What's the difference between candela and lumen?

Candela measures light intensity in one direction. Lumen measures total light output in all directions. A focused flashlight may have high candela (intense beam) but lower lumens than a bare bulb with low candela but light going everywhere.

Why is candela an SI 基本単位?

Candela measures visible light's effect on human vision, which can't be derived from other physical units alone. It connects photometry (light as humans see it) to radiometry (raw electromagnetic power). The weighting for human eye sensitivity makes it fundamentally different from watts.

How do I compare flashlight brightness?

Use candela for throw distance (focused beam brightness). Use lumens for overall output. A thrower flashlight has high candela, low lumens; a flooder has high lumens, lower candela. Beam distance in meters ≈ 2 × √candela.

光度 Converter

About Luminous Intensity Conversion

Luminous intensity measures the amount of light emitted in a specific direction—how bright a light source appears from a particular viewing angle. Unlike luminous flux (total light output), intensity is directional and describes the concentration of light within a cone of specific solid angle. A laser pointer has extremely high intensity in its narrow beam despite low total lumens, while a frosted bulb has low intensity spread across a wide angle. This directional nature makes candela the key specification for searchlights, stage spotlights, and any focused light application.

The SI単位 is the candela (cd), one of the seven SI 基本単位s—the only one that directly incorporates human visual perception. It's defined by the luminous efficacy of 540 THz (green-yellow) radiation at exactly 683 lumens per watt. Luminous intensity is crucial for flashlight throw distance, vehicle headlight design and regulation, signal beacons, and any application where directional light output determines performance. The relationship lumen = candela × steradian connects intensity to total flux.

Our converter handles all standard luminous intensity units used in photometry and lighting specifications.

Common Luminous Intensity Conversions

変換元	変換先	乗数
cd	lm/sr	1 (equivalent)
cd	candlepower (cp)	1 (equivalent)
cd	mcd (millicandela)	1,000
mcd	cd	0.001
cd	Hefnerkerze (HK)	1.106
Hefnerkerze	cd	0.904
cd	carcel	0.104
carcel	cd	9.6
cd	bougie décimale	1.02

Luminous Intensity 単位リファレンス

Candela (cd) – The SI 基本単位 of luminous intensity, defined as one lumen per steradian. Originally based on the light output of a standard candle, the modern definition references monochromatic 540 THz radiation with luminous efficacy of exactly 683 lm/W. This makes the candela the only SI 基本単位 that directly incorporates human visual perception. Typical values: candle flame ~1 cd; desk lamp ~100 cd; car headlight 20,000-200,000 cd; lighthouse beam millions of cd.

Candlepower (cp) – Historical term now standardized as exactly equal to the candela. Originally, candlepower varied by country—the British standard candle, German Hefnerkerze, and American standard all differed slightly. International standardization in 1948 unified these under the candela. The term persists in flashlight marketing and colloquial usage, particularly in the US.

Millicandela (mcd) – 1/1000 candela (10⁻³ cd), the standard unit for LED specifications. Indicator LEDs typically range from 20-100 mcd (visible but not bright) to 5,000-20,000 mcd for high-brightness types. mcd specifications assume a specific viewing angle—a 5° LED appears brighter than a 60° LED of equal lumens because the light is concentrated.

Hefnerkerze (HK) – The historical German unit of luminous intensity, based on the Hefner lamp burning amyl acetate. 1 HK ≈ 0.903 cd (or 1 cd ≈ 1.107 HK). Used in German and Austrian standards until the international candela was adopted. Occasionally encountered in historical technical literature.

Carcel – French historical unit based on an oil lamp. 1 carcel ≈ 9.6 cd. Rarely encountered except in 19th-century French technical documents.