The <strong>optical density</strong> of a material (also known as its absorbance) gives
the percentage of transmittance of light of a specific wavelength, a
value of 2.45 corresponds to a percent transmittance between 1 and
0.1 percent.  That is to say, nearly all of the light of this wavelength is
absorbed (rather than transmitted) by the film (which makes sense,
because this is looking at a wavelength of 478 nm, and "the light
harvesting efficiency [reads] almost 100% in the whole visible region
below 550 nm").