The working plane is the theoretical plane to which service illuminance En relates. In the normal case the working plane is treated as a horizontal plane 0.85 m above the floor. Service illuminance can also relate to work surfaces lying in other planes (e.g. switchgear cubicle assembly, drawing boards, desks). In the case of indoor circulation areas, service illuminance relates to the midline at a max. height of 0.2 m above the floor (EN 12464 Part 1).