Industry Regulations & Standards

candela (cd)

The Candela: A Light Measurement for the Ages

In the realm of electrical engineering, light, like electricity, is a fundamental force that requires precise measurement. One of the most important units in this field is the candela (cd). The candela represents the luminous intensity of a light source, essentially quantifying its "illuminating power."

Defining the Candela:

The candela, or "candlepower," is defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^12 hertz (corresponding to green light) and has a radiant intensity in that direction of 1/683 watt per steradian. This sounds complex, but it essentially boils down to measuring the light emitted by a specific area of a blackbody radiator at a particular temperature.

A Historical Perspective:

Historically, the candela was based on the light emitted by a standard candle. This method, while straightforward, lacked precision. Today, the candela is defined using a more rigorous scientific approach, but its name still reflects its origins.

Understanding the Candela:

To visualize the candela, think of a light bulb emitting light in all directions. The luminous intensity is measured in a specific direction, like the beam of a flashlight. This intensity is measured in lumens per steradian (lm/sr). A steradian is a unit of solid angle, like a cone of light, so the candela measures the amount of light emitted within a specific solid angle.

Applications of the Candela:

The candela is widely used in various applications, including:

  • Lighting design: Engineers use candelas to calculate the required luminous intensity of light fixtures for different spaces, from homes to stadiums.
  • Photography: Camera settings, particularly aperture and shutter speed, are adjusted based on the luminous intensity of the light source to capture the desired image exposure.
  • Automotive lighting: Candelas are crucial in the design of headlights and taillights to ensure optimal visibility and safety.
  • Scientific research: The candela is essential for measuring light in a wide range of scientific disciplines, such as astrophysics and optics.

In Conclusion:

The candela is a vital unit of measurement that allows engineers, designers, and scientists to accurately quantify the illuminating power of light sources. Although it has evolved from its humble candle-based origin, its importance remains steadfast in the world of electricity and light.


Test Your Knowledge

Candela Quiz:

Instructions: Choose the best answer for each question.

1. What does the candela measure? a) The total amount of light emitted by a source. b) The intensity of light in a specific direction. c) The brightness of a light source. d) The energy of light waves.

Answer

b) The intensity of light in a specific direction.

2. What is the historical basis for the candela? a) The brightness of a standard light bulb. b) The light emitted by a star. c) The light emitted by a standard candle. d) The intensity of sunlight.

Answer

c) The light emitted by a standard candle.

3. Which of the following is NOT a direct application of the candela? a) Designing headlights for cars. b) Measuring the luminosity of a distant galaxy. c) Determining the resistance of a wire. d) Adjusting camera settings for optimal exposure.

Answer

c) Determining the resistance of a wire.

4. What is the unit of solid angle used in the definition of the candela? a) Radian b) Degree c) Steradian d) Lumen

Answer

c) Steradian

5. What is the frequency of light used in the modern definition of the candela? a) 540 x 10^6 Hertz b) 540 x 10^9 Hertz c) 540 x 10^12 Hertz d) 540 x 10^15 Hertz

Answer

c) 540 x 10^12 Hertz

Candela Exercise:

Task: A street lamp emits 2000 lumens of light in all directions. Assuming the lamp distributes light evenly, calculate the luminous intensity in candelas.

Hints:

  • Remember that a steradian represents a solid angle.
  • The total solid angle of a sphere is 4π steradians.

Exercice Correction

Here's how to solve the problem:

1. **Calculate the solid angle:** Since the lamp emits light evenly, the solid angle is 4π steradians.

2. **Divide the total lumens by the solid angle:** 2000 lumens / 4π steradians ≈ 159.15 lumens/steradian

3. **Convert lumens/steradian to candelas:** 1 lumen/steradian = 1 candela. Therefore, the luminous intensity is approximately **159.15 candelas**.


Books

  • "The International System of Units (SI)": This official publication by the International Bureau of Weights and Measures (BIPM) provides the most comprehensive definition of the candela and its relation to other SI units.
  • "Light Measurement and Photometry: An Introduction to the Science and Technology of Light" by John H. Poynton: This book covers various aspects of light measurement, including the candela and its applications in different fields.
  • "Handbook of Optics, Volume I: Fundamentals, Techniques, and Design" (edited by Michael Bass): This multi-volume handbook offers in-depth information on light and optics, including sections on photometry and the candela.

Articles

  • "The Candela: A Historical Perspective" by John V. McDavid: This article explores the historical development of the candela from its candle-based origins to its modern definition.
  • "The SI Unit of Luminous Intensity: The Candela" by R. W. G. Hunt: This article published in the Journal of the Illuminating Engineering Society provides a detailed explanation of the candela and its relationship to luminous intensity.
  • "The Candela and its Applications in Lighting Design" by J. D. Goodman: This article discusses the practical applications of the candela in lighting design and the challenges associated with its use.

Online Resources

  • NIST Website: The National Institute of Standards and Technology (NIST) website offers comprehensive information on the candela, including its definition, history, and applications.
  • BIPM Website: The International Bureau of Weights and Measures (BIPM) website provides official information about the SI system, including the definition of the candela.
  • Wikipedia Page for Candela: This page offers a concise and readily accessible overview of the candela, its history, and its use in different areas.

Search Tips

  • "Candela definition": This search will provide numerous definitions of the candela from various sources.
  • "Candela applications": This search will highlight the different uses of the candela in various fields, like lighting, photography, and scientific research.
  • "Candela history": This search will reveal information about the origins and evolution of the candela as a unit of measurement.

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