Time-delay and relativistic correction f(β) as a function of expansion velocity β. For non-relativistic velocities the integral over solid angle reduces to π, but decreases at greater velocities due to the geometrical effect in Rees (1967). The blue dashed line indicates the geometrical correction only, while the red line includes the full correction of both time-delays and relativity. In the highly relativistic limit the time-delay effect implies the observed surface area approaches 0, as only the ejecta expanding towards the line of sight is visible.