Precision Lighting of LED Array using an Individually Adjustable
Color Temperature and Luminous Flux Technology
Min-Wei Hung, Wen-Ning Chuang, Cheng-Ru Li, Kuo-Cheng Huang and Yu-Hsuan Lin
Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu, Taiwan
Keywords: White Light Led, Color Temperature, Pulse Width Modulation.
Abstract: This paper presents a novel white light LED array equipped with a 3-pulse-width-modulation (PWM)
control module that could separately adjust either the color temperature or luminous flux without influence
on the other. An optical measurement system was set up to provide the complete information of color
temperature, illuminance and spectrum of the LED array for analysis. The radiometry quantity of spectral
data was converted into photometry quantity. The average percent deviations of color temperatures with a
fixed illuminance, and illuminances with a fixed color temperature were estimated, respectively. Comparing
with the existing commercial products, the developed LED array has better adjustability, stability and
precision. The proposed innovations represent a novel solution for white light LED lighting technology and
related applications.
1 INTRODUCTION
Light emitting diode (LED) is a kind of semicon-
ductor light source, which emits light when a driving
voltage is applied. The optical properties of LED
light are monochromatic, non-coherent, non-polari-
zed and divergent (Nadarajah and Yimin, 2005; Ye,
et al., 2010). In recent years, LED industry becomes
increasingly popular because of energy lack and
carbon emission problems. Due to the advantage of
long lifetime, low power consumption, good
luminous efficiency, faster switching and small size
of LED, it has been regarded within the scope of
green lighting. Nowadays, LED has been widely
used in light bulb, car headlight, photography lights,
billboards and versatile environmental lighting
applications. In order to increase the brightness of
LED lighting for practical usage, the LED chips are
usually arranged as an array device. In the field of
photography, white light LEDS are often used in
supplementary light of portrait photography.
Conventional bulb-type lamps might be replaced
because of LED light’s lightweight and portability
characteristics. Color temperature is a characteristic
of white light, which could be described by the
temperature of an ideal blackbody radiator that
radiates light. Traditionally, the color temperature of
LED photography lights is variable by simply
changing the color filters. However, the filter types
are limited and spectral discontinuous. Because the
kinds of ambient light are various and the human eye
is very sensitive to the wavelength and brightness of
lights, white balance in continuous regime is hard to
be achieved by using the color filters.
To overcome the problem described above, a
LED light source that can spectral-continuously
compensate the white balance should be developed.
However, multiple pieces of the same white light
LED chips assembled in a lamp can only increase
the total brightness of light, but cannot change the
original spectral characteristics. Fortunately, the
commercial white light LED product is composed of
the blue light LED and yellow phosphor. According
to the demand of application, the phosphor ratio in
LED is adjustable. It means that the white light LED
products with various color temperature are
individually manufacturable (Ingo and Marc, 2006,
(Sheu, et al., 2003, Yoshi, 2005, Guoxing and
Huafeng, 2011, Guoxing and Lihong, 2010, Jung-
Chieh and Chun-Lin, 2009, Elodie, et al., 2009).
Typical white light LED products have several types
of color temperature: 2700K 3000K 3500K
4000K4500K 5000K5700K6500K. Color
temperatures over 5000 K are called cool white, and
color temperatures lower than 3000K are called
warm white. With these color temperatures, the
optical lighting with spectral-continuous color
temperature could be achieved by mixing the various
Hung M., Chuang W., Li C., Huang K. and Lin Y.
Precision Lighting of LED Array using an Individually Adjustable Color Temperature and Luminous Flux Technology.
DOI: 10.5220/0006112001830190
In Proceedings of the 5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2017), pages 183-190
ISBN: 978-989-758-223-3
Copyright
c
2017 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
183
LEDs. The contribution of each kinds of LED could
be individually controlled by their own driver
circuit. Actually, some of the products developed by
the above-mentioned method have been successfully
applied to the household lighting. However, the
stability and precision of the color temperature and
brightness are not good. Although these products
can be effectively applied to the field of photogra-
phy and home lighting, they cannot be applied to the
field of spectral optics.
