Color	Wavelength (nm)	Frequency (THz)	Photon energy (eV)
Ultraviolet	10-400	750-30k	124-3.3
Violet	380–450	670–790	2.75–3.26
Blue	450–485	620–670	2.56–2.75
Cyan	485–500	600–620	2.48–2.56
Green	500–565	530–600	2.19–2.48
Yellow	565–590	510–530	2.10–2.19
Orange	590–625	480–510	1.98–2.10
Red	625-750	400-480	1.65–1.98
Infrared	700-999	300-430	1.70–1.24 m

Yes, this is nothing new. We just repeat it here, because the functionality of those LEDs acting like photodiodes depends on the relation of the wavelength's of the transmitter and the receiver.

This Experiment uses twice the WL-TIRC IR LED (Mouser 15400585F3590) running at 845 nm. The Transmitter is described in detail here. The RX-diode was terminated with a 50 Ω, therefore the slopes are steep, but small. We can see, that without any active element, a voltage of 2.4 mVpp is generated, just by the photons emitted from our testsource in a distance of 9 mm.

Here we wanted to know how a 50 Hz powered fluorescent lamp injects photons into our 845 nm LED. The 50 Ω termination was removed for that. We can see that a voltage of ≈ 2 mVpp is visible. As this does not change when switching the lights on or off, we assume that this is a stray coupling from the wires of the diode. Another possible reason mayst be, that a fluorescent lamp does not emit photons in the region where our LED is sensitive enough.

Color	RX Level [mVpp]
Green	4.52
Yellow	6.72
Orange	8.08
Red	3.04
Infrared	2.64
None (Noise)	2.88

Here we wanted to know what wavelength range from the transmitter λ_TX can be seen by our receiver λ_RX. The receiving LED is red, whilst the transmitting LED is changed. The receiving signal level is measured with a High - Impedance Scope.

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