Candela - SI Unit of Measurement

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What is Candela?

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Vision is one of the most important senses that allow humans to interact and perceive the world. Vision detects light from the surroundings to gain information about the environment around oneself. Candela is the SI unit used to measure the visual intensity of a light source (luminous intensity). This is a unique SI unit, which is highly dependent on human perception.  

The Candela is the SI unit for photometry and should not be confused with the total light emitted by an object, which is given by lumen. Candela merely represents the amount of light perceived by the human visual system. 

Origin of Candela

Candela Latin word ‘Candle’ is a word that refers to an ancient light source, still in use today. Before 1948, there was no standard measuring system for luminous intensity around the world. The luminosity measure was based on the brightness of a candle/ incandescent lamp of certain composition or design respectively. 

Back then, Candlepower was used to define luminous intensity depending on the candle. The English standard of candlepower was defined as the light produced by a pure spermaceti candle weighing 1/6th pound with a burning rate of 120 grains per hour whereas countries like Germany, Scandinavia, and Austria use the Hefner lamp as a luminous object. 

The candela was first defined by French Physicist and inventor, Jules Violle. He launched a unit called Violle, which is the light emitted by 1cm2 of platinum at its melting or freezing point. This intensity of emitted light was caused by Plank radiation (due to the black body effect) and thus eliminated the dependency of the unit on the construction of the device. This also made it easy to measure the quantity as the device was easy to manufacture and high-quality platinum was readily available back then. 

Definition of Candela

In 2018, the 26th general conference on weights and measures redefined candela, which came into effect from 20th May 2019, defined one candela as the luminous intensity in a given direction produced by monochromatic radiation of frequency  540 × 1012 Hz and has an intensity of 1/683 watts per steradian.  

The frequency of around 555nm is chosen because the human eye is most sensitive towards this frequency under bright conditions. In this condition, photopic vision prevails over scotopic vision. 

Candela Expression

One candela is mathematically expressed as: 

1cd= (\[\frac{Kcd}{683}\]) Kgm2 s-3 sr-1

The unit is dependent on Luminous intensity and radiant intensity for a given wavelength of light. A common candle emits candela power 1 cd of luminosity on average. Luminance is measured in Lux, which is defined as one candela per square metre. It is also called nits in some cases. 

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Figure: Graph of Photopic Luminous Intensity

Other Measures of Light

Lumen is also a measure of light intensity. Lumen defined the total luminous flux of a light-emitting body by multiplying the angular span of the emitted light with intensity (expressed in candela) If we take an example of an LED, the total light energy produced by different types of LED is almost the same, but the apex angle of emission varies. The LED, which emits at a narrow-angle, will have greater intensity, than the same energy emitted over a broader angle. 

Lux is the measure of luminance on a surface. Lux is expressed in units of candela per square meter. On a sphere of radius 1 meter, one steradian represents an area of 1 m2. Thus as a distance of 1 meter, one candela is equal to the values for lux. 

Foot Candela

Foot-candle is a unit of illuminance/ light intensity, which is not recognized as a SI unit. One foot-candle is defined as one lumen per square foot. If a point source of light of 1 candela is placed in the centre of a sphere of 1-foot radius, then the illumination on the inside surface of the sphere is 1 unit of foot-candle. One foot-candle is about 10.76 lux. A foot-candle is a common unit of measurement around the world In the architectural industry to map and plan spatial lighting systems. It is also essential for monitoring lighting in places like museums and galleries where there may be photosensitive objects. It is also required in the maintenance of greenhouse or artificial plant culture environments as light intensity is a major factor in the rate of photosynthesis in plants. 

Did You Know?

  • The candela is the one and only SI base unit used to quantify our perception of a piece of objective physical reality. 

  • The sun is one of the brightest objects we see in our daily life. It has been found that illuminance due to the sun on earth is about 133000 lux. Upon conversion to candlepower, the luminous intensity of the sun becomes equivalent to 3.0 x1027 or 3 octillion candles. 

So, that is all about Candela, an important unit and that has immense significance in the world of science. 

FAQ (Frequently Asked Questions)

Q1. How Accurate is the Measurement of Candela?

Ans. Candela is an SI unit that somewhat depends on human perception, thus its accuracy is not that high. The nation’s measurement laboratory is trying to reduce this uncertainty in luminous intensity management. The accuracy for measuring a second, is at a range of a billionth second, whereas candela has an accuracy of less than one in a thousand (0.1%). It is important to improve the accuracy of this SI unit as it has many industrial applications.

Q2. How to Increase the Accuracy of Candela?

Ans. The national measurement laboratory, NIST is trying to improve ways to measure candela. Up till now, measurement of photometer responsivity was dependent on splitting of light with a prism and running each colour through a photometer. This method causes narrowing of the spectrum lines due to an increase in slice width. NIST has developed a pulsed laser that can be tuned to particular wavelengths of light, to extrapolate pure colours of about 0.1nm width. This will allow us to create photometers that mimic the human vision mechanism, removing subjective vision from the measurement.