The candela

The candela (cd) defines luminous intensity (kukū whakaputa tūrama).

The candela was a unit first defined by the amount of light emitted by a common candle, and it is the only SI unit that has a definition based around human perception. It is used to measure the effectiveness of lighting systems made for the human eye.

The SI definition for the candela is the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency 540 × 10¹² hertz and that has a radiant intensity in that direction of 1/683 watts per steradian (W/sr).

In other words, the candela describes how bright a source of light will be when looking directly at it from a particular direction. There are a number of other – often more useful – photometric quantities than can be measured. These include illuminance (measured in lux), which describes how bright a surface illuminated by a source will be, and luminous flux (measured in lumens), which quantifies the amount of visible light emitted by a source.

Point of discussion: The candela is the only SI unit that is tied to human perception. Part of the definition (540 x 10¹²Hz) refers to visible green light. Why do you think this frequency was chosen?

Transcript

FARZANA MASOULEH

Candela describes how a source of light appears to the human eye. And one candela was defined to be as bright as a standard candle at the time of the definition. The constant unit that is used to define the candela is Kcd, which is a conversion factor for the original or absolute power divided by the power of that source that is perceived by the eye. So that is a constant conversion factor and is shown with that number [K_cd = 683 cd sr W^-1 at 540 x 10¹² Hz].

Also, MSL keeps candela for New Zealand so that the operating theatres are bright enough for a surgeon to see the patient.

Acknowledgements

This video clip is from a recording of a presentation by the Measurement Standards Laboratory of New Zealand (MSL) in celebration of the redefinition of the International System of Units (SI), which happened on 20 May 2019. The presentation by Peter Saunders and Farzana Masouleh of MSL was filmed at Unleash Space, Faculty of Engineering, Auckland University.