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White Light = Right Light “Some of us will compare this to the ‘Chip’ revolution in the computer industry which has changed the way we think and communicate as a society. That comparison may not be far off from reality”

Why LED?

off from reality since LEDs, unlike tradi-

three summations of light intensity across

tional light sources, are, in essence elec-

the visible spectrum, the number of com-

tronic components, as the name gives

binations of different wavelengths of light

away, ‘Diodes’, albeit of a different kind;

that produce the sensation of white are

ones that emit visible light among other

practically infinite. While describing the

odes as part of our first electronics ex-

forms of radiation. In a study conducted

quality of white light from LEDs, we usu-

periment. Some of us later than others

by the Polytechnic Institute of New York

ally refer to the consistence of color in

but we all remember those blinking red

University along with Regnant Lighting,

the beam of light as well as the Corre-

LEDs from our middle school experimen-

we have tried to examine the current and

lated Color Temperature (CCT) variations

tal kits. Music enthusiasts will appreci-

upcoming methods of generating White

over (a) the life of the LED and (b) the

ate the dancing ‘LED’ displays that were

Light using Light Emitting Diodes (LEDs).

variations in the junction temperature of

popular in stereo systems.

This paper will also examine the life of

the LED chip.

Most of us having even the slightest link to the electrical and electronics world have tinkered around with Light Emitting Di-

White LEDs along with some of the chalHowever, it is only in the latter half of the decade ending next year, that we see a worldwide move towards using the same

lenges and issues facing White LEDs.

The methods of producing White LEDs that are going to be covered in this article in-

Why White?

clude:

LEDs (just in a different package) for general lighting applications. Some of us will

White is a color tone that is evoked by

compare this to the ‘Chip’ revolution in

light that stimulates all the three types

the computer industry which has changed

of color sensitive cone cells in the human

the way we think and communicate as a

eye in almost equal amounts. Since the

society. That comparison may not be far

impression of white light is obtained by

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Blue LEDs coated with Yellow Phosphors Near UV wavelength emitting LEDs coated with RGB emitting Phosphors


FEATURED TECHNICAL COLUMNS

Figure 1. Different Colors and Wavelengths as per the CIE Chromaticity Diagram

Figure 2. Configuration of a Blue LED with yellow phosphor coating for white light generation. Phosphor layer absorbs part of the Blue light and emits Yellow Light

Near UV (n-UV) LEDs with RGB emitting phosphors So called, n-UV LEDs are coated with High Efficiency Europium based Red and Blue emitting phosphors plus Green emitting Copper and Aluminum doped Zinc Sulfide (ZnS:Cu,Al). In this case, the efficiency of the LED package is lesser as compared to the blue light with yellow phosphor approach since the Stokes shift is larger (since UV radiation has a significantly shorter wavelength as compared to visMulti-Chip LEDs that combine Red, Green and Blue LEDs LEDs using Zinc Selenide Quantum Dots LEDs

Blue LEDs Coated with Yellow Phosphors

to a 580 nm Yellow light through a phenomenon known in Physics as the ‘Stokes Shift’ where shorter wavelength radiation is transformed to longer wavelength radiation. This converted Yellow light stimulates the red and green receptors of the eye, and along with the unconverted, leaked blue light, gives the appearance of white light.

ible blue radiation). However, the Color Rendering is better in this approach since we are creating the appearance of white using 3 distinct colors, namely, red, green and blue as compared to blue and yellow.

Multi-Chip LEDs using multiple light sources

Phosphor based LEDs have a lower effia.

Dichromatic Light Source

Commercially available Blue LEDs were

ciency than normal LEDs due to the heat

first produced by Shuji Nakamura of

loss from the Stokes shift along with phos-

Nichia Corporation in Japan in 1993. They

phor-related degradation issues. How-

It uses a 445 nm (Violet-Blue) and 570-

were based on wide band semiconduc-

ever, the phosphor method is still the

590 nm (Yellow-Green) spectral lines to

tors – Gallium Nitride (GaN) and Indium

most popular technique for manufactur-

create White light. Peak wavelengths of

Gallium Nitride (InGaN). This method of

ing high intensity white LEDs. The design

500 – 505 nm are least desirable for high

producing white light utilizes a 450-

and production of a light source or light

output. It has a low CRI but a high Lumi-

470nm wavelength emitting Gallium Ni-

fixture using a monochrome emitter with

nous Efficacy.

tride LED that is covered with a yellow

phosphor conversion is simpler and

phosphor coating of Cerium Doped Yt-

cheaper than a complex Multi-Chip RGB

trium Aluminum Garnet crystals

system discussed ahead, and the major-

(Ce3+:YAG). The crystals are powdered and

ity of high intensity white LEDs presently

It uses 450 nm (Blue), 540 nm (Green)

bound in the form of a viscous adhesive.

on the market are manufactured using

and 610 nm (Red) spectral lines. There is

The blue ‘pump’ light is then converted

phosphor light conversion.

observed a 20% gain in Luminous Effi-

b.

