I am mentoring a trainee who is writing a dissertation on phototherapy for neonatal jaundice. She has been using a spectroradiometer and has noticed that the spectra from LED-based units are all slightly asymmetric. I have also noticed this in publlished papers. In the attached drawing I have exagerated the effect, where the emission of wavelengths longer than the peak emission, but many spectra that I have seen show this effect. Anybody know why this may be?
Thanks in advance
Mike
Michael Lynn
CIE rapporteur
First, LEDs are not laser diodes; therefore, they emit a relatively broad spectrum around the dominant color. The actual spectrum is a function of the material used in the diode.
Also bear in mind that the spectrum emitted by a black body radiator always has a tail extending to longer wavelengths; so this may also be a factor.
LED emits non-coherence light, Commercial LED has a layer of phosphor-compound material deposited on the emitting area to convert and control the light output spectrum, Without knowing how the emitted light is measured, and what type of LED is under test (What type of active materials, power output, etc.) it is difficult to explain, yet the light output spectrums is not going to be symmetric
One possible reason is that there is more lower energy carriers in the conduct band so the spectrum has a tail extending to longer wavelength.
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