Office lighting does not have to be static, light sources do not have to be white, and lamps can be individually addressable. The key to this variety is to assign a dimming ballast with an individual address for each lamp. This is the advantage of digital addressable lighting control, or DALI, as the concept has come to be called.

DALI was born in Europe. The Austrian firm Tridonic and others developed the digital serial interface ballast in 1991; by 1998, a consortium of European lighting manufacturers had developed the Digital Addressable Lighting Interface. A language or protocol for ballasts and relay switches, DALI was devised to control a small set of up to 64 intelligent luminaires per DALI controller or 'busmaster.' A busmaster is the size of a cigarette pack and connects to the serial interface of a PC, which means the control panel is small in size.

What Is DALI Good For?

Using this technology, intricate color and luminance patterns can be achieved without applying individual circuits for each lamp to be dimmed. Contrary to analog controls, power is supplied in the traditional way with a minimum number of circuits. However, all chips are connected with a two-wire bus that carries both power and standardized control signals on low-voltage wiring to every device on a network, thereby reducing cabling costs. The messaging system complies with an open-source protocol, so every building automation system can use programs to send commands, and building engineers and lighting designers can write programs to transmit commands. Every ballast and relay switch on the network gets its unique address stored in the ballast and can be controlled via software, and from any PC that has a web browser. Users in Great Britain, Austria and Germany have controlled the DALI Lighting Laboratory at Penn State University in University Park, Pennsylvania, from their desktop. System performance and energy consumption can be monitored from anywhere in the world. This is an invaluable information and maintenance tool (although it may also be the nightmare of facility managers).

DALI control systems can also be an asset in the current race for green building points. The LEED rating system assigns credits directly to lighting, namely for the following design accomplishments: a daylight factor of 2 percent in over 75 percent of the occupied work spaces, no light pollution from excessive façade lighting, and a direct line of sight to windows. Since DALI software performs automatic monitoring of both light energy use and ballasts, a building on a DALI system can obtain additional LEED credits for these points: in the Indoor Environmental Quality category, 'Controllability of Systems, Perimeter' (credit 6.1), and 'Controllability of Systems, Non-Perimeter' (credit 6.2); and in the Energy and Atmosphere category, 'Optimize Energy Performance' (credit 1.3), and 'Additional Commissioning' (credit 3).

Digital versus Analog

DALI complies with an open-source protocol based on standard IEC60929, which specifies performance requirements for electronic ballasts for use on AC supplies up to 1000V at 50Hz or 60Hz with operating frequencies deviating from the supply frequency. It only applies to electronic ballasts. A small built-in chip acts as the de-central controller in the electronic ballast or relay switch. One small controller can manage up to 64 ballasts. DALI commands are sent not in analog, but digital format between 0 and typically 16V DC. This ensures reliable, unambiguous command transmission, which is not the case with analog lighting controls operating between 0 and 10V DC.

The advantages of digital over analog controls are many. Unlike analog controls, DALI-based systems operate with two-way communication-receiving informa-tion as well as sending it-and therefore, they are able to accept error feedback and lamp and ballast operating informa-tion. Furthermore, DALI systems enable programmability of ballasts, disturbance-free transmission, a logarithmic dimming curve based on brightness perception, and insensitivity to polarity (that is, the ability to switch the two low-voltage DALI wires going into the ballast). DALI systems facilitate exact dimming levels for all ballasts regardless of manufacturer, and can handle up to 64 addresses per controller and 16 groups and scenes per busmaster. Ballasts are approved for Class I and Class II wiring installations. Low-voltage control wires can share the same conduit as power wiring without interference. The control of lighting fixtures is independent of power circuit wiring, thus simplifying electrical design, reducing wiring complexity and lowering costs. DALI commands can be sent at three communication control levels (broadcast, group and address), and pro-grammability includes groups, scenes, maximum and minimum light levels, fade times, and emergency light levels. Other advantages include time-of-day schedules, status reporting and feedback on activity levels, and energy monitoring.

A cost comparison for Penn State's lighting laboratory with 90 fluorescent lights controlled by a conventional 0–10V DC system versus a DALI system shows that the cost of the latter is about 64 percent of the former. While conventional dimming ballasts are approximately 20 percent cheaper, the cost for the software, the PC interface, control panels, and the enclosure is much higher. In addition, the DALI solution requires 6 circuits versus the other solution's 90 circuits.

What are DALI's limitations? It requires wiring, making it better suited for new construction than for renovation. It is strictly a lighting protocol: Interfaces must be used to translate the protocol and communicate with other control protocols, such as BACnet for HVAC and building automation. In order to maximize energy savings, it will be necessary for lighting controls such as DALI devices to communicate with HVAC controls.

Despite this short list of limitations, DALI is a convenient tool for lighting designers to make dynamic lighting happen. Ongoing research into this technology will further expand its capabilities. Current developments in DALI include window blind control, dimming for tungsten halogen loads and radio control through new and emerging wireless 'mesh' networks (using radio frequency). A mesh network employs one of two connection arrangements: full mesh topology or partial mesh topology. In the full mesh topology, each node is connected directly to each of the others. In the partial mesh topology, nodes are connected to only some, not all, of the other nodes. It handles many-to-many connections and is capable of dynamically updating and optimizing these connections. Future develop-ments will include software that performs purpose-driven lighting control based on load shedding, time of day, temperature, occupancy, user preferences, and input from the Internet.

Dr. Martin Moeck is an assistant professor in the Department of Architectural Engineering at Pennsylvania State University. Since July 2002, Dr. Moeck has worked on developing a digital addressable lighting laboratory with Internet access and web control. It is the first known light lab of its kind in the United States.