. Int. J. Environ. Res.
Public Health 2022, 19, 1849. https://
doi.org/10.3390/ijerph19031849
Academic Editors: Irena Fryc and 1. Introduction
Tomasz Ści˛eżor In 1609, Galileo Galilei pointed a telescope at the night sky. His inspection of the
pathways of stars and planets brought new and profound knowledge of celestial movement.
Received: 27 December 2021
Accepted: 3 February 2022
His discovery that the Earth is not the center of the universe propelled revolutions in science
Published: 7 February 2022
and religion [1].
Galileo’s telescopic discoveries, and the cultural commotion they caused, were remark-
Publisher’s Note: MDPI stays neutral able. However, Galileo’s night sky was no less spectacular than his discoveries. From a
with regard to jurisdictional claims in
contemporary perspective, it is difficult to imagine that the nocturnal world was once a
published maps and institutional affil-
central part of human existence. Starlight has aroused curiosity, inspired art, and formed
iations.
the basis of countless creation stories for thousands of years. Stars have guided exploration,
navigation [2], and astronomy [3], compelling some of the most significant technological
leaps in human innovation.
Copyright: © 2022 by the authors.
The night sky’s transformation also serves a crucial biological purpose. Darkness
Licensee MDPI, Basel, Switzerland.
helps regulate sleep patterns in humans and provides wildlife migration, feeding, and
This article is an open access article mating cues. Even trees depend on the length of the day to guide the timing of leaf molt.
distributed under the terms and Cycles of light and dark have always determined the rhythms of life [4].
conditions of the Creative Commons In large cities worldwide, millions of artificial lights have replaced natural darkness
Attribution (CC BY) license (https:// with the haze of a perpetual glow that hides all but the brightest stars. In short, light
creativecommons.org/licenses/by/ pollution is an excessive and inappropriate artificial light at night. Light pollution is
4.0/). increasing, covering nearly 80% of the globe [5]. Astronomers were the first to call attention
Int. J. Environ. Res. Public Health 2022, 19, 1849. https://doi.org/10.3390/ijerph19031849 https://www.mdpi.com/journal/ijerph
Int. J. Environ. Res. Public Health 2022, 19, 1849 2 of 11
to the purloined night. However, they were far from the only group affected. Our overall
lives and health have also been altered by the disappearance of natural darkness [6].
Human beings are diurnal organisms whose biological clocks are governed by natural
light and dark cycles [7]. We lived in harmony with these natural cycles for millennia.
Throughout our evolution, our human ancestors, similar to other mammals, were also diur-
nal. They were generally active during daytime and rested at night under dark conditions.
Bearing in mind that human evolution took place near the Equator, our ancestors followed
cycles of 12 h of light and 12 h of darkness (12L–12D).
Humans have always tried to prolong daylight by whatever local means available, in-
cluding burning wood, animal fat, organic and mineral oils, etc. (especially after migrating
from equatorial areas to places with shorter days and longer nights). However, nighttime
activities under such limited lighting sources were rather limited.
The invention of the electric light bulb changed this situation dramatically. English
physicist Joseph Wilson Swan and American inventor Thomas Alva Edison publicly demon-
strated this new technology almost simultaneously in 1879.
Since then, electrical light bulbs and electricity have become more reliable and af-
fordable as energy sources for illumination. Following these technological developments,
electric lights proliferated worldwide. As a result of this proliferation, humans became no
longer “tied” to the traditional 12L–12D cycles. Instead, we could be active around the
clock at will. Supported by artificial illumination, we can be active at night and rest during
the day, contrary to our diurnal nature acquired through years of evolution.
The invention of electric light created new opportunities by allowing the extension
of daylight beyond what the Sun dictated. However, exposure to artificial light at night
(ALAN) inhibits the secretion of melatonin (a hormone associated with sleep) and alters our
biological rhythms, causing what is known as circadian disruption or chronodisruption [8].
A growing body of scientific and clinical evidence suggests that chronodisruption is associ-
ated with numerous diseases [9], such as cancer, Alzheimer’s, and cardiovascular diseases.
In addition to its important role as a sleep regulator, melatonin has other beneficial
properties for our health. It is a powerful antioxidant, an antioncogenic agent, and enhances
and regulates our immune system’s proper functioning, among other properties [10]. There-
fore, melatonin plays a fundamental role in the protection against diseases such as COVID,
whose harmful effects on humans can be minimized with an optimal immune system [11].
Decades of research on the human eye and the discovery of a new photoreceptor in
the retina, not associated with vision but with the regulation of circadian rhythms, allow us
to know and understand the effects of light on our biological clocks. This photoreceptor is
known as ipRGC (intrinsically photosensitive retinal ganglion cell) [12] and is sensitive to
both light intensity and the spectral distribution of light. Specifically, this photoreceptor
contains the pigment melanopsin, which has an absorption peak in the blue part (around
480 nm) of the visible spectrum. These ipRGC cells also receive synaptic inputs from
rods and cones [13]. The results of these interactions define the spectral sensitivity of the
circadian system, which reaches its maximum in the wavelength range from about 430 nm
to 500 nm.
White light-emitting diodes (WLEDs) are energy efficient and more environmentally
friendly than their predecessors. In any case, the white light they emit contains a large
proportion of blue light in the spectral range between 430 nm and 500 nm. This blue light
interacts with the ipRGC photoreceptors and inhibits the secretion of melatonin with a
consequent alteration of biological rhythms [14].
