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Candlepower

Candlepower is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Candlepower rather than just read about it. In short: Candlepower (abbreviated as cp or CP) is the unit of luminous intensity in the United States customary systems of measurement. It is currently defined as being equal to 0.981 candela.

Candlepower — main illustration
Candlepower — illustration

Key takeaways

  • Candlepower belongs to science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Candlepower to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Candlepower from memory before moving on to harder problems.

Reference excerpt

Candlepower (abbreviated as cp or CP) is the unit of luminous intensity in the United States customary systems of measurement. It is currently defined as being equal to 0.981 candela.

History The term candlepower was originally defined in the United Kingdom, by the Metropolitan Gas Act 1860, as the light produced by a pure spermaceti candle that weighs 1⁄6 pound (76 grams) and burns at a rate of 120 grains per hour (7.8 grams per hour). Spermaceti is a material from the heads of sperm whales, and was once used to make high-quality candles.

Calibration of lamps To measure the candlepower of a lamp, a person judged by eye the relative brightness of adjacent surfaces—one illuminated only by a standard lamp (or candle) and the other only by the lamp under test. They adjusted the distance of one of the lamps until the two surfaces appeared to be of equal brightness. Then they calculated the candlepower of the lamp under test from the two distances and the inverse square law.

See also Candela Lumen (unit)

Notes and references

Further reading Rowlett, Russ (2001-07-13). "How Many? A Dictionary of Units of Measurement". University of North Carolina at Chapel Hill. Retrieved 2007-07-10. International candle at Sizes.com Last revised: 27 June 2007. Accessed July 2007 Candle History - Candlepower 2003 Bob Sherman at Craftcave. Accessed July 2007. Brief History Of Lighting 2004 by The Wolfstone Group. Accessed July 2007. A History of Light and Lighting by Bill Williams Edition: 2.3 - (2005) Accessed July 2007. Coaton, J. R. (February 2002). "The genesis of incandescent lamp - manufacture". Engineering Science and Education Journal. 11. IEEE: 21. doi:10.1049/esej:20020103 (inactive 11 July 2025). Archived from the original on September 9, 2017. Retrieved 2007-07-10. Measurement of the candlepower of lamps{{cite journal}}: CS1 maint: DOI inactive as of July 2025 (link) Cottington, Ian E. (February 1986). "Platinum and the Standard of Light - A Selective Review Of Proposals Which Led To An International Unit Of Luminous Intensity" (PDF). Platinum Metals Review. pp. 30, (2). Archived from the original (PDF) on 2013-01-31. Retrieved 2007-07-10. Metropolitan Gas Act of 1860

Illustrations

Candlepower illustration

Worked examples

Example 1 — a first encounter with Candlepower

Start with the simplest possible case. Write down what Candlepower claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Candlepower before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Candlepower ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Candlepower

In research
Candlepower appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Candlepower in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Candlepower is common in secondary-school and first-year university syllabi. It links to neighbouring topics Candles, Lighting, Obsolete units of measurement, so understanding it makes those chapters shorter.
In everyday life
Look for Candlepower outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Candlepower in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Candlepower means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Candlepower out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Candlepower in simple terms?

Candlepower (abbreviated as cp or CP) is the unit of luminous intensity in the United States customary systems of measurement. It is currently defined as being equal to 0.981 candela.

Why does Candlepower matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Candlepower?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Candlepower.

Tags

  • Candles
  • Lighting
  • Obsolete units of measurement
  • Units of luminous intensity

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