color stability feedback schemes.pdf

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Achieving color point stability in RGB multi-chip LED modules

using various color control loops

P. Deurenberg*, C. Hoelen, J. van Meurs, J. Ansems

Central Development Lighting, Philips Lighting, Eindhoven, the Netherlands

ABSTRACT

The continuing research effort in high power LEDs will allow their use in high quality lighting systems in the (near)future. There are still a number of issues to tackle, for instance the LEDs (strong) temperature dependence. This

dependence will change the emitted flux and the spectral distribution of the LED. In addition, these parameters will also

change as the LED ages. When creating white light by mixing red, green and blue LEDs, the temperature effects

described above will already result in a visible color difference after a small rise in temperature.

To overcome this issue, a number of LED color control loops have been developed. These loops can be based on: theheat sink temperature, flux measurements of each primary color, a combination of these last two and an integrated color

point. For this purpose, an RGB test set up has been built, equipped with a temperature sensor and various photo-sensors.

The appropriate color control loops have been implemented and tested in software. Some control loops use empiricallydetermined LED parameters (d/dT or T0), the value of these parameters has been determined for a different set of LEDs.In addition, initial optical LED (and sensor) calibration has been performed at a single temperature only.

The color stability of the various color control loops has been measured for a temperature increase of about 50degrees Centigrade. In this range, we find that, on short term, all color control loops show a significant improvement in

the color error, except for the color control loop based on flux measurements of each primary color, which performs

nearly as mediocre as open loop. However, the color control loop based on the heat sink temperature cannot offer color

stability when the LED ages, which is expected to be significant. The color control loop based on an integrated color

point seems the most expensive one.

Keywords: LED, RGB, color control, color feedback, color feed forward

1.INTRODUCTION

LEDs for lighting have great future potential. Their luminous efficacy has already reached the luminous efficacy of thecompact-fluorescent lamps. Apart from the advantages with respect to efficiency and lifetime, using LEDs in general

lighting also provides the opportunity to instantaneously change color and dimming level.Two approaches can currently be discerned to create white LED light: 1) by using phosphor based white LEDs, and

2) by using multi-color LEDs (e.g. by additive mixing of red, green and blue)2. An advantage of the second solution is a

theoretically higher efficiency, however, this option still has many challenges, especially with respect to color stability:LEDs have the unfortunate property that their output varies with their temperature. Not only does the flux output change,

but also their peak wavelength of emission increases for rising temperatures. In addition, for option 2, the non-constant

maintenance causes color point shifts during ageing. An RGB LED system without compensation for any of these effectswould already produce clearly visible color differences after a temperature rise of about 20 C. The CIE1976 uniform

color space (UCS)5 is usually used to analyze just-noticeable-color differences. In this color space, color differences,

between two color points (u1, v1) and (u2, v2), can be calculated by equation (1).

( ) ( )2'

2

'

1

2'

2

'

1'' vvuuvu += (1)

The maximum allowed color deviation (or target color deviation) depends strongly on the application. In this research,the target is set at uv=0.010, because lamps are usually specified within 10 points in the CIE 1931 color space

5and

because this target also coincides with display specifications. However, a color difference ofuv=0.0035 is assumed as

a just noticeable difference5.

*[email protected]; phone +31 40 27 57990; fax +31 40 27 56693

Fifth International Conference on Solid State Lighting, edited by Ian T. Ferguson,John C. Carrano, Tsunemasa Taguchi, Ian E. Ashdown, Proc. of SPIE Vol. 5941

(SPIE, Bellingham, WA, 2005) 0277-786X/05/$15 doi: 10.1117/12.623020

Proc. of SPIE Vol. 5941 59410C-1


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Unlike earlier publications2, 3, 4discussing a single color control loop, this paper gives an overview of four possible

LED color control loops to counter the effects of temperature (and ageing). These loops are described and tested on anexperimental set up (with an LED source consisting of two red, six green and two blue Lumileds Luxeon emitters 7);

measurement results are presented and discussed. This overview can be used as a basis for selecting a suitable color

control loop. The actual choice depends on the application and its requirements.

