Isolux charts represent the beam pattern and the distance illuminated on a flat road surface using a single lamp.
The colour shades represent the distance in metres from the lamp. The dark blue shade indicates an illumination level of one Lux (for example 320 metres in the shown diagram).
One Lux represents the intensity of the light of a full moon (under clear atmospheric conditions) or just sufficient light by which to read a newspaper.
You may also be interested in information about Isolux lighting diagrams and Illumination charts.