A few short years ago, LEDs were like the Jessica Simpson of the live event production industry; they were pretty but not too bright. Thanks to Haitz’s law, that’s changing faster than executive salaries on Wall Street. But what does the technology have in store for us in the future? For answers, we asked Chad Stalker of Luminus Devices where all this will lead. PLSN: There’s been a lot of discussion about the greening of the lighting industry and a lot of data has been tossed around about the efficacy of LEDs. At the same time, there are a number of new lamp technologies like the CFL, T5 fluorescent, LiFi plasma, etc. What’s the straight dope on LEDs regarding the luminous efficacy in lumens per watt?
Chad Stalker, Luminous Devices: LEDs have had a lot of press due to the significant promise that they hold in outperforming many other light sources in the marketplace. Leading LED performance, available in high volume, is at over 100 lumens per watt from the source. However, many things affect the total efficiency of any lighting system. It is easy to get caught up in the efficiency of the light source, but neglect the efficiency of the whole lighting system. Many traditional and new light sources (e.g. lamps, LEDs, etc.) have very high light source efficiency. However, the light system efficiency — basically how much light comes out of the fixture, isn’t always very high.
In the case of LEDs, especially when used in directional light applications, the overall lighting system efficacy are equal or better than traditional light sources because the light is only sent where it is desired instead of being sent backwards and into the fixture. In the end, the lighting system efficacy, characterized in lumens/watt, is on par with other traditional light sources.
The combination of a directional light source with directional application is the reason why LEDs are discussed as a leading, eco-friendly, lighting solution.
We’ve heard a lot about Haitz’s law (LEDs have doubled in brightness and dropped in price every 16-24 months over the past 40 years), and we in the entertainment production industry have witnessed firsthand the exponential increase in intensity of LEDs and simultaneous drop in their price. But RGB luminaires have a discontinuous spectrum and the Color Rendering Index (CRI) is low compared to incandescent. Some fixtures, like the 7-color LED Seladors, have a much better Color Rendering Index. Is the color in LEDs being improved? Can the full spectrum be realized with LED sources?
Over the last handful of years, LEDs have been rapidly improving. Luminus, for example has demonstrated steady brightness improvements of 30 to 50 percent year-over-year and steady improvements that follow this trend are expected. Color is a unique topic for LEDs. With fairly narrow spectral bands (of 20-40nm full-width-half-max), it has always been difficult to create full spectrum light with LEDs. When combining just red, green and blue to make white, the color rendering may only reach 20 or 30, a level that is unacceptable for good quality lighting. In order to enhance the CRI, more colors must be added. Some companies tile many different colors together to create the appropriate spectral profile, but the end result is a fixture that has significant color pigmentation in the near field. An added problem of doing this is that when an object is placed in front of one of these fixtures, the spatial separation of the different colors causes color breakup in the shadow.
To address this, Luminus created the PhlatLight CBM-380 — a four primary LED module capable of delivering 2500+ white lumens at a CRI of greater than 90. The result is an LED module that can track the black-body curve with high color rendering as well as match the chromaticity points of 97 percent of the standard theatre gels. Additionally, the close proximity of the chips in the CBM-380 — only the width of a few human hairs — allows the fixture to look extremely uniform in the near field. Since all of the light is coming from a single source, color shadowing becomes a thing of the past.
We’re just starting to see some LED hard-edge focusable fixtures on the market, but as of right now, they are primarily lower end products. Is there a future for LED profile fixtures (with gobo projection)? If so, when will they be more common?
An important limiting factor for high-end LED profile fixtures is the source etendue limitations that aren’t seen in conventional arc-based projection systems. Source etendue can be defined as the product of the area and solid angle of the source. For example, lamp-based projection systems use a small and efficient light source that allows the system to output high amounts of light flux in a very tight beam. Typical LEDs are larger than most lamp sources and do not allow the system designer to efficiently focus enough light through the gobo system from a single chip. The lack of light has been addressed by tiling many small LEDs into tightly packed arrays and focusing them all onto a single gobo, but due to etendue, the light flux out of the projection lens is very low. Eventually, LEDs will improve in efficacy enough to compete with high power lamp-based sources from an etendue standpoint, but it is going to happen with different system architectures due to the differences in which lamps and LEDs emit light. Lamps emit light over a small area but into all directions, and LEDs emit light over a larger area but only in the forward direction. Luminus LEDs were originally designed for microdisplay projection systems, so they are well-suited for gobo projection systems due to the attention paid to maximizing the brightness per etendue of the source.
We’re also just starting to see some larger LED fixtures. Vari-Lite has a prototype 600-watt LED color wash fixture in development and more manufacturers are starting to produce RGB color wash fixtures in the 300-watt range. What is the limiting factor at the moment? How big can they ultimately get?
When looking at the limitations that can affect how big these next generation high output fixtures, thermal management is a major factor. How a fixture manufacturer manages this has a direct impact on how well such systems can scale in size. In order to deliver the light intensity and beam control these next generation fixtures are delivering, they utilize specialized LEDs. These LEDs are typically specialized high power packages that deliver significant light output as well generate considerable heat.
In order to deliver the tens of thousands of hours of operating life expected by LED-based lighting systems, the fixture designers need to implement fairly aggressive cooling solutions. The complexity of these thermal management solutions can present the fixture designers some challenges when you have to take into consideration size, power and noise constraints. In the end, these constraints directly impact how big these systems can be.
But there are factors that are in play that are working towards making the system less complex, and in turn more scalable, when you look at such things as efficacy improvements and advances in thermal management.