In this paper, a white light LED array that can be
separately adjusted color temperature and luminous
flux was developed. The device is composed of two
kinds of LED chips and a corresponding driving
circuit. The color temperatures of two kinds of LED
chips are 2800K and 4900K, respectively. The
driving circuit using 3-PWM modules to interacti-
vely control two kinds of LEDs. The resulted
complex frequency of PWM has much shorter
trigger time. Therefore, the deviations of color
temperature at low duty cycle could be significantly
reduced. The continuously-adjustable color tempera-
ture range of LED array is from 2800K to 4900K.
An optical system was also setup to measure the
illuminance and optical spectrum of the LED array
simultaneously. The measured spectral information
with various color temperature and illuminance
could be used as a basis for qualifying the
independent-adjustability of LED array. By using
the developed driving method and calibration
process, the deviation of color temperature and
illuminance could be rapidly minimized. This
technology provides a solution for precise optical
lighting and related spectral applications.
2 EXPERIMENTAL SETUP AND
WORKING PRINCIPLE
The experimental setup for color temperature,
illuminance and spectrum measurement of the
developed white light LED array is shown in Fig.1.
The LED array is propped up by a metal plate and
lighting down from top through a circle hole. The
light beam is divergent and diffused by an optical
diffuser, which has a thickness of 0.15 mm. The
photographs of the front and rear side of the LED
array are shown in the embedded pictures of Fig. 1.
The LED array contains 12 × 8 pieces of white light
LED chips. The 2800K and 4900K LEDs are equally
divided and interlaced arranged. By adjusting the
light contributions of two kinds of LED chips, the
lighting with spectral-continuous color temperature
could be achieved. The product models of the LED
chips are SMD 5050 of 2800K and 4900K. The
color temperature and luminous flux of the LED
array could be manually or automatically controlled
by the circuit board. Because the driving circuit
using three PWM modules to interactively control
two kinds of LED chips, the frequency of the PWMs
have been mixed and generate a much shorter trigger
time. Therefore, the adjustability, stability and
precision of color temperature and luminous flux
would much better than the traditional driving circuit
that has only two PWM control modules
(Prathyusha, 2004) (Montu and Regan, 2007)
(David, et al., 2012). The divergent white light was
partly collected by a chroma meter and a
spectrometer. The instrument model of the chroma
meter and spectrometer are CL-200A of Konica
Minolta and SD1200 of OTO respectively. The
receptor head of the chroma meter and integrating
sphere of the spectrometer are both located near the
center of the LED array in x-axis. The detection
distance in z-axis between the LED array and
sensing heads is about 60 mm.
Figure 1: Experimental setup of the measurement system.
The working principle of three PWM is shown in
Fig. 2. Traditionally, the color temperature of the
LEDs can be arbitrarily regulated by means of two
PWM controllers. As shown in Fig.2 (a) and 2(b),
the PWM-1 and PWM-2 controlled the duty cycles
of the 4900K and 2800K LEDs, respectively. The
expected color temperature of the LED could be
achieved by linearly combining the products of duty
cycle and color temperature of two kinds of LEDs.
However, This method has a drawback. The
minimum duty cycle of PWM cannot be lower than
the operation threshold of LED microcontroller. It
means that the resolution for LED brightness
adjustment will be limited. In other words, the color
temperature and brightness will be roughly tied to
PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology
184
each other. The solution of the problem is mixing a
low-frequency PWM-3 with origin PWMs. The
figure 2 (c) shows the spectrum of the PWM-3. The
frequency is only 1kHz, which is one twentieth of
PWM-1 and PWM-2. The figure 2 (d) and 2 (e)
show that the new waveforms were accordingly
generated by PWMs-mixing. As a result, the
frequency of PWM becomes smaller and complex,
which leads to the high-adjustability of the duty
cycle. For a fixed luminance condition, the total
pulse width per unit duty cycle is constant.
