Here comes the new class of optical TEMPEST attacks: recovering sound by analyzing optical emanations from a LED power indicator.
There are 2 classes of optical TEMPEST attacks against the confidentiality. 1) recovering content from monitors, 2) recovering keystrokes from keyboards.
By analyzing the response of the power indicator LED of various devices to sound and show that there is an optical correlation between the sound that is played by connected speakers and the intensity of their power indicator LED due to the facts that:
(1) the power indicator LED of various devices is connected directly to the power line,
(2) the intensity of a device's power indicator LED is correlative to the power consumption, and
(3) many devices lack a dedicated means of countering this phenomenon.
Based on these, here comes the Glowworm attack, an optical TEMPEST attack that can be used by eavesdroppers to recover sound by analyzing optical measurements obtained via an electro-optical sensor directed at the power indicator LED of various devices (e.g., speakers, USB hub splitters, and microcontrollers).
This is a very interesting attack, and you can read the full article at https://www.nassiben.com/glowworm-attack.