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LED filament
Jan 23, 2018

An LED filament light bulb is an LED lamp which is designed to resemble a traditional incandescent light bulb with visible filaments, for aesthetic purposes. It produces its light by LED filaments, multi-diode structures that resemble the filament of an incandescent light bulb. They are designed as drop-in replacements for conventional clear incandescent bulbs used in decorative light fixtures, such as table lamps, pendant lights, chandeliers, and bulb-based candelabras. They have the appearance of traditional incandescent bulbs but the higher efficiency of light emitting diodes (LEDs).

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 one 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 standard incandescent bulb.

LED filament bulbs are patented by Ushio and Sanyo in 2008. Panasonic described a flat arrangement with modules similar to filaments in 2013. Two other independent patent applications were filed in 2014 but were never granted.

The LED filament consists of multiple series-connected 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.

Flickering of dimable filament LED lamp as interference with a digital camera frame rate

Lifespan of LED emitters is reduced by high operating temperatures. Lacking a heat sink, LED filament bulbs may contain a high thermal conductivity gas (helium) blend to better conduct heat from the LED filament to the glass bulb.

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 < V < 3.7. Some types may additionally use red LEDs (1.63 < V < 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 a higher color rendering index (which is a separate issue from colour temperature) than the early blue LEDs with yellow only phosphor.

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