We cannot regress to Edison’s incandescent light bulb, which is inefficient from an
energy perspective. However, we can move toward lights that simultaneously preserve
energy efficiency, spectral quality, and human health. In this article, we propose a light bulb
that meets these characteristics. We developed a blue-free WLED to avoid chronodisruption
while maintaining high quality light parameters, such as the color rendering index (CRI).
To our knowledge, this is the blue-free WLED based on organic materials with the highest
CRI. This is a proof of concept which allows us to conclude that it is possible to design truly
Int. J. Environ. Res. Public Health 2022, 19, 1849 3 of 11
efficient and healthy WLED lights that support the body’s biological needs (non-visual
effects of light) and are good at rendering the color of objects (visual effects of light).
2. Materials and Methods
Most white LEDs on the market use blue LED chips manufactured from GaN-based
materials and a phosphor coating placed either directly on the chips or separated from
the chip (remote phosphor configuration) [15]. A phosphor is defined as a luminescent
material that absorbs and reemits energy over time, which is associated with the lifetime of
the excited electron. A fraction of the blue light passing through the phosphor is absorbed,
experiences the Stokes shift down-shifting, and appears as white light when mixed with
the remaining unabsorbed blue light. Shuji Nakamura first demonstrated this and was
awarded with the Nobel prize for his blue and white LED inventions [16,17].
These WLED sources emit light with characteristic spectral distributions that depend
on the nature of the excitation source (blue LED chip) and the phosphors (spectral con-
verters) used. Most phosphors are inorganic, i.e., carbon-free crystals. Furthermore, many
of them are based on rare earths subject to geopolitical tensions. Therefore, the search for
alternative luminescent materials is a promising alternative. Quantum dots [18–21] and
organic dyes [22] attract attention due to their high photoluminescence quantum yields
and tunable light emission. They may be used as an alternative to traditional phosphors
for allowing a fine adjustment of CRI and CCT.
By showing a narrow emission leading to pure colors, quantum dots have been
implemented in commercial flat panel displays. Organic luminescent dyes show a broader
emission. Therefore, they can be very useful for general lighting in order to cover the whole
visible spectrum. Usually, the absorption spectrum of quantum dots is broader than the
absorption spectrum of organic dyes. However, this is not a problem for organic dyes if
their absorption spectrum covers the emission range of the LED chip acting as a source
of excitation.
In summary, quantum dots and organic dyes are promising materials as color convert-
ers for lighting. However, the most efficient QDs are based on toxic Cd. Organic dyes are
sustainable, environmentally friendly, and cheaper than quantum dots.
We present a study of WLEDs using organic fluorescent dyes to replace the con-
ventional phosphor. These organic luminescent materials have already been tested as
spectral converters in the energy sector (photovoltaics) [23–25], in the agrifood sector
(greenhouses) [26], and even in the medical sector (ophthalmology) [27] to achieve a spe-
cific spectral redistribution. The lighting industry may benefit from these types of materials,
which are sustainable, environmentally friendly, and free of geopolitical tensions.
We developed different blue-free WLEDs for this article. They are based on blue LED
chips and luminescent organic materials in a remote phosphor configuration acting as
spectral converters. These luminescent organic materials combine in the same layer or
different layers in order to have greater versatility in the spectral conversion, thus achieving
an emitted light with the desired properties.
Selected Excitation Source and Materials
As excitation source, we used an LED whose relative spectral power distribution
(SPD) chart is shown in Figure 1. The SPD provides emitted power as a function of the
wavelength. The LED emits light in the wavelength range between 410 nm and 500 nm,
corresponding to the blue range of the spectrum, with an emission peak at 450 nm.
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Figure 1. Spectral power distribution (SPD) chart of the selected blue LED.
Figure
Figure 1.
1. Spectral
Spectral power
power distribution
distribution (SPD)
(SPD) chart
chart of
of the
the selected
selected blue
blue LED.
The luminescent
The luminescent organic
luminescent organic molecules
organic molecules
molecules are are incorporated
are incorporated
incorporated into into
into aaa polymer
polymer matrix
polymer matrix with
matrix with high
with high
high
The
optical transparency,
optical transparency,
transparency, such such
such as as PMMA.
as PMMA.
PMMA. These These composite
These composite systems
composite systems (polymeric
systems (polymeric matrix
(polymeric matrix
matrix andand
and
optical
luminescent
luminescent species)
species) are
are deposited
deposited on
on glass
glass in
in the
the form
form of
of thin
thin films.
films. These
These thin
thin films
films
luminescent species) are deposited on glass in the form of thin films. These thin films
function
function as as luminescent
as luminescent filtersthat that spectrallyconvert
convert light and expand the spectrum of
function luminescent filtersfilters thatspectrally
spectrally convertlight lightandandexpand
expand thethe
spectrum
spectrumof the
of
the emitted
emitted lightlight
withwith respect
respect to the
to the blue
blueblue
LED LED
chip,chip, in order
in order to cover
to cover a larger
a larger fraction
fraction of
of the
the emitted light with respect to the LED chip, in order to cover a larger fraction of
the visible
visible part part
of of the
the spectrum.
spectrum.
the visible part of the spectrum.
