The basic photometry quantities of a dual-panel backlight are investigated from an energy flow viewpoint. A Gaussian-like distribution of the main panel and subpanel luminances is assumed and the luminous flux, illuminance, and luminance of the two panels are solved analytically. The influence of the luminance cone angle on the photometry quantities of the main panel and the subpanel is carefully examined. Finally, an assessment of the attainability of either equal or nonequal luminance conditions of the main panel and the subpanel is presented. The results show that the upward luminance cone angles of the subpanel region and the main panel area minus the subpanel region play an important role in determining the luminous flux and illuminance in these regions. The luminance cone angles of the upward and downward directed fluxes govern the luminance difference between the main panel and the subpanel. Finally, the relationship between the luminous flux ratio and the subpanel luminance cone angle ratio suggests that a maximum luminance difference of 70% can be obtained between the main panel and the subpanel when the dual-panel backlight is designed with a symmetrical layout about the light guide.