In typical colliding experiments, the luminosity is measured by counting the number of small angle Bhabha scattering events. This is a technique used under the assumption that the collision energy is reasonably monochromatic, except for the effect of the initial state radiation.
However, this is no longer a valid assumption in high energy linear colliders, including JLC. This is because of the significant energy spread in collisions associated with beamstrahlung energy loss. Rather, the differential luminosity with respect to the effective center-of-mass energy (hereafter called ``luminosity spectrum") needs to be measured. Miller et al. have proposed an idea for measuring the luminosity-spectrum by using the acollinearity angle of the large angle Bhabha events. The energy difference between - and beams appears in the acollinearity angle of the Bhabha scattering. The potentiality of this idea has been studied by means of a luminosity-spectrum generator and a Bhabha event-generator.