Regarding
Regarding the the luminescentspecies, species, wesought
sought organic dye molecules whose excita-
Regarding the luminescent
luminescent species,we we soughtorganic
organicdye dye molecules
molecules whose
whose excitation
excita-
tion spectral
spectral range range
coverscovers the
the emissionemission spectrum
spectrum of the selected
of theofselected blue
blue LED. LED. The
The Coumarin Coumarin
6 dye
tion spectral range covers the emission spectrum the selected blue LED. The Coumarin
6meets
dye meets
this this requirement.
requirement. Figure Figure
2 2 shows
shows its its excitation
excitation and and emission
emission spectra.
spectra. The The vis-
visible
6 dye meets this requirement. Figure 2 shows its excitation and emission spectra. The vis-
ible excitation
excitation spectrum
spectrum (Figure
(Figure 2a) extends
2a) extends from
fromfrom ultraviolet
ultraviolet to 500to 500 with
nm, with an excitation
ible excitation spectrum (Figure 2a) extends ultraviolet tonm,
500 nm, with an excitation peak
an excitation
peak
at 457atnm,
457very
nm,close
verytoclose
the to the emission
emission peak of peak
the of the
blue LED.blue LED. Likewise,
Likewise, the the spectrum
emission emission
peak at 457 nm, very close to the emission peak of the blue LED. Likewise, the emission
spectrum under
under the under
excitationthe excitation wavelength of 457 nm presents
at 506aa nm
peak at 506 nmallowing
(Figure
spectrum the wavelength of 457 nm presents
excitation wavelength of 457 nm a peak
presents peak (Figure
at 5062b),
nm (Figure
2b), allowing the
the extension extension
of the blue LED of +the blue LED
phosphor + phosphor
system’s emissionsystem’s
rangeemission
with respectrange withpure
to the re-
2b), allowing the extension of the blue LED + phosphor system’s emission range with re-
spect to the pure blue LED.
blue LED.
spect to the pure blue LED.
Figure 2. Photoluminescent
Figure 2. excitation
Photoluminescent excitation (a)
excitation(a) and
(a) and emission
and emission (b)
emission (b) spectra
(b) spectra of
spectra of Coumarin
Coumarin 666 green-emitting
of Coumarin green-emitting
green-emitting
Figure Photoluminescent
converter embedded in a PMMA matrix.
converter embedded
converter embedded in
in aa PMMA
PMMA matrix.
matrix.
Figure
Figure 333 shows
shows the3D3D photoluminescent (PL) spectra under the excitation wave-
Figure showsthethe 3D photoluminescent
photoluminescent (PL)(PL)
spectra under
spectra the excitation
under wavelengths
the excitation wave-
lengths
from 410 from 410
nm 410 nm
to 500 to 500 nm (corresponding to the emission range of the blue LED
lengths from nmnm to (corresponding to the emission
500 nm (corresponding range of the
to the emission rangeblue
of LED chip).
the blue LEDBy
chip). By
observing observing
the the
spectra, spectra,
we can we can
conclude conclude
that that
Coumarin Coumarin
6 6
embeddedembedded
in PMMA in PMMA
can be
chip). By observing the spectra, we can conclude that Coumarin 6 embedded in PMMA
can be effectively
effectively excited excited
by the by the selected
selected blue blue
LED LED chip.
chip.
can be effectively excited by the selected blue LED chip.
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Figure 3. 3D photoluminescent spectra of Coumarin 6 embedded in a PMMA matrix under the
excitation
Figure
Figure wavelengths
3.3.3D
3D from 410
photoluminescent
photoluminescent nm toof
spectra
spectra500
of nm.
Coumarin 6 embedded
Coumarin in ain
6 embedded PMMA matrix
a PMMA under
matrix the the
under
excitation
excitationwavelengths
wavelengths from
from 410
410 nm
nm to
to 500
500 nm.
nm.
We then sought a second luminescent species that allowed us to extend the spectral
rangeWe
We ofthen
emitted
then light
sought
sought further.
aa second Lumogen Red
second luminescent
luminescent is a molecule
species
species that allowed
that withus
allowed a broad
us to extend
to excitation
extend spec-
the spectral
the spectral
trum that
range
range can partially
of emitted light absorbLumogen
lightfurther.
further. light emitted
Lumogen Redisby
Red athe
isa moleculeblue LED
molecule with
withand light
a broad
a broad corresponding
excitation
excitationspectrum
spec-to
the Coumarin
that
trum can
thatpartially6 emission
absorb
can partially spectrum.
light
absorb emitted
light Figure
by the
emitted 4 shows
by blue
the blue its LED
LED excitation
andand and
lightlight emission spectra.
corresponding
corresponding to the
to
TheCoumarin
the visible6excitation
Coumarin emission
6 emission spectrum
spectrum.
spectrum. (Figure
Figure 44a)
Figure 4extends
shows shows from
its excitation
its ultraviolet
and and
excitation toemission
emission 600 spectra.
nm spectra.
withThe
an
excitation
visible peak
excitation at 450
spectrumnm, which
(Figure overlaps
4a) extendswith
from the emission
ultraviolet
The visible excitation spectrum (Figure 4a) extends from ultraviolet to 600 nm with an peak
to 600 of
nm the
withbluean LED. An-
excitation
peak
otheratexcitation
excitation450peak
nm, at
which
peak overlaps
450 at
nm, nmwith
570which the emission
overlaps
overlaps with
withthe peak
the of the blue
Coumarin
emission LED.
