Interesting:

Chief Security Engineer Andrea Barisani and hardware hacker Daniele Bianco used a handmade laser microphone device and a photo diode to measure the vibrations, software for analyzing the spectrograms of frequencies from different keystrokes, as well as technology to apply the data to a dictionary to try to guess the words. They used a technique called dynamic time warping that’s typically used for speech recognition applications, to measure the similarity of signals.

Line-of-sight on the laptop is needed, but it works through a glass window, they said. Using an infrared laser would prevent a victim from knowing they were being spied on.