LEDs for Flash Applications

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September, 2010 Page 1 of 14

LEDs for Flash Applications

Application Note

Abstract

This application note introduces LEDs withoptimized characteristics which are primarysuitable for use as a camera flash.In addition to a short summary of thecommon advantages of LEDs and the re-quirements of camera flashes, the mostimportant parameters are described withreference to the operating mode.Beyond that several assembly possibilitiesare shown including their thermal descrip-tions and some simulation results for theLUW FQ6N in flash operating mode areprovided.

Introduction

The ambient light available for taking apicture often is insufficient in everydaysituations, so it requires the use of a flashunit as an additional light source.

Due to their increasing brightness, LEDs aresuitable to replace, for example, the conven-tional flash tubes used in flash units ofmobile phones or digital cameras. During thelast few years LEDs as camera flashesbecame more and more state-of-the-art inmobile phone applications.

In comparison to flash tubes, LEDs provideseveral advantages. A Traditional flash unitconsists of a flash tube in which a flash is

created by means of a gas discharge. Theflash tube contains an inert gas, usuallyxenon or krypton.Using a suitable circuit, the battery chargesa capacitor to a level of a few hundred volts.This is then stepped up to a secondaryvoltage in the kV range by means of anignition coil. This ignition voltage is releasedin the flash tube, causing the gas to ionize.

The flash arises through recombination andlasts only a fraction of a second. During thistime a few hundreds amperes of currentflow.The light emitted from the flash tube exhibitsa continuous spectrum which is similar to thesunlights spectrum (a Planck emitter in thecolor temperature range of 5500 6500K).Modern flash units contain a sensor, inwhich the reflected light from the subject ismeasured by means of a photodiode. The

flash is automatically switched off after apredetermined amount of light is sensed.

In this case, LEDs offer a particularly optimallight source for mobile devices. Due to therapid development in the area of semicon-ductor technology in recent years, LEDspossess a very high brightness and addi-tional key features:

Advantages of LEDs

High mechanical stability

Small dimensions

Low voltage required to create a flash,compared to flash tubes

No charging time the flash is immedi-ately available

Longer lifetime than conventional flashtubes


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Longer flash duration possible, up tocontinuous mode

Multichip-LED adjustable color tempera-ture, adaptable spectrum

Flash RequirementsDepending on the application, variousdemands are placed on the camera flash inorder to achieve a correct exposure. Thisleads to differing requirements which mustbe fulfilled, however.

1. Conventional Xenon Flash

Xenon photographic flash units are capableof illuminating subjects up to 45 meters

away. The coverage range is regulated bythe flash power.Figure 1 shows the discharge curve for atypical conventional flash unit at maximumpower.

Figure 1: Light output over time of aXenon flash unit at maximum power

A sharp rise in light intensity is visible,followed by decay. Depending on thedistance between the camera and thesubject, a particular quantity of light is

required for a proper exposure.The quantity of light is defined to be theproduct of the illuminance and the flashduration, which corresponds to the integralof the area under the discharge curve. Thequantity of light (flash power) can becontrolled by the flash duration. For thatpurpose, the flash discharge and thus thedischarge curve is prematurely interrupted.

Conventional flash units illuminate a subjectwith an illuminance of more thanEv > 1000 lx. The flash duration varies from15s to 2ms, depending on the coveragerange. The period between two flashesranges from 1s to 5s. This period isnecessary in order to recharge the capacitor.

The color temperature of the flash isbetween 5500K and 6000K.Conventional flash units have a lifetime ofabout 5,000 flashes. Afterwards, the bright-ness is reduced to a level of 90%.

Table 1 summarizes the requirements of aflash unit used for conventional applications.