6 emission
peak of theAnother
spectrum.excitation
blue LED. Like-
An-
peak
wise, at
other 570emission
the nm overlaps
excitation with
peakspectrum
at 570 nmthe
underCoumarin 6 emission
the excitation
overlaps with spectrum.
wavelength
the Coumarin ofLikewise,
570 nm
6 emission the emission
presents
spectrum. two
Like-
spectrum
peaks at under
608 nm the
and excitation
649 nm wavelength
(Figure 4b), of 570
allowing nmthe presents
extension
wise, the emission spectrum under the excitation wavelength of 570 nm presents two twoof peaks
the at 608
spectral nm
rangeandof
649
the nm (Figure
emitted 4b),
light. allowing the extension of the spectral range
peaks at 608 nm and 649 nm (Figure 4b), allowing the extension of the spectral range of of the emitted light.
the emitted light.
Photoluminescent excitation
Figure 4. Photoluminescent excitation (a) and emission (b) spectra of Lumogen Red red-emitting
converter
converter embedded
embedded in
in a
a PMMA
PMMA matrix.
matrix.
Figure 4. Photoluminescent excitation (a) and emission (b) spectra of Lumogen Red red-emitting
converter embedded in a PMMA matrix.
Figure 5 shows the 3D PL spectra under the excitation wavelengths from 410 nm to
740 nm (corresponding
Figure 5 shows the to
3Dthe
PLemission of the blue
spectra under LED chipwavelengths
the excitation and the Coumarin 6 dye).
from 410 nm Byto
observing the spectra, we can conclude that Lumogen Red embedded in PMMA can
740 nm (corresponding to the emission of the blue LED chip and the Coumarin 6 dye). By be
excited by the blue LED + Coumarin 6 system.
observing the spectra, we can conclude that Lumogen Red embedded in PMMA can be
excited by the blue LED + Coumarin 6 system.
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Figure 3Dphotoluminescent
5. 3D
Figure 5. photoluminescentspectra
spectraLumogen
Lumogen Red
Red embedded
embedded in ainPMMA
a PMMA matrix
matrix under
under the the
excitation wavelengths from 410
410 nm
nm to
to 740
740 nm.
nm.
Another
Another keykey property
property of of aa luminescent
luminescent material
material acting
acting as
as spectral
spectral converter
converter is
is the
the
quantum yield (QY), which is defined as the ratio of emitted photons to absorbed
quantum yield (QY), which is defined as the ratio of emitted photons to absorbed photons. photons.
These
These luminescent
luminescent organic
organic dyes
dyes (Coumarin
(Coumarin 66 andand Lumogen
Lumogen Red)
Red) possess
possess high
high quantum
quantum
yields both in solution and solid state. QY of Coumarin 6 and Lumogen Red
yields both in solution and solid state. QY of Coumarin 6 and Lumogen Red embedded embedded in in
PMMA
PMMA is is 76%
76% and
and 97%,
97%, respectively.
respectively.
3. Results and Discussion
3. Results and Discussion
By combining the luminescent species Coumarin 6 (C6) and Lumogen Red (LRed) in
By combining the luminescent species Coumarin 6 (C6) and Lumogen Red (LRed) in
the same layer or different layers and varying their concentrations, it is possible to achieve
the same layer or different layers and varying their concentrations, it is possible to achieve
different spectral conversions. The different concentrations are expressed as percent by
different spectral conversions. The different concentrations are expressed as percent by
weight with respect to the host material (PMMA).
weight with respect to the host material (PMMA).
We developed 19 different WLEDs from the previously described blue LED chip
and 19Wedifferent
developed 19 different
spectral WLEDs
converters. We from the previously
grouped described
the different blue
spectral LED chip into
converters and
19 different
three spectral
categories: converters.
monolayer We grouped
molecular systems thewith
different spectral converters
one luminescent into three
dye (Section 3.2),
categories: monolayer molecular systems with one luminescent dye
monolayer molecular systems with two luminescent dyes (Section 3.3), and multilayer (Section 3.2), mono-
layer molecular
luminescent systems
molecular with two
systems luminescent
(Section dyes
3.4). It is (Section
possible 3.3), andthe
to compare multilayer
differentlumines-
WLEDs
cent molecular systems (Section 3.4). It is possible to compare
and optimize their configuration to achieve a blue-free WLED with high light the different WLEDs and
quality
optimize their configuration to achieve a blue-free WLED with high light
by measuring their optical properties. In Section 3.1, we describe and justify the optical quality by meas-
uring their opticalthat
characterizations properties. In Section
will be carried out in 3.1,
thewe describesections
following and justify
withthetheoptical character-
different WLEDs
izations that
we developed. will be carried out in the following sections with the different WLEDs we
developed.
3.1. Optical Characterization
3.1. Optical
3.1.1. Characterization
Spectral Distribution Power (SPD)
3.1.1.The
Spectral Distribution
spectral Power (SPD)
power distribution (SPD) chart visually represents the light spectrum
emitted
Theby a light power
spectral source. distribution
It provides emitted
(SPD) chartpower as a function
visually of thethe
represents wavelength. SPD
light spectrum
charts
emittedarebyaapractical wayItto
light source. compare
provides the quality
emitted powerand as atype of light
function created
of the by different
wavelength. SPD
light sources. The chart shows which wavelengths of light (measured
charts are a practical way to compare the quality and type of light created by different in nanometers) the
illuminated regions receive. Therefore, it can be useful to visualize the
light sources. The chart shows which wavelengths of light (measured in nanometers) the fraction of blue light
that differentregions
illuminated LEDs emit.
receive. Therefore, it can be useful to visualize the fraction of blue
light that different LEDs emit.
