What Is a Candela? The SI Base Unit of Luminous Intensity Explained

Walk into any lighting shop and you'll see boxes covered in numbers: watts, lumens, kelvins, and sometimes candela. Most people skip past that last one. But if you work with LEDs, flashlights, automotive lamps, or signal lights, the candela is the number that tells you how bright a light looks in one specific direction.

So, what is a candela? This guide covers the definition, where the unit came from, how it connects to lumens and lux, and how it's measured in practice. It's one of the core intensity of light units, and once it clicks, the rest of photometry gets much easier.

What Is a Candela?

The candela, symbol cd, is the SI base unit of luminous intensity. Luminous intensity describes how much visible light a source sends out in a particular direction, weighted by how sensitive the human eye is to different colors.

Two parts of that sentence matter:

• In a particular direction. Candela isn't about total      light output. It's about how concentrated the light is. A bare bulb and a      spotlight can produce the same total light, but the spotlight packs it      into a narrow beam, so its candela value in that direction is much higher.

• Weighted for the human eye. Our eyes respond most      strongly to green light around 555 nm and much less to deep red or violet.      The candela accounts for that, so it measures brightness as people see it,      not just raw energy.

A common household candle gives off roughly one candela. That's where the name comes from: "candela" is Latin for candle.

The Modern Candela Definition

For a long time, the candela was defined through physical objects like standard candles and, later, a glowing block of platinum at its freezing point. Those references were hard to reproduce, so the definition kept evolving.

The current candela definition, adopted in 2019 along with the other SI redefinitions, ties the unit to a fixed number. The luminous efficacy of monochromatic light at 540 × 10¹² hertz (a green wavelength of about 555 nm) is set at exactly 683 lumens per watt. From that, the candela follows: one candela is the luminous intensity in a given direction of a source that emits that green light with a radiant intensity of 1/683 watt per steradian.

You don't need to memorize that. The takeaway is that the candela is now based on a fixed constant of nature rather than a physical artifact, which is why labs anywhere in the world can reproduce it with confidence.

Candela vs. Lumen vs. Lux

This is where most people get tangled up. The three units are related, but they answer different questions.

The link between candela and lumen is the steradian (sr), the unit of solid angle:

1 lumen = 1 candela × 1 steradian

A full sphere covers 4π steradians, about 12.57 sr. So a light that shines equally in every direction at 1 cd produces roughly 12.57 lumens in total.

Lux connects through distance. For a point source, the illuminance on a surface facing it is:

Lux = candela ÷ distance² (in meters)

That's the inverse square law. A 100 cd light gives 100 lux at 1 meter, but only 25 lux at 2 meters.

A Quick Worked Example

Say an LED spotlight is rated at 100 cd with a 60° beam angle. To estimate its total lumens, first find the solid angle of the cone:

Ω = 2π × (1 − cos(30°)) ≈ 0.84 sr

Then multiply: 100 cd × 0.84 sr ≈ 84 lumens.

Now compare that with a bulb that puts out 84 lumens in all directions. Its intensity would be only about 6.7 cd. Same lumens, very different candela. This is why a spotlight looks so much sharper than a general-purpose bulb.

Where the Candela Unit Is Used

You'll come across candela values more often than you might expect:

• LED datasheets: Manufacturers often list      intensity in millicandela (mcd) for indicator LEDs and in candela for spot      and beam products.

• Automotive lighting: Headlamps, brake lights,      and turn signals must hit minimum and maximum intensity values in specific      directions.

• Aviation and marine signals: Navigation lights are      specified by candela so they're visible from a set distance.

• Emergency and exit signs: Compliance often depends on      intensity at defined angles.

• Stage and architectural lighting: Designers use candela      distribution to plan beam shape and coverage.

If you only look at lumens for these products, you miss the directional picture, and that's usually the part that matters.

How Luminous Intensity Is Measured

You can't measure candela reliably with a basic phone app or a cheap lux meter. Accurate results need proper photometric equipment and a controlled setup.

The main tool is the goniophotometer. It rotates the light source, or a detector, through a range of angles and records intensity in each direction. The result is a luminous intensity distribution curve, often exported as an IES or LDT file that lighting designers use in their software.

For total light output, labs use an integrating sphere to capture the lumens. A spectroradiometer adds color and spectral data so the readings can be properly weighted for the human eye. Together, these give a complete picture of a light source's performance.

Distance also matters. Candela values are defined for a point source, so measurements must be taken far enough away that the source behaves like one. Skip that rule and your numbers will be off, which is one of the most common photometric mistakes.

Why Accuracy Matters

A small error in photometric readings can lead to failed compliance tests, mislabeled products, or lighting designs that don't perform as promised. That's why calibrated laboratory testing equipment and a traceable measurement process are so important for manufacturers and test labs.

Lighting products also need to pass safety checks, not just brightness checks. LED luminaires go through electrical testing for insulation, earth continuity, and dielectric strength, alongside their photometric evaluation. A complete lab covers both.

Key Takeaways

• The candela (cd) is the SI      base unit of luminous intensity.

• It measures brightness in a      specific direction, adjusted for how the human eye sees light.

• One candela equals one lumen      per steradian.

• Lux depends on candela and      distance, following the inverse square law.

• Accurate candela measurement      needs a goniophotometer and controlled conditions.

Understanding the candela makes datasheets easier to read, product comparisons fairer, and test results easier to trust. Once you know the difference between intensity, flux, and illuminance, the rest of the intensity of light units start to make sense.