Critical Aspects of Led Selection

7/31/2019 Critical Aspects of Led Selection

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CRITICAL ASPECTS OF LED SELECTION

Mohammed Haseeb O, M.Tech Energy Management, Auditing & Lighting, MIT Manipal

Key words: LEDs, Solid State Lighting, Color Rendering, Color Consistency, Color Rendering Index (CRI), PhilipsLumileds, CREE, NICHIA, OSRAM, Competition Analysis, Luminous Flux, Luminous Efficacy, Junction Temperature, Energy

Efficient Lighting, MacAdam Ellipse, Hot testing, Binning

The competition in the field of solid state lighting has peaked in such a way that the customers

often get confused. Different claims made by Competitors add complexity. The main reason behind this

scenario is the lack of awareness regarding solid state lighting among the public. Before Entering into

the field of LED Technology, a customer should be in a position to understand the LED Datasheet.

The Customer's LED selection should be application specific. LED Luminous flux, Luminous

efficacy, Color Rendering, color Consistency, Color Over angle and lumen maintenance are the

parameters which are worth noting before selecting any LEDs. Other important factors that must beconsidered are forward voltage and forward current.

The tendency to go after an LED, which delivers more lumens, is a common practice among

customers, worldwide. It should be noted that the high luminous efficacy is one of the driving

parameters of booming Solid State Technology. So luminous efficacy must be considered along with

luminous flux for proper selection of LEDs. Another point to be noted is the current at which the

datasheet specifies the lumen output & luminous efficacy. If you are not able to find the luminous flux at

your desired drive current refer the plot of "Normalized Luminous Flux VS Forward Current". From the Y-

axis of the plot you can obtain, a multiplication factor corresponding to your drive current. This factor is

called Flux Fraction Ratio (FFR) w.r.t forward current. Multiply the FFR with the Datasheet flux at the testcurrent, and then you will get the luminous flux at your drive current. The procedure is not over yet.

Next you have to take the FFR corresponding to Junction temperature (Tj). Its worth noting because

only few LED Manufacturers like PHILIPS Lumileds test the LEDs in real world conditions (i.e.; hot tested -

tested at 85 deg. C). Most of the other manufacturers test LEDs at 25 deg, which is way below the real

operating Tj. Don't get cheated by the luminous flux at 25 deg. C as LED's lumen output degrades at

higher temperatures. In order to make proper analysis, take FFR corresponding to Tj from the plot

"Relative Luminous Flux VS Thermal pad temperature" and multiply with Luminous flux obtained at your

drive current. So as a whole, exact luminous flux is:

LUMINOUS FLUX= Luminous Flux at test current*FFR corresponding to drive current*FFR

corresponding to Tj.

The Luminous efficacy mentioned by most of the LED manufactures is at lowest possible test

current (e.g. 350mA) but your drive current may be even higher. So don't expect the same luminous

efficacy at higher currents too. Moreover Luminous Efficacy Highlighted in the front page of the LED

datasheet is for cool white (CCT > 5000K). Luminous efficacy will comes down for neutral white and

warm white variants. So select the LEDs based on your application. If your application requires higher


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drive current and warm white, don't get cheated by the highlighted information in the data sheet front

page. PHILIPS Lumileds datasheet is very user friendly and specifies lumen output and luminous efficacy

at different drive currents and different CCTs.

Color Rendering is another property of LEDs which is widely misunderstood. Most of the LED

manufacturers consider Color Rendering Index (CRI) as the only parameter for measuring ColorRendering. But actually its not so. According to ANSI, Color Rendering Index is the measure of how

similar colors appear under illumination by a test source, compared to under a reference source that has

the same correlated color temperature. In practice its known as color accuracy, a numerical rating of

the color quality produced by a light source. CRI is determined by measuring the color of eight samples

and then taking the average of those ratings. While that number is representative of overall color

accuracy, this testing method is not perfect. By using an average it means that a particular light source

might be very strong in some areas and very weak in others. So for accurate color rendering

measurement, more saturated colors like Strong Red can also be considered. It is a generally accepted

fact that humans prefer over saturated currents to under saturation. So considering CRI along with R9

(deep RED) will give a better Color Quality assessment at least in retail applications. Philips Lumiledsspecifies R9 value for its midpower and high voltage LED. The accurate way of color quality assessment

can be done by adopting Color Quality Scale developed by National Institute of Standards & Technology

(NIST). Color Quality Scale (CQS) uses 15 color samples, many of which are more saturated than the

eight CRI color shades. So don't get carried away with the high CRI values specified in the datasheet.

