Candela vs. Lumens: What Is The Difference?

Candelas and lumens are used to describe the performances of various light sources, making it very useful to know what they actually mean.

Both candelas and lumens are units of the International System of Units, and both refer to (simplified) the amount of light. However, ...

Updated: November 21, 2024.

 Candela And Lumens Defined

Simplified, candela (cd) measures the amount of light in a particular direction given by a light source, while lumens (lm) measure the total amount of light given by a light source.

"Particular direction" is a solid angle (also known as a spatial angle) given in steradians (sr) and labeled as Ω.

For example, a sphere features a solid angle of 4π sr (~12.56 sr), while a cone with a 60° angle features a solid angle of one steradian (1 sr).

Candela and Lumens Relation

The lumen (lm) is related to the candela (cd) and steradians (sr) as:

1 lm = 1 cd sr

Since a sphere has a solid angle of 4π sr (~12.56 sr), a light source that radiates 1 (one) candela of light in all directions features a total luminous flux of:

1 cd x 4π sr = ~12.56 cd sr = ~12.56 lm

Few Examples

Candle

An ordinary candle emits light with the luminous intensity of roughly 1 cd - hence the name "candela."

If that light is radiated in all directions uniformly, the light (luminous) flux is:

Φ_v = 1 cd * 12.56 sr = 12.56 lm

14W LED Bulb

14W LED bulbs are very often used in homes to replace old 100W incandescent bulbs and 25W fluorescent lights.

As such, they are able to provide a light output of ~1400-1700 lumens, depending on the model, light temperature, and similar.

If we assume that a 14W LED bulb provides 1500 lumens, then if the light is radiated in all directions, the intensity is:

I_v = 1500 lm / 12.56 sr = ~ 119.4 candela

If the light is focused in a 20° beam (0.095 sr), the light intensity is:

I_v = 1500 lm / 0.095 sr = ~ 15789 candela

So, depending on the beam angle, the very same 1500 lm provides the light intensity of ~119 and ~15789 candelas.

How does candela affect the distance a light beam travels?

Candela affects the distance a light beam travels by determining the intensity of the beam in a specific direction. Higher candela values indicate a more concentrated beam of light, which can travel farther compared to a lower candela light source.

This is because candela measures the luminous intensity - how bright the light appears in a given direction - rather than the total amount of light emitted (measured in lumens).

For example, a flashlight with high candela and a focused beam will project light farther, making it ideal for activities like search-and-rescue or outdoor navigation.

Conversely, a low-candela light with a wide beam will illuminate a broader area but won’t reach as far. This makes candela a critical factor for applications where beam distance is essential.

How do beam patterns influence candela and lumens?

Beam patterns significantly influence the distribution of candela and lumens in a light source. Candela measures the luminous intensity in a specific direction, while lumens represent the total light output in all directions. The shape and focus of the beam pattern determine how the light's total lumens are concentrated or dispersed, directly affecting the candela value.

• Focused Beam Patterns: Narrow or spot beams concentrate light into a smaller area, resulting in higher candela values. This means the light appears brighter in its targeted direction, making it ideal for tasks like long-distance illumination or spotlighting.
• Wide Beam Patterns: Flood or diffuse beams spread light over a larger area, distributing lumens across a broader space. While this reduces the candela value, it enhances overall area coverage, making it suitable for close-range tasks like reading or illuminating a room.

In essence, the beam pattern determines whether the light is optimized for intensity and distance (high candela) or for wide, even illumination (higher practical use of lumens).

Understanding this relationship helps in selecting the right lighting for specific applications.

Candela vs. Lumens vs. Watts

Note: These values change over time with technological advancements, especially for LED light sources. Also, they depend on the LED light color, if the light is dimmed and similar.

So, if You have a 1000 lumens LED flashlight, depending on the exact LED light source, You can expect a power requirement of 10-15 Watts from the flashlight's battery pack.

Which is more important when choosing a flashlight, candela, lumens, or Watts?

When choosing a flashlight, the importance of candela, lumens, or watts depends on how you plan to use it:

• Candela is more important if you need a flashlight with a long-range or focused beam. High candela values indicate a more intense light in a specific direction, making it ideal for tasks like search-and-rescue, spotting objects at a distance, or navigating outdoors in the dark.
• Lumens matter if you want an overall brighter flashlight for illuminating a large area. Higher lumens mean more total light output, which is beneficial for general-purpose use, such as lighting up a campsite or a room.
• Watts are less relevant today for choosing a flashlight, as they measure energy consumption, not light output. With advancements in LED technology, energy-efficient flashlights can produce high lumens with relatively low wattage. Instead of watts, focus on the lumens-to-watts ratio to assess energy efficiency.

Long Story Short: Lumens is a measure of total light output, while candela is a measure of light intensity.

Also, prioritize candela for long-range applications, lumens for general brightness, and consider watts only when evaluating energy efficiency in combination with the other metrics.

An LED flashlight, for example, with variable zoom and variable LED output, can change zoom (increase or decrease the light intensity - candela) or can change the LED total light output (increase or decrease the total amount of light - lumens), which also changes the required Watts.