3.1.2. Color Rendering Index (CRI)
3.1.2.The color
Color rendering
Rendering index,
Index also known as the CRI or Ra value, is defined by the
(CRI)
Commission Internationale de l’Éclairage (CIE, International Commission on Illumination)
The color rendering index, also known as the CRI or Ra value, is defined by the Com-
as a quantitative indicator of a light source’s ability to correctly reproduce the Ri test colors
mission Internationale de l’Éclairage (CIE, International Commission on Illumination) as
of any object in comparison to a reference light source, such as sunlight. The standard
a quantitative indicator of a light source’s ability to correctly reproduce the Ri test colors
CRI points, namely R1 −R8 , derive from CIE 1974 test color samples. The value of CRI is
of any object in comparison to a reference light source, such as sunlight. The standard CRI
calculated as CRI = ΣRi/ 8. CRI ranges from 0 (no color rendering) to 100 (perfect color
Int. J. Environ. Res. Public Health 2022, 19, 1849 7 of 11
rendering). The special CRI points R9 –R15 are also color rendering markers represented in
the CRI points diagrams.
3.1.3. Correlated Color Temperature (CCT)
The emission spectrum of a blackbody radiator is a function of its temperature. An
arbitrary white light source can be characterized by finding the temperature of the black-
body radiator that radiates light of a color comparable to that of the light source. This
temperature is called the correlated color temperature (CCT) and is measured in Kelvin (K).
For example, the color of a candle’s flame is similar to a black body heated to about 1800 K.
The flame is said to have a “color temperature” of 1800 K. Similar to other incandescent
bodies, the black body changes its color with increased temperature. It first acquires a red
tone, then changes to orange, yellow, and finally white, bluish-white, and blue.
Although the correlation is not always perfect, the lowest color temperatures generally
correspond to warm lights with low blue content, and the highest color temperatures corre-
spond to cool lights with high blue content. Therefore, color temperature is a parameter we
can also consider when designing LEDs with low blue content.
3.2. Monolayer Molecular Systems with One Luminescent Dye
We developed four WLEDs from the blue LED chip (See Figure 1) by integrating an
organic luminescent dye into a single layer and using different concentrations of the dye
for each WLED. Table 1 summarizes the characteristics of the WLEDs we developed.
Table 1. Description of the WLEDs (1, 2, 3, and 4) and their optical properties.
WLED % Dye (Layer 1) CRI CCT (K)
WLED1 C6 1.5% - -
WLED2 C6 2.3% 70.6 7997
WLED3 C6 2.5% 73.7 8627
WLED4 C6 3% 73.9 6895
As shown in Table 1, white light with a CRI above 70 was achieved using Coumarin 6
dye with a concentration of 2.3%. However, the blue component is still significant, even
in WLED4, which has the highest dye concentration and therefore the greatest capacity to
absorb blue light. This result is shown in Figure 6a with the spectral distribution power of
WLED1. The SPD shows two peaks in the spectrum, corresponding to the emission peak
Int. J. Environ. Res. Public Health 2022, 19, x 8 of 12
of the blue LED chip and the emission peak of the Coumarin 6 dye. Figure 6b shows the
corresponding CRI points.
(a) (b)
Figure6.
Figure 6. Spectral
Spectral power
power distribution
distribution chart
chart (a)
(a) and
and CRI
CRI points
points (b)
(b) of
of WLED4.
WLED4.
3.3. Monolayer Molecular Systems with Two Luminescent Dyes
We developed three WLEDs from the same blue LED chip (see Figure 1) and two
luminescent species (Coumarin 6 and Lumogen Red) integrated into the same layer. By
using two luminescent dyes, instead of just one luminescent dye, we pursued a double
Int. J. Environ. Res. Public Health 2022, 19, 1849 8 of 11
3.3. Monolayer Molecular Systems with Two Luminescent Dyes
We developed three WLEDs from the same blue LED chip (see Figure 1) and two
luminescent species (Coumarin 6 and Lumogen Red) integrated into the same layer. By
using two luminescent dyes, instead of just one luminescent dye, we pursued a double
objective: on the one hand, to improve the CRI by expanding the emission spectrum; on
the other hand, to reduce the blue peak with the contribution of two luminescent species,
which present some absorption around the blue LED emission peak (450 nm). We also
increased the concentration of the Coumarin 6 dye to help further reduce the blue peak of
the spectrum.
Table 2 shows different formulations for these organic dyes and the CRI and CCT val-
ues of the corresponding WLEDs. WLED6 achieved a CRI higher than 75, thus improving
the CRI of the WLEDs developed with a single luminescent species in the previous section.
Table 2. Description of the WLEDs (5, 6, and 7) and their optical properties.
WLED % Dye (Layer 1) CRI CCT (K)
WLED5 C6 4% LRed 0.75% 73.6 2388
WLED6 C6 5% LRed 0.75% 75.8 2292
WLED7 C6 6% LRed 0.75% 72.8 2369
WLED6 reduces the intensity of the blue peak of the spectrum most out of all the
WLEDs. Figure 7 shows the SPD chart and the CRI points of WLED6. As shown in Figure 7a,
Int. J. Environ. Res. Public Health 2022, 19, x 9 of 12
WLED6 reduces the intensity of the blue peak by more than half with respect to WLED4. It
also improves its CRI by almost two units (see Figures 6b and 7b).
(a) (b)
Figure 7.
Figure 7. Spectral
Spectral power
power distribution
distribution chart
chart (a)
(a) and
and CRI
CRI points
points (b)
(b)of
ofWLED6.
WLED6.
3.4.
