Dear Martin,
dλ refers to the spectral width of the band. It represent the spectral range that is measured by each band, according to the Spectral Response Function of the MSI sensor. For more details see the updated Sentinel-2 Spectral Response Functions at this link:
https://earth.esa.int/web/sentinel/user-guides/sentinel-2-msi/document-library/-/asset_publisher/Wk0TKajiISaR/content/sentinel-2a-spectral-responses
The parameter dλ generally corresponds to the Full width at half maximum (FWHM) of the response curve:
Full width at half maximum (FWHM) is an expression of the extent of function given by the difference between the two extreme values of the independent variable at which the dependent variable is equal to half of its maximum value. In other words, it is the width of a spectrum curve measured between those points on the y-axis which are half the maximum amplitude.
Half width at half maximum (HWHM) is half of the FWHM.
FWHM is applied to such phenomena as the duration of pulse waveforms and the sp...
Federico