2.MEASUREMENT OPTIONSThe color control loops introduced here, strongly relate to the optical variations that can be expected for LEDs whentheir temperature changes. As can be found in literature6, 7, a rising LED junction temperature causes the flux output to

decline (e.g. -1d/dT = -0.2 K-1), the peak wavelength to increase (e.g. d/dT = 0.10 nm/K for a red LED) and the

forward voltage to drop (dVF/dT -2 mV/K). If also the LEDs forward current is varied, the flux output (d/dI > 0) and

the peak wavelength (d/dI < 0 nm/K) change. To simplify LED color control loops, the LEDs are operated using aPWM (pulse-width-modulated) forward current, driving them at a constant current and changing their on-time (duty

cycle) to vary the average flux.

Keeping these temperature dependent optical variations of LEDs in mind (e.g. d/dT or -1d/dT), the color control

loops below can maintain a constant color point up to a certain degree. For each color control loop, a color error

prediction is formulated, based on an RGB LED system at a junction temperature of 60 C and 3500 Kelvin on the blackbody line, gathered from literature2. As will be shown in subsequent sections, not all measurement possibilities will offer

an equally stable color point.

2.1Temperature measurement (TFF)

Most of the LED output variations are caused by a change in junction temperature. Therefore, temperature measurements

form a reasonable basis for a compensation scheme. Unfortunately, it is not practical to directly measure the junctiontemperature and for this reason, an indirect measurement of the junction temperature is made through the heat sink

temperature. As it is not the junction temperature that is controlled, this color control loop is called temperature feed

forward(abbreviated as TFF).

The flux output and peak wavelength of an LED both change as a function of temperature. Starting from a correct

color point at an initial temperature and a suitable model with accurate prediction of the LED output characteristics, thiscolor point can be maintained. Unfortunately, these dependencies are not precisely known and may vary over batches

and manufacturers. This can result in significant color errors. In addition, this scheme cannot correct for maintenance

issues. Given the variability in LED ageing, a simple counter cannot yet adequately address this issue. Simulationsindicate that long-term color error uv can be much larger than 0.0052.

2.2Flux measurement (FFB)

A single photodiode can be used to obtain the LED flux of each color component. The controller can subsequently

maintain the preset flux to preserve the color point. This measurement scheme will be able to correct for flux variations

due to temperature and ageing. Unfortunately, it cannot correct for peak wavelength shifts caused by temperature

changes. This can already result in a color point deviation uv > 0.005 for temperature changes of T=20 C2. This

color control loop is called flux feedback (abbreviated asFFB).

2.3Flux and temperature measurement (FFB&TFF)

Combining the last two measurements, thus flux feedback combined with temperature feed forward (abbreviated as

FFB&TFF), can make a significant improvement, as the combination is able to determine all variations in LED output:flux changes due to temperature and ageing through the photodiodes, and wavelength changes through the temperature

sensor.However, it still relies on data describing the relation between wavelength shift and temperature (d/dT). Therefore,

it also suffers from uncertainties in this relation. The degree of uncertainty will determine the accuracy improvementover flux feedback only. However, the color error uv will be smaller than 0.01 for temperature changes up to 50 C2.

2.4Color coordinates measurements (CCFB)

An LED color control loop can also be achieved by directly feeding back the color coordinates of the mixed light using a

color sensor. Hence, this color control loop is called color coordinates feedback (abbreviated as CCFB). In this case, the

spectral response of the sensors must match the CIE 1931 2color matching functions5. The feedback signal would result

in X, Y and Z color values. In principle, the sensors could be photodiodes covered by an appropriate optical filter.



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As the system would directly control the white light, a high degree of color accuracy is possible. All above-

mentioned variations in LED output are measured and can thus be compensated for. Errors are mainly introduced bymismatches between the sensor sensitivity curves and the color matching functions. Simulations indicate that with

adequate sensors it should be possible to achieve a color accuracy uv of about 0.0062. Using color sensors that are

identical to the color matching functions, this can drop to a color differenceuv of about 0.0022.

