In a video uploaded to Vimeo, the duo reveal aspects of their method and their results. But for a small slit, the screen of the Android smartphone has been entirely covered. With a camera set to long exposure, the handset is slid over the device (a Macbook for example), and the visible slit shows a series of colored dots which apparently relate to the EMF field.
Using what appears to be a separate program, a circle on the phone's screen appears to expand according to EMF intensity. The pair used the Processing open source programming language, which was designed for creative and visual projects.
Without knowing exactly what the dots mean, perhaps one shouldn't take the pictures as a literal visualization of the fields' forms, especially since the patterns created will be highly dependent on how the phone has been moved. But as a work of art, this is hardly the point.
Here's that video ...