Photoluminescent exit signs (PL) are one of two types of code-compliant non-electrical exit signs (Self-luminous/Tritium/nuclear powered exit signs are the other). The photoluminous exit signs work by using premium-grade glow-in-the-dark pigments that are either screen-printed, sheet, or mold casted to form a contrasting EXIT legend and background. The installed signs require a continuous charging light source of a required minimum of 5fc (foot candles) of ambient lighting source during normal business hours and building occupancy. When the lighting levels drop beyond a threshold, such as in a power outage, the stored light energy in the sign is then visibly contrasting creating a glow in the dark effect. Note: either the letters or the background of the sign are glow-in-the-dark but never both. PL signs have a "rated viewing distance" between 50ft to 100ft which means these signs must not exceed this distance within their spacing from one another or the exit door. These ratings are determined in accordance with the Underwriters Laboratory standard (UL924), the gold standard for testing and certifying emergency lighting equipment performance.