The illuminance or light level is the amount of light energy reaching a given point on a defined surface area, namely the luminous flux (i.e. lumens) per square meter.
Illuminance is invisible! It is light passing through space and not seen unless you look at the source (e.g. a light bulb) or a surface it reflects off. In other words we can only see "luminance" (see next section).
Illuminance is measured in lux and the measurement will include all light reaching the point of measurement from a 180o hemisphere.
Lux levels relevant to the perception of light range from below 0.1 lux to above 120,000 lux.
The lux value is used to describe the amount of light in a given environment. A living room may have an illuminance level of 100 lux, an office 500 lux and direct sunlight greater than 120,000 lux. The eye is actually capable of sensing luminances spanning from below 10-6 to 108 cd/m2, i.e. a total range of 1014 or 10,000 billion times!