According to its definition, a lumen (unit symbol lm) is the physical unit of measure of the light stream that is emitted by a light source. The higher the lumen value of a light, the higher is also its brightness.
However, lumen, as against the specification of the pure output in Watts, takes into consideration the actual perception of the human eye. The human eye perceives light of different colours differently. Thus, a source of light that radiates green light at an output of 1 Watt, is far better perceived by the human eye than, for example, a light source that transmits red light with the same output. As against that, light with a brightness of 1 lumen is perceived as being bright by the human eye, no matter which colour it has.