A lamp includes a tube, a first inner electrode and a second inner electrode. The tube has a cavity, a first sealed portion and a second sealed portion. The cavity is located between the first and the second sealed portions. The first and the second inner electrodes are disposed at the first and the second sealed portions respectively and extend into the cavity. The first inner electrode has a first rod and a first head portion disposed at a first end of the first rod and located within the cavity. A volume of the first head portion is between 1.0 and 30 mm3. The second inner electrode has a second rod and a second head portion disposed at a second end of the second rod and located within the cavity. A volume of the second head portion is between 0.1 and 4.0 mm3.