Archive for March, 2009

render21

Under that LED die lies a reflector surface and a thermal slug to manage the loads these powerful products demand.

There is one universal truth in all solid-state components, LEDs included – HEAT is the enemy. There is no escaping the loss in performance and life that results from heat within an LED device. The reason packaged High Brightness LED devices do as well as they do comes down to one key factor – thermal management. Inside those innocuous looking devices is a slug of copper that conducts heat from the LED and delivers it to the PCB, or MCPCB directly. This is considerably more effective than the 5mm (and similar through hole design) products, which have virtually no heat sink beyond the legs and one small bit of metal under the reflector cup. The illustrations here show this in better detail.

5mm2

The relative lack of thermal control (other than the standoff legs and a small metal bar under the reflector) limits these through hole devices to very low loads or intermittant use.

The standard 5mm, or through hole style LED is just fine for what it was invented for – loads of a fraction of a watt, and use for indicators that were operated intermittently. It’s when this design is loaded with high output LED die driven at higher currents, then operated continuously, that causes these things to fall apart. Thermal design is all about staying ahead of the heat gain. Fail that and the microchip can’t survive. First lumen output suffers, then ultimately the die begins to break down. In the worst cases, the bond wires melt or break from thermal expansion and contraction.

HB Packaged LED devices are designed specifically to manage the additional heat of high current operation and continuous duty that general illumination requires.

In addition, this thermal conductor design allows luminaire engineers to draw heat away from the LEDs through metal circuit boards, heat sinks, and other designs that pull heat from that internal slug and distributes it. This cannot be done effectively on through hole LEDs (5mm or others), as the stand off legs simply cannot conduct heat effectively from the die inside.

HB package devices also include considerably more reflector area and optical control around the LED die. This amplifies the performance of the die inside, as well as producing control from more precise placement of the LED inside luminaire optical systems.

As a general rule, surface mount LED devices should be held to elss than 1/4 watt, and driven at no more than 10 to 30 mA. Common package devices using high quality LED die and proper internal and external thermal management can reach power levels up to 5 watts at drive currents of 700 mA. At the extreme end, LED packages have been deployed that exceed 200W, while delivering reasonable life and lumen depreciation – although these devices require expert engineering development and are generally very specific in application. Try any of this with a through hole device and the result will be a little pop and  barely visible flash as it expires.

In response to a growing sentiment that “playful” design is doomed due to the demand to cut energy use, that we must give up quality to cut watts quantity. I could not disagree with this more. In fact design plays a much larger role in cutting energy use than anyone is, giving it credit for. In fact, I contend that design of an application plays a larger role than reaching for some ultimate efficiency number. The two renderings shown here were created in AGI32 using photometric data from available LED products. The effect of design on the quality of a space and its energy use are clearly illustrated. The only factor changed between these two renderings is the lighting system applied. All other factors are identical.

This "efficient" design uses products that generate high efficiency ratings, and meets the illuminance requirements necessary for vision. Yet, there is not a lot of interest here - the space just looks flat.

72Watts - This "efficient" design uses (6) 12W"high efficiency" luminares rated at 52 lumens per watt to meet the illuminance requirements necessary for the space - an average of 12 Fc throughout. Yet, there is not a lot of interest here - the space just looks flat. There is very little focus or dimensional definition beyond the physical objects themselves.

    40 Watts - This space uses more luminaires with narrower beam spreads, and lower efficiency - but of smaller wattage each. The result is greater definition of the space, more visual interest, and an energy savings of 48%!

40 Watts - This space uses (10) 4W medium efficiency luminaires at 39 lumens per watt, but much narrower beam spreads. The result is lower average illuminance (5Fc) but greater definition of the space that reinforces vision within the 3D space. Further, there is more visual interest and a central focus. Best of all - there is a realized energy savings of 48%!

Don’t buy into the baloney that energy conservation requires one must eschew design. Instead embrace the use of artful application of light to reduce the amount of light being thrown into a space from luminaires with poor control but high efficiency – use less light, with greater focus onto target surfaces. The art of lighting design is about design for vision, not meeting prescribed illuminance levels on some plane above the finished floor. The best designs create the most interesting and visually attractive space with the least amount of energy. We do not want to live in a world where the only qualifier of efficiency is the luminaire manufacturers data sheet above all else. Not only will this lead to greater energy consumption, it will reduce the quality of space we all live within for no reason. We need more design, more interest in the application of light, and less influence of purely empirical calculation.

Design has been devalued by those who believe lighting is something to be applied to meet minimum standards, that “effect” is a luxury that we can live without. This is why we live in offices with uniform illumination levels and flat surface rendering that gives us all headaches and eye strain. We know we don’t like it, ut live withit it. Why? Dynamic vision is created as much from the design of dark releif within a space, as it is from applying light onto horizontal planes within a calculation tool. The difference in these renderings are subtle in 2 dimensional presentation. In the 3D space we all live within, the effect is far greater and more readily felt and seen. With a greater degree of design expertise, we can realize greater energy savings AND an improvement in visual performance and quality.