The best replacement for transmittance is absorbance.

The best alternatives to transmittance are rated as following:
★★★★★  absorbance
★★★  reflectance
★★  concentration
★★  transmission
★★  transmissivity
★★  absorbance graph
★★  wavenumber
★  absorbance calculator
★  transparency
★  optical density
  absorption
  opacity

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Speaking about transmittance, one probably shouldn't forget about absorbance.

Other search terms related to transmittance and their ratings are:
★★★★★ absorbance
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transmittance
absorbance
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