3.CALIBRATION ISSUESOne of the key issues for all color control loops is the calibration of the unit. Calibration is needed for accurate colorsetting because the LED binning and sensor characteristics do not provide enough detail. However, it also should not be

required to determine all LED parameters for a range of temperatures. The calibration can be split into two parts: 1) the

optical characteristics of the LEDs (C-matrix), and 2) the optical characteristics of the (color) sensors used in CCFB (S-

matrix). With these matrices, target color points can be translated to LED output power and sensor set points.A target color point and light level with tristimulus values5XT, YTand ZTcan be expressed as:

( )[ ]

=

B

G

R

T

T

T

D

D

D

TC

Z

Y

X

(2)

where matrix C describes the CIE set points of the LEDs as a function of the duty cycle Difor LED color i. The C-matrix

contains the CIE 1931 tristimulus values for each LED color (Xi, Yiand Zi) on a column basis:

( )[ ]

=

BGR

BGR

BGR

ZZZ

YYY

XXX

TC

(3)

Unfortunately, this matrix is temperature dependent as the flux output and peak wavelength change as a function of

temperature. The inverse of the C-matrix, also called Calibration matrix, can be used to determine the required duty

cycles for a certain target color point. Similarly, the (target) sensor values SAT, SBTand SCTcan be expressed as:

( )[ ]

=

B

G

R

T

T

T

D

D

D

TS

SC

SB

SA

(4)

where the S-matrix contains the sensor output values (SAi,SBiand SCi) for each LED color (again on a column basis):

( )[ ]

=

333

222

111

SCSBSA

SCSBSA

SCSBSA

TS

(5)

This matrix is also temperature dependent, although for small sensor temperature changes this can neglected. The S-

matrix should be mostly diagonal, determined by the degree of coupling between sensors and LEDs. When using a singlephotodiode to determine the flux output of each LED color, the sensor is time-multiplexed and therefore the S-matrix

already is diagonal by design.

As the CIE 1931 x, y chromaticity coordinates are the ratio of the tristimulus values, ( )ZYXXx ++= etc.5, asimilar (calibration) matrix can be used to calculate the (sensor) set points from these color coordinates, however, the

outputs need to be normalized with respect to the maximum (sensor) set point. An additional parameter LT(0 100 %)can be used to indicate the relative amount of light with respect to maximum possible light output at the chosen color

point. Flux output can either be set through a scaling factor for the (sensor) set points, or it can be implemented through

scaling the maximum flux output (being the sum of YR, YGand YB). The flow from user set points (xT, yT, LT) to duty

cycles (DR, DG, DB) and, depending on the used color control loop, (color) sensor set points would then be



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U S E R D O M A I N

C a l i b r a t i o n m a t r i x

U s e r

I I

I n t e r f a c e L - . 1 e 4 C I E x y , L - - >

9 O l E x y . L

L E D d u t y c y c l e s a t

_ _ _ _ _ _ L

: T _ T r 1 e r s n u u

A c T U A T O R D O M A I N

L i g h t i n g s y s t e m

. . I

( m d . d r i v e r )

'

d u t y c y c l e - - >

l i g h t o u t p u t

e x p

D C n o m , T , l f e ) D C n o m

e f

1

T

r e f

l f e %

m o d e l

a c t u a l s y s t e m

( )[ ] ( )[ ]

++

SC

SB

SA

D

D

D

Z

Y

X

YYY

y

x

TS

B

G

R

TC

T

T

T

BGR

T

T

T

1

Lapply

normalize

(6)

Note that there exist other, equally suitable, matrices to determine the same information.

4.COLOR CONTROL LOOPS

This section will discuss the different color control loops. Each loop tries to maintain the color point based on a

calibration matrix valid at a single junction temperature for each LED color. Depending on the characteristics of thecolor control loop, it uses certain sensor values and/or (empirical) LED parameters. These empirical LED parameters

must be determined for a different set of LEDs! These systems do notregulate the system temperature!

Figure 1: Block diagram for an open loop system

4.1Open loop (OL)

The basic building blocks, present in every color control

loop, are introduced by means of a system without

compensation. The block diagram can be found in figure 1.

Note that, without compensation, a rise in system

temperature will lead to a deviation in color point.The target color point is set by the user in the user

domain(User interface-block) and converted to actuator

domainby the Calibration matrix-block using an invertedC-matrix, equation (3). The Lighting system converts the

duty cycles to light. In all of the loops below, the LEDs,

PWM amplifier, optics and sensors (incl. possible ADconversion) are unified in the Lighting system-block.

