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、

4000K、4500K、 5000K、5700K、6500K. 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

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

(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

/nm. The unit of total

irradiance is W/m

, 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

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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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