LEDs have come on in leaps and bounds. But is the incandescent form factor making a delayed comeback?
For a number of years, LEDs have been heralded as the new, energy efficient, sexy and must-have lighting source. Encouraged by technical development and legislative drivers, LED lights can be purchased at a relatively low cost from your local corner store about as readily as you could purchase a candle and a box of matches. White LEDs are available in a whole range of temperatures, and colour changing LEDs can produce every wavelength in the visible spectrum.
However, despite these advantages, in most instances LED light sources are used simply as replacements for existing incandescent bulbs and compact florescent tubes. Where are all the innovative luminaires, Li-Fi networks and the mood sensitive light walls we were promised?
Consumer technology is driven by technical advances and fashion trends. While it is relatively easy to sell a new product on the basis that it is better/faster/smaller than the previous model, fashion is far more fickle. Flip-phones, push-button digital watches and the Soon-To-Be-Consigned-To-The-Thrift-Store Selfie-Stick seemed very desirable at the time, but are now faintly embarrassing.
Thus far development of LED light sources has been driven solely by technical advancements (if you ignore the compulsory requirement for every LED Christmas tree light now manufactured to flash). Modern LED light sources achieve impressive levels of brightness, consistency of hue and lifetime, all accomplished at the same time as incredible reductions in price. However a walk through the halls of any lighting show in 2015 must have left the CFOs of quite a few LED companies in a state of shock. Incandescent bulbs are back in fashion! Big glass globes with glowing filaments on show everywhere.
Of course these are not traditional incandescent lamps. The “filament” is actually a string of LEDs. They range from simple linear pieces to works of art with beautiful shapes, curves and turns. LED filament light bulbs, as they are known, are the very latest in LED fashion. Indeed, some lamps on show had three operating modes –“off”, “light emitting” and “glow” where the LEDs are turned on just enough that the form of the filament is visible and can be admired.
LED filament bulbs may look retro, but actually contain some advanced technology. Like their incandescent halogen ancestors LED filament bulbs are filled with gas at a specific pressure. The gas is formulated to optimise the thermal conductivity and viscosity as both influence the ability to transfer heat by conduction from the LEDs to the glass envelope.
The substrate for the LEDs is a thermally conductive and formable dielectric. Good thermal conductivity is needed to spread the heat from the LEDs. The dielectric potential permits electrical connection between LEDs and the substrate needs to be formable so it can fit through the neck of the bulb, like the ship-in-a-bottle trick. Few materials are able to meet all of these requirements so if the fashion for LED filament bulbs persists, further innovation in this area should be expected.