4.2Temperature feed forward (TFF)

As mentioned earlier, the junction temperature of each LED cannot be measured directly. Therefore, these temperatures

are determined through the system (or heat sink) temperature and a thermal model of the system. The compensation can

then be implemented through an inverted LED model (model prediction control). As a model for the flux output, the

influence of temperature on an LED based unit can be split into two parts. In the first part, the light output () of each

LED color decreases as a function of temperature (if the reference temperature (T ref) is below present junctiontemperature Tj), for example:

( ) %exp%%,%,0

lifeT

TTDClifeTDC

refj

refj

= (7)

where DC% represents the duty cycle (0%-100%), T0 a characteristic LED parameter and life% describes the LED

maintenance. In the second part, a different temperature changes the LED spectrum (e.g. peak wavelengths shifts), which

results in a different color impression. This last influence will be compensated for by a change in set point through the

Calibration matrix-block (middle of figure 2 below). The change in flux output is dealt with through the outside loop,in which the nominal duty cycles from the Calibration matrix-block are increasedwith the same factor as the flux

output decreases or vice versa (see equation (7)):

(8)

If the Lighting system is well modeled by these functions, the actual system and the model in the above equation

exactly cancel and the flux output will remain constant for every temperature.



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u s e r

Figure 2: Block diagram for temperature feed forward system (the main feed forward loop is indicated by the thick lines)

Describing figure 2, the user domainset points are converted to actuator domain duty cycles for each LED color(Calibration matrix-block). This conversion now depends on the junction temperature of each LED as the peak

wavelength may have shifted (indicated by the arrows form the Calculate junction temperature for each LED color-

block to the Calibration matrix-block). Depending on the junction temperature of each LED, the decreased flux outputis compensated for by an appropriate multiplication factor supplied by the ref/ (Tj,i)-blocks. This results in adapted

duty cycles, which are filtered through the Rescale duty cycle-block (described in section 4.6). After a small delay

(indicated by the z-v-blocks), these values are fed into the Lighting system-block, which generates the light and the

heat sink temperature based on the applied duty cycles. Using a thermal model of the system, the LED junctiontemperatures can be derived from the dissipated power and the heat sink temperature. These are passed to the

ref/ (Tj,i)-blocks, which completes the loop.

Essentially, a quasi-static situation is assumed every time the duty cycle is changed. This assumption is valid when

the sample period of the feedback is much shorter than the thermal time constant of the lighting system. Comparing thissituation to a classic approach, the controller actually has proportional feedback with a variable gain.

4.3Flux feedback (FFB)

A system using optical feedback with a single optical sensor multiplexed over multiple LED colors is depicted in figure 4

below. With this approach, it is possible to detect and compensate for flux changes caused by ageing and junction

temperature changes. Unfortunately, wavelength shifts due to temperature changes, cannot be detected and will generate

a color error.

Figure 3: Implemented measurement approach featuring a

single measurement for each LED color and an additionalmeasurement to determine the additive background level

As said, a single sensor is used to determine the light

output of each LED color; this is achieved by time-

multiplexing the sensor over all LED colors. This means

that the instantaneous radiative power is determined of all

switched-on LED colors during each measurement. Thereis one measurement for each LED color and an additional

one to determine the additive background light componentfalling onto the sensor. This multiplexing requires a largesensor signal bandwidth, because the measurements are

performed in rapid succession, see figure 3. The

multiplexing and the inherent decoupling are indicated by

the Time multiplexer- and Color signal extractor-blocks, both in the top right corner of figure 4.



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w S ) L E D

b l L E O

C O L O R E D P A R T S A R E

A C T U A T O R D O M A I N

The main control loop (indicated by the thick lines), starts with a sensor set point for each LED color at a certain

reference temperature (Tref). The difference between set point target (S-matrix, equation (5)) and actual state is calculatedand passed to a Proportional-Integral-Differential-controller (PID), which determines the change in duty cycles. As the

measurements determine the light output amplitude, as a function of the applied forward LED current and junction

temperature, the measured signals no longer depend on the duty cycle. Therefore, in order to facilitate the feedback loopwith a duty cycle dependent sensor signal, the sensor values are multiplied with the previous iteration of the PID

controllers output (indicated by the z-1

-blocks). As such, the PID controller is designed to determine the relativeamount of power, which needs to be applied to maintain the flux amplitude at a desired level.

