We measured the effect of a transient glare source on t heperceived brightness of astandard luminance (L(STD)) patch (0.5 cd/m2) as a function of the surround luminance (Ls). In the experiment, both increment and decrement stimuli were dependent on the value of the Ls (0.01, 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, or 2.0 cd/m2). We adopted a magnitude comparison paradigm using constant stimuli to determine the test matching luminance (L(M)). When L(S) was lower than the luminance of the patch, which corresponds to increments, LM was lower than L(STD), and this effect was highest for the lowest L(S). There was a small but noticeable cusp as increments shifted to decrements. As L(S) increased further (i.e., as the decrement grew), L(M) flattened out below L(STF). The overall pattern of results could be interpreted in terms of the concept of contrast brightness, with consideration of the intrinsic differences in brightness evaluations between decrements and increments.