Therefore, time of the single PWM become longer
that could be effectively processed by the
microcontroller. It means that the brightness and
color temperature of the LEDs could be controlled in
a more precise and flexible way.
Figure 2: Working principle of the 3-pulse-width-
modulation (PWM) control module.
In order to verify the efficacy of our developed
LED array, two commercial LED products were
selected for conducting the quality comparison. The
item models of them are LL-162VT of Viltrox and
PW-96LED of Paniko, respectively. Both of them
are mixed LED array and designed for the
photography application. In the measurement
experiment, the chroma meter was first used to
roughly measure the stability and adjustability of our
developed LED array and these two typical
products. Figure 3 shows the relationship between
the measured color temperature and illuminance of
the LL-162VT LED. The main feature of the LL-
162VT is that the color temperature is non-
adjustable and the luminous flux is adjustable.
However, the measured result shows that the LED
color temperature gradually increases with
increasing illuminance. Although the labelled color
temperature of this product is 5600K (Dashed blue
line), all the measured data (Red curve) are less than
this value in practice. The maximal percent devia-
tion ratio of the color temperature is 3.13%. We
believe that the result is good enough for the
photography application because it is hard to distin-
guish such a small difference by the human eye.
Figure 3: Deviations of the color temperature of a typical
luminous-flux-adjustable product (LL-162VT).
Figure 4: Deviations of the illuminance of a typical color
temperature-adjustable product (PW-96LED).
Figure 4 shows the relationship between the
measured color temperature and illuminance of the
PW-96LED LED. The main feature of the LL-
162VT is that the color temperature is adjustable and
the luminous flux is non-adjustable. The estimated
illuminance of this product is 7000 lux (Dashed
green line), which is calculated by the labelled
luminous flux. The measured result shows that the
illuminance is much weaker than the labelled value
when the color temperature is lower than 3614K.
The illuminance becomes more stable when the
color temperature is larger than 5000K. The
maximal percent deviation ratio of the illuminance is
12.59%. It could be easily found that these two
existing products have similar shortcomings: The
Precision Lighting of LED Array using an Individually Adjustable Color Temperature and Luminous Flux Technology
185
changed illuminance always leads the change of
color temperature, vice versa.
Figure 5: The photopic weighting function over the white
light LED’s spectral range.
In this study, the white light LED array using the
developed 3-PWM control technology can overcome
the problem described above, that is, either the
adjustments of parameter will not obviously affect
the other. The measured results of our LED device
will numerically show in the next chapter.
Furthermore, although the commercial chroma meter
could quantify the color temperature and
illuminance of LED array, the precision is much
poorer than an optical spectrometer (or power
meter). Accordingly, an optical spectrometer was
subsequently used to finely measure the spectral
distribution of our developed LED array. The
purpose of this measurement is to know the power
contribution at each wavelength of light. However,
the quantity standard is different between the
chroma meter and spectrometer. The measured unit
of chroma meter is lux (illuminance), which belongs
to photometric quantity. The measured unit of
spectrometer is W/m
2
(irradiance), which belongs to
radiometric quantity. The relationship between them
can be written as follow:
𝐸
𝑣
= 683
𝐸(𝜆) ∙ 𝑉
(
𝜆
)
𝑑𝜆 (1)
where 𝐸
(
𝜆
)
is the irradiance, which could be
measured by the spectrometer. The unit of irradiance
at each wavelength is W/m
2
/nm. The unit of total
irradiance is W/m
2
, which could be obtained by
integrating the entire spectrum; 𝐸
𝑣
is the illuminance,
which could be measured by the chroma meter. The
operator of the 683
𝑉
(
𝜆
)
𝑑𝜆 is photopic weighting
function of human eye, as shown in Fig 5. The
equation is obviously shows that the relationship
between irradiance and illuminance. Therefore, the
measured LED radiometric data could easily convert
to the illuminance data. It means that the
replacement of measurement instrument does not
affect the result for the comparison.
