SI Units – Luminous Intensity

The candela (cd) is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 10¹² Hz, K_cd, to be 683 when expressed in the unit lm W⁻¹, which is equal to cd sr W⁻¹, or cd sr kg⁻¹ m⁻² s³, where the kilogram, meter and second are defined in terms of h, c and ∆ν_Cs.

Radiation at frequencies other than 540 x 10¹² Hz is also measured in candelas in accordance with the standard luminous efficiency, V (λ), curve that peaks at 540 x 10¹² Hz (yellow-green). Learn more about the realization of the candela and luminance (candela per square meter).

Resources for Students and Teachers

• SI Base Units Relationship Poster (SP 1247) – a colorful poster illustrating the relationships of the International System of Units (SI) derived units with special names and symbols and the seven traditional base units. (NIST)
• SI Units Card Deck (SP 1297) – this activity offers a fun way to enhance understanding of the International System of Units, including the defining constants, base units, derived units with special names, and prefixes. (NIST)
• Candela – Vision is perhaps the most powerful of human senses, and light one of humanity’s age-old metaphors for understanding, wisdom and truth. (NIST)
• Tour of the Electromagnetic Spectrum – Visible Light. (NASA)
• Lights on the International Space Station – Exploring Space Through Math. (NASA)
• Science of Light – Learning more about the science of light and human vision will help us understand the value and fragility of natural lightscapes. (National Park Services)
• Measures of Brightness. (Institute of Physics)
• Top 10 Tips for Teaching the Metric System. (NIST)

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Credit: J. Wang and B. Hayes/NIST ©2020 U.S. Secretary of Commerce. All Rights Reserved.

League of SI Superheroes – Candela

This comic book-style video animation series has been developed to help middle school students learn about the 7 SI base measurement units. Don't let her small size fool you, Candela's power over light helps to brighten the whole world. A candela is a measure of brightness about equal to the light given off by a single candle.

Navigate to more SI base unit information

• SI Units
• Fundamental Physical Constants
• Length – meter (m)
• Time – second (s)
• Amount of substance – mole (mole)
• Electric current – ampere (A)
• Temperature – kelvin (K)
• Mass – kilogram (kg)

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