How to compute reflectance from an image given a wavelength (python)

Question

I have recently come across a paper entitled, "Portable Multispectral Imaging System Based on Raspberry Pi".

In this work, the authors have presented a low-cost spectral imaging system using a raspberry pi. This is done by light multiplexing through a LED panel where 8 selected wavelengths covering the spectrum UV to NIR is included. The camera used is a Raspberry Pi NoIR, RGB camera add-on for raspberry pi that does not have any IR cut filter installed.

When the sample is situated in the corresponding position inside the prototype, the system takes a photograph at each available wavelength sequentially.

Here's an example of an image captured at different wavelengths:

But I do not understand how they were able to compute the reflectance spectra in the plot.

Or did I understand it correctly that the reflectance spectra is the computed intensity, using the equation below?

Why would that be? Can anyone please direct me to the proper readings or source materials in order to understand this concept? Please enlighten me. I am new to this. Thank you!