Philips Lumileds Specifies CRI plus R9 value for some of its products like LUXEON H and Midpower LEDs

like Lumileds 3535 and Lumileds 5630.

The next important factor to be addressed is Color Consistency. Color consistency can be

addressed in two ways. They are color consistency among different LEDs (within the same variant) and

Color Consistency over time. Most of the LED manufacturers follow ANSI Compliant Binning. PhilipsLumileds ensures Color Consistency among the LEDs by employing "Freedom from Binning. Freedom

from Binning LEDs can be used for applications which demands high color consistency. Philips Lumileds

LUXEON LEDs for outdoor applications falls in 5 SDCM (Standard Deviation Color Matching), whereas

LEDs for Indoor applications falls under 3 SDCM. According MacAdam, human eyes are not able to

detect color variations within a single 3 step MacAdam ellipse. Even 5 step MacAdam ellipse is a luxury

for outdoor lighting. According to ASSIST, Industry should strive towards 2 step MacAdam ellipse. But 3

step MacAdam ellipse itself provides higher color consistency and is cheap as compared to 2 SDCM

product. Color Consistency over time is achieved by Philips Lumileds through Unique Manufacturing

Process.

In order to create customer awareness in Color over angle, it is important to understand LED

Anatomy regarding phosphor coating. Two common methods used in Industry for phosphor coating are-

coating phosphor directly on the die, which in turn enhance more blue to white light conversion but

results in rapid phosphor degradation, the second method is stirring phosphor uniformly into silicone

dome, but the blue light absorption by phosphor will be less and color over angle will be poor. Philips

Lumileds employs LUMIRAMIC Technology, applying phosphor uniformly over a ceramic plate which is


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placed directly over the die. Using LUMIRAMIC Technology Philips Lumileds is able to rectify the

phosphor degradation and improves color over angle.

The boom in SSL industry is mainly due to the long lifespan of LEDs. One has to replace LED

lamps, only once or twice in his lifetime. Most of the local manufactures claim one lakh hours of life or

more without any standards. But with the aim of standardization, now each LED manufacturer has to

generate TM-21 report. The Lumen Maintenance of LEDs is specified in L70, B50/L70, B10.L70, B50

means the duration (hours) at which the lumen output of 50% of the sample of LEDs depreciates by

30%.L70, B10 can also be explained in a similar way. The main thing you have to note while reading

lumen maintenance report is the conditions at which it is specified (i.e.; drive current and Tj).Unlike

other manufacturers Philips Lumileds has a wide database to claim lumen maintenances per TM-21,the

extrapolation is possible only up to 6 times the test data. Since Philips Lumileds has wider database,

LUXEON LEDs have a projected life of not less than 50,000 hours on data sheet at extreme conditions

(max.drive current and max.Tj).

Unlike OEMs Philips Lumileds manufacture application specific LEDs. OEMs suggest their all LEDS

for each and every application including Indoor Lighting, Outdoor Lighting, and portable lighting and so

on. Philips Lumileds makes the LED selection easier for customers by producing application specific LEDs.

LUXEON REBEL & LUXEON REBEL ES are well suited for all most every application due to their

excellent performance. Philips Lumileds suggests LUXEON A for indoor lighting, whereas LUXEON R for

outdoor lighting and tunnel and underpass lighting applications. LUXEON K and LUXEON H are

recommended for retrofits. LUXEON H is well suited for compact product design due to its ability to

minimize driver complexity due to its high forward voltage design. LUXEON S is well suited for Spot

lights, Whereas LUXEON C is recommended for portable lighting. LUXEON M is a multiple dieconstruction and its high lumen output and luminous efficacy makes it suitable for outdoor lighting

along with LUXEON REBEL ES and LUXEON R. Philips Portfolio ranges from Midpower (3535 & 5630) to

high power LUXEON K.

Disclaimer: Document is written after carefully analyzing most of the available LED Datasheets

(including Philips Lumileds, CREE, NICHIA, and OSRAM). Thin Film Flip Chip Packaging (TFFC), which

makes LUXEON LEDs unique in SSL industry, is out of the scope of this paper. This Paper solely

represents the view of the author, does not represent the views of the SSL Industry or any Manufacturerin Particular.

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Critical Aspects of Led Selection