In principle, these values should now be transformed to the actuator domain; however, this is not necessary because

the sensor measurements are already completely decoupled by the chosen measurement method (also indicated by the

unity gain matrix I-block. To implement the color point chosen by the user, the PID outputs are multiplied with thenominal duty cycles for the chosen color point at the reference temperature provided by the Calibration Matrix-block

(C-matrix, equation (3)), thus providing corrected signals in the actuator domain. After passing these duty cycles throughthe Rescale duty cycle-block (described in section 4.6), they are delayed and passed to the Lighting system-block,

which generates the light and the sensor values.

Figure 4: Block diagram for flux feedback system (the main feedback loop is indicated by the thick lines)

4.4Flux feedback and temperature feed forward (FFB&TFF)

A system using optical feedback combined with a temperature feed forward utilizing a single optical sensor multiplexed

over multiple LED colors is depicted in figure 5 below. With this approach, it is possible to detect and compensate forflux decreases and maintenance issues through the optical sensor. Color point changes due to wavelength shifts (d/dT)

can be compensated for via feed forward through the temperature sensor. Assuming an adequate model of the

wavelength shifts is available, (very) accurate color stability should be possible.

The optical feedback loop is already described in the previous section (flux feedback) and will not be discussed againhere. In this case, the Lighting system-block also provides a heat sink temperature, from which the three junction

temperatures can be derived (bottom right block). These temperatures are passed to the Calibration matrix-block to

account for the peak wavelength shifts. Additionally, the flux references for the flux feedback loop need to be altered, as

the flux sensitivity of the photodiode is wavelength dependent. Note that if the temperature of the photodiode changes as

well, this sensitivity change needs to be accounted for as well (which may require an additional temperaturemeasurement in the Lighting system-block).



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0

b I L E D

E l i l l i i i

A C T U A T O R D O M O I D -

A C T U A T D O M A I N

Figure 5: Block diagram for flux feedback and temperature feed forward (the main feedback loop is indicated by the thick lines)

4.5Color coordinates feedback (CCFB)

The most general, but probably most expensive, approach to solve color accuracy issues is color coordinates feedback,

which controls the integrated color point of the mixed light. This approach measures the color properties of the mixed

light of all LED colors through separate, optically filtered, photodiodes. The relative output of the separate photodiodes

provides information regarding the change in color coordinates and light output of the mixed light. Changes due to

temperature rise, maintenance issues and even an LED failure can be detected and compensated for.

Figure 6: Block diagram for color coordinates feedback (the main feedback loop is indicated by the thick lines)

By integrating the sensor signal over multiple PWM periods, high frequency noise and turn-on/turn-off driver

behavior can be neglected, as all these signal disturbances are accounted for in the (integrated) measurement. However,the integration or filtering, introduces low frequency sensor dynamics. This makes it necessary to control in sensor

domainand reduces the MIMO (multi input, multi output) system to multiple SISO (single input, single output) systems.

Each SISO system can simply be controlled by a PID-controller, if the coupling between the different SISO systems is

not too big (indicated by a diagonally dominant S-matrix (5). Controlling in the sensor domain also implies converting



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the user inputs (e.g. in CIE 1931 2 x, y chromaticity coordinates5 and relative luminous output) to corresponding sensor

set points (arrows from the Calibration matrix to the Color sensor references-blocks).The block diagram can be found in figure 6. It shows that the user domainuser inputs are converted to the actuator

domainby the Calibration matrix-block. The set points for the feedback loop are converted from the duty cycles to the

sensor domain through equation (4). The sensor measurements are compared to the set point and the difference is passedto a PID controller. The resulting PID action (still in sensor domain) must then be converted to actuator domain(duty

cycles for the driver) through the decoupling matrix (the inverted sensor matrix, S-1

described in equation (5)),implemented by the Sensorduty cycle- block. After multiplication with the nominal duty cycles (for the chosen color

point at the reference temperature provided by the Calibration Matrix-block), the duty-cycles can be filtered through

the Rescale duty cycles-block (described in section 4.6). After a short delay, the duty cycles are converted to light and

corresponding sensor values by the Lighting system-block.

