LED Lights

[jayway]

The lifetimes of traditional light sources are rated through established test procedures. For example, CFLs are tested according to LM-65, published by the Illuminating Engineering Society of North America (IESNA). A statistically valid sample of lamps is tested at an ambient temperature of 25° Celsius using an operating cycle of 3 hours ON and 20 minutes OFF. The point at which half the lamps in the sample have failed is the rated average life for that lamp. For 10,000 hour lamps, this process takes about 15 months. Full life testing for LEDs is impractical due to the long expected lifetimes. Switching is not a determining factor in LED life, so there is no need for the on-off cycling used with other light sources. But even with 24/7 operation, testing an LED for 50,000 hours would take 5.7 years. Because the technology continues to develop and evolve so quickly, products would be obsolete by the time they finished life testing. The proposed method involves operating the LED component or system at rated current and voltage for 1,000 hours as a “seasoning period.” This is necessary because the light output actually increases during the first 1,000 hours of operation, for most LEDs. Then the LED is operated for another 5,000 hours.

Electrical and thermal design of the LED system or fixture determine how long LEDs will last and how much light they will provide. Driving the LED at higher than rated current will increase relative light output but decrease useful life. Operating the LED at higher than design temperature will also decrease useful life significantly. Most manufacturers of high-power white LEDs estimate a lifetime of around 30,000 hours to the 70% lumen maintenance level, assuming operation at 350 milliamps (mA) constant current and maintaining junction temperature at no higher than 90°C. However, LED durability continues to improve, allowing for higher drive currents and higher operating temperatures. Specific manufacturer data should be consulted because some LEDs available today are rated for 50,000 hours at 1000 mA with junction temperature up to 120°C.2

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[jayway]

1) Lumen (lm)



Lumen is the international(SI) unit of luminous flux, a measure of overall light emitted by a light source.



2) Candela (cd)



Candela is the international(SI) unit of luminous intensity, a measure of light emitted by a light source in a particular direction.

Any given light source will have different luminous intensities in different directions considered.

[candela = lumen/solid angle]



3) Beam Angle [Unit : degree]



Beam angle is the angle between two directions for which the luminous intensity is 50% of the maximum intensity as measured in a plan through the nominal beam center-line. The beam angle of a lamp gives an idea of how the light beam spread away form the beam center.



4) LUX (lx)



Lux is the international(SI) unit of illuminance, a measure of light arriving at a surface, 1 lumen per square foot equals 1 foot-candle, while 1 lumen per square meter equals 1 lux.

[lux = lumen/m2]



5) Color Temperature / Correlated Color Temperature (CCT)



A black-Body radiator will have different color when it is heated up to different temperature(in kelvin). The temperature at which the heated black-body radiator matches the color of the light source is the color temperature of the light source.



6) Color Rendering Index (CR)



Color rendering index, ranged from 0-100, measures a light source's ability to reproduce the color of various objects being lit by reference light source, black body radiator (CRI=100)



7) Efficacy (lm/W)



Efficacy is the efficacy of a light source to convert electricity power to light expressed in lumen output per unit power input (watt)

[Efficacy = lumen / wattage]



8) Dimming



Not all lighting products are designed for dimming function. None-dimming LED lighting product will cause unpredictable result and may affect the life time of the products. The below figure shows the typical input current variation while adjusting the DC voltage input to dimming and none-dimming product.