Abstract The research team has designed optical structure to be used as tiles on the outsides of buildings for indoor illumination. To cover an entire building, the team has designed optical units to be used together that can compress light and the recommended thickness of units is around 20 mm. The important factor in our design is producing parallel exit beam from our single units. The research team has used two different optical surfaces, saw-toothed and elliptical, and the con-focus principle to design three optical units. The team has designed three types, a saw-toothed unit, an elliptical unit, and a modified elliptical unit. Finally, the team analyzes the efficiency and the beam divergence angle to compare these optical structures. The modified elliptical type offers the best performance because it has higher efficiency of parallel exit beam and higher utilization rate.