Lighthouse lights

Candela lightpower

The candela (symbol cd) is the SI base unit of light intensity. The light intensity indicates how much light is in each piece of a light beam. The candela is one of the seven basic units of the SI (International System of Units). The luminous intensity of a light, or its candlepower (old definition), is now expressed in international units called candela (cd). Intensities of lighthouse beams can vary from thousands to millions of candelas.

Origin of the Candela

Prior to 1948, various standards for luminous intensity were in use in a number of countries. These were typically based on the brightness of the flame from a "standard candle" of defined composition, or the brightness of an incandescent filament of specific design. One of the best-known of these was the English standard of candlepower. One candlepower was the light produced by a pure spermaceti candle weighing one sixth of a pound and burning at a rate of 120 grains per hour. (spermaceti is the name for a waxy substance from which the spermatozoa exist in the skull of the porpoise. A pot fish has about 3000 liters of it. It consists largely of acetyl palmitate.).

It became clear that a better-defined unit was needed. Jules Violle had proposed a standard based on the light emitted by 1 cm2 of platinum at its melting point (or freezing point), calling this the Violle. The light intensity was due to the Planck radiator (a black body) effect, and was thus independent of the construction of the device. This made it easy for anyone to measure the standard, as high-purity platinum was widely available and easily prepared.

Examples of the Candela

⛆ The light intensity of an ordinary flashlight or candle is about 1 candela
⛆ A light bulb of 100 watts has a light intensity of approximately 120 candelas.
⛆ A fire alarm light emits approximately 75 candela depending on the settings.
⛆ A 25W compact fluorescent lamp will have an intensity of approximately 135 cd.
⛆ The light intensity of an LED lamp is measured in milli candela (mcd), or thousands of candela and will emit about 50 mcd.

Definition of the Candela

Due to the big differences it became evident that a better defined unit was needed. The official definition in the SI is as follows: "The candela is the light intensity in a given direction of a source which emits monochromatic radiation at a frequency of 540 × 1012 Hz and whose radiation intensity in that direction is 1/683 watts per steroid."

Monochromatic means light of one colour, or more precise: light of one wavelength. The frequency of 540 × 1012 hertz corresponds to green light (wavelength of approximately 555 nm). Steradial is the unit of space angle. The space angle indicates how fast a light beam flares out. For a cone-shaped light beam, this is related to the opening angle. If a light source is equally strong in all directions, the space angle is 4π steradials.