So basically, if you need a light bulb that emits light in all directions (for example ceiling light in a house) look at lumens when comparing various bulbs, however, if you need a light that can focus the maximum amount of brightness in a smaller beam such as a spot light or a flashlight, look at candelas when comparing such lights. Luminous intensity measures how much light is emitted by a light source in a single direction, the unit of luminous intensity is candela (cd). To recap, luminous flux measures how much total visible light is emitted by a light source, the unit of luminous flux is lumen (lm). The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. One regular candle emits approximately 1 candela of luminous intensity, this is why candela was also called candle in older times.īecause candle was not the most precise light source to measure light intensity, a lot stricter rules and definition for measuring light intensity were defined, the official definition of candela: Candela replaced the older unit that was used to express luminous intensity – candlepower.

Basically, it measures the quantity of visible light emitted in one specific angle from the light source, which is a useful measurement when comparing devices that produce focused beam of light such as spot lights, flashlights and laser pointers.Ĭandela definition – candela (cd) is a measurement unit in SI of luminous intensity. Luminous intensity is measured in candela (cd) which is a SI base unit. Luminous intensity is the luminous power or the quantity of visible light emitted by a light source in a given direction per unit solid angle. To describe this, a term called Illuminance and units called lux or foot-candle are used. However, because lumens are measured at a specific distance in all directions from the light source, it isn’t the best measurement to describe how bright a light is going to be on a specific area. Luminous flux (in lumens) is usually indicated on the packaging of a light bulb (or can be found in special catalogs of light bulbs) and is used as an objective measurement of the light output of a light source to better compare various types of light bulbs. One lumen equals the amount of light emitted by a light source (radiating equal amount of light in all directions) through a solid angle of one steradian with an intensity of 1 candela. Lumen definition – lumen (lm) is a measurement unit of a luminous flux or luminous power. Luminous flux only measures light radiated over visible wavelengths to a human eye from about 400-750 nm. In simple words, luminous flux tells how much light a lamp puts out in all directions per second, luminous flux is expressed in units called lumens (lm). Luminous flux or luminous power measures the total amount of light emitted by a light source over a period of time.

Equation to calculate light intensity