The presence of a glare source in the visual field produces a veiling luminance ($L_v$), which generates a brightness reduction that can be expressed in terms of the glare index ($V$). The relation between the veiling luminance caused by glare and the apparent brightness reduction of a reference target has already been established for steady conditions. In this paper, the relationship is derived for transient glare. First, the relation is tested empirically, and then previous results concerning the effect of transient glare on brightness are summarized and analyzed. From this analysis, a power function relation between $L_v$ and $V$ is encountered.