What is the Candela Unit of Measurement?

Kate Onissiphorou

by Kate Onissiphorou

4th May 2022

The candela is the base unit in the International System of Units (SI) that measures how bright or radiant a light source is, given certain wavelengths that are visible to the human eye. It’s measured in terms of the luminous intensity of a point light source per unit solid angle, known as steradian. Its abbreviated form is ‘cd’.

As a base unit of measuring luminosity, the candela is also related to other units of measurement, like the lumens and lux. The main differences between the three concepts are outlined below:

  • Lumen – a measure of the total amount of light given off by a source when uncovered
  • Lux – the brightness of the illuminated surface area at a given distance
  • Candela – the light intensity as seen from the point source of light but at a specific wavelength of visible light.

While they might appear similar, they are distinct units with their own specific parameters, uses and related concepts. For instance, if you were to buy a projector, you’re more likely to encounter lumens. However, if you wanted to find out the optimal distance of the projector from the wall, lux would be a good concept to consider.

What is the use of candela?

Candela (cd) serves as the base unit of measuring luminosity as perceived by the human eye. It’s useful in calibrating artificial light sources like LED bulbs. 

As the name suggests, the candela unit of measurement is based on the luminous intensity of a wax candle. The common wax candle has a luminous intensity equivalent to approximately one candela. However, the scientific definition is more nuanced and technical.

Three white vessel candles on a pile of books

A wax candle has a luminous intensity equivalent to one candela

A candela measures the luminosity of visible light from a point source in a given direction and area called steradian. A steradian is an area of a sphere that is equal to the square of the radius of the sphere. Commonly, it is the unit sphere with a radius of 1.

More precisely, a candela is the measure of intensity in a given direction of a monochromatic, which is green, having the frequency of 540 × 1012 Hz with the radiant power of 1/683 watt per steradian.

The lumen, which is derived from candela, is a measure of luminous flux, which differs from radiant flux. It is the total quantity of light emitted by a source per unit time. Meanwhile, lux is equivalent to lumen per square metre.

An infograogic explaining lumens and candelas

Source

You can use these concepts to calculate the amount of light necessary to illuminate a room or an open space. From there, you can then work out the number of LED bulbs, fluorescent tubes, incandescent bulbs, or other artificial light sources that are required to light an area. 

Calculating illumination is very useful when designing building interiors and open public places. It enables you to precisely determine the optimal position of street lights, for instance. It’s also vital to the filmmaking industry, as the right lighting can set the mood and storytelling. With one film scene or succeeding scenes often requiring several takes, it’s sometimes necessary to use photometers to recreate the precise lighting conditions to achieve consistency. Similar concepts are used in photography, such as dynamic range and exposure.

How do you measure candela?

You can measure candela by using a digital photometer or other similar instruments. In some cases, you may need to do some calculations to convert the units from lumens and lux.

Person holding a digital light lux meter to read the intensity of light

Digital photometers measure the illuminance of an indoor or outdoor area

You must remember the following conversions:

  • 1 lumen (lm) = 1 candela (cd) ⋅ streadian (sr)
  • 1 sphere = 4π sr
  • 1 cd × 4π sr = 4π cd⋅sr ≈ 12.57 lm

Hence, 12.57 lumens is the total luminous flux of a light source that uniformly radiates 1 cd in all directions.

How is candela used in chemistry?

Candela as a unit of measurement has no direct use in chemistry. It does, however, have some importance when analysing the chemical compositions of substances based on their spectroscopic signatures.

You can also compare the energy released by some chemical reactions based on the intensity of light they produce.

Is candela a base unit?

Candela is an SI base unit that’s used to derive other units, such as lumens and lux. Before the standardisation of measuring light intensity, the Hefnerkerze or Hefner lamp output was used as a standard.

One Hefnerkerze is equivalent to about 0.920 candelas, which is almost a 1:1 ratio.

What is the relationship between candela and lumens?

As a base unit, the candela is equivalent to one lumen per steradian. This translates to 12.57 lumens for a sphere, assuming that a source emits light equally in all directions.

Original post: What is the Candela Unit of Measurement?. No Republication or Redistribution allowed without written consent. Contact ReAgent Chemical Services for more information.

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