When choosing a lighting fixture, understanding brightness measurements is essential. In today's market, three key metrics help describe 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 to each other. Read on to make an informed choice!
Most light bulbs today are rated in lumens. This information provides data about how much luminous flux is available in the product, referring to its total visible light.
Candela ratings remain unchanged when part of the light is obscured, but lumen ratings decrease because less total light is emitted.
For example, a light bulb rated at 3 cd. 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.
The lumen has been an official SI unit since 1930 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).
How to Calculate Lumen:
To work out lumens with a lux meter, remember that lux measures how much light hits an area, and lumens are lux multiplied by that area. First, use the lux meter to check the light level and write down the reading. Then, measure the size of the surface the light is shining on in square meters. Multiply the lux reading by this area to get the lumens. For example, if your meter shows 500 lux and the light covers 3 m², you have 500 × 3 = 1500 lumens. This works best if the light spreads evenly; if not, or if you want the bulb’s total output, you’ll need more measurements or special tools. Keep in mind that how far you are from the light will change the reading.
Candela, sometimes expressed as ‘candlepower,’ is abbreviated as cd. It's the base measurement used to determine the product's luminous power intensity. It indicates how bright a light source appears from a given distance.
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 high candela ratings 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.
How to Calculate Candela:
You can calculate it if you know the luminous flux and the solid angle over which the light is emitted. The formula is candela = lumens ÷ steradians. To do this, first measure or estimate the total lumens of the light source, then find the solid angle it covers, which depends on how wide the beam spreads. For example, a light emitting 100 lumens evenly over 1 steradian has an intensity of 100 candela. This method assumes the light is evenly distributed within the measured angle; for real-world lights, beam patterns may require more detailed measurements.
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 example, if a 500-lumen spotlight illuminates one square meter, the surface receives 500 lux. 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 to 25,000 lux. On a bright, sunny day, direct sunlight can soar up to around 100,000 lux.
How to calculate lux:
The basic way to calculate lux is by dividing the total luminous flux incident on a surface by the area of that surface, using the formula: Lux (lx) = Luminous Flux (lm) / Area (m²).
For a point light source that emits light uniformly in all directions, you can also determine illuminance at a specific distance using its luminous intensity (in candelas, cd) and the inverse square law, which states that illuminance decreases with the square of the distance from the source. This formula is: Lux (lx) = Luminous Intensity (cd) ÷ Distance² (m²).
Practical factors like the angle of incidence matter too. If light hits a surface at an angle, illuminance is reduced by the cosine of that angle, meaning light at 60° would cut illuminance in half. It’s important to note that non-uniform sources, such as spotlights, don’t follow the inverse square law as cleanly as uniform point sources.
Candela, lux, and lumens are the three ways to measure light.
Lumens measure 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 lumens to candela directly is not meaningful; you should compare products using the same unit. You'd better compare two candela or two lumens ratings.
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 light source is placed far away, it won't provide much lux on a surface, but when it is close by, the lux increases because more light is concentrated 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 measure how much total light a source emits.
Once you know those descriptions, finding an appropriate replacement solution is much easier.
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