What are LED filament light bulbs?
LED filament light bulbs are a new lighting technology developed by Ushio Lighting in 2008 with the intention of mimicking the appearance of standard vintage light bulbs. By mid 2010, LED filament bulbs were being introduced into the market by several manufacturers including Edison Mills.
The LED filament is composed of a series of LEDs on a transparent substrate, referred to as Chip-On-Glass (COG). These transparent substrates are made of glass or sapphire materials. This transparency allows the emitted light to disperse evenly and uniformly without any interference. An even coating of yellow phosphor in a silicone resin binder material converts the blue light generated by the LEDs into white light. Degradation of silicone binder, and leakage of blue light are design issues in LED filament lights. Positive benefits of the LED design are potential higher efficiencies by the use of more LED emitters from lower driving currents. A major benefit of the design is the ease with which near full "global" illumination can be obtained from arrays of filaments.
Lifespan of LED emitters is reduced by high operating temperatures. In the absence of a heat sink, LED filament bulbs may use a high thermal conductivity gas inside the bulb to aid heat dissipation.
The large number of LEDs (typically 28 per filament) simplifies the power supply compared to traditional LEDs as the voltage per blue LED is 2.48 < ΔV < 3.7. Some types may additionally use red LEDs (1.63 < ΔV < 2.03). Two filaments with a mix of red and blue is thus close to 110 V, or four close to 220 V to 240 V, compared to the 3 V to 12 V needed for a traditional LED lamp. Typically, four filaments are used and the appearance is similar to an overrun carbon filament lamp. Typically, a mix of phosphors are used to give better color rendition (which is a separate issue to color temperature) than the early blue LEDs with yellow only phosphor.
- US 2009184618, Kunihiko Hakata & Tomomi Matsuoka, "Light-emitting device and lighting apparatus incorporating same", published 2009-07-23, assigned to Ushio Denki KK
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