Abstract A local‐dimming control is one of the advanced ways for backlighting an LCD. It balances less power consumption with higher contrast than other ordinary backlighting systems by independently controlling each luminance of backlights according to pictures. On the other hand, a new defect arises; that is, all the backlight units are not completely uniform and luminance of each backlight varies as an input signal changes. We define this defect as “Halo”. There have not been presented any particular methods to evaluate “Halo” so far. In this paper, we propose a new method on how to evaluate Halo defect in local‐dimming backlight systems with explaining the mechanism why Halo defect arises.