Flash unit for conventional applications

Subject illuminance Ev > 1000lx

Flash duration 15s 2ms

Flash coverage 2m 35m

Lifetime 5,000 flashes

Time between flashes 1s 5s

Viewing angle 100

Color temperature 5500K 6500KTable 1: Flash unit for conventional

applications

2. Flash units for mobile phones

In mobile phones, the minimal illuminancedepends on the optical resolution of usedcamera chip (Fig. 2).Nowadays, camera modules with between 3and 5MPixel are used as standard for mostmobile devices. For these, the minimalcenter illuminance should be from 80lx to

200lx at 1m.For the currently high end devices with8MPixel or more, the light requirements areeven higher, starting at 300lx.However depending on the customizeduniformity demand of the illuminated targetarea the light level in the center can varyfrom the given values.


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Figure 2: Typical center illuminance in 1m vs. resolution of the camera chip

Moreover in most applications, the flashshould cover a rectangular field of view, e.g.55 x 43. In the center of this field, therequested illuminance level should beachieved. The illuminance in the corner ofthis field of view is, dependent on thedesired homogeneity, around 20% to 40%.

The required flash duration is in the range ofup to 400ms. Depending on the processing

rate of the mobile phone, the time betweenflashes is usually about 2.5s, although thiscan be shorter. The duty cycle of a flash isgiven by pulse duration divided by the cycletime (pulse duration plus break).Due to the long integration time of typicalCMOS image sensors (around 300 ms), anappropriate light source should ideally becapable of outputting a flash in the form of asquare impulse (Fig. 3).

0

20

40

60

80

100

0 200 400 600 800 1000 1200

Time [ms]

rel.Brightness[%]

Figure 3: Ideal square impulse of a flashmodule

The lifetime of the flash unit is assumed tobe higher than 30,000 flashes.

LEDs for Flash Applications

Currently in the market only white LEDs fromthe multitude of available LED-types areused for camera flashing.White LEDs are typically based on the

principle of color addition, in which theprimary color blue (blue semiconductor chip)and the appropriate complimentary coloryellow (yellow converter) are used to createwhite light. The resulting color mixturerespectively the color temperature is therebyalready specified during production. Thetypical color temperature of white LEDs is inthe range of 5000K to 7000K.

In addition to the function of digital imagesensors (CCD or CMOS), multi color LEDs

may be also suited for use as camera flash.

In the following, white LEDs which can beconsidered for use as a substitute for flashtubes are presented.

Like all LEDs from OSRAM OptoSemiconductors, these LEDs fulfill theapplicable RoHS guidelines and containneither lead nor other banned substances.


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All shown LEDs are compatible with existingindustrial SMT processing methods, so thatall current populating techniques can beused for the mounting process. The individ-ual soldering conditions according to JEDECcan be found in the respective data sheet.To reach the optimal performance of the

LEDs, thermal management should beconsidered.

Since OSRAM Opto Semiconductors con-tinually makes improvements to the semi-conductor chip technology, especially at theluminous intensity of LEDs, please check thedata sheets of the following LED types forfurther details and the latest performancedata (www.osram-os.com).

OSLUX - LUW FQ6N

The LUW FQ6N is especially developed forcamera flash applications with high de-mands on brightness combined with limiteddimensions (4mm x 3.9mm x 2.45mm).

The LED is constructed with a metal leadframe (Cu-Alloy) in contact with a semicon-ductor chip and housing with an integratedlens (Fig.4). The electrical contacts are

located underneath the lead frame.

The chip bases on the newest ThinGaNtechnology and provides excellent color uni-formity as a result of the front emitterbehavior combined with color conversion atthe chip level.

Figure 4: OSLUX LUW FQ6N

The integrated optics consists of a moldedlens which is fixed to the LED frame.According the specification the target of thelens design is thereby aligned to maximalilluminance in the center with adequacyuniformity of the viewing area (Fig. 5).

Figure 5: Rectangular Illumination patternof the OSLUX LUW FQ6N at 1m distance

Table 3 shows the electrical and opticalcharacteristics of the LUW FQ6N.

