One lumen is the Luminous Flux or expressed more simply, the total amount of light energy emitted into one solid angle or steradian (see definition below), by a point source having a luminous intensity of 1 candela.
One steradian solid angle is the three dimensional angle with its vertex in the centre of a sphere which covers ¼p of the surface of the sphere. If we place a point light source with the luminous intensity of 1 candela at the centre of the sphere and measure the light being emitted into one steradian, we get 1 Lumen.
The lumens value for projectors describes its luminous flux – or the total amount of light energy emitted from the lens.
Lumens values for video projectors are typically in the range of 200 to 27,000 lumens at time of writing. For large format slide projectors the lumens value can reach 100,000 and for light sources alone, such as xenon arc lamps, the lumens value can be several 100,000s.
If the light source is isotropic, i.e. radiating the light evenly in all directions, a light source of 1 candela will emit 4p lumens into space as this equals 4p steradians. The relation between lumens and candelas is 1 cd = 4p lumens or 1 candela = 1 lumen/str.