4.6Some general properties

The above-presented methods have at least three things in common. First, all color control loops explicitly result in

positive feedbackwith respect to the system temperature (this is required by the LED characteristics). This implies that

limits must be set to the power dissipation and measures should be taken to cope with unusual circumstances (e.g. a

considerably elevated ambient temperature). Second, the chosen block layout and by implementing the PID output as amultiplication factor, all color control loops show (nearly) open loopbehaviorwith respect to chosen color point and flux

level when looking from the user point of view through the system. Consequently, the color point and flux level of the

system can be changed very dynamically.

Third, in order to ensure a constant color point in different circumstances, information exchange between the

independent color loops is required as each loop individuallyregulates its state to set point. If one of the color loops canno longer reach its set point, the desired color point cannot be maintained. A situation like this can readily occur and

some measures need to be taken. In such circumstances, the set points and present duty cycles are scaled down in

conjunction with the eyes sensitivity to color differences versus flux differences. The required algorithm thus prioritizesthe correct color point in case of insufficient flux. This is implemented and indicated by the Rescale duty cycle-blocks,

the Scale factor-arrow provides the Color sensor references-block with the required reduction factor.

Also, a similar approach must be applied when transforming the user input to the sensor domain. The user can requesta flux output at a certain color point, which is simply not possible for the light fixture, as its maximum flux output

strongly depends on the set color point. For instance, compare the flux outputs at fully saturated blue and white light.

Therefore, the Calibration matrix-block also prioritizes the color point above the flux output when transforming the

user set points to the sensor domain.

5.EXPERIMENTAL SET UP

In order to verify the presented color error expectations for each color control loop, an experimental set up has beenbuilt. This section describes some of the aspects of the experimental set up. The total set up is shown in figure 7:

Figure 7: The experimental set up used to determine theperformance of each color control loop

For accurate measurements, the LED light needs to be

mixed; this can be achieved using an integrating sphere. For

calibration and reference measurements a PHOTO

RESEARCH PR-650 SpectraScan SpectraColorimeter(mounted on tripod) has been used. This portable colorimeter

offers an accuracy of 2nm and 0.006 x, y color accuracy fortypical CRT phosphors.

The various color control loops have been implemented ona Texas Instruments TMS320LF2407 16-bit integer DSP with

10 bit AD converters. This processor is used to generate PWM

signal at a 14.3 bits resolution and has (more than) sufficient

calculating power to fulfill all possible requirements.

Lumileds Luxeon emitters7are used and their specifications at350 mA forward current and 41 C heat sink temperature can

be found in the table 1. LED parameters T0and dpeak/dT have

been determined empirically for a different set of LEDs.



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1 < 2

T I F T 6 I D I

R I

P W M 1 n

M 2 0 . O p n 1 2 . 5 M R R S O . 0 0 0 5 0

T s R R u n

R o n r u n e

n o n n o u n

O O j n i S R 0 0 0 0 0 0

P u s i n i n

0 0 . 0 0 0

R i o n 0 0 0 0

0 5 0 n o

I I I I I I I I I I I I I

7

l R 5 0 I I I .

T s R R u n A s e r u n e 5 6 0 R U n s

O R 1 1 : 4 1 : 3 6 s l T n

P u s i n i n

2 4 . 4 1 1

I I I I I I I I I I I I I I I I I I I

u s g s

M R R . R n g l R . 5 M R R R R . R T S T I

LED color # x y

peak

[nm]

dom

[nm]

dpeak/dT *

[nm/K]

FWHM

[nm]

[lm]

T0*

[K]

Prad

[W]

Pel

[W]

Rth

[K/W]

Red 2 0.7005 0.2981 640 626 0.15 20 31 90 0.25 2.17 18

Green 6 0.2027 0.7225 526 533 0.05 35 37 300 0.26 7.60 15

Blue 2 0.1458 0.0449 457 463 0.02 25 24 1300 0.17 2.25 15Table 1: Overview of LEDs properties (at 350 mA forward current) used in experimental setup at 35 C heat sink temperature

(* determined empirically using a different set of LEDs)

5.1Drivers

LEDs are essentially current driven light sources. Unfortunately, the LEDs peak wavelength tends to shift as a function

of this forward current. If not taken into account, this can provide substantial color errors. To deal with this problem,

LEDs are driven with a PWM current signal. The LED driver can therefore be seen as a PWM amplifier.