OSLUX LUW FQ6N

If 350mA 500mA 700mA 1000mA 1.5A 2.0A

v (typ.) 98lm 130lm 169lm 218lm 300lm 350lm

Ev avg. at 1m 120lx 159lx 206lx 266lx 366lx 427lx

Uf (typ.) 3.25V 3.3V 3.45V 3.55V 4.0V 4.2V

Max. Pulse duration[Ta=25C, D=5%)]

> 10s 8s 2,5s 600ms 200ms 20ms

Table 3: Characteristics of OSLUX LUW FQ6N


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Due to the optimized low thermal resistance,the LED can be driven with a current of up to2 A in pulse mode.

In line with demand the OSLUX LED is alsoavailable in a second version with variedlens design and adapted optical

characteristics. The target of the additionallens shape is aligned to a good uniformity ofthe viewing area with adequacy illuminancelevel (Fig. 6)

Figure 6: OSLUX with homogenizedillumination pattern at 1m distance

Table 4 shows the characteristics for thisversion of the OSLUX LED.

Overall, the OSLUX LUW FQ6N exhibits asuperior efficiency and an excellent thermalcharacteristic.

CERAMOS - LUW CAEP

This LED is a combination of minimizedpackage and the newest high efficientThinGaN chip technology with excellentcolor homogeneity.

Figure 7: CERAMOS LUW CAEP

Especially designed for applications withextremely limited space the LED exhibits avery high luminous brightness with a dimen-sion of 2.04mm x 1.64mm x 0.75mm.

The LUW CAEP consists of a ceramicsubstrate with the bonded chip on it, and anencapsulant of silicone. The electricalcontacts are located underneath the ceramicsubstrate (Fig. 7).

The LED is ruggedized and suitable forpulse currents up to 1000mA.Table 5 shows the optical and electricalcharacteristics of the LUW CAEP.

OSLUX with homogenized illumination pattern

If 350mA 500mA 700mA 1000mA 1.5A 2.0A

v (typ.) 98lm 130lm 169lm 218lm 300lm 350lm

Ev avg. at 1m 86lx 114lx 149lx 192lx 264lx 308lx

Uf (typ.) 3.25V 3.3V 3.45V 3.55V 4.0V 4.2V

Pulse duration[Ta=25C, D=5%)

> 10s 8s 2,5s 600ms 200ms 20ms

Table 4: Characteristics of OSLUX with rectangular illumination pattern


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CERAMOS LUW CAEP

If 350mA 500mA 700mA 1000mA

v (typ.) 107lm 142lm 185lm 238lm

107lx 142lx 185lx 238lxEv avg. at 1mWith OSRAM OS reference design lens

Uf (typ.) 3.25V 3.3V 3.45V 3.55V

Max. Pulse duration[Ta=25C, D=5%)]

DC DC 2,5s 600ms

Table 5: Characteristics of CERAMOS

Similar to other toplookers without lens, theCERAMOS LED has a viewing angle of 120with a Lambertian characteristic (Fig. 8).

Figure 8: Radiation characteristic ofLUW CAEP

The LED can be easily combined withsecondary optics e.g. a Fresnel lens to focusthe light in the center of the viewing field.This optics is commonly fixed in the cover ofthe mobile phone.

LED Characteristics Related toFlash Operation

In order to determine whether a LED issuitable for use as a camera flash, variouscharacteristic optical properties should beconsidered. These include

Luminous flux of the LED

Illuminance

Radiation characteristics

Flash Duration

Brightness behavior with respect toflash duration

Switching time

Color coordinates

In comparison to other LEDs the interactionof the individual values has also to beobserved.

Brightness and Illuminance

When characterizing LEDs, the brightness isusually stated as one of two values -

luminous flux v (units of lm) or luminousintensity Iv (units of cd).The luminous flux of an LED is defined asthe total light output, independent ofdirection (Fig. 9).Luminous intensity reflects the amount oflight within a specified solid angle in the

direction of radiation (e.g. 0.01 sr = 3.2,see Fig. 9).


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Figure 9: Definition of luminous flux and

luminous intensity

The two characteristic values v and Iv areonly conditionally suitable for the characteri-zation of flash LEDs.

