The absorption maxima of the blue Bodipy B1, B2, B3 and B4 are 413 nm, 422 nm, 426 nm and 431 nm, respectively (fig. ). The four blue fluorophores possess the same backbone but the additional alkyl substituents attached to the Bodipy core shift the absorbance towards the longer wavelengths, while the extinction of the dyes remains nearly constant. The absorption spectrum of the green dye displays a 523 nm absorbance and another (weaker) one at 375 nm, while the 415 -435 nm region between those two maxima is characterized by weak absorbance. To characterize the spectral behavior, the ratio of the absorbances for the four blue dyes relative to the green dye are shown in fig.