  A formula for the collider luminosity is derived for a head-on collision of
two beams with different parameters. Three particular cases are presented: the
collision of two identical axially symmetric bunches, the collision of a bunch
with a coasting beam, and the collision of two coasting beams. The colliding
beams have coinciding longitudinal axes. The formula is valid for colliding
both counter propagating and co-propagating ("merging") beams.
