SI unit of measurement

Definition of Brightness

Candela is a unit that measures the brightness of a light source in a particular direction, also known as luminous intensity.

It tells us how bright a light source appears to the human eye, which is crucial for understanding how well a light will perform in a given situation. It’s the most useful metric for determining overall brightness.

Candela as a measurement has replaced candles, which were historically the measure of luminous intensity. A common wax candle emits approximately one candela of luminous intensity.

Candela = Lumens ÷ Solid Angle


  • Candela is the luminous intensity of the light source in the given direction, measured in candela (cd).
  • Lumens is the total amount of light emitted by the source in that direction, measured in lumens (lm).
  • Solid Angle is the angle in steradians (sr) that the light is emitting into. This represents the portion of the total sphere of emission that the light is occupying.

Thankfully, digital photometers do all the hard work of calculating candelas.

However, the formula helps us to understand that the angle of light does affect the candela of a lamp. A narrow spot beam will have a higher candela than a wide angle beam as it concentrates the light more. Distance doesn’t change the candela of a light source.

diagram showing how light angle affects candles

In boat lighting, candela is an important consideration for navigation lights and other signaling lights that need to be visible over long distances and in all weather conditions.

The brightness of the light source, measured in candela, determines how far the light will be visible and how well it will stand out against background light.

For example, sidelights that need to be visible from dead ahead and up to 22.5 degrees abaft of the beam, must produce 4.3 candelas of light to meet the U.S. Coast Guard rating of 2 nautical miles visibility.

Example of Candela in a Sentence

"The lighthouse lamp has a candela rating of 1,000,000, allowing it to emit a powerful beam of light that could be seen by ships up to 20 nautical miles away."

Synonyms: luminous intensity, luminous flux density

Related Terms for Candela