LED CONSTRUCTION::
GENERAL FAQs:~
General LED Fundamentals
Topic 1: What is an LED?
An LED is a light source made from semiconductor materials
An LED is a light source comprised of a small chip covered with transparent resin. The chip is composed of semiconductor materials, and emits light at the junction/connection area between the materials when current flows through. A semiconductor possesses a conductivity intermediate between a conductors and insulator. When a p-type semiconductor and n-type semiconductor are placed together and voltage is supplied, current flows through the movement of electrons/holes. When an electron meets a hole, it falls into a lower energy state and releases energy, in the form of light (photon).
Topic 2: How is the color of an LED determined?
The emitted color of an LED is determined by the constituent elements
The types of materials that comprise the LED determine the color within a specific range, based on the resulting energy (band) gap. A wider gap will create shorter wavelengths while a smaller bandgap will produce longer wavelengths. For example, in the diagram below we can see that for an AlGaInP system the emitted color can range from yellow to red and beyond.
Topic 3: The color of an LED is expressed in units of nm (nanometer).
Light is a component of electromagnetic radiation
Visible light is a part of the electromagnetic spectrum, situated between ultraviolet and infrared rays, and is perceived as color.
Topic 4: Types and structures of LEDs
Two types of LEDs
LEDs are available in 2 basic types, a surface mount chip type and a leaded (through-hole) lamp style. Select the ideal solution based on intensity/brightness, directivity, and mounting constraints.
Topic 5: How is an LED made?
The main component is an ultra-compact chip on the order or microns
LED chips are made from a wafer base consisting of SiC (Silicon Carbide) and Al2O3 (Sapphire). This LED element is enclosed with resin together with a bonding wire and electrode.
Topic 6: White LED Structure
There are 3 main ways to make a white LED
(1) Red LED + Green LED + Blue LED
White light can be emitted using an RGB LED. These units are also currently used in full-color displays since the red, green and blue elements can be used to make a wide range of colors.
(2) Near-Ultraviolet LED + Red/Green/Blue Fluorescence
This method uses a near-ultraviolet LED with a short wavelength similar to blue. The three different wavelengths of light resulting from the red, green, and blue phosphorescence creates white light with good color rendering properties.
(3) Blue LED + Yellow Phosphorescence
This
method
is the
most
widely
used for
lighting.
A blue
LED and
yellow
fluorescence
are used
to make
white
light,
with red
and
green
fluorescence
added to
maintain
color
rendering
characteristics.
Topic 7: What is the fluorescent substance used for white LEDs?
It is not possible to make white LEDs without flourescence
The
fluorescent
substances
are
extremely
small
particles
that,
when
exposed
to
certain
wavelengths,
emits
visible
light.
Yellow,
green,
and red
fluorescence
are
added
with a
blue or
near-ultraviolet
LED to
emit
white
light.
Two
techniques
are used
to
combine
the blue
LED and
fluorescence:
1) The
fluorescence
is mixed
with the
resin
and
encloses
the blue
LED
2) The
fluorescence
is
placed
directly
on the
chip
itself
Topic 8: Faster than any current light source
20 million times faster response speed than incandescent bulbs
LEDs provide excellent responsiveness, making them ideal for stop lamps in cars that require instantaneous response. LEDs feature a response of 100ns or less, compared to incandescent light bulbs that require anywhere from 0.15 to 0.25 seconds to reach the specified light intensity.
Topic 9: LED light distribution
Broad directivity is possible with LEDs
The directivity refers to the change in brightness based on the variation in angle from the central axis. LEDs typically feature strong directivity with narrow light distribution. However, this can be overcome by using a lens, which can widen directivity to 120º.
Topic 10: What is the light emission spectrum?
This refers to the range of wavelengths included in light
In particular, a Light Emission Spectrum displays the distribution of light intensities in relation to wavelength. Monochrome LEDs show a single peak wavelength based on the color. White LEDs, on the other hand, will show two or more peaks, one for the LED and one for each of the phosphorescent substance used.
Topic 11: What is color rendering?
Including multiple color components is the key to good color rendering
Color rendering is a quantitative index of the ability of a light to faithfully reproduce the true color of an object compared to natural lighting, and is often represented by an index (Color Rendering Index Ra).
