Update on how LEDs are driving lighting control system technology forward
The call for improved energy efficiency and government/energy code requirements continues to drive the need for greater use of lighting control. Effectively, flexible lighting control delivers the necessary amount of light for a given task in a safe manner. The two strategies used to accomplish this today are:
1) occupancy-based control — turning lighting on when a space is occupied and reducing it when it’s not (using either time-based scheduling or occupancy/vacancy sensors); and
2) daylight harvesting, which increases or decreases electric lighting, depending on how much natural light is available.
Solid-state lighting (SSL) — light-emitting diodes (LEDs) and organic LEDs — the light sources of the future, don’t use gases, plasma, or filaments, so they are not bound to the limitations of legacy light sources. This opens up the lighting field to new form factors (how a fixture looks) as well as new ways for control and delivery of power to the lighting fixture.
An interesting aspect of the LED chip is the relative ease with which it may be controlled: There is no warm-up time or hot-restrike issue to contend with, and changes in light levels or color output are easy to handle. All of the components (LED drivers and controls) are now essentially digital, solid-state devices that can work together easily, delivering performance not previously achievable. This means we are at the forefront of complete digital control that will not only allow for individual lighting fixtures to be controlled, but also enable communication between devices and a high degree of interaction with the environment.
Let’s look at one of the strongest markets today for LED lighting as an example: streetlighting and other outdoor area lighting. Leaving aside the continuously increasing lm/W efficiency and high color rendering properties of LED chips, an outdoor luminaire can be dynamically controlled for appreciable energy savings. The electronic circuitry in the chip driver can include a microcontroller, temperature and humidity sensors, input from a pavement-embedded induction loop, or other vehicle sensor. This information can be communicated to a central location by using an RF or powerlinenetwork. As a result, the light output of a roadway luminaire can be changed from sunset to midnight and midnight to dawn, or at other intervals in between. During inclement weather, such as fog or rain, the light output can be adjusted for maximum safety, and a similar scenario is possible during an accident/ emergency or police activity.
On a smaller scale, one manufacturer has introduced an LED outdoor luminaire that can be controlled by an onboard, camera-based occupancy sensor and video processor. Motion detection can be confined to any part of the camera’s field of view for operation, using specialized microchips as part of the control system.
Beyond the outdoor area lighting market is an even larger growth sector. The commercial market is poised to become the biggest user of digital controls. The ceiling and walls of a typical office should be able to mimic the outdoor environment regarding the amount of light as well as spectral content. Manufacturers now offer autonomous and semi-autonomous light control systems that place ambient light sensors in individual luminaires and even LED-based replacement lamps themselves. This improvement in sensing and control will mean that, just as a mobile phone automatically detects available networks, an integrated lighting system should be able to deliver a preselected setting of dimming and color temperature when you enter a room. A key element in the success of these newer systems is the development of low-cost, chip-based sensors, first used in cell phone/ computer display equipment.
For example, an ambient light sensor (ALS) should offer a performance that matches the technology of other control system components. In the past, these sensors were either a simple photoresistor or a photodiode. Now, a system-on-chip light sensor serves as a complete light-sensing subsystem, including conversion of analog readings to a digital I2C output signal. This includes correction for any effects caused by light flicker on an integrated circuit as small as 2 mm square, and the sensor costs about the same or even less than a simple photosensitive component alone.
Additionally, an ambient light sensor in an interior space should measure only the visible portion of the electromagnetic spectrum, and it should weigh the various colors to roughly match the sensitivity of the human eye, meaning it should have a photopic response. Being able to distinguish between daylight, incandescent, CFL, and LED lighting is also useful, because the sensor can aid a controller in reducing energy consumption by automatically switching to the most efficient light source(s).
Another manufacturer in the lighting industry, with wide experience in data networking, now offers a sensing and control system for commercial lighting that places a variety of such sensors in a single, small module so that a number of ambient conditions can be measured. The module can be located adjacent or within a luminaire. In addition to detecting the light level at each fixture, other ambient conditions, such as temperature, carbon dioxide level, carbon monoxide level, air pressure, air velocity, humidity, particulate, and color, can be recorded. The company developed this sensor based on the idea to shift from simply using energy to managing it.
A sensor built into the luminaire that automatically responds to its environment — whether it’s occupancy, available daylight, time of day, or other variables — and delivers just the right amount of light when and where it is needed is the perfect solution for reducing energy consumption and costs. Rather than applying controls as an afterthought, this built-in approach maximizes energy efficiency.
In the simplest case, as more or less ambient light is sensed by the lamp, it responds in the opposite direction with more or less output in order to maintain a preset target illumination level. When communication and networking functions are wrapped around the sensor/microcontroller combination, a more sophisticated set of interactions is enabled. For example, as daylight increases, theluminaire would not only respond autonomously, but it also would report a response to nearby luminaires. This would enable the device to act in coordination to “balance” the light seamlessly across not only the space directly affected by the daylight, but also in adjoining areas that are more influenced by the dimming luminaires. With low-cost and low-power wireless networking (such as ZigBee), or wired networks, group intelligence and centralized control systems can be implemented readily.
The author wishes to acknowledge the assistance of Sajol Ghoshal, director, Sensor Driven Lighting, austriamicrosystems, in the writing of this article.