Guide

Light quantities: lumen, lux and candela

Three units that are constantly confused although they measure three different things. UnitCloud deliberately does not convert them into each other — and explains why.

Three different quantities

Lumen describes luminous flux, the total amount of light emitted by a source. Lux describes illuminance, how much light arrives on a surface. Candela describes luminous intensity in a specific direction. These are not three notations of the same quantity but three different physical quantities.

The formulas that connect them

Lux is lumen per square metre: spreading 1000 lumen evenly over 10 square metres yields 100 lux. Candela is lumen per steradian, i.e. per solid angle. And luminance in candela per square metre, also called nit, describes how bright a surface itself appears. Because every one of these formulas contains an area or a solid angle, none of the quantities can be converted without that additional figure.

Why there is no lumen-watt formula

Watt is the electrical power consumed, lumen the luminous flux emitted. Their ratio is called luminous efficacy and is not a fixed number but depends on the technology. That is why there is no normative conversion from watt to lumen. To compare brightness, read the lumen figure on the packaging, not the watt figure.

Practical orders of magnitude

A typical office workplace sits in the range of a few hundred lux according to common planning practice, a living space well below that, an operating theatre a multiple above. Those values come from planning standards and depend on the task; they are not a specification. UnitCloud deliberately states no numbers here, because lighting design is a professional task.

Sources