An alternate application of dichroic mirror for the improvement of luminance and luminous efficacy in an ac plasma display panel (PDP) is suggested. Since only part of the vacuum ultraviolet (VUV) generated in the PDP cell is used to excite the phosphor, we propose deploying a dichroic mirror that increases luminance and efficiency by reflecting part of the VUV toward the phosphor. The optical constants of the dielectric layers of the dichroic mirror and a protecting layer of a PDP were determined from the measurements of reflectance and transmittance of the corresponding films and they were used to design the dichroic mirror including the protecting layer. The 147 nm xenon resonance line is usually used to activate the phosphor in the PDP cell. Thus, we designed the dichroic mirror to have a high reflectance for the VUV centered around 147 mn and a high transmittance for the visible light. We investigated the usefulness of the dichroic mirror by comparing the luminance and luminous efficacy of test panels with and without the dichroic mirror and found it could increase the luminance by 20%-31% and the luminous efficacy by 25%-34%.