With regard to the application, the photomet-ric value for luminous flux density Ev (units oflx = lm / m) is most often used. Illuminancedescribes the luminous flux for a specificarea at a specific distance (Fig. 10).

Figure 10: Definition of illuminance Ev

When comparing illuminance values fromvarious LEDs, the distance at which thevalues were obtained must be taken intoaccount, since illuminance is reciprocalproportional to the square of the distance.

2)(

r

IrE

v

v=

(photometric distance law)

This means for example, that when thedistance is doubled, the illuminance de-creases by a factor of four.

Due to the physical behavior of thesemiconductor diode, the luminous flux of anLED does not increase or decrease linearlywith the forward current applied and is alsotemperature-sensitive.This means that if the luminous flux at aspecified value is to be doubled, for

example, the forward current must beincreased by an additional factor.Temperature dependency means that athigher temperatures, less light is producedby the LED.The impact of both effects can be seen infollowing diagrams (Fig. 11 & 12).

Figure 11: Relative luminous flux vs.current (e.g. LUW FQ6N)

Furthermore, it should be noted that themeasured illuminance only represents the

brightness at the center of the LED orillumination field. Outside the center,illuminance level falls off more or lesssharply, depending on the radiation charac-teristics of the particular LED or theadditionally used lens.

In order to achieve a uniform illuminationand thus positively influence the imagequality, the entire image area should benearly homogeneously illuminated.


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Figure 12: Relative luminous flux vs.temperature (e.g. LUW FQ6N)

Radiation characteristics

Detailed information about the angle-dependent distribution of the luminous inten-

sity respectively radiation is given by theradiation characteristic of the LED (Fig. 13).

Figure 13: Radiation characteristic of theOSLUX LUW FQ6N

The radiation characteristic of a LED isaffected by its respective structure and iscomponent specific for this reason.Thereby the form of the radiation pattern canbe influenced within a certain range by alens directly on the top of the LED or by aseparate secondary optics.

For instant SMD LEDs without lens showusually a Lambert' radiation characteristic,SMD LEDs with a simple spherical lensfeature a more or less focused radiation.

Generally concerning the lens effect, twodifferent aspects can be purposed -

homogenization or focusing.

During the homogenization by an appropri-ate lens design a leveling of the radiationbehavior is strived within a certain anglerange. The larger the homogeneous rangethereby is the lower is the light and/ordensity of light in the center.In contrast during the focusing a collimatingof the radiated light is sighted.In Figure 14 the interrelation between ho-mogeneity and illuminance in the center isonce more illustrated on the basis of oneLED with different lens characteristics.

Flash Duration

The quantity of light produced by a flash isdetermined from the product of the flashduration and illuminance Ev. With a higherilluminance of the LED, a shorter flashduration is required for a sufficient exposure.

In order to reduce blurring, the flash durationshould be kept as short as possible.

Brightness behavior within the flashduration

When power is applied to a LED, the forwardvoltage reaches a maximum followed by arapid drop. Initially, the LED is dark andbegins operation at room temperature. Dueto the current, the brightness decreases as

the LED becomes warmer. The slightdecline starts when the LED warms up andthe heat is transferred within the PCB. Thebehavior becomes saturated when thermalequilibrium is achieved between the PCBand the surrounding environment.The higher the LED current is the sharper isthe decrease in brightness. The timerequired to reach thermal equilibrium isdependent on the PCB material used.


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Figure 14: The trade-off between center illuminance and homogeneity

Nevertheless, the drop of brightness duringthe entire flash is around 10%, resulting in anearly constant light level (Fig 15). The slightdecrease can be compensated by thedriving circuitry.Because brightness decreases slightly overtime, it is important to specify at what time

the LED is measured when comparingdifferent flash LEDs.OSRAM Opto Semiconductors LEDs aremeasured as follows: After waiting around5 ms for the current to stabilize, thebrightness is measured for a short timeperiod, typically 25 ms.