Figure 6: Deviations of the color temperature with
increasing illuminance of the developed LED array.
Figure 7: Enlarged view of the color temperature curves in
Fig.6.
3 RESULTS AND DISCUSSION
The developed LED array using the 3PWM control
technology could separately adjust the color
temperature and luminous flux without influence on
the other one. There are four kinds of color
temperature, 3000k, 3500K, 4000K and 4500K,
were created by mixing and adjusting the light
contributions of 2800K and 4900K LEDs. In fact,
the color temperature is continuously adjustable
from 2800 to 4900K. Figure 6 shows the relationship
between four color temperatures and illuminances of
the developed LED array. It could be found that
there is no significant change of the color
temperature whatever the illuminance is. In fact, the
changes are very slight and almost negligible. Figure
7 shows the enlarged picture for clearly discerning
PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology
186
the change of each color temperature. A dotted grey
line marked the setting value of each color
temperature. Although the curves look dramatic
changing, the amplitudes are very small. The
average percent deviations of the color temperatures
are only 0.15 % (3000K), 0.28 % (3500K), 0.2 %
(4000K) and 0.17% (4500K), respectively. The
result indicates that the stability of our
developed LED array hardly deteriorate with the
increasing color temperature. In other words, a
luminous-flux-adjustable LED device with a
constant color temperature was successfully
achieved.
Figure 8: Measured spectra of the LED array with various
illuminances at color temperature of 3000K.
Figure 9: Relationship between relative total irradiance
and illuminance at various color temperatures.
To understand the contribution of each
wavelength, the optical spectrometer was used for
the measurement. The color temperature is fixed at
3000K. The spectrum curves were captured at
various illuminance of LED array. The measured
result is shown in Fig.8. Because the white light
LED is composed of blue light LED and yellow
phosphor, the spectra have two obvious peaks. The
axis of ordinate indicates the relative irradiance.
Qualitatively, it is reasonable that the spectral curve
become taller with the increase of LED illuminance.
Quantitatively, the relative total irradiance could be
obtained by integrating the curves. The optical
spectra were individually measured at fixed color
temperature of 3500K, 4000K and 4500K. The
relationship between the estimated relative total
irradiance and adjusted illuminance at various color
temperature is shown in Fig. 9. It indicates that there
is a proportional relationship between them.
However, the slopes are slightly different. The
reason is that the two parameters belong to different
physical quantity. According to the equation (1), the
relative irradiance should be weighted by the
photopic function and integrated over the white light
LED’s spectral range. Then the calculated results
would be comparable with the illuminance. Because
the low color temperature light emitted by the LED
array has more power contributions in red and
yellow regime, the value of the integrated spectrum
curve weighted by the photopic function would
become larger. It means that the variation of
illuminance would slightly larger than that of the
relative total irradiance. Therefore, the slope would
be relative smaller at lower color temperature.
Figure 10: Measured spectra of the LED array with
various color temperature at fixed illuminance of 5000
lux.
On the other hand, the developed LED array
should also have the color-temperature-adjustable
characteristics without influence on the luminous
flux. This time, the luminous flux of LED array is
fixed and the spectrum curves were captured at
various color temperature. Although we had in fact
measured seven sets of spectra from 2000 to 14000
lux, only the spectra at 5000 and 8000 lux are shown
in this paper as the representatives. Figure 10 and 11
Precision Lighting of LED Array using an Individually Adjustable Color Temperature and Luminous Flux Technology
187
shows the measured spectra of the LED array with
various color temperature at fixed illuminance of
5000 and 8000 lux, respectively. The spectral
distributions and relations are similar to each other.
Both figures indicate that lower color temperature
light emitted by the LED array has more power
contributions in red and yellow regime, and fewer
contributions in blue regime. If the developed LED
array has the advantage of stable luminous flux
whatever the color temperatures, the values
calculated by integrating the spectrum curves
weighted by the photopic function must be nearly
equal to each other. To demonstrate this
characteristic, the spectrum curves in Fig. 10 and
Fig. 11 were first weighted by the photopic function
(Fig. 5). The corresponding results are shown in Fig.
