Operating temperature and drive current are two well‑known variables that affect the long‑term lumen maintenance of high‑power LED chips. Over thousands of hours of use, high temperatures reduce the efficiency of the quantum wells in the chip causing a slow loss in light output. This is why precise temperature control during LM‑80 testing is so important. In addition to the LED chip, the manufacturing methods and the materials used in the construction of the LED lamp components are also critical variables that are affected by temperature and drive current and are the primary factors that impact the lumen maintenance behavior of LEDs. As all of these variables are interrelated, the topic of LED lumen maintenance is quite complex.

The primary factors that can influence the lumen maintenance of this type of product include (but may not be limited to) the following.

• The silicone material used as the lens on the LED lamp

• The LED chip materials and fabrication technology

• The phosphor used and the phosphor application method

These factors are illustrated in the high‑power LED lamp cross-section drawing in Figure below. Taken as individual sub‑components within the LED lamp, they each are influenced by the operating conditions (temperature and current) and each might degrade differently over time.

The silicone encapsulants are a good example of this degradation. The silicones used in the LED industry are polyorganosiloxanes.Depending on their exact chemical composition, these siloxanes may be very sensitive to operating temperatures. At higher temperatures the transparent nature of the materials can quickly degrade causing an overall loss in light output from the LED lamp. The energy of the photons emitted from the chip itself can also damage the siloxane material, further reducing its transparency, causing additional lumen depreciation. Thus, the higher the drive current, the more light being emitted from the chip and the faster the lumen depreciation of the lamp. Higher drive currents also result in higher operating temperatures of the LEDs, compounding the depreciation issue.

Like the high‑power LED lamps previously discussed, mid‑power LED lamps, shown in cross-section in Figure 3, also use siloxanes as encapsulants and so are similarly sensitive to heat and photonic energy. However, unlike high‑power LED lamps, which are constructed with ceramic substrates, most mid‑power (and low‑power) LEDs are packaged in polyphthalamide (PPA) plastic polymers. PPA polymers are even more susceptible to damage from heat and photonic energy than some of the most sensitive silicones used as LED encapsulants.Exposure to temperature and light causes these PPAs to darken. Since much of the light emitted from plastic‑packaged LEDs is reflected off the interior walls of the package, the inevitable darkening of the PPA results in rapid lumen depreciation. The quick lumen depreciation that results from operating mid‑power LED lamps at higher temperatures and drive currents limits their use to non‑critical lighting applications, where long lumen maintenance is not necessary.

An understanding of the construction of LED components, their sensitivities to operating conditions and the desired long‑term lumenmaintenance are important in selecting the correct LED to be used in a specific application.