RED Superluminescent LEDs
The best wavelength for human eyes
As a result of million years of evolution, the human eye is most sensitive for wavelengths around 555 nm (see Fig. a), where the luminous efficiency is the highest. However, the sensitivity is rapidly decreasing for the red spectral region with a slightly increasing wavelength. The luminous efficiency in the lower red spectral range at 625 nm is almost three times higher than at 650 nm.
For typical end user applications, luminous flux values of about 10 lm are desired. Hence, for the red spectral region light sources are required that deliver output powers between 40 mW and 120 mW (Fig. b). If taking the example of a 10 mW light source, and considering the photopic response of the eye, the luminous efficiency at 630 nm emission is 2.5x higher than at 650 nm.
As a consequence, red SLEDs should operate at wavelengths below 640 nm, where the luminous efficacy of the human eye is much higher. 2
Fig.(b): Luminous efficacy for human eyes. The luminous efficacy is obtained from the luminous efficiency by multiplication with the scaling factor Km = 683 lm/W, which is the maximum luminous efficacy at 555 nm.
2 N. Matuschek and M. Duelk, “Highly efficient superluminescent light-emitting diodes (SLEDs) at 625-650 nm,” NUSOD ’17, Paper TuC6, Copenhagen, Denmark, July 24-28, 2017.
RED SLEDs for the visible spectral range
Superluminescent light emitting diodes (SLEDs) are an interesting alternative to laser diodes (LDs) for applications where short coherence lengths or low speckle noise are required. They combine the directionality typical of LDs with the spectral width of LEDs.
Their beam-like output enables efficient coupling to external elements while keeping a broadband emission spectrum. These characteristics stem from amplifying the spontaneous emission by stimulated emission within a single or double pass in the waveguide. 1
Therefore, visible SLEDs targeting the red, blue, and green primary colours represent an ideal light source candidate for compact and efficient projection systems with low power consumption and for sequential lighting of DLP-based micro-displays.
Red lasers for the red spectral range are available since decades, red SLEDs following several years
After more than 15 years of experience of producing industry-leading GaAs-based light sources, EXALOS has successfully managed to deliver high-performance SLEDs with 650 nm spectral emission to the global market in recent years. With 635 nm EXALOS pioneered a high-performance SLED – designed for AR and head-up displays: VISIIIBLES RED (read more).
Visible RED SLEDs. Made by EXALOS.
Available with 650 nm and 635 nm.