12 and Fig. 13, respectively. Due to the unit
converting, the axis of ordinate becomes relative
illuminance from relative irradiance. It could be
found that the contributions of LEDs in blue regime
have been substantially weakened because the
human eye is less sensitive to blue light. The larger
contributions are generated from the region of
wavelength between 520 and 620 nm. Then, the
values of relative total illuminance could be obtained
by integrating the curves in Fig. 12 and 13. The
percent deviation of total illuminance of each value
could be subsequently estimated. The tables
embedded in Fig. 12 and 13 shows the accordingly
results. When the reference is defined as the relative
total illuminance value at 3000K, the maximal
percent deviations are less than 1.35% at 5000 lux
and 7.6% at 8000 lux, respectively. The results
indicate that the change of color temperature affect
slightly the illuminance. Therefore, the stability of
the luminous flux of the color-temperature-
adjustable LED array is numerically better than that
of the existing products.
Figure 11: Measured spectra of the LED array with
various color temperature at fixed illuminance of 8000
lux.
Figure 12: Spectrum curves weighted by photopic function
with various color temperature at fixed illuminance of
5000 lux.
Figure 13: Spectrum curves weighted by photopic function
with various color temperature at fixed illuminance of
8000 lux.
Figure 14 shows the suggested classification of
LED array, calibration of measurement system and
possible applications. Because the operation of
chroma meter is simple, rapid and high compatible
with environments, the color temperatures and
illuminances of the tuneable white light LED array
could be first measured. In this step, the poor-
stability ones would be classified and applied to the
daily lighting because the general people's tolerance
for color temperature variety is relatively high.
Optical spectrometer has much better precision than
the chroma meter, and could subsequently provide
the power contribution at each wavelength. The
better-stability LED array could be applied to the
professional visual domain, such as photography,
display manufacturing and color management.
Finally, the best-stability LED array qualified
strictly by the spectrometer could be applied to the
research domain, especially in spectral optics,
PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology
188
sample excitation, photochemical reactions and bio-
optical response. Furthermore, the spectrometer
could calibrate the optical source, even the LED
array equipped with various optical filters, such as
bandpass filter, polarizer and attenuator. The
radiometric and photometric quantities could be
freely converted by using the Eq. (1). After
performing the calibration, the LED array driven by
3-PWM control modules would provide the best
adjustability, precision and stability.
Figure 14: Suggested procedure for the LED classification
and optical system calibration.
4 CONCLUSIONS
This study succeeded in developing a 3-PWM
control module that can separately adjust the color
temperature and luminous flux of a white light LED
array. The breakthrough is that either the
adjustments of parameter will not obviously affect
the other one. The color temperature of the LED
array is continuously adjustable by mixing and
adjusting the light contributions of 2800K and
4900K LED chips. An optical measurement system
composed of a chroma meter and optical
spectrometer was set up. The measured results show
that the average percent deviations of the color
temperatures are at least smaller than 0.28%, in spite
of the illuminance of LED array. With the
converting of radiometry and photometry quantity,
the measured integrated spectra weighted by
photopic function can be treated as a reference of
illuminance. For the adjustment of color temperature
with a fixed illuminance, the percent deviations of
the illuminance are between 1.35% and 7.6%.
Therefore, the developed LED array equipped with
the 3-PWM control module is numerically better
than that of the existing commercial products. The
LED driving circuit concept, measurement
procedure and analysis method are compatible with
various kinds of white light LED. We believe that
this study provides a new solution and applications
for white light LED lighting technology.
ACKNOWLEDGEMENTS
The authors would like to express their appreciation
for financial aid from the Ministry of Science and
Technology, R.O.C under grant numbers 104-2622-
B-492-001-CC3. The authors would also like to
express their gratitude to the Instrument Technology
Research Center of National Applied Research
Laboratories for the support.
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