Solid-state lighting, or SSL, is the future of museum lighting—if we can learn to control it. Common SSL sources, like LED lamps and luminaires, are on the rise in museum settings and are poised to eclipse many common incandescent lighting sources in terms of energy-efficiency, longevity, and long-term cost savings. What’s more, both white and RGB LED sources offer designers near-endless possibilities for exciting, new museum exhibit and gallery designs. However, the rapidly ascending technology isn’t without its challenges, as anyone who has ever tried to dim an LED lamp can attest. Flicker, color shift, and an inability to dim below 10% are common issues for SSL sources, such as LEDs. These issues raise the question: Can we really control LEDs?

The short answer is yes. In this article, I’ll address these common concerns and will explore various approaches to controlling LED lamps and luminaires. I’ll talk about how those approaches differ from how we control the incandescent lamps. Lastly, I’ll share some recent examples of LED lighting controls successfully implemented in museum settings.

Why controlling LEDs isn’t the same as controlling incandescents

The first key to understanding how to control LEDs is realizing that the common methods we use to dim traditional incandescent lamps don’t necessarily apply to LEDs. That’s due to the fact that LEDs and incandescents produce light differently. When we dim an incandescent source, we are using a dimmer from (insert your favorite company) to reduce the power to a lamp. Some of these companies use Silicon Controlled Rectifier (SCR) dimming technology, while others use Triode for Alternating Current (TRIAC) technology, or perhaps even traditional resistive dimming. Dimming in this way reduces the power on the hot and neutral wires, the two wires feeding the lamp, which causes the filament to produce less light. A filament does not have any electronics; it does not have circuitry. It is designed to turn current through resistance into light.

In comparison, LED sources have quite a bit of electronics to make light from power. They typically include some kind of driver with a capacitor to provide constant current and some kind of transformer and/or inverter to reduce voltage and/or change AC to DC power. LED sources may also include additional electronic components based on their shape, size, voltage, and power requirements. At a low, dimmed level on a power-line dimmer, the source’s electronics can’t sustain sufficient power to run. As a result, the lamp’s light cuts out at low levels. How the electronics are made and designed will affect that cut-off point. Most new dimmer-rack controllers will allow a user to adjust this point, but there may still be a harsh point at which the light just turns off, rather than dims to zero.

LEDs’ dimming curve is another issue. We have seen some products set to a level of 50%, but they have only reduced 10% of their light output. Then the source stays at this level until about 10% and turns off. This is what some SSL LED manufacturers call dimming, although I would argue that it’s not. Why this happens is all in the manufacturing of the driver and its relationship to the LED array. Let’s just say it’s possible and not desired.

A third common complaint with two-wire power-line LED dimming is flicker. This can occur for many reasons, but it’s usually seen one of two ways. First, we see the 60 Hz (in North America and some parts of the world, 50 Hz elsewhere) AC cycle of power that passes through the driver directly into the LED light engine. Since this is mainly due to lower cost drivers, an easy solution is to upgrade to a better quality driver. The second occurrence is harder to fix, and occurs when “dirty” power from the local utility passes through the driver and causes flicker. It can affect even the best-quality drivers. It may only impact the SSL product in certain areas of a facility. For example, while using the same technology in the same building I have seen one area with no flicker and another with massive flicker. The only fix for this is to use a different control strategy. (Please note, some products just flicker, dimmed or not. Dimming is not going to help this issue and is beyond the scope of this discussion.)

LEDs’ color output may also have a strange reaction to dimming. An incandescent or halogen lamp will produce an amber shift to warmer color temperatures, as it dims. In comparison, most LED technology does not produce this amber shift when dimmed, although some do. The sources that do not produce an amber shift can react to dimming in a few different ways. Some keep their color temperature, but the color gets muddy, while some dim and shift their Correlated Color Temperature (CCT) to the blue and others shift to the warm. Then there are lamps that produce any or all of these effects when dimmed. A lighting designer needs to be aware of this issue as the items the designer is lighting may shift in color. If it’s lighting a graphic or an artifact in a museum, this shift may become a problem.

