An exemplary holding frame (20) is for fixing lamp pairs (31) having electrode holders (311) and lamps wires (312), and includes a top surface (201), a bottom surface (202), an inner surface (203), and an outer surface (204). The top surface defines plural electrode receiving openings (2011). The bottom surface defines plural wiring openings (2021). The outer surface defines plural electrode groove pairs (2041), and each electrode groove of each of the electrode groove pairs communicates between a respective one of the wiring openings and a respective one of the electrode receiving openings. The electrode groove pairs are configured for receiving the electrode holders of the lamp pairs. The bottom surface defines plural wiring channels (2022), and each wiring channel communicates between the wiring openings at a corresponding electrode groove pair. The wiring channels configured for receiving selected portions of the lamps wires.