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An LED filament light bulb is an electric light bulb which produces light with an LED that is shaped to look like the filament of an incandescent light bulb, sometimes an Edison light bulb. The light bulb is supplied with electric current by feed-through terminals or wires embedded in the glass. Most bulbs are used in a socket which provides mechanical support and electrical connections.
History and design
A LED filament type design light bulb was produced by Ushio Lighting in 2008, intended to mimic the appearance of a standard light bulb. Contemporary bulbs typically used a single large LED or matrix of LEDs attached to a large heatsink – as a consequence these bulbs typically produced only a 180 degree range of light. By the mid 2010s LED filaments were being introduced into the market by several manufacturers – these designs used several LED filament light producers arranged in the same or similar pattern to that found in the wires of a standard incandescent bulb.
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 – 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 numbers 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 110V, or four close to 220V to 240V, compared to the 3V to 12V 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 colour rendition (which is a separate issue to colour temperature than the early blue LEDs with yellow only phosphor.
LED filament lamps are patented.