The lighting power is an important factor of light. Each bulb and each lamp radiates and illuminates the space in different ways. Four characteristics are use to describe the lighting power. The luminous flux, the luminous intensity, luminance and illuminance.
The basic unit of luminous flux is the lumen (lm). This unit gives the measure of total amount of light emitted by a light source in all directions. This measure is use to evaluate the efficiency of the lamp and allows us to determine its actual brightness.
When we want to know the light intensity emitted by the light source in a given direction, candela is use. the more these are concentrated (candela per square meter), higher is the luminance. Thus, two bulbs of 800 lumens emitting light in the same direction but don't having the same size will have a different luminance. More candela we have for a given area, more it will be dazzling.
In a lighting project, the most important is the lighting level the user will perceive. Regardless of the characteristics of the light source, it's the light the user receives at a certain distance which is important. This level of illumination is calculated in lux. Lux represent the amount of light received on a surface (in lumen per square meter).
The human eye can accommodate to varying levels of illumination, from 100'000 lux for a day of summer sun, to less than 1 lux for a night of full moon. A hallway or stairway generally receives 100 lux, bathroom and toilet are 200 lux, living rooms range from 100 to 400 lux, and working places from 200 to 800 lux. More precise is the application work, much more it needs illumination.
Lumens = total amount of light
Candelas = total amount of light in a certain direction
Lux = total amount of light in a certain direction at a certain distance.