3.4. Multilayer
Multilayer Luminescent Molecular Systems Systems
We
We developed multilayerWLEDs
developed multilayer WLEDs to to reduce
reduce further
further andand practically
practically eliminate
eliminate the
the blue
blue
peakpeak
of theofWLEDs
the WLEDs and improve
and improve CRI values.
CRI values. The molecular
The molecular systemssystems of 3.3
of Section Section
(corre-3.3
(corresponding
sponding to WLED5, to WLED5,
WLED6, WLED6, and WLED7),
and WLED7), formed by formed by two luminescent
two luminescent species,
species, combine
combine
with the with the molecular
molecular systems of systems
Sectionof3.2Section 3.2 (corresponding
(corresponding to WLED1,toWLED2,
WLED1,WLED3,WLED2,
WLED3,
and WLED4),and WLED4),
formed by formed by a luminescent
a luminescent species, tospecies,
create 12tonewcreate 12 new
WLEDs. WLEDs.
The molecular The
molecular
systems fromsystems from
Section 3.3Section 3.3 into
integrate integrate
layer into layer to
1 (closest 1 (closest
the bluetoLED)
the blue LED)
of the newofWLEDs
the new
WLEDs
and theand the molecular
molecular systemssystems from Section
from Section 3.2. integrate
3.2. integrate into2 layer
into layer 2 (thedistant
(the most most distant
from
from the LED).
the blue blue LED).
TableTable 3 shows
3 shows different
different WLED WLED configurations
configurations and theandcorresponding
the correspondingCRI
CRI
and and
CCTCCT values.
values. CRICRI values
values improve
improve significantly,
significantly, surpassing
surpassing thethe value
value of of
8080ininall
all
WLEDs.
WLEDs. It It also
also significantly lowers the the value
value of of the
thecolor
colortemperature,
temperature,whichwhichisisgenerally
generally
correlated
correlated with
with aa lower
lower proportion of blue light, as mentioned
mentioned before.
before.
Table 3. Description of the WLEDs (8–19) and their optical properties.
WLED % Dye (Layer 1) % Dye (Layer 2) CRI CCT (K)
WLED8 C6 4% LRed 0.75% C6 1.5% 82.9 2187
Int. J. Environ. Res. Public Health 2022, 19, 1849 9 of 11
Table 3. Description of the WLEDs (8–19) and their optical properties.
WLED % Dye (Layer 1) % Dye (Layer 2) CRI CCT (K)
WLED8 C6 4% LRed 0.75% C6 1.5% 82.9 2187
WLED9 C6 4% LRed 0.75% C6 2.3% 81.6 2124
WLED10 C6 4% LRed 0.75% C6 2.5% 81.5 2113
WLED11 C6 4% LRed 0.75% C6 3% 81.0 2080
WLED12 C6 5% LRed 0.75% C6 1.5% 82.9 2167
WLED13 C6 5% LRed 0.75% C6 2.3% 81.3 2110
WLED14 C6 5% LRed 0.75% C6 2.5% 81.2 2111
WLED15 C6 5% LRed 0.75% C6 3% 80.2 2078
WLED16 C6 6% LRed 0.75% C6 1.5% 82.1 2174
WLED17 C6 6% LRed 0.75% C6 2.3% 81.3 2096
WLED18 C6 6% LRed 0.75% C6 2.5% 81.4 2107
WLED19 C6 6% LRed 0.75% C6 3% 80.9 2083
Figure 8 shows the SPD chart and CRI points of the WLED with the best CRI
Int. J. Environ. Res. Public Health 2022, 19, x
value
10 of 12
(82.9), corresponding to WLED8. There are high CRI values (Figure 8b) and hardly any
blue component of emitted light (Figure 8a).
(a) (b)
Figure 8.
Figure 8. Spectral
Spectral power
power distribution
distribution chart
chart (a)
(a) and
and CRI
CRI points
points (b)
(b) of
of WLED8.
WLED8.
It is
It is possible
possible to lower the blue component
component and and color
color temperature
temperatureusing
usingWLED11,
WLED11,
which marginally
which marginally sacrifices the CRI a little bit.
bit. The CRI assumes a value of81.0,
The CRI assumes a value of 81.0,and
andthe
the
color temperature
color temperature has a value of 2080 K. K. Figure
Figure 99 shows
shows the
the SPD
SPDchart
chartand
andCRICRIpoints
pointsofof
WLED11.The
WLED11. Theblue
blueemission
emissionisisminimal
minimal(Figure
(Figure9a),
9a),
andand the
the CRICRI values
values areare high
high (Figure
(Figure 9b).
9b). Therefore, two WLEDs with high CRI values and minimal blue color emissions (corre-
sponding to a very low CCT) were achieved. WLED8 had the best CRI value (82.9) and a
color temperature of 2187 K. WLED11 had the lowest CCT value (2080 K) and a CRI of 81.0.
(a) (b)
Figure 9. Spectral power distribution chart (a) and CRI points (b) of WLED11.
Figure 8. Spectral power distribution chart (a) and CRI points (b) of WLED8.
It is possible to lower the blue component and color temperature using WLED11,
which marginally sacrifices the CRI a little bit. The CRI assumes a value of 81.0, and the
color temperature has a value of 2080 K. Figure 9 shows the SPD chart and CRI points of
Int. J. Environ. Res. Public Health 2022, WLED11.
19, 1849 The blue emission is minimal (Figure 9a), and the CRI values are high (Figure
10 of 11
9b).
