LED Light is Directional

 

While FL tubes and conventional light bulbs distribute their light all around, LED light is directed to an angle of 120°. Luminaries are designed to shade off the non-useful portions.

For example:

For a lamp that illuminates a working area from above, only the fraction of the light that hits the working area is useful. All other light is lost – radiated in other directions.

When comparing the lumen, an FL tube needs to produce much more light to achieve the same illumination on a flat surface.

Since in most cases 2/3 of the light of an FL tube is lost that way, an LED tube only needs 1/3 of the lumens to achieve the same illumination.