Figure 15: Typical brightness behavior within the flash duration


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Switching Time

White LEDs contain semiconductor chipsbased on InGaN technology. The switchingtime of InGaN dies is a few tenth of ns.The yellow converter responds approxi-mately a factor of 10 later. After this time,

the light appears white to the eye.Since the switching time of the converter is afactor of 106 shorter than that of the flashduration, the switching time of the converterdoes not need to be considered. Thus, it canbe assumed that during the entire durationof the flash, white light is measured by thedetector.

Color Coordinates

For most areas of photography, the colorrendering index of white LEDs (typ. 80) issufficient.Figure 16 shows the spectrum of a typicalultra white LED. The dashed line indicates

the standard eye response curve V().

Figure 16: Spectrum of typical ultra whiteLED (e.g. LUW FQ6N)

Within the professional sector, a higher color

rendering index is required.For these applications, the use of severaldifferent single-color or multi color LEDs, aswell as white LEDs with multiband convert-ers, is recommended.By enhancing the chromatic spectrum, thecolor rendering index can be significantlyimproved.

The forward current of standard white LEDsinfluences the chromaticity coordinate,however. This relation can be seen in

Figure 17. With increased forward current,the chromaticity coordinate shifts further intothe blue range.

Figure 17: Chromaticity coordinate shiftvs. forward current (e.g. LUW FQ6N)

Systems comparison

In the system the two presented LED typesshow their individual advantages andspecifics, wherein they fulfill differentrequirements and conditions in the end.

Related to the set-up, the OSLUX LEDshave the substantial advantage that due tothe lens design no additional optics inapplication is needed. Thus the assemblyand alignment to the window aresubstantially simplified. However the spacerequirement for the set-up is larger and thetwo LED versions provide only two specifiedradiation characteristics.The CERAMOS LED in contrast scores inthe set-up with its very small dimension andenables a higher flexibility by the determina-tion and definition of the radiation charac-teristic for the system due to the externalauxiliary optics (Fig.18).If the lens is fixed in the cover, specialattention however must be given here to the


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alignment of the LED to the lens. A smalloffset of the LED for example is sufficientthat the desired performance is not reached.

In comparison of the total efficiency of bothsystems, it arises that in a set-up withintegrated lens, as with the OSLUX LED,

overall a higher luminous flux will be emittedto the target area, than in a system withexternal secondary lens (Fig. 19).The main cause is that due to the Lambert'radiation of the toplooker LED only a limitedportion of the available light can be collectedinto the external lens.

Thermal characteristics

An important feature of the LUW FQ6N isthat the chip is directly mounted on the Cu-Alloy lead frame. Thus, the heat generatedby the chip is transferred through the leadframe and can subsequently flow throughthe PCB to the environment.This setup leads to a low thermal resistancefor the LED of RthJS= 10.5 K/W (typical).Furthermore, due to the optimized lowthermal resistance, the LUW FQ6N can bedriven with currents up to 2 A in pulse modein special cases.

In order to achieve optimal performance,thermal management should be considered.

The assembly methods presented here werefurther examined with respect to theirthermal properties and were thermallysimulated in various modes of operation:

Figure 18: Comparison of assembly systems of the flash LEDs

Figure 19: Comparison of system efficacy for the different set-ups


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With regards to mounting on circuit boards,three approaches were considered:

1. LED mounted on FR4 main PCB2. LED mounted on Flex PCB3. LED mounted on Flex on Al PCB

This resulted in specification of the followingboundary conditions for the thermalsimulation:

Ambient Temperature: Tamb = 25C

Heat transfer coefficient of mobile phonecover: = 8 Wm-2K-1

Figure 20, 21 and 22 show the simulationsetup of the three different PCB materials on

which the LUW FQ6N is mounted. Thematerial characteristics of the PCBs aregiven below.

