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”
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-
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
Blue LEDs coated with Yellow Phosphors Near UV wavelength emitting LEDs coated with RGB emitting Phosphors
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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
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-
Trichromatic Light Source
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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.
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
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
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 email@example.com
Published on Feb 16, 2010