The smartphone's ambient light sensor has been used in the literature to study different physical phenomena. For instance, Malus's law, which involves the polarized light, has been verified by using simultaneously the orientation and light sensors of a smartphone. The illuminance of point light sources has been characterized also using the light sensor of smartphones and tablets, demonstrating in this way the well-known inverse-square law of distance. Moreover, these kinds of illuminance measurements with the ambient light sensor have allowed the determination of the luminous efficiency of different quasi-point optical sources (incandescent and halogen lamps) as a function of the electric power supplied. Regarding mechanical systems, the inverse-square law of distance has also been used to investigate the speed and acceleration of a moving light source on an inclined plane or to study coupled and damped oscillations. In the present work, we go further in presenting a simple laboratory experiment using the smartphone's ambient light sensor in order to characterize a non-point light source, a linear fluorescent tube in our case.