The candela (symbol: cd) is the base unit of luminous intensity in the International System of Units (SI). Luminous intensity is a measure of the amount of light emitted by a source in a particular direction. The candela is crucial for the science and technology of lighting, including both the design and the assessment of lamps, lighting fixtures, and other sources of illumination.

Here’s a detailed overview:

  1. Historical Evolution:
    • Initially, light was measured in terms of the apparent brightness of a source to the human eye. However, this approach was subjective and didn’t allow for precise, reproducible measurements.
    • The definition of the candela has evolved over the years to provide a more objective and accurate measure of luminous intensity.
  2. Current Definition (as of 2019):
    • The candela is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540×1012540×1012 Hz (which corresponds to a wavelength of about 555 nm, near the peak of human visual sensitivity) to be 683 cd/W683cd/W, when expressed in the unit lm/Wlm/W, which is equal to cd⋅sr⋅W−1cd⋅sr⋅W−1, or cd⋅sr⋅J−1cd⋅sr⋅J−1, where the joule and the watt are defined in SI units.
    • This definition provides a precise, objective standard for measuring luminous intensity.
  3. Significance in Science and Technology:
    • The candela is crucial for the design, evaluation, and quality control of lighting products and systems, allowing manufacturers, regulators, and consumers to communicate accurately about levels of illumination.
    • It is fundamental to fields like vision science, photometry, colorimetry, and optical engineering.
  4. Industrial and Everyday Applications:
    • The candela is used in a wide range of industrial applications, including automotive lighting, street and highway lighting, architectural lighting, and consumer electronics.
    • In everyday life, the candela is relevant wherever light levels are important, such as in home lighting, office lighting, and public spaces.
  5. Standards and Regulations:
    • Accurate measurement of luminous intensity, expressed in candelas, is essential for ensuring compliance with many national and international standards and regulations regarding lighting performance and efficiency.
  6. Technological Advancements:
    • The precise definition of the candela supports the development of new lighting technologies, such as LED and OLED lighting, and helps drive advancements in energy efficiency, optical design, and smart lighting systems.
  7. Education and Research:
    • The candela is a fundamental concept taught in education and research related to light and optics, enabling students, scientists, and engineers to explore and understand the behavior and control of light.
  8. Global Standardization:
    • The candela facilitates global standardization and interoperability in the lighting industry, enabling international collaboration, trade, and communication in science, technology, and industry.

The candela, as defined in terms of fundamental constants, represents a critical step forward in the international system of units, ensuring that luminous intensity can be measured with high precision and consistency across a wide range of modern scientific, industrial, and everyday contexts.