The thermal simulation was done for 5 flashcycles with a flash pulse condition of

tP = 300 ms

If= 1 A

Interval 2s off

1. FR4 PCB

LED on multilayer main board

PCB with 8 layers

PCB thickness 1.15 mm

Figure 20: LED on multilayer main PCB

2. Flexible PCB (15x8mm)

LED on separate Flex PCB

Flex PCB with 2 layer

35 m Cu, 50 m PI, 35 m Cu

Figure 21: LED on separate Flex PCB

3. Flexible PCB on Aluminum (10x8mm)

LED on Flex PCB with Al

PCB with 1 mm Al and 50 madhesive

Flex PCB with 35 m Cu, 50 m PI

Figure 22: LED on separate Flex on AlPCB

The results of the thermal simulation areshown in Figure 23.The simulation shows that after 5 pulses thejunction temperature of the LED is still belowthe max. specified value of 175C for allthree set-ups.Thereby the set-up on flexible PCB with1mm Aluminum plate features the bestthermal behavior and keeps the temperatureof the chip nearly constant.A similar behavior provides the multilayer

main PCB with a good thermal conductor forthe flash operation, but on a highertemperature level.In comparison with that the junctiontemperature of the LED mounted on a FlexPCB increases continuous during the flashoperation. Nevertheless after 5 pulses, themaximum allowable junction temperature of175Cis still not exceeded.


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Figure 23: Comparison of different PCB materials for flash operation (LUW FQ6N)

Conclusion/Summary

In general, the requirements for the use ofan LED as a camera flash can already be

fulfilled and/or exceeded by current LEDtechnology, especially for applications inmobile phones.Furthermore, in contrast to conventionalflash tubes, LEDs exhibit significant advan-tages such as improved shock resistance,small dimensions, low energy requirements,and a higher lifetime. In addition, no charg-ing time is required for the LED flash.For best optical and electrical performanceof LED camera flashes, the typicalproperties of the semiconductor chips such

as thermal behavior and effects should betaken into account.

The presented LEDs, OSLUX andCERAMOS, are exceptionally suited for useas a camera flash in mobile phones.

Especially developed and optimized for thisapplication, the OSLUX fulfills the require-ments regarding brightness, color homo-geneity and uniform illumination. With itsintegrated lens, it exhibits the best optical

performance as well as system efficiency.

Depending on the requirements of theapplication, the CERAMOS LUW CAEP isalso suitable for a use as camera flash. Dueto its individual advantages, e.g. smallerspace requirements, highest luminance andthe possibility to generate individual illumina-tion patterns with auxiliary optics it fulfillsmany requirements for a wide range ofapplications (e.g. mobile and video).

Besides their use in flash units, the LEDsare also well suited as a flash lamp for videocameras. The advantage in this case is thatthe flashes can be synchronized to the videoframes; the flash only activates during framecapture. Between frames, the flash is turnedoff. Compared to common video lamps for

video cameras, this results in a lower energyusage.

The further development of LEDs will lead tohigher efficiency and more light output. Atthe same time, the required forward currentand the dimensions can be reduced.

As OSRAM Opto Semiconductors willcontinually develop improvements to theLED, please check the data sheets of theLED types for the latest performance data

(www.osram-os.com).


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Appendix

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www.ledlightforyou.com

Authors: Andreas Stich, Roland Fischl, Rainer Huber

ABOUT OSRAM OPTO SEMICONDUCTORSOSRAM is part of the Industry sector of Siemens and one of the two leading lightingmanufacturers in the world. Its subsidiary, OSRAM Opto Semiconductors GmbH in Regensburg(Germany), offers its customers solutions based on semiconductor technology for lighting, sensorand visualization applications. OSRAM Opto Semiconductors has production sites in Regensburg(Germany) and Penang (Malaysia). Its headquarters for North America is in Sunnyvale (USA), andfor Asia in Hong Kong. OSRAM Opto Semiconductors also has sales offices throughout the world.

For more information go to www.osram-os.com.

All information contained in this document has been checked with the greatest care. OSRAM OptoSemiconductors GmbH can however, not be made liable for any damage that occurs in connectionwith the use of these contents.

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LEDs for Flash Applications