Screen content shown alongside streaming EMI. Brighness changes lead to differences in power consumption (of the TV's backlight) and this in turn modulates the electromagnetic interference (EMI) signal emitted by the TV.
The left plot shows frequency shifting EMI (which fluctuates left to right between higher and lower frequencies), the right plot shows energy density EMI (which fluctuates in amplitude).
The paper (link below) suggests that it is possible to tell what someone is watching on a TV by collecting a short period of unintentionally generated EMI from any wall socket in a home (not just the socket connected to the TV). This research was based on in-lab and in-home experiments with 8 TVs ranging in size, technology, and manufacturer, and a dataset of 20 movies plus over-the-air broadcasts.