Luminous flux (Lumen)

The luminous flux of a source of light refers to the ‘quality of light’ emitted by the source. The luminous flux is measured in Lumen with an integrating sphere.Le fl ux lumineux d’une source de lumière est représentatif de la ‘quantité de lumière’ qu’émet la source. 


Light intensity (in Candela)

This quantity is only interesting if it is accompanied by the beam angle. For a given flux, the intensity in the light bulb axis is as high as the beam angle is small and the relation between these three quantities is non linear.
Example: a light bulb of 20 W with a beam angle of 12 ° has a maximum intensity of 3250cd whereas a 70 W of 42 ° will only have an intensity of 2000 cd. If we only take intensity into consideration, it would seem that a 70 W bulb illuminates less than a 20 W bulb. This is because it covers a larger surface.



The illumination is measures in Lux using a light metre and is directly dependent on the luminous flux. It refers to the spread of the luminous flux on the illuminated surface (reminder). In practice, this is the only information of interest to the ‘lamda’ user with regard to visual comfort. This is also the quantity indicated on the lighting specifications.



A dozen lux on a book suffice for reading
A hundred lux on the ground correspond to the ‘standard’ lighting of a flat,
400 to 500 lux on a table are required for detailed tasks,
200 000 lux correspond to solar lighting.


In all cases and for any given luminous flux, the lighting depends on the distance between the source of light and the surface it illuminates: