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Illuminance - the concept and unit of lighting

The illuminance E at a point on a plane is equal to the luminous flux ⊿Ф incident on the surface area of the element containing this point divided by the surface area ⊿A of the element:

E =⊿Ф/⊿A (1.9)

For the radiant flux, the corresponding irradiance can be defined similarly to the above. The unit of illuminance is lumen/square meter (lm/m2), or "lux"; irradiance is expressed in watts/square meter.

It must be emphasized that the illuminance referred to here is the illuminance of a point on a given plane, so the term "plane illuminance" is sometimes used. The given plane generally refers to a horizontal plane, but it does not necessarily refer to a horizontal plane. If the direction of the plane changes, the illuminance of a point on this plane will generally change. The illuminance of a point in space is meaningless without further additional conditions. The illuminance of a point on a curved surface must be understood as the illuminance on the section passing through this point on the curved surface.

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