How and why does the wavelength shift for my Luminus LED?

LEDs have slight shifts in the shape of the spectral power distribution (SPD) curves when they are operated at different conditions.  This is characterized as a shift in the peak wavelength for monochromatic (color) LEDs and shifts in the CIEx and CIEy coordinates for white LEDs.

In the case of GaN-based LEDs, there are two competing causes for wavelength shifts, current injection level, and junction temperature.

• The injection level effect causes the peak wavelength to shift to shorter wavelengths at higher currents.
• The temperature effect causes the peak wavelength to shift to longer wavelengths at higher temperatures.

These effects are due to device physics.  LEDs are semiconductor quantum well devices.  Higher current levels increases the percentage of higher energy levels populated by minority carriers so the proportion of higher energy (shorter wavelength) photons increases. Higher temperature operation lowers the bandgap of the semiconductor materials which results in a shift towards lower energy (longer wavelength) photons.

These two effects are measured during the product development process and incorporated in many of our datasheets.  For some LED products, the shift is slight and are not included in our datasheets for brevity.  These data can be supplied on request if you have an application that is sensitive to minute shifts in wavelength.  Contact [e-mail] if you need unpublished data.

Example of the wavelength shift for a monochromatic LED (SST-10-G)

The plots above are presented as relative deltas with the reference point defined in the Optical and Electrical Characteristics table in the datasheet.  For this device, the reference point is 350 mA and 25 C for a nominal 527 nm peak wavelength. As you can see, if the current is increased, the delta is negative and there is a shift towards higher energy (shorter wavelength).  If the temperature is increased, the delta is positive and the shift is towards lower energy (longer wavelength). The final wavelength shift is calculated using both of these charts and is generally pretty small due to these competing effects. The current level is usually known but the junction temperature must be estimated with some knowledge of the application specific thermal design. If this temperature is completely unknown, 85 C is a good starting point.

Example of the wavelength shift for a white LED (CXM-3 White LED)

White LEDs use combinations of blue LEDs and phosphors to produce white light with tailored characteristics.  One important characteristic in the color point of the LED which is expressed in general terms as a color temperature (CCT) and in exact terms as a pair of CIEx and CIEy color coordinates in the CIE 1931 color space. White LED designers don't care about details of the peak wavelength shifts of the blue LED and phosphor, they want to know how the color point shifts under different operating conditions. The figure below shows these data where again, both the operating temperature and the current need to be considered to calculate the final shift.

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