diff --git a/spec/quantities.yaml b/spec/quantities.yaml
index 4a6202db..3babc060 100644
--- a/spec/quantities.yaml
+++ b/spec/quantities.yaml
@@ -78,3 +78,5 @@ relation:
 resistance:
   definition: Property of a resistive element, equal to the scalar
             quantity given by the quotient of voltage u and electric current i (IEC 80000-6:2022)
+illuminance:
+  definition: Illuminance is the total luminous flux incident on a surface, per unit area, usually measured in Lux
diff --git a/spec/units.yaml b/spec/units.yaml
index ddfb393c..5f32a026 100644
--- a/spec/units.yaml
+++ b/spec/units.yaml
@@ -398,3 +398,11 @@ Ohm:
   unit: Ohm
   quantity: resistance
   allowed-datatypes: ['numeric']
+
+# Illuminance
+
+lx:
+  definition: Light intensity in Lux
+  unit: lux
+  quantity: illuminance
+  allowed-datatypes: ['numeric']