Question

LED Lighting BASIC PHYSICS:

a) Give typical forward I-V curves for R-G-B LEDS. Be specific in your three explanations and use manufacturers spec sheets. Also explain why LED lights are both Hg (think bulb breakage effects and land fill issues with dead fluorescent bulbs) and UV free as compared to fluorescents. UV rays cause colors to fade in retail clothing stores, art work to fade in museum lighting and UV accelerates the rancidity of meat fat. UV accelerates the rancidity of meat fat.

b) Explain how and why LED emission takes place over a small solid angle or in simple terms in beam like and unidirectional. Use manufacturer spec sheet plots of emission angles. In contrast fluorescent and incandescent light is omni-directional spread evenly over all 4π sterradians.

Answer 1

The way colors of individual LEDs are determined is the energy an electron loses when the electron moves from one side of the LED to the other. The amount of energy the electron sheds in the form of light is determined by the materials in the LED. The photon that is generated will have a characteristic wavelength and manufacturers have chosen materials to arrive at the desired colors. The red to blue characteristics deviate as the band gap increases from red to blue which in turns increases the forward voltages.

LED is a electronic diode that does not contain Mercury unlike in Fluorescent bulbs. They does not break apart in its lifetime as in bulbs and so it does have any environmental challenges while dumping or disposals.

they also does not radiate UV rays which have harmful radiations , The Phosphors present in the LED convert the UV into visible white light

(B) :

The explanation am providing here by assuming that you know the basic principle of LED and its working. Basically there are two ways to fabricate LEDs.in both the cases p-n junction grown on sapphire substrate.if the LEDs directly manufactured from the substrate the radiation angle will be wider and emit light in all directions.T get the emission in desired direction with a narrower angle,a reflector is attached to light-emitting layer and both these are connected to a common substrate like germanium. After that we removed original substrate and this reflector than prevents the light emission into the substrate.In this case maximum angle of radiation is 90 degree.

the bottom diagram shows emission characteristics for different LEDs , compare with Lambertian emission. The first dotted line(------)shows emission for oslon square LED, the middle one is for oslon SSL 150, and the upper one(....) is for oslon 80. You can see a different angle at different intensities.So LEDs whose characteristics fall on the Lambertian curve emits all its light less than 90-degree angle and those characteristics which deviated from Lambertian cure they reflect some percentage of light into the substrate.

LEDs emission characteristics are selected according to requirements and applications.

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