Figure 8: PWM amplifier for the LED color control loops

In particular for the time-resolved

measurements, a fast and well-defined turn-

on and turn-off behavior is required, on the

other hand, when the sensor signal isintegrated over the PWM period deviations

will be within the measurement sample.

As no out-of-the-box driver solution is

available, three independent current sourcesare created. Each current source is based on

the TI PT6101 buck converters. TI

guarantees that these converters produce afully regulated output voltage within 1 ms of

either the release of the Inhibit pin or the

application of power. As this is too slow, the

buck converters have been modified.The new driver circuitry is displayed in figure 8. The resulting rise and fall times are depicted in figure 9.

Figure 9: I-source rise time (left) and fall time (right) for an LED load in experimental set up

(Ch1: LED current, Ch3: PWM input signal)

The rise and fall times are typically about 5 s, however these times are load dependent.

5.2SensorsDepending on the activated color control method, a certain type of sensor is required. Color coordinates feedback

requires filtered sensors, which (more or less) match the Color Matching Functions, whereas flux feedback simply

requires a photodiode. Other color control loops, like temperature feed forward and the combination of the latter two also

require a temperature sensor. Therefore, the experimental set up has been equipped with several optical sensors (attachedto the integrating sphere, but on the opposite side of the LEDs) and a temperature sensor on the heat sink of the LEDs.

This position of the light sensors makes this setup independent of the types of LEDs (packaged or chip-on-submount).



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3 5 0 4 0 0 4 5 0 5 0 0 5 5 0 6 0 0 6 5 0 7 0 0 7 5 0

W a v e l e n g t h ( n m ]

There is a shield inside the integrating sphere to prevent direct LED light from reaching the sensors. The optical

measurements are oversampled extensively to overcome the effects of the drivers switching-frequency.

5.2.1Temperature sensor

A simple and effective measurement of the LEDs junction temperature can be obtained through a temperature dependentresistor (NTC) thermally connected to the MCPCB that the LEDs are mounted on. An NTC sensor is recommended

because it has a suitable sensitivity curve, is available in quite small sizes, has reasonable accuracy (1% thru 10%) and awide range of resistances (100 Ohm thru 470 kOhm).

As the NTC is directly connected to the MCPCB (or heat sink), the junction temperature (Tj) can simply be estimated

as the sum of the heat sink temperature (Tb) and the thermal resistance (Rj2b) multiplied by the dissipated power (Pdiss)(assuming the LED junction heats up almost instantaneously with respect to PWM period):

dissjbbjbbj PRTTTT ++= 22 (9)

5.2.2Flux sensor

A cheap solution for flux sensors is a photodiode; these are widely available in many different packages. A photodiode

can usually be modeled as a number of parallel current sources (e.g. a signal (I s), leakage (Il) and noise (In) current

source) and a parallel shunt resistor (Rd). This resistance (Rd) is very large (Rd>>1 M), so the current of the parallelcurrent sources (Is, Iland In) add up and Iland Incan be considered as additive noise.

Unfortunately, the photodiodes sensitivity is not uniform over the visible spectrum: very sensitive for red, but quiteinsensitive for blue. On the positive side, the spectral sensitivity is almost linear within the visible range. Combined with

the shifting peak wavelength of the LEDs as their junction temperature rises, the set point can be adapted using a linearfit of the sensors sensitivity. In figure 10 below, a spectral overview of the LED output and a standard photodiode

response is depicted. The used photodiode is a Siemens SFH213. This is a Silicon-based photodiode with a very short

switching time. It is especially suitable for radiation between 400 nm and 1100 nm. Note that the spectral sensorsensitivity will change when the sensor temperature changes.

5.2.3Color-filtered flux sensors

For color coordinates feedback (CCFB), the red, green and blue ratios of the mixed light must be measured. To get an

accurate reading of human color impression, the CIE1931 2 color matching functions (CMF)5 must be used.Unfortunately, color filters adjusted to this specific response are available, but rather expensive. In a future lighting

application, cheaper filters must be used and, therefore, an Agilent HDJD-S831-QT3339 sensor array is used in thisexperimental set up. Agilent also utilizes this sensor for their color management system HDJD-J822-SCR0011.

