Candela vs. Lux vs. Lumens: Differences and Comparison 2024
Imagine you just burnt out a light bulb and you’ve gone to the store to find a replacement. Once you’re in the lighting aisle, you’re likely to encounter thousands of light bulbs that look incredibly similar and boast a wide array of different numbers, stats, and units on the side of their packaging. Maybe you remember that you’re supposed to look for the proper wattage; maybe you know that you should keep an eye out for lumens. Maybe all you know is that you’re supposed to find a similar-size light bulb!
From candelas to lux to lumens, there seem to be a billion ways to measure light output. But what do they mean, and when should you expect to use each term?
In this guide, we’ll explain the difference between candela, lux, and lumens, and how you might apply these new terms – hopefully making your next trip to IKEA or the hardware store that much smoother.
Candela, lux, and lumens are all SI units used to measure light, and they are all mathematically related to one another. However, they all measure subtly different aspects of light, and they are all somewhat important measurements.
1. What are lumens?
Lumens are an SI unit used to measure the amount of visible light emitted from a source. Lumens are, essentially, a very basic measure of brightness – the higher the lumens, the brighter the lamp, bulb, or light fixture. It’s a measure of total light, rather than a measure of the specific quality of the light, like how concentrated it is or what color it is.
General use household light bulbs usually produce light measured between 300-1000 lumens, though most fall in the 350-500 range. Lights used for other purposes – say, outdoor floodlights – often have a different frame of reference, since they need to produce such a high volume of light.
Lumens measure light, but common wisdom has it that you should shop based on watts instead of lumens when looking to replace a bulb. But is that true?
Well, not really – at least, not for most shoppers. Wattage (measured in watts) is a measure of power, not necessarily light. It can be really important to make sure you don’t exceed the maximum wattage of your light bulb, or else risk creating a fire hazard inside your home. Fortunately, most lightbulbs you find in the store do not use more than 60 watts – the “limit” of most general-purpose lamps.
Back when incandescent bulbs were the more prevalent bulb type, it was common for people to worry much more about watts than lumens. For incandescent bulbs, wattage was a pretty good indicator of how much light the bulb could produce – incandescent use a lot of energy, and the ratio of wattage to lumens is relatively stable in these bulbs.
However, LED bulbs changed this paradigm. Now, it’s much more relevant to shop with lumens in mind for brightness rather than watts. LED bulbs don’t need that much energy to create a lot of light, so wattage isn’t a good measure of light output.
If you’re shopping for LED bulbs, just make sure you don’t exceed the max wattage of your socket. Otherwise, keep an eye on the lumens – that should be a better indicator of brightness.
If you’re shopping for incandescent bulbs, the wattage-brightness measurement may still be helpful, though most light bulbs should also be labeled with their lumens. Make sure you don’t exceed the max wattage of your socket either – this can be unsafe.
Additionally, some LED bulbs will list their actual wattage – usually a fairly low number – alongside an “equivalent” wattage that is much higher. This “equivalent” number is just an indicator used to tell people who are used to the wattage system the amount of light it would produce if it were incandescent. Some people worry that it’s not okay to put an LED with a high equivalent wattage into a socket, but it’s fine. The lower number is the real amount of power the light bulb will draw.
For instance, if you have a 16w bulb that is equivalent to a 100w incandescent and a socket that says you can’t exceed 60w, don’t worry. You’re only actually using 16w, and the bigger number is just to assist some consumers with understanding the amount of light the bulb produces.
2. What is lux?
Lux is the measure of how much light is falling on a surface – also called illuminance. As the term “illuminance” suggests, lux measures how much a surface is being illuminated by a light source. As a result, it is also a measure of light intensity. The higher the lumens and/or the closer the surface is to the light source, the higher the lux. The inverse, too, is true: the lower the lumens and/or the further away the surface is, the lower the lux. Additionally, the larger the surface you’re illuminating, the more lumens you’ll need to achieve the same amount of lux.
3. What are candelas?
Candelas, on the other hand, are concerned with the amount of light within a beam. Candelas essentially measure the “casting distance” of the light – how close the light source needs to be to an object in order to illuminate it. Two light sources with similar lumen levels can have very candela measurements, meaning that the maximum distance that the light source can illuminate a surface at is different.
Consider a laser pointer and a candle. While the two might emit the same total amount of visible light, the narrow concentration of a laser pointer means that the light can travel very far – out a window, onto other buildings outside, or any other extreme distance. By contrast, candles have a very diffuse concentration of light and thus their light does not travel very far. They can illuminate everything immediately around them, but the light from a candle usually doesn’t illuminate an entire room – meaning the light can’t even travel far enough to illuminate the walls. In this example, laser beams emit a ton of candelas, while candles emit relatively few. Candelas are measured with the unit abbreviation CD.
4. When would you use candela, lux, or lumens?
So you understand what these measurements are in the abstract, but when would you actually use each of them?
Lumens are used generally to describe the amount of light something emits. They will be the measurement you pay attention to when shopping for lamps, bulbs, strip lights, under-cabinet lighting, and any other general purpose household lighting. Specialty lighting will also likely include their lumens, though shoppers looking for particular kinds of lighting will also be paying attention to other metrics, too. If you’re looking to know the basic brightness of a light bulb, look for the lumens – they’ll be measured in “lm.”
Lux can be a good way to measure how bright a room is or will be based on the lumens emitted. Lux typically describes light in and around areas. For instance, movie theaters sit around 100 to 200 lux, while lux levels in the home can sit between 100 and 1000. Additionally, lux can be helpful for measuring the perceived brightness of a light source to the human eye. Lux is a derived measurement, so it is easy to calculate lumens from lux – one lux is equal to one lumen per meter squared. If you’re interested in knowing how bright a surface is, use lux, which is abbreviated as “lx.”
Candelas will be relevant when you’re looking to purchase a light source that is designed to travel very far. Tactical flashlights, floodlights, spotlights, and laser beams typically boast high candela ratings, and shoppers are usually concerned about purchasing beams of light that can travel far enough for their purposes. If you’re looking for the brightest beam of light money can buy, look for the candelas of the light source, which will be abbreviated as “cd.”
Many light sources will list all three measurements on their packaging or in their product description, which can be somewhat confusing. Just be aware of what you’re looking for, and don’t worry as much about the others. If you’re shopping for light bulbs, worry about lumens. For flashlights, lumens and candelas will be what you’re looking for.
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