In astronomy, the optical window is the portion of the electromagnetic spectrum that passes through the atmosphere all the way to the ground. It is called "optical" because the wavelengths we can see are all in this range. The window runs from around 300 nanometers (ultraviolet-B) at the short end up into the range the eye can use, roughly 400-700 nm and continues up through the visual infrared to around 1100 nm, which is in the near-infrared range.
In this installation, four laptops are configured to track a different color (red, yellow, green, and blue). Depending on whether the laptop sees its assigned color, and where that color is in the installation space, the laptop will play recently-heard sounds.