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The lighting industry uses several different units to measure light, depending on what information is needed.Below are a few of the most common units and terms:Flux (Luminous Flux) - Originating from the Latin word 'Fluxus,' meaning flow, flux is the amount of energy a light emits per second, measured in lumens (lm).When it comes to lighting, you need to consider watts (W) (energy used) versus lumens (lm) (brightness). Or electricity consumption versus light output. Lumens are weighted for human perception where as watts are not.Lumen (lm) - The SI unit of luminous flux, this is a unit of light flow.Watt (W) - The unit of measuring electrical power, this is a radiometric measurement.Intensity of Light - the Quantity of visible light that is emitted in unit time per unit solid angleCandela (cd) - The SI base unit of luminous intensity. It is a unit of luminous intensity of a light source in a definitive direction. 1 lumen = 1 candela x steradian (the SI unit of solid angle).Illuminance - the amount of luminous flux per unit areaLux (lx) - Lux is a standardized unit of measurement of the light intensity. The SI unit of illuminance and luminous emittance. One lux is equal to one lumen per square meter - formula: Lux - Lm/m^2Footcandle - A non-SI unit of light intensity. While lux is lm/m^2, a footcandle is lm/ft^2.What is Illuminance?Luminance is the intensity of light from a surface per unit area in a given direction.cd/m^21 cd/m^2 = 1 nitNit (nt) - A name given for a unit of luminanceFor an easier understanding, think of a lamp that produces light.The light from a lamp is measured in lumens (measure of light intensity)The light that falls on a surface is expressed as luxThe human eye sees this visually in terms of brightness, or luminance, that is measured in candelas |