NanoTemper Technologies offers protein labeling kits with different fluorophores: NT-495 (BLUE), NT-647 (RED) and RED 2nd Generation. All fluorophores have a molecular weight of <1 kDa. RED 2nd Generation fluorophores are specifically developed to be Spectral Shift and TRIC (Temperature Related Intensity Change) sensitive, thus yielding higher binding signal amplitudes and improving signal-to-noise (S/N) ratios in binding assays. When choosing a fluorophore for your experiment, make sure it matches the filter set of your instrument. 

εmax and cf280 are necessary values for concentration determination and the calculation of the degree of labeling according to the labeling manual. For molar extinction coefficients k is short for 1000 (e.g. 70k = 70000) 

Note that absorption and emission spectra can be strongly buffer dependent. Because of high spectral similarity, the absorption and emission spectra of NT 647 and RED 2nd Generation are depicted in the same figure.