In its simplest form, a dim-bulb tester puts an incandescent lamp in series between a device — like an old radio — and the AC power from a wall socket.

Xiaoxue Guo, Jiapu Zou, Zihua Wu, Yuanyuan Wang, Huaqing Xie. Experimental study on the power losses of a single photovoltaic cell and two series and parallel connected cells with partial shadows.