When choosing the lighting fixture, understanding brightness measurements is essential. In today's market, three key metrics help define brightness: candela, lux, and lumens.

In this guide, we’ll break down the differences between lumens, lux, and candela, explaining each unit and how they relate. Read on to make an informed choice!

Part 1. What is Lumen?

Most light bulbs offer measurements in lumens today. This information provides data about how much luminous flux is available in the product, referring to its total visible light. 

When candela ratings remain the same when light becomes obscured, a lumen rating decreases because less total light is available.

For example, a light bulb turned on with a 3 CD rating. That product would provide approximately 38 lumens when it offers uninterrupted illumination. Let's say that half of the bulb gets obscured, leaving a single hemisphere to provide illumination. The 3 CD rating remains unchanged because the parts left unobscured are still visible from the same distance. But if you measured the lumens in that situation, it would cut the rating by 50%. That's because you're only looking at the visible light from the source.

Since 1930, the lumen has been an established unit in the International System of Units (SI). Today, it serves as the reference for measuring light intensity in candela. One candela represents the intensity of a light source that emits monochromatic light at a wavelength of 555 nm with a luminous flux of 1 candela per square meter (cd/m²).

Part 2. What is Candela?

Candela is often seen as a candlepower or “CD" rating for lights. It's the base measurement used to determine the product's luminous power intensity. It lets you know how bright the light is based on how far away you are while still seeing it.

Candlepower and horsepower are structured similarly from a measurement point of view. If one horsepower is equivalent to the performance of a single horse, one candlepower equals the light output from a single candle.

Any light eventually becomes too dim to see when you're far enough away from it. That means the CD rating is different from lumens because you're looking at an intensity rating vs. a total light output measurement.

Products like spotlights and laser pointers tend to have the industry's highest candlepower rating because most of the illumination focuses in a single direction. Even if you block a portion of the light, the same intensity is seen from the areas that aren't obscured.

Part 3. What Is Lux?

Lux measures how much light illuminates a specific area, making it an essential unit for understanding lighting levels in a space. One lux is equal to one lumen spread over a square meter. This unit helps determine the intensity of light reaching a given space, making it useful for assessing lighting conditions in a room or outdoor area.

Lamps displaying lux measurements for brightness usually list a distance from the bulb as part of the information. That info is necessary because any proximity alterations change the perceived lux level. For instance, a spotlight shines only in one square meter with a 500-lumen rating, the surface area has a 500-lux rating. Now let's say the spotlight gets backed up so that it now shines over five square meters. Even though the lumens stay the same (500), the lux rating decreases to 100 because of the extra space that requires illumination.

Although this information isn't always helpful for indoor lighting needs, it does provide data about replicating daylight inside or providing appropriate task light installation.

What does lux look like in real life? In perspective, 1 lux is similar to the light level during twilight or moonlight. The office lighting is around 400-500 lux in general. A dim, overcast day might measure about 100 lux, while full daylight reaches 10,000 and 25,000 lux. On a bright, sunny day, direct sunlight can soar up to around 100,000 lux.

Part 4. The Relation Between Candela, Lumen, and Lux​

Candela, lux, and lumens are the three ways to measure light.

4.1 Candela VS. Lumens

Lumen measures the total brightness across a wider area, while candela focuses on the intensity of light in a specific direction. This is the main difference that often leads to confusion between the two units. Besides, lumens can quantify the light emitted by a source. In other words, lumens tell you how much total light is from a source, while candela shows how concentrated that light is in one spot. For example, a high-lumen floodlight means it can light up a large area, while a spotlight with a high candela rating directs a focused beam of light. One candela equates to 12.57 lumens. Since candela and lumens measure different intensity aspects. Comparing one with the other information between two products won't provide an accurate result. You'd better compare two candela or two lumens ratings.

4.2 Lux VS. Lumen

The main difference between lux and lumens is what they measure: lux measures how much light lands on a specific surface, which indicates how well-lit a particular area will be. Lumens measure the total light output from a source. For instance, when a high-lumen is placed far away, it won't provide much lux on a surface, but when it is close by, the lux would be increased since it concentrates more light on that spot. Therefore, lux can tell how well a specific space will be illuminated.

Every light contributes to the candela, lux, and lumens ratings achievable in that space. From accent to task lighting, you'll want to consider each specific need to understand what to install or use as a replacement.

Here's an easy way to remember the different ratings so that you get the correct bulbs for your space.

Candela measures the levels of visible light intensity from the source.
Lux describes how bright surfaces are once the bulbs are correctly installed.
Lumens looks at how much light occurs within the environment.

Once you know those descriptions, finding an appropriate replacement solution is much easier. ​​