Everything about LEDs you wanted to know

An LED is a semiconductor device that emits light on certain wavelengths. A semiconductor chip (die) mounted on a printed circuit board (PCB) with power, driver, heat sink, lens, reflector, and diffuser, comprises an LED fixture or LED luminaire. An LED emits both heat and light. Light is emitted in the form of particles called photons.

 

Why do I need a lens, reflector and diffuser in an LED fixture?

An LED mounted on a PCB emits light in nearly 180 degrees. The optical lens acts as a collimator, transmitting light in a more aligned direction.  A reflector increases efficiency by ensuring as much light as possible is leaving the fixture. A diffuser spreads out or scatters light and hides individual LEDs. A heat sink provides a path for heat to travel from the LED to the exterior of the fixture where it dissipates in the surrounding air, thus cooling the system.

 

What is the life time of LED?

The Lifetime of an LED is generally considered to be the number of hours it takes to reach lumen depreciation by 30%, as suggested by the Illuminating Engineering Society (IES). The human eye can notice a difference in light output when it drops to 70% of the original lumen output. This metric is indicated as L70.

Which factors determine an LEDs’ lifetime?

The lifetime of an LED product is mainly determined by the luminaire design. Factors that affect the LED lifetime are thermal management, amount of drive current, quality of the heat sink and even ambient factors such as room temperature and humidity. Of these, thermal management is perhaps the most important factor in the life of an LED.

The resins which encapsulate the LEDs force most of the heat to be transferred through the back of the LED array and then conducted out of the fixture through the heat sink where heat dissipates into the surrounding air. Without proper thermal management, the junction temperature of the LED rises, which results in colour shifting, reduction in light output levels, and a short LED life.

 

What are LM-79 and LM-80 tests?

LM-79 and LM-80 are testing standards developed by IES in order to ensure the specification integrity of lighting systems.

LM-79: This test provides for the total luminous flux, power, efficacy and chromaticity of an LED luminaire.

LM-80: This test measures the lumen depreciation of a luminaire.

Advantages of LED

·         LEDs offer some significant advantages over incandescent and flourescent lighting.

·         Less power consumption

·         High lifetime

·         Small in size

·         Focus light in specific direction

·         No UV/IR radiation

·         Excellent colour quality

·         No flicker

·         Switches on instantaneously

·         High efficacy

 

LED terminology

Solid state lighting (SSL) term being used for light being emitted from a semiconductor source rather than a gas or filament

Light emitting diode (LED) produces light by the movement of electrons inside a semiconductor chip. LEDs are PN junctions

Driver is a simple electronic circuit used for connecting an LED to the mains. It converts AC to DC while reducing the drive currents for the LEDs

Lumen depreciation is the reduction in light output (lumens) over the lifetime of the LED.

Junction temperature is the temperature at the PN junction due to heat.

PCB stands for printed circuit board. It is a thin plate on which chips and other electronic components are placed.