Light Transmission
The percentage of light that passes through a scope to reach the eye, the figure that largely determines how well the optic performs in low light.
Light transmission is the share of the light entering the front of a scope that actually reaches your eye, expressed as a percentage. Every glass element and every internal