What feature allows the dual site glass window to be visible in the dark?

The correct answer highlights that the dual site glass window has a property that allows it to emit light in dark conditions. This feature is particularly useful in emergency situations where visibility is compromised, such as in smoke or darkness typically encountered in firefighting scenarios. The ability of the glass to glow in the dark ensures that users can quickly read vital information displayed on the gauge, contributing to operational safety and effectiveness.

This characteristic enhances situational awareness without requiring additional light sources, which might be impractical or dangerous to utilize in certain environments. The glowing ability is specifically designed to be part of the equipment, ensuring that it remains functional even when visibility is low.