Figure 10: LED output (normalized to 1), CMF filter, Agilent sensor (green filter

normalized to 1) and photodiode characteristics (max sensitivity 1)5, 8, 9

As the sensors do not match the

color matching functions, they needto be calibrated to match the CMF

responses. In figure 10 below, a

spectral overview of the 2 degreeCMF, LED output, Agilent sensor

response and a standard photodiode

response is depicted. For color

coordinates feedback, the average

light per PWM period is required, so

the sensor signals must be integratedover each PWM period.



11/12


12/12

distribution and average value of these parameters is very important e.g. for reliability figures. Besides the statistical

properties of these empirical parameters, the accuracy of the thermal model of the system will also influence theperformance of certain color control loops. In all cases, integrating Silicon based sensors into a lighting fixture influences

the sensors spectral sensitivity, because they also heat up, in turn, this influences the performance of the color control

loop. Therefore, an improvement can be made by also compensating for temperature-induced changes to the sensorsspectral sensitivity.

Also, ageing will have an effect on the performance of the temperature feed forward method. If ageing only changesthe lumen output, all other methods should be able to handle this. However, if also the peak wavelength changes, of the

presented methods only the color control loop based on an integrated color point (CCFB) will be able to cope with this,

although other suitable sensor solutions are likely to exist. Additionally, the effects of sensor ageing may influence the

color stability.

7.CONCLUSIONS

Although LEDs promise a new era in lighting, their color instability needs to be solved by using different LED types or

by using an additional color control loop. In this paper, four methods have been presented and compared with respect to

their short-term performance. The color errors for each of the color control loops have been determined for a temperature

range of about 50C and for color temperature between 2500 Kelvin and 6000 Kelvin. In this range, we find that, onshort term, all color control loops show a significant improvement in the color error, except for the color control loop

based on flux measurements of each primary color (FFB), which disregarding ageing effects performs nearly as

mediocre as open loop. However, the color control loop based on the heat sink temperature (TFF) cannot offer colorstability when the LED ages, which is expected to be significant. On average, FFB&TFF shows the best results. These

results have been obtained using LED tristimulus data and sensor data at a single (junction) temperature. In addition,

LED parameters d/dT and T0have been determined empirically on a different LED set.It is also apparent that the color error is not constant for all colors; we already see larger differences between color

points on the blackbody line from 2500 Kelvin to 6000 Kelvin. This means that the color stability of a color control loop

must be specified for a certain color point range. In relation to this, FFB&TFF shows opposite behavior with respect to

the other methods, which is most likely caused by the chosen sensor; using a blue enhanced may result in a decreased

color error.The two color control loops based on temperature feed forward (TFF and FFB+TFF) both require prior knowledge of

empirical LED properties, therefore a color control loop based on an integrated color point (CCFB) is the most robust

method as no empirical LED properties are required, however, it does need rather expensive sensors. FFB+TFF issuitable as less expensive alternative for CCFB, especially when the LEDs have matured and their parameters show less

spread from batch to batch. A less expensive color sensor may also yield an acceptable color error for the CCFB method,while still offering robustness.

REFERENCES

1 A. Zukauskas, Quadrichromatic white solid state lamp with digital feedback, Proc. Of the Soc. of Photo-Optical Instrumentation, 5187, pp 185-198, 2004.

2 S. Muthu and F. J. P. Schuurmans, Red, green, and blue LEDs for white light illumination, IEEE Journal in

Quantum Electronics, 8, pp 333-338, 2002.

3 S. Muthu, F. J. P. Schuurmans and M. D. Pashley, Red, green and blue LEDs based white generation: issues andcontrol,Proc. of the 2002 IEEE Industry of Applications, 1, pp 327-333, 2002.

4 A. Perduijn, S. de Krijger, J. Claessens et.al., Light output feedback solution for RGB LED backlight

applications, SID2003.

5 G. Wyszecki and W.S. Stiles, Color science. New York: Wiley, 1982.

6 A. Zukauskas,Introduction to Solid-State Lighting. New York, Wiley, 2002.7 Datasheet Lumileds Luxeon emitters, Lumileds website [online]: http://www.lumileds.com

8 Datasheet Siemens photodiode SFH213, Siemens website [online]: http://www.siemens.com

9 Datasheet Agilent color sensor module HDJD-S831-QT333, Agilent website [online]: http://www.agilent.com11 Datasheet Agilent color management system HDJD-J822-SCR00, Agilent website [online]:

http://www.agilent.com

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