(a) (b)
Figure9.9. Spectral
Figure Spectral power
power distribution
distribution chart
chart (a)
(a) and
and CRI
CRI points
points (b)
(b) of
of WLED11.
WLED11.
4. Conclusions
Therefore, two WLEDs with high CRI values and minimal blue color emissions (cor-
responding
In recenttodecades,
a very low CCT)
overall wereconditions
living achieved.and WLED8 had the
healthcare best
have CRI value
improved (82.9) and
considerably
a color temperature of 2187 K. WLED11 had the lowest CCT
in many regions of the world. On the other hand, new emerging factors, such value (2080 K) and a CRI of
as air
81.0.
pollution, seriously endanger our health. In this article, we consider and draw attention to
potentially harmful agents such as light. How can light become a risk factor for human
4. Conclusions
beings? We are certainly not talking about natural light provided by the Sun, which
livingInbeings
recenthave become
decades, accustomed
overall to after years
living conditions of evolution.
and healthcare Instead,
have improvedwe refer to the
consider-
potentially
ably in many harmful
regionseffects
of theofworld.
exposureOn theto artificial
other hand, light at night
new (ALAN).
emerging ALAN
factors, suchdiffers
as air
from sunlight
pollution, in several
seriously important
endanger our ways:
health.the
In time and duration
this article, of exposure,
we consider and drawintensity, and
attention
the spectral distribution
to potentially or range
harmful agents suchofaswavelengths
light. How can of emitted light. a risk factor for human
light become
We are
beings? We currently searching
are certainly for lighting
not talking deviceslight
about natural that provided
provide lighting efficiency
by the Sun, whichand are
living
energy-saving.
beings have become Simultaneously,
accustomedwe to should consider
after years the biological
of evolution. Instead,effects of these
we refer lights.
to the po-
The light harmful
tentially emitted effects
by theseof luminaires
exposure tocontains
artificial alight
largeat fraction of blueALAN
night (ALAN). light despite
differs being
from
perceived as white. This blue light interferes with our circadian rhythm during the night,
causing misalignment and depriving us of restful sleep. To overcome this problem, we
developed an LED luminaire that emits high-quality white light with hardly any blue
component. The color temperature is 2080 K, which corroborates that the emitted light is
warm with very low blue radiation. This luminaire also has a high CRI (81.0), guaranteeing
good visual perception. Furthermore, it is manufactured from sustainable organic materials
that respect the environment. Contrary to many rare-earth-based LED luminaires, it is
free of geopolitical tensions. To the best of our knowledge, this is the first luminaire to
simultaneously comply with these characteristics, care for human health and the planet.
Author Contributions: Conceptualization, A.M.-V.; investigation, A.M.-V., D.M. and A.B.G.-D.;
writing—original draft preparation, A.M.-V.; writing—review and editing, A.M.-V., D.M. and
A.B.G.-D. All authors have read and agreed to the published version of the manuscript.
Funding: This research and the APC were funded by Consejería de Ciencia, Innovación y
Universidad—Gobierno del Principado de Asturias through the 2018–2022 Science, Technology
and Innovation Plan and by the European Union through the European Regional Development Fund
(ERDF). Grant number AYUD/2021/57246.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data supporting the findings of this study are available within
the article.
Conflicts of Interest: The authors declare no conflict of interest.Int. J. Environ. Res. Public Health 2022, 19, 1849 11 of 11
References
1. Grego, P.; Mannion, D. Galileo and 400 Years of Telescopic Astronomy; Springer: New York, NY, USA, 2010; ISBN 978-1-4419-5570-8.
2. Burch, D. Celestial Navigation: A Complete Home Study Course, 2nd ed.; ISBN 9780914025467. Available online: https://www.
starpath.com/catalog/books/1887.htm (accessed on 20 December 2021).
3. Dickinson, T.; Dyer, A.; Seager, S. The Backyard Astronomer’s Guide; Firefly Books: Buffalo, NY, USA, 2021; ISBN 978-0-228-10327-1.
4. Saini, R.; Jaskolski, M.; Davis, S.J. Circadian Oscillator Proteins across the Kingdoms of Life: Structural Aspects. BMC Biol. 2019,
17, 13. [CrossRef] [PubMed]
5. Nighttime Light Pollution Covers Nearly 80% of the Globe. Available online: https://www.science.org/content/article/
nighttime-light-pollution-covers-nearly-80-globe (accessed on 23 December 2021).
6. Irwin, A. The Dark Side of Light: How Artificial Lighting Is Harming the Natural World. Nature 2018, 553, 268–270. [CrossRef]
[PubMed]
7. Roenneberg, T.; Kumar, C.J.; Merrow, M. The Human Circadian Clock Entrains to Sun Time. Curr. Biol. 2007, 17, R44–R45.
[CrossRef] [PubMed]
8. Erren, T.C.; Reiter, R.J. Defining Chronodisruption. J. Pineal Res. 2009, 46, 245–247. [CrossRef]
9. Erren, T.C.; Reiter, R.J. Light Hygiene: Time to Make Preventive Use of Insights—Old and New—into the Nexus of the Drug
Light, Melatonin, Clocks, Chronodisruption and Public Health. Med. Hypotheses 2009, 73, 537–541. [CrossRef]
10. Posadzki, P.P.; Bajpai, R.; Kyaw, B.M.; Roberts, N.J.; Brzezinski, A.; Christopoulos, G.I.; Divakar, U.; Bajpai, S.; Soljak, M.;
Dunleavy, G.; et al. Melatonin and Health: An Umbrella Review of Health Outcomes and Biological Mechanisms of Action. BMC
Med. 2018, 16, 18. [CrossRef]
11. Shneider, A.; Kudriavtsev, A.; Vakhrusheva, A. Can Melatonin Reduce the Severity of COVID-19 Pandemic? Int. Rev. Immunol.
