I have little knowledge on light metering, but this is backed up with Wikipedia and a quick chat with our devs:
lux is illuminance (luminous flux incident on a surface)
you can, for example, spread a flux of 1000 lumens over some area
full moon has specific lux, overcast day, office area, etc
lumen is luminous flux
it is the total quantity of emitted light
Lumens are usually used to measure "light intensity" (how much light a light source emits).
Lux is usually used to measure the amount of light on some area.
Generally, lux as a unit makes little sense as a light source intensity unit.
From Wikipedia: "The difference between the units lumen and lux is that the lux takes into account the area over which the luminous flux is spread. A flux of 1,000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1,000 lux. The same 1,000 lumens, spread out over ten square metres, produces a dimmer illuminance of only 100 lux. Mathematically, 1 lx = 1 lm/m2."
^The above is also true for a Corona Light.
Let's imagine a rectangular surface which has some width and length. This surface reflects all the light it receives. We measure illuminance over that surface and we get a lux measure of X lux.
Now, if we create a rectangle Corona Light, set its width and length to the same values as that surface from previous example and set its intensity to the same number of X lux, then we can say that the this light has the same "brightness" as the surface from the previous example.