How to properly control LEDs

Despite these common setbacks, the good news is that there are ways to dim LEDs smoothly. The key is using digital control technologies to control LED sources. Put behind you the days of dimmer racks. You just don’t need them anymore. Instead, DMX512 or DALI controls enable much better control of LED sources. However, before we talk about the best ways to control an LED source, let’s discuss the current most popular method.

Four-wire, low-voltage, 0-10 V DC fluorescent dimming, a practice outlined in NEMA ANSI C82.11, has become the most popular dimming option for LED sources. (It is important to note that the NEMA ANSI standard does not define or address the minimum light output or shape of the dimming curve.) In my opinion, this is merely a lazy solution touted by luminaire manufacturers . . . but I digress. Using this control strategy, a relay is used to turn power on and off to the fixture while a dimming controller sends 10 V (lowest level) up to 1 V (highest level) to the driver. The LED then reacts. When no voltage is on, the control wires, the LEDs are to be at full brightness. The problem with this method is that if the driver does not get 10 V due to voltage drop, the source will not dim low enough. We have also experienced issues with cross-talk from line voltage wire to the low voltage control wires, causing LEDs to flicker or have odd dimming curves. What’s more, 0-10 V DC dimming is one of the most costly dimmer modules to install. (Connect with your favorite lighting control system manufacturer and ask for the pricing. Remember you need the relay for the power and the 0-10 V controller per zone of control.)

Not surprisingly, unless there is no other option, I recommend avoiding 0-10 V dimming. Instead, I suggest using digital controls to control LED sources. In most scenarios you will get smooth dimming, from full light to off levels, as well as many control features you don’t get with any of the other control strategies, such as individual luminaire control and status information like voltage and temperature. The key question is which digital control protocol to use: DMX512 or DALI?

ANSI E1.11 - USITT DMX512-A is a fast, well-known protocol. It is most often used in theatrical and entertainment applications. It has moved into the architectural environment with the use of color changing LED technology. Through low-voltage cable that is daisy-chained, or by using opto-splitters, all fixtures are connected back to a controller. Power and data are required. DMX512 is a popular protocol, but it also has some limitations, such as 32 maximum unit loads per line, a low-voltage signal, and the fact that it must be daisy-chained. Fixtures must be wired to the DMX512 control system, though there are some wireless technologies. And, there is also the issue of emergency (EM) lighting. DMX512-controlled fixtures need a control signal to know what to do. To use a DMX-controlled fixture as an EM fixture you need to have a DMX512 controller properly listed for EM applications. There are a few on the market that work quite well and meet most code requirements, but the DMX512 control system must be designed appropriately for this. Another thing to be mindful of, is that the DMX512 driver in an “off” state still consumes power. On one project we are working on, we figured a cost of approximately $3,000 a year in power. So we installed a $3,000 relay panel to make sure when the source is off, the driver is off as well.

DALI (Digital Addressable Lighting Interface) (IES 60929 Annex E, IEC 62386 Parts 101 & 102) is the other digital protocol that works very well with LEDs. DALI is a non-proprietary standard that sends control signals to an LED driver via two wires that can be installed as low voltage or line voltage wires. In a line-voltage scenario the wiring can typically be pulled in the same conduit as your power wires. A DALI network consists of a controller, power supply, and a maximum of 64 devices, though I suggest limiting this to 56 so you have some room to grow. The controller assigns the addresses, so you cannot pre-address drivers. This makes replacing the drivers a bit more complex and requires training maintenance staff. One particular limitation of DALI is speed: Do not expect to get zero-count bump cues on a DALI system. This slowness is offset by the ease of installation for the DALI control wires. Another great feature is that the control wires can be wired in a star, “T,” or daisy-chain. This makes installation much easier than DMX512.

