they produce. This can either be in the form of a plastic cover over the fixture, or a reflector placed behind the light source. Most individual light sources can be rotated and aimed for where light is needed most in an emergency, such as toward fire exits. Modern fixtures usually have a test button of some sort which temporarily overrides the unit and causes it to switch on the lights and operate from battery power even if the main power is still on. Modern systems are operated with relatively low voltage, usually from 6-12 volts. This both reduces the size of the batteries required and reduces the load on the circuit to which the emergency light is wired. Modern fixtures include a small transformer in the base of the fixture which steps-down the voltage from main current to the low voltage required by the lights. Batteries are commonly made of lead-calcium, and can last for 10 years or more on continuous charge. U.S. fire safety codes require a minimum of 90 minutes on battery power during a power outage along the path of egress. Compliance Codes New York City requires emergency lights to carry a Calendar Number signifying approval for local installation, Chicago requires emergency lighting to have a metal face plate, and Los Angeles requires additional exit signs be installed within of the floor around doors to mark exits during a fire, as smoke rises and tends to block out higher installed units. As there are strict requirements to provide an average of one foot candle of light along the path of egress, emergency lighting should be selected carefully to ensure codes are met. In recent years, emergency lighting has made less use of the traditional two-head unit - with manufacturers stretching the concept of emergency lighting to accommodate and integrate emergency lighting into the architecture. An emergency lighting installation may be either a central standby source such as a bank of lead acid batteries and control gear/chargers supplying slave fittings throughout the building, or may be constructed using self-contained emergency fittings which incorporate the lamp, battery, charger and control equipment. Self-contained emergency lighting fittings may operate in "Maintained" mode (illuminated all the time or controlled by a switch) or "Non-Maintained" mode (illuminated only when the normal supply fails). Some emergency lighting manufacturers offer dimming solutions for common area emergency lighting to allow energy savings for building owners when