Light transmission specifies an amount of light that is let through the build of optical product. Every crossing through each lens means a certain loss of light (0.1% with best coatings, up to 5% without coatings). Higher light transmission rate is very important when using optics at dawn or twilight. Good optics normally have light transmission up to 90%, whereas top-notch ones have even 95% and more light let through.
Although the quantity of light reaching the eye depends on the size of an exit pupil, light transmission determines transparency of the lenses, whether the image is dark and cloudy or bright and clear.
Light transmission can be increased with applying different coatings on the glass surfaces. However, it depends on the coating type and number of layers. Multi-layered coatings mean higher light transmission.
Uncoated glass reflects about 5% of the light (entry-level price range), single coating reduces the reflectivity to approx. 1.5% (middle price range), multi coating reduces the reflectivity to approx. 0.1 to 0.2 % (premium price range)