Hi Simon,

When you take a look at the table of bands for a sensor (e.g. Sentinel-2), you will see that each band has a wavelength. For instance the Red band on Sentinel-2, B04 is indicated as 664.6 nm (S2A) or 665.0 nm (S2B). This is actually not quite true…

A satellite sensor doesn’t exactly measure at a single wavelength but integrates the incoming electromagnetic (EM) radiation over multiple wavelengths: this range is called a band. There is a nice explanation here. The width of a band (the part of the EM spectrum covered) is a trade-off between being narrow enough to measure accurately phenomena (think of a type of light, such as Red or Blue light) but wide enough to let enough energy reach your sensor to get a clear signal.

Each band has a spectral response, which determines how much each wavelength is contributing to your signal. For Sentinel-2 you can find the data in the technical handbook. The central wavelength defines the wavelength around which your interval of EM spectrum (band) is centered. ESA provide such a table on their website, shown below:

Some sensors have a lot of narrow bands (hyperspectral), others cover a wide part of the EM spectrum (broadband). In your case, I imagine that the equation means that you need to fetch the closest band to wavelength:

842 nm in your example would correspond to Band 8 on Sentinel-2.

Great explanations thanks !

My problem here is that if i remplace what they call “the central values of wavelength” by the band values, I got 0 …

so either they are lying in the paper or there is another method that they din’t explain.

I’ve find this : Sentinel-2 Spectral Response Functions (S2-SRF) - Sentinel Online

If I multiply the band value by the coefficient this function gives me for a particular “spectrum” nm (sorry I can’t find the right word for a particular part of the spectrum…) ,

it gives me the value of for the length in question (example: 665) ?

For example :

line 367 of the S2A spectral reponse (in the document) they gave me a coefficient of 0.81 for the band 4/ “spectrum” 665nm.

s if I compute 0.81*B4 ,this gives me the value of the central wavelength or not at all…?

I am not familiar with the methods in the papers that you cite. It would probably be a good idea if you contact the authors to ask them for more information about their approach: in my experience, authors are generally quite open to discussing their science.

Ok, thanks you for your answer!

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