A lumen is a measure of the amount of brightness of a lightbulb; the higher the number of lumens, the brighter the lightbulb ; 1 lumen = foot candle 30cm candle light.
The lux (symbol: lx) is the unit of illuminance; or luminous flux per unit area in the International System of Units (SI); lx is equal to one lumen per square metre.
Lux is Photometric measure of illumination in an area; Lumens or illumination received.
Lumens is used for light emitting devices; such as light bulbs.
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lumen unit for light emitting devices, while
lux unit for measuring light received.
Is there a question in your post? The title says what’s the difference, but then you go on to state the difference, so I’m not sure what you are asking. Well, I’ll just discuss and hope it’s helpful.
First, I have to mention the photopic scaling. Lumens and Lux describe the amount of light produced by a source. You would think that might be described in units of power (Watts or Watts per square meter), but Lumens and Lux describe the PERCEIVED amount of light. (I‘d say “brightness” which is a perfectly good English word for what I mean, but, unfortunately, in optics that word is used to describe a specific quantity. So using it in the common English sense might confuse) So, Lumens give the power emitted scaled by a function that describes how well the human eyes can see various wavelengths. Thus, for example, a kilowatt infrared source emits zero lumens because we can’t see it. It wouldn’t light up a room (well, not before it sets fire to the curtains anyway).
Now let’s discuss the difference between Lumens and Lux. Lumens describe the total amount of visible light emitted per second. That says nothing about where the light is going. A 100 lumen light bulb puts out the same amount of perceivable light as a 100 lumen laser. The light bulb sprays the light all over the room, while the laser shines it all on a tiny spot on the wall.
To describe the difference between the laser and the light bulb, you would want to say how concentrated the light is at the wall. Looking at a small region on the wall the laser puts much more light per square millimeter than the light bulb. For describing that concentration of light on the wall we have Lux. Lux is Lumens per square meter.
So, for things that are intended to spray light everywhere, like light bulbs, nobody cares how concentrated that light is on the wall. The expectation is that the light will uniformly illuminate the scene. That is never strictly true, but perfection isn’t too important. In that case, you only care about the total light produced, and you would specify it in Lumens.
On the other hand, for things which are intended to project power somewhere distant, we care about the intensity at the distant target or wall. In that case you might use Lux. That would be the case for, say a spotlight.
I should note that concentrating light at a distance is exactly what lasers do. However they are never specified in Lumens or Lux probably because even if they are in the visible, you aren’t supposed look at them with the naked eye.
Dear Doctor
"In layman's terms, the main difference between these two figures is that lumens is a measurement of how bright a light is going to be, whilst lux tells us where that amount of light output is going to be in a specific area.
Lux is a measure of illuminance, the total amount of light that lands on a surface. It takes into account the area over which the luminous flux (lumens) is spread. A flux of 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 Lux.
Lumens is a measure of luminous flux, the total amount of light emitted in all directions."
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