There are two types of emergency signals that are most effective - visual and audio. Emergency lights come in a variety of types and application purposes. Alarm lights are activated when control system detects a malfunction or anomaly, which requires personnel attention or evacuation. They are usually bright red or orange, flashing or pulsating to attract more attention. Backup emergency lights are triggered when the main power supply is cut off. They provide illumination so the workers may finish their work or leave the facility. Sirens provide a loud repeating audio signal, with the similar function to an alarm light. They are often used in combination with the latter, indicating danger or emergency. Commonly sirens and alarm light are controlled by one system to provide a better indication of danger.