An LED filament light bulb is an LED lamp which is designed to resemble a traditional incandescent light bulb with visible filaments, for aesthetic and light distribution 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).
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.
Lifespan of LED emitters is reduced by high operating temperatures. LED filament bulbs have many smaller, lower power LED chips than other types, avoiding the need for a heatsink, but they must still pay attention to thermal management; multiple heat dissipation paths are needed for reliable operation. The lamp may contain a high thermal conductivity gas (helium) blend to better conduct heat from the LED filament to the glass bulb. The LED filaments can be arranged to optimise heat dissipation. The life expectancy of the LED chips correlates to the junction temperature (Tj); light output falls faster with time at higher junction temperatures (the “Droop Effect”). Achieving a 30,000 hour life expectancy while maintaining 90% luminous flux requires the junction temperature to be maintained below 85°C.
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.