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Over view of an LED;
The basic LED comprises a diode chip mounted in the coined reflector cup of a lead frame, connected to electrical wires, and encased in a solid epoxy lens. It operates as a PN junction semiconductor diode - 'P' representing the material that conducts by virtue of an electron deficiency; and 'N' that conducts by virtue of free electrons. The LEDs emit a monochromatic light when energy levels change in the semiconductor diode, the shift in energy generating photons emitted as light energy. The specific colour wavelength of the light depends on the difference in energy levels and the type of semiconductor material used to form the LED chip - primarily compounds, formed by elements from groups III & V of the periodic table, which emit light when current is passed through them. LED luminosity increases as current increases depending on the semiconductor material used, and up to some maximum rating beyond which the LED will break down. To guard against breakdown, current limiting devices such as resistors, constant current ICs, or current limiting power supplies, are used in all applications.
The wavelength for visible colours such as blue, green, yellow and red fall into the spectral range from 400 to 700 nanometers (nm). Infrared LEDs reach wavelengths from 830 to 940 nm and higher. The colour of the LEDs is determined exclusively by the semiconductor compound used and not by the lens colour.
It is in colour applications where LEDs really shine. White light from
a typical incandescent bulb must be filtered so that only light from a
particular part of the spectrum is made visible. That takes its toll in
wasted energy and colour purity. Incandescent bulbs can waste 90% and
more of their energy in light blocked by the lens of the filter. By
contrast, because it is monochromatic, a light emitting diode is able
to deliver 100% of its energy as brilliant, pure unfiltered light in
the full spectrum of colour, and without any colour shift.
Multicolour LEDs are created by combining different LED chips within a
common LED housing and applying positive and negative voltages to turn
on the individual colours.
Lenses are available in several different configurations:
1. Clear types having no tint of diffusion and producing the greatest
light output and narrowest viewing angle. They are designed for
applications requiring very high intensity, or colourless LEDs in the
"off" state.
2. Tinted types to indicate in the "off" state of what the lamp's
colour will be when it is in the "on" state.
3. Diffused types having tiny glass particles embedded in the epoxy
lens. This spreads the light to a viewing angle of approximately ± 35
degrees from centre. These types of LEDs are often used for
applications in which the LED protrudes through a hole in the front
panel of the equipment.
4. Non-diffused types with no glass particles in the epoxy lens and
producing a narrow viewing angle of ± 12 degrees from centre. They are
often used in backlighting applications in which the LED is focused on
a translucent window of a panel.
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