Dear All,
I'm trying to measure the frequency and wavelength of the Red LED of MAX30102 connected to Arduino Nano.
I checked sparkfun library for MAX30102 and examples provided in the library but did not get any information to measure frequency and wavelength.
I got your point. What about the frequency of reflected light ?
why would it be any different?
what could possibly have changed its wavelength (??)
Maybe @engr_khalil is making a Doppler LIDAR 
The wavelength can change by flourescence.
(But then it is not being reflected, but re-radiated.)
the speed of the reflecting body occurred to me, but by how much would the wavelength change due to a speed of 10 mph for example? wouldn't the difference be 10 / c?
Yes, I think that's correct frequency shift for speeds << C. It's the same as it is for Radar (traffic speed gun, weather, etc).
Correction:
The shift is f0 * v / C for v <<C.
So, for red light and 10Km/h ---> ~4Mhz, mas o menos.
what is frequency of the LED light? and % of that freq
what is the change in wavelength?
Roughly 1.0e-6%
Let's say a red LED is 685 nm. That's 438 THz. Which is what I used to get delta f of ~4 MHz at 10 Km/h.
so what % is 4 Mhz / 400 THz ?
are they measuring the doppler shift or using travel time to measure distance and then calculating the speed from the change in distance
First, everything I've said applies to a coherent light source (i.e. Laser diode). I don’t think it stands a chance of working with an LED.
Regardless of % change, you’d detect it (in theory) by mixing the local oscillator (the laser) with the reflected signal in a non-linear element (i.e. photodiode). That will produce sum (out of band) and difference (4 Mhz) frequencies. It’s the 4 Mhz IF that you’d measure.
Of course, hand waiving, easier said than done. The intensity of the local laser will be much higher than the reflected signal. They’d need to be somewhat equalized somehow. Then you’d have to worry about SNR etc. But that’s theory.
i think everything you've said is correct for an LED.
the LED is a single wavelength, but not coherent.
are you suggesting the output of the photodiode will be a 4 Mhz voltage?
In theory. 4 Mhz current actually, photodiodes are current sources.
Hopefully, engr_khalil will reply and give the real reason for the question.
Thanks for your kind reply and discussion @Paul_KD7HB @gcjr @gfvalvo @EmilyJane
The MAX30102 is working based on the amount of reflective light. As far as my project concerned, I need to measure the frequency of the reflected light to check either its shifted or not.
Can you disclose the percentage of frequency change you are interested in?
Why?
