Basic Lighting Knowledge 1) Lumen (lm)   Lumen is the international(SL)unit of luminous flux, a measure of overall light emitted by a light source.   2) Candela (cd)   Candela is the international(sl) unit of luminuous intensity,a measure of light emitted by a light souce in a particular direction. Any given light source will have different luminous intensities in different direction considered. [candela = lumen/solid angle]   3) Beam Angle [Unit : degree]   Beam angle is the angle betweent two directions for which the luminous intensity is 50% of the maximum intensity as measured in a plan through the nominal beam centerline.Beam angle of a lamp give an idea of how the light beam spread away form the beam center.   4) LUX (lx)   Lux is the international(SL)unit of illuminance, a measure of light arriving at a surface, 1 lumen per square foot equals 1 footcandle, 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 whent it is heated up to different temperature(in kevin). 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 ranges from 0-100 ,measures a light source's ability to reproduce the color of varies objects being lit by reference light source, black body radiator (CRI=100) 7) Efficacy (lm/W)   Eifficacy is the effcience of a light source to concert electricity power to light which is expressed in lumen output per unit power input (watt) [Efficacy = lumen / wattage] 8) Design Life Time   Design life time is the average life of lighting product when it is operated at nominal lamp voltage,current and controlled working environment.Actual lamp life will varies because of any different from those rating condition. 9) Dimmability   Not all lighting prodcuts is designed for dimming function.Dimming a non-dimmable LED lighting product will cause un-predictable result and may affect the life thime of the products.The below figure ahows the typical input current variation while adjusting the DC voltage input to dimmable andnon-dimmable product.