The required lighting level in a certain situation is expressed in terms of illuminance.

Reference surface

Reference surface of an indoor area is the surface where the recommended appropriate illuminance is supplied, The reference surface does not need to be reduced to a single surface area, but it may include a number of separate areas. Indoor lighting specifications must always include a clear definition of the reference surface.
In indoor working areas, the reference surface will normally be the working plane. For indoor areas where tasks are not restricted to fixed places, the working plane is considered to be the horizontal plane limited by indoor walls at a height of 0.85 m. above the floor. For indoor areas where task localizations are known and clearly specified, the reference surface may consist in specific areas of working or task areas.
When the task is not performed in a horizontal plane or is at a different height, the reference surface will have the angle of the task plane and be at its height.
In indoor areas where work is not done, the reference surface may be the floor, the wall, or any important plane.

Illuminance uniformity

Illuminance given on the reference surface by a lighting installation will never be totally uniform, either in space or in time.

Measurement of illuminance uniformity on the reference surface is the ratio between minimum illuminance and average illuminance.
In general lighting, illuminance uniformity on the reference surface must not be lower than 0.8 to provide possible locations of equivalent tasks in all the indoor areas.
In localized general lighting or lighting of general areas, average illuminance in areas surrounding tasks must not be lower than one third of the level for task areas.
Ratio between average illuminances for two adjacent indoor areas (for example, an office and a corridor) must not exceed 5:1.