A typical blue LED with yellow fluorescence will exhibit an Ra of about 70. However, mixing the blue LED with red and green fluorescence improves color rendering to an Ra=90 or more. However, light emission efficiency will decrease slightly in order to increase light absorption.
Topic 12: Color temperature
The color temperature varies based on the type of fluorescence
The color temperature is a quantitative value (indicated in Kelvins) derived from the wavelength of light emitted from a blackbody radiator (a hypothetical object that completely absorbs external light) at different temperatures. In general, light at a lower temperature has a reddish tinge while at a high color temperature it appears bluish.
The color temperature of LEDs can be changed by changing the amount and type of fluorescence (in the case of a blue/near-ultraviolet chip).
Topic 13: What is a multichip?
As its name implies, it contains multiple chips
A multichip LED integrates several LED elements into a single package. This is often required for more powerful devices, such as ROHM's 10W LED. Advantages include reduced color variation and a larger surface emitting area.
Topic 14: The importance of heat radiation
A key factor in the lifespan of an LED
The high efficiency of LEDs results in far smaller heat generation than other devices, ensuring longer life while eliminating the need for thermal countermeasures.
Topic 15: Light emission efficiency of LEDs
70% of power supplied to an LED is converted to heat
Light emission efficiency indicates the amount of luminous flux (lm) emitted from a light source based on the amount of power (W) used, and is normally given in lumens per watt (lm/W).
Recent white LEDs feature an efficiency of over 100lm/W. Although LEDs are considered high efficiency light sources compared with traditional systems, only around 30% of the supplied power is converted to light. The rest is lost as heat due to voltage, internal (quantum), and optical inefficiencies.
Topic 16: Overall LED efficiency
Thermal and power supply designs are the key to increasing efficiency
Overall efficiency denotes the amount of efficiency of the entire system, including the power supply, after the total luminous flux is removed from the power consumption. Previous LED systems experienced a decrease in efficiency of 30-50% due to power source losses, thermal generation, and diffusion cover effects. However, overall efficiency has increased due to recent improvements.
Topic
17:
Lumen
and Lux
Lumens measure the
total amount of light
output from a particular
source. Lux measures the
intensity of the light
hitting a specified
area. For example, an
ordinary household
lightbulb generates
about 1000 lumens, but
the intensity of its
light at a particular
point, such as on a book
you're reading, will be
comfortably low. Almost
all flashlights rely on
an optical device such
as a reflector or lens
to squeeze most of their
output into a small
area, which allows you
to illuminate a point of
interest with enough
intensity, but without
requiring a lot of
power. To illustrate
this concept, try the
following: First, look
at your room's ceiling
light. In all
likelihood, you can
stare at it without much
discomfort. Now, try
looking into a weak
flashlight like a
traditional incandescent
Mini-Maglite. You'll
notice that it seems
very bright. This is
lux. Now, remove your
Minimag's head to put it
into candle mode. Try
switching between its
output and your ceiling
light's output. Since
they're now illuminating
approximately the same
area, the much higher
lumen value of the
ceiling light will
provide much higher lux
values at a chosen
point. An extreme
example of high lux and
low lumens is a laser,
which doesn't really
create that much light,
but focuses it into a
tiny, brilliant point.
Topic
18: CCT and CRI
A: CCT stands for
"Correlated Color
Temperature" and CRI
stands for "Color
Rendering Index." CCT is
expressed in terms of
degrees Kelvin,
corresponding to the
temperature of a
black-body radiator
(such as the Sun) at
that color temperature.
For example, a
black-body radiator
heated to about 8,000
degrees Kelvin would
appear slightly bluish,
so a light (an MH HID,
for example) with a CCT
of 8000K would have a
bit of a bluish tint to
it. CRI is expressed as
a number from 1-100 and
refers to how well a
light source reveals
colors regardless of its
CCT. For example, a
power LED that creates
blue light that is
filtered through a
yellow phosphor to end
up with white has no red
component, leaving red
and yellow objects
slightly faded. This
will lead to a low CRI
rating. Incans, on the
other hand, generally
have a CRI of around
100, as they emit all
spectra of visible light
(as well as IR and UV
light).
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