EL, or electroluminescent signage, is essentially a flat light bulb sandwich consisting of layers of conductive and non-conductive plastic and a layer of phosphor.
The phosphor is laminated between two conductive layers and, as a voltage is applied between the two conductive layers, or electrodes, the phosphor emits light energy.
When a high-quality, high-resolution image is printed over the light, it behaves like a backlit sign