What Is The Difference Between Lumen And Lux?

Lumens and lux are easy to confuse if you aren’t a lighting expert. Both are useful measurements when you’re planning how to illuminate a space, whether that’s your home, garden, or workplace.

They are related terms, but that doesn’t mean they’re interchangeable. 

They measure light differently and knowing the difference will make it much easier to understand when you’re planning a new lighting setup or replacing a bulb.

Lumens is the measurement of light emitted by a bulb – it is only concerned with the light source. Lux instead measures the surface illuminated by the bulb, so it takes environmental factors, such as distance from the light source and direction of the light, into account.

In this article I’m going to explain:

  • The difference between lumens and lux, in more detail
  • Why lux is an important measurement
  • The correlation between lux and lumens
  • How to convert lux into lumens

Lumen and Lux Explained

lux explained

If you buy a light bulb, place it in a lamp and then place that lamp in a small box, it’s going to create a lot of light in a small space.

Place that same lightbulb in a fitting inside a huge office building, and it isn’t going to do much at all.

The bulb itself is creating the same amount of light – but the impact of that light on the environment depends on many other factors.

That’s the easiest way to explain the difference between lumens and lux. Lumens are how you measure the brightness of a bulb. No matter where you use that bulb, it’ll be the same brightness.

Lux is a measurement of light on a surface – how effectively is it being lit?

One lux is equivalent to one lumen of brightness per square meter – so over a space of one meter squared, a surface with one lux is as bright as a bulb giving off one lumen at point-blank range.

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You can recreate this test with a torch or your smartphone. Either mark up space on a wall or use a part of a wall as a focal point. Sit one meter away from the wall and shine the light onto it.

Now move back another two meters. See how the same torchlight is now much dimmer? The bulb hasn’t got any dimmer. But the light is being spread further around the room, and so the surface is benefiting less.

Now return to being one meter away, but shine the light through a cardboard tube. See how the brightness has increased on the wall?

You’re removing the light dissipation, so the light is more concentrated onto the wall – meaning the lumens are the same, but the lux is now higher.

It’s worth noting at this point that you may also have heard of foot candles. Simply, foot candles and lux do the same job, but one is imperial (1 foot candle = 1 lumen per square foot) while one is metric (1 lux = 1 lumen per square meter).

Why Is Lux An Important Measure?

lighting in the bedroom

Lux matters because it tells you how effective a light source is. When you’re planning your lighting at home or work, you don’t plan for how bright the bulb is but how much it illuminates your room.

That doesn’t mean lumens aren’t important – they’re related, as you will learn in a second. But lux is the measurement that tells you whether a room is bright enough.

Lux is so important that the Illuminating Engineering Society of North America has produced an entire handbook with a recommended list of lux levels for different requirements.

All kinds of residential and commercial scenarios are covered. As a guide, you’ll want somewhere between 100 and 300 lux for living spaces in your home, but a brighter light around the 750 lux mark for kitchens or workbenches where you’ll potentially be working with sharp tools.

Are Lux And Lumens Correlated?

While lux is arguably the more important measurement in real-life applications, you still need to take lumens into account.

Why?

Because the lighting in your home feels too dim, are you more likely to buy a brighter bulb or lower the entire ceiling?

Exactly – the lumens rating of a bulb helps you determine how bright the light is, and it directly impacts the lux measurement.

Buy a brighter light, and the total amount of light reaching a surface will typically increase. And alternatively, if an area is too bright, you can buy a lower-lumen bulb to reduce the lux.

It’s not guaranteed though; some light fittings are just poorly designed, either through bad placement, being too recessed or directed incorrectly. In these cases it may be a bigger job that’s needed to replace or relocate the fitting, rather than simply getting the brighter bulb.

How To Convert Lux Into Lumens

light meter to measure foot candle

Every home, office, or commercial space is different. That’s why you can’t shop for light bulbs based on lux. You can’t predict how your particular environment is going to affect light.

Again, let’s use the torch example. I couldn’t sell you a torch saying that it is 500 lux, because that would depend entirely on where and how you used the torch.

That’s why you can’t convert lumens into lux. It’s impossible to know exactly how bright your surfaces will be, depending on the bulb alone.

You can convert lux into lumens, though. Once you know a lux rating (which is measurable with a light meter), then you can work out the lumens of the light. However, doing it practically can be tricky.

One lux equals one lumen per square meter. So, you can reverse this and multiply the lux by the area in meters squared illuminated by the bulb to work out the lumens.

I said this is tricky because if you do this practically, you need to work out the different lux levels and areas for each surface within a room.

It’s very rare for a room to be a completely empty box. So the lux will be slightly higher on an elevated worktop than it would be on the floor, and it’ll be much lower behind a sofa.

Final Words

Understanding the difference between lux and lumens is essential when you’re optimizing your lighting.

On a simple level, more lumens will give you more lux. You can still look at how light is directed or where fittings are placed if you want to change how a room or space is illuminated.

How easy do you find it when shopping for a bulb based on lumens?

And have you ever used a light meter to check how well-lit your home or workspace is?

Let me know in the comments.

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