In full sunlight you measure 100,000 lux or more, on an overcast day about 1,000 and at sunset around 400. Dusk sits around 10 lux and a full moon at 0.1. Indoor lighting is far below that: an office reaches 500 lux.
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What lux actually is
Lux is lumens per square metre. One lux is one lumen spread over a square metre of surface. It measures what arrives on a surface, not what a lamp emits.
That is why you can reach any lux value you like with the same lamp: hold it close and the lux shoots up, hang it high and it drops again. More on that in What is the difference between lumens and lux?
The values in a row
- 100,000 to 130,000 lux: direct sunlight on a clear day
- 10,000 to 20,000 lux: daylight in the shade or under light cloud
- 1,000 lux: an ordinary overcast day
- 400 lux: sunrise or sunset in clear weather
- 100 lux: a very dark winter day
- 10 lux: dusk
- 1 lux: deep dusk
- 0.1 lux: full moon under a clear sky
What stands out is the scale. Between full sunlight and a full moon there is a factor of a million, and yet you can see in both. Your eye adapts over a range no camera can match.
Why indoor lighting looks so low
An office sits at 500 lux, a living room often at 50 to 150. Next to those 100,000 lux outdoors that looks absurdly little, and yet it works fine.
That is because indoors you have nothing to compare it with. As long as everything in the room is of the same order, your eye settles in and sees enough. It goes wrong the moment a bright window sits next to a dark corner, because your pupil then has to keep switching back and forth.
You can measure that at a window: put your phone with a lux meter app on your desk by the window and then in the corner of the room. If you see a factor of ten, that is where an extra light point genuinely helps.
What to set on a dusk switch
This is where those numbers come in handy. A dusk switch or sensor operates at a lux value you set yourself, and most models run somewhere from 2 to 100 lux.
What that means in practice:
- Around 10 lux: it comes on when dusk genuinely sets in. The right choice for most gardens and driveways.
- Around 20 to 50 lux: it comes on earlier, including on a dark rainy afternoon. Handy for a dark passage or under a dense canopy.
- Below 5 lux: it waits until it is almost completely dark. Frugal, but you spend the first half hour of dusk in the dark.
Two things that often go wrong. Do not put the sensor in the light of the lamp it switches, because it will come on, measure light, and switch straight off again. And allow for street lighting: a lamp post nearby lifts the measured value, so your light comes on later than you expect.
A lux meter app is an estimate
A phone's light sensor is meant to dim the screen, not to measure. It sits behind glass, looks in one direction only and is not calibrated. For comparing two spots in the same room that is fine; for an absolute value it is not.
If you want to know how much light you need in a room, work in lumens per square metre instead. That is set out in How many lumens do you need per room? The spots are at recessed spotlights, the outdoor lights at wall lights.
How many lux is daylight indoors?
Right by a window you easily pass 1,000 lux on a clear day. Two metres further in it has often already dropped to a few hundred.
What lux do you set a dusk switch to?
Around 10 lux is the usual choice: that is the moment dusk genuinely sets in. Higher means on earlier, lower means later.
How many lux is a lumen?
That depends on the area. One lumen spread over one square metre is one lux. So the same lamp gives more lux the closer it hangs.