Illuminance
In SI derived units these are measured in lux ( lx ) , or equivalently in lumens per square metre ( lm·m−2 ) . 
<triplet> lux <subj> SI derived units <obj> instance of <triplet> lumens <subj> SI derived units <obj> instance of <triplet> square metre <subj> SI derived units <obj> instance of <triplet> lm <subj> SI derived units <obj> instance of