Trichromatic Light Source

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29


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cacy at the same CRI as compared to Dichromatic Light Source. It has a good

Figure 4. Example of the T-point for two different types of LEDs

Figure 3. InGaN/Quantum Dots (QDs) White LED structure

CRI, good luminous efficacy. c.

Tetrachromatic

Light

Source It uses 440-450 nm, 495-595 nm, 555565 nm and 610-620 nm spectral lines. It has an excellent CRI and a very low luminous efficacy.

LEDs using Zinc Selenide

associated optical losses. Optical back-

decay. Even if an LED is technically oper-

scattering losses using larger conventional

ating and producing light, at a certain

phosphors reduce the package efficiency

point in time, the light produced will be

by as much as 50 percent.

insufficient for the application for which it is being used. So, the life of an LED should

This method of producing white light using LEDs was developed by Sumitomo

While the optical properties of conven-

be based upon the time it can produce

Electric Industries, Japan. It consists of a

tional bulk phosphor powders are deter-

sufficient light for the intended applica-

homoepitaxial ZnSe blue LED grown on a

mined solely by the phosphor’s chemical

tion.

ZnSe substrate. It produces greenish-blue

composition, in quantum dots the optical

emission (480-490nm) from the active

properties such as light absorbance are

Recent studies define the life of an LED

region (the homoepitaxy) and an Orange-

determined by the size of the dot. Chang-

device or system in general lighting ap-

Yellow light (ranging from 510-780nm, but

ing the size produces dramatic changes

plications as the time it takes for the light

centered around 585-600nm) from the

in color. Their emission color can be tuned

ZnSe substrate. The emission form the

from the visible throughout the infrared

substrate is known as self-activated

spectrum. This allows quantum dot LEDs

emission and is obtained due to excita-

to create almost any color on the CIE dia-

tion of the substrate due to emission of

gram. This provides more color options

the active layer.

and better color rendering white LEDs.

This method of producing white light is

Life of White LEDs

output to reach 70% of the initial value (commonly referred to as L70). Light output reduction in LEDs occurs primarily due to yellowing of the epoxy surrounding the die which leads to the Color Shift to yellow. The yellowing of the epoxy is caused due to excessive heat at the p-n junction of the LED. The junction tem-

phosphor free. Also, the ZnSe substrate is conducting and allows for top and bot-

Long life is one of the key features of LEDs

tom contacts unlike GaN devices. It can

and is one of the primary reasons for

operate at a low voltage of 2.7V and has a

switching from other light sources, along

wide range of color temperature from

with reduced energy use. LEDs don’t fail

2800 – 13300 K, as compared to 6000 –

catastrophically and they suffer a gradual

perature is measured at T-point which is usually outside the LED since that can be used as a standard by all manufacturers and is a direct indicator of the junction temperature as it is very close to it.

8500 K for most GaN devices by controlling the wavelength of active layer and substate emissions.

Quantum Dots LEDs

Figure 5. Light Output as a function of time for high-power white LEDs operated at various ambient temperatures from L100 to L70. The lines are regression fits for the data collected. T-point temperatures for each array are shown.

This approach is based on encapsulating semiconductor quantum dots — nanoparticles approximately one billionth of a meter in size — and engineering their surfaces so they efficiently emit visible light when excited by near-ultraviolet (UV) LEDs. The quantum dots strongly absorb light in the near UV range and re-emit visible light that has its color determined by both their size and surface chemistry. The small size of the quantum dots, even smaller than the wavelength of visible light, eliminates light scattering and the

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FEATURED TECHNICAL COLUMNS

Figure 6. An illustration of Haitz’s Law

The Road Ahead - White LEDs compared to other traditional White Light sources Moore and Haitz – Similarities between LEDs and Transistors in computer hardware? Moore’s Law describes a long-term trend in the history of computing hardware, in which the number of transistors that can be placed inexpensively on an integrated circuit has doubled approximately every two years for more than half a century now.

Figure 7. Evolution of luminous efficacy performance of various white light sources. Commercially available high-power white LED performance is indicated by the points along the solid blue curve. The purple dotted line shows the United States Department of Energy Roadmap for the developments in Luminous Efficacy of LEDs in the coming years.

Whereas, Haitz’s Law is an observation/prediction about the steady improvement in LED technology over the years. It states that every decade, the cost per lumen (unit of useful light emitted) falls by a factor of 10, the amount of light generated per LED package increases by a factor of 20, for a given wavelength (color) of light.

Acknowledgements The author is grateful to Professor Valery Sheverev of Polytechnic Institute of New York University for the guidance and continuing support in preparation of this article. Academic articles from various journals and research papers have been used in the preparation of this article and references can be provided upon request. Author can be reached at rsood@regnant.co.in

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