2020, 39, 153–162. [CrossRef]
12. Mure, L.S. Intrinsically Photosensitive Retinal Ganglion Cells of the Human Retina. Front. Neurol. 2021, 12, 300. [CrossRef]
13. Schroeder, M.M.; Harrison, K.R.; Jaeckel, E.R.; Berger, H.N.; Zhao, X.; Flannery, M.P.; St Pierre, E.C.; Pateqi, N.; Jachimska, A.;
Chervenak, A.P.; et al. The Roles of Rods, Cones, and Melanopsin in Photoresponses of M4 Intrinsically Photosensitive Retinal
Ganglion Cells (IpRGCs) and Optokinetic Visual Behavior. Front. Cell Neurosci. 2018, 12, 203. [CrossRef]
14. West, K.E.; Jablonski, M.R.; Warfield, B.; Cecil, K.S.; James, M.; Ayers, M.A.; Maida, J.; Bowen, C.; Sliney, D.H.; Rollag, M.D.; et al.
Blue Light from Light-Emitting Diodes Elicits a Dose-Dependent Suppression of Melatonin in Humans. J. Appl. Physiol. 2011, 110,
619–626. [CrossRef]
15. Lee, H.-Y.; Le, P.; Nguyen, D. Selecting a Suitable Remote Phosphor Configuration for Improving Color Quality of White Led. J.
Adv. Eng. Comput. 2019, 3, 503. [CrossRef]
16. The Nobel Prize in Physics. 2014. Available online: https://www.nobelprize.org/prizes/physics/2014/nakamura/facts/
(accessed on 24 December 2021).
17. Nakamura, S. Background Story of the Invention of Efficient Blue InGaN Light Emitting Diodes (Nobel Lecture). Ann. Phys. 2015,
527, 335–349. [CrossRef]
18. Erdem, T.; Demir, H.V. Color Science of Nanocrystal Quantum Dots for Lighting and Displays. Nanophotonics 2013, 2, 57–81.
[CrossRef]
19. Yu, B.; Huang, Z.; Fang, D.; Yu, S.; Fu, T.; Tang, Y.; Li, Z. Biomimetic Porous Fluoropolymer Films with Brilliant Whiteness by
Using Polymerization-Induced Phase Separation. Adv. Mater. Interfaces 2022, 9, 2101485. [CrossRef]
20. Wang, S.; Kershaw, S.V.; Rogach, A.L. Bright and Stable Dion-Jacobson Tin Bromide Perovskite Microcrystals Realized by Primary
Alcohol Dopants. Chem. Mater. 2021, 33, 5413–5421. [CrossRef]
21. Dey, A.; Ye, J.; De, A.; Debroye, E.; Ha, S.K.; Bladt, E.; Kshirsagar, A.S.; Wang, Z.; Yin, J.; Wang, Y.; et al. State of the Art and
Prospects for Halide Perovskite Nanocrystals. ACS Nano 2021, 15, 10775–10981. [CrossRef] [PubMed]
22. Nyalosaso, J.L.; Boonsin, R.; Vialat, P.; Boyer, D.; Chadeyron, G.; Mahiou, R.; Leroux, F. Towards Rare-Earth-Free White Light-
Emitting Diode Devices Based on the Combination of Dicyanomethylene and Pyranine as Organic Dyes Supported on Zinc
Single-Layered Hydroxide. Beilstein J. Nanotechnol. 2019, 10, 760–770. [CrossRef]
23. Menéndez-Velázquez, A.; Mulder, C.L.; Thompson, N.J.; Andrew, T.L.; Reusswig, P.D.; Rotschild, C.; Baldo, M.A. Light-Recycling
within Electronic Displays Using Deep Red and near Infrared Photoluminescent Polarizers. Energy Environ. Sci. 2013, 6, 72–75.
[CrossRef]
24. Mulder, C.L.; Reusswig, P.D.; Velázquez, A.M.; Kim, H.; Rotschild, C.; Baldo, M.A. Dye Alignment in Luminescent Solar
Concentrators: I. Vertical Alignment for Improved Waveguide Coupling. Opt. Express 2010, 18, A79–A90. [CrossRef]
25. Delgado-Sanchez, J.M.; Lillo-Bravo, I.; Menéndez-Velázquez, A. Enhanced Luminescent Solar Concentrator Efficiency by Foster
Resonance Energy Transfer in a Tunable Six-Dye Absorber. Int. J. Energy Res. 2021, 45, 11294–11304. [CrossRef]
26. Sánchez-Lanuza, M.B.; Menéndez-Velázquez, A.; Peñas-Sanjuan, A.; Navas-Martos, F.J.; Lillo-Bravo, I.; Delgado-Sánchez, J.M.
Advanced Photonic Thin Films for Solar Irradiation Tuneability Oriented to Greenhouse Applications. Materials 2021, 14, 2357.
[CrossRef] [PubMed]
27. Menéndez-Velázquez, A.; Núñez-Álvarez, C.; del Olmo-Aguado, S.; Merayo-Lloves, J.; Fernández-Vega, A.; Osborne, N.N.
Potential Application of Photoluminescent Filters for Use in Ophthalmology. Opt. Mater. 2018, 86, 505–511. [CrossRef]You can also read
