In an ideal AR device, only the AR object image should appear in the real environment. However, certain AR devices, such as those using a half mirror, also present the AR background surrounding the AR object due to the transmittance of the half mirror. This might be perceived as a two-dimensional display image overlapping onto the real environment. In this scenario, the perception of brightness might differ from cases where only an AR object is superimposed on the real-world background. To explore the brightness perception of AR images presented by AR glasses with half mirrors, we conducted two psychophysics experiments. In Experiment 1, using a single pair of AR glasses, we observed the brightness perception of AR images under various correlated color temperatures (CCTs) for exterior lighting. Participants perceived the AR image to be brighter than the reference image with the same luminance. The CCT of the exterior lighting had no significant effect. In Experiment 2, we investigated the effect of the AR background on the brightness perception of AR images using two pairs of AR glasses with different transmittances (35% vs. 70%). Participants perceived AR images to be brighter than the reference when using glasses with higher transmittance. These results suggest the presence of an additional contrast effect on brightness perception in AR glasses with half mirrors.