It is important to know how your lighting control system will control the DALI drivers in your installation. According to the IES TM-23-11 standard, “DALI uses three types of addressing—broadcast, group, or individual, coupled with a command, to communicate specified actions.” In broadcast mode all drivers on the controller act as one zone. Using groups set in the drivers, those groups act as one zone. Individual allows you to control each driver independently. The lighting controller can control DALI devices either by sending DALI commands to the controller and drivers to turn on, or go to a specific level. The light controller can also program scenes into the drivers and then just trigger scenes in the drivers. When mixed loads exist in a project, such as 0-10 V, DMX512, and DALI loads, the latter control scenario is preferred.

Please note, there is typically an upcharge to get a fixture with DALI or DMX controllable drivers. However, you will typically save money on your lighting control system and the added cost should be worth the flexibility you gain from it, not to mention the headaches you may avoid with other control protocols.

LEDs and digital controls for museums

These options are all well and good if you are doing a new project, or have open spaces where you can run some new wiring, but what if you have to work with an existing lighting system? One of the most common existing lighting systems in museums is track lighting with PAR lamps and two-wire dimming, a set-up that we frequently encounter in our work lighting museums at Lightswitch. We’ve experienced many issues when converting these systems to use LED or LED replacement lamps (LEDi), including flicker. However, several new technologies have enabled us to successfully retrofit existing museum track lighting systems and avoid these issues.

In the Art Institute of Chicago’s Japanese Art Galleries in the Roger L. and Pamela Weston Wing of the Art Institute of Chicago, for example, new lighting was needed to light artifact cases and the museum’s popular Ando Gallery. These galleries have pieces that are constantly rotated. As the art changes, light levels need to be adjusted to meet the new artifacts’ requirements. To fulfill these requirements, the track system we installed used a power line carrier control signal. The control system sends a DMX512 signal to a power-line signal generator. The system then sends the control signal over the power wires to the controller inside each track head. The museum can use a touchscreen to set levels on each LED fixture. This technology was retrofitted into the existing building wiring and allowed for great flexibility and dimming of the sources with no flicker. At its core the technology is using DMX512, but it’s being used in different ways.

At the Museum of Science and Industry (MSI), in Chicago, Lightswitch renovated three exhibits using another new LED technology, a LEDi lamp that is wirelessly controlled. The LEDi lamps replaced 90 W, 130 V PAR38 lamps that were not on dimmers. Using a laptop and software by the lamp manufacturer, we were able to individually control each new lamp’s color and intensity. An interface box was used when color or intensity needed to be changed via cues. This interface box translates a DMX512 signal into a wireless signal to the lamps. By using this lamp we have added dimming capabilities to the three exhibits, and can set the light’s color without the use of dichroic glass or gels.

As a result, MSI’s “Colleen Moore’s Fairy Castle,” an exhibit with lighting that used to be too moody and dark, is now light and alive. Rooms are highlighted with slight changes in color temperature, and the more than 80-year-old artifact no longer has UV or IR projected onto it. MSI’s new exhibit “Numbers in Nature,” the museum’s first all-LED exhibit, features 110 of these lamps that were used in place of incandescent lamps and a traditional dimming system. The exhibit’s lighting load was reduced by 70% and the museum reduced 50% of the projected project cost using this technology. After the success of these two projects, the museum’s “U-505” exhibit was also retrofitted with the same LED lamps. We replaced 220 90 W, 130 V PAR38 lamps, some of which were dimmed, with switching modules in the existing dimmer racks. The replacement system meets the museum’s need for low-maintenance fixtures. It will no longer need technicians to change lamps frequently and risk potentially changing the focus of the track heads.

Given the correct digital controls, LED luminaires can be controlled better than (or, in some cases, as well as) traditional sources. It takes a bit more work, and it takes testing, experience, and some thought, but it’s possible. Digital controls may give designers and museum staff opportunities they did not have with the sources of the past. To harness those opportunities, we need to change the way we think about dimming and LEDs and open ourselves up to new possibilities and technologies.