  An absolute luminosity can be measured in an accelerator by sweeping beams
transversely across each other in the so called van der Meer scan. We prove
that the method can be applied in the general case of arbitrary beam directions
and a separation scan plane. A simple method to develop an image of the beam in
its transverse plane from spatial distributions of interaction vertexes is also
proposed. From the beam images one can determine their overlap and the absolute
luminosity. This provides an alternative way of the luminosity measurement
during van der Meer scan.
