LED Overview
LED- Light Emitting Diode is a semiconductor light source.
What is Electroluminescence?
When a LED is forward biased (switched on), electrons recombine with electron holes within the device, releasing energy in the form of photons. This effect is called electroluminescence.
The semiconductor materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light. Energy gap of the semiconductor determines the color of the Light.
Construction
LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. LED’s especially with GaN/InGaN, use sapphire substrate. Most materials used for LED production have very high refractive indices. This means that much light will be reflected back into the material at the material/air surface interface. The refractive index of silicon is 4.24, while air is 1.0002926.
Color Spectrum and Materials
White Bright LED
Two ways of producing high intensity white-light using LEDs.
- One is to use individual LEDs that emit three primary colors RGB and then mix all the colors to form white light.
- The other is to use a phosphor material to convert monochromatic light from a blue or UV LED to broad-spectrum white light.
LED Lifetime
In a standardized manner lifetime has been suggested to use the terms L75 and L50 which is the time it will take a given LED to reach 75% and 50% light output respectively. The rated life time is the time till then the output light intensity remains same. Generally, the typical life time of an LED is more than 50,000 hours.
L70: It refers to lumen depreciation to 70% of initial lumen output; stated conversely, it indicates 70% lumen maintenance. “B” specs add a target statistical confidence interval. Thus, B50 indicates no more than 50% of a sample of LED devices would be expected to have their light output drop below a target lumen maintenance level. B10 would mean no more than 10% of the sample met that L standard within the given time.
Advantage
- Lower Power consumption
- Longer Life
- Higher Efficiency
- Uni-directional
- Specific color
- Eco-friendly
Application
- Indicators and signs - The low energy consumption, low maintenance and small size of modern LEDs has led to uses as status indicators and displays on a variety of equipment and installations.
- Lighting - LED lights provide an efficient light source with less heat output.
- Smart lighting - Light can be used to transmit broadband data.
- Light sources for machine vision systems - Machine vision systems often require bright and homogeneous illumination.
- Remote controls, etc.,
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