White light emitting LEDs (“white LEDs”) include one or more photoluminescence materials (typically inorganic phosphor materials), which absorb a portion of the blue light emitted by the LED and re-emit light of a different color (wavelength). The portion of the blue light generated by the LED that is not absorbed by the phosphor material combined with the light emitted by the phosphor provides light which appears to the eye as being white in color. Due to their long operating life expectancy (>50,000 hours) and high luminous efficacy (100 lm/W and higher) white LEDs are rapidly being used to replace conventional fluorescent, compact fluorescent and incandescent lamps. LED lamps (bulbs) are typically constructed from a small number of high-intensity white LEDs.
Recently, LED-filament lamps have been developed that comprise LED-filaments that closely resemble the filament of a traditional incandescent lamp. The LED-filaments, which are typically about an inch long, comprise COG (Chip-On-Glass) devices having a plurality of low-power LED chips mounted on a transparent glass substrate. The LED-filaments are encased in a phosphor-impregnated encapsulant, such as silicone. Typically, LED-filament lamps are configured to generate “warm white” light with a CCT (Correlated Color Temperature) of 2700 K to 3000 K with a General Color Rendering Index (CRI Ra) of up to about 80.
While the CRI Ra of packaged white LEDs can be increased by including a longer wavelength red emitting phosphor while experiencing only a small reduction in performance, when a longer wavelength red emitting phosphor is included in a LED-filament to increase CRI Ra from 80 to 90 this results in a substantial reduction in performance, in particular luminous efficacy, of the LED-filament in a range from 15% to 20%. The reduction in efficacy results in greater heat generation within the LED-filament. Since there is no way of readily managing an increase in heat in an LED-filament, this makes it impractical to produce high lumen (>800 lm) CRI Ra 90 LED-filaments with an acceptable luminous efficacy. There is thus a need to provide LED-Filaments and LED-filament lamps that have a CRI Ra of at least about 90 and which have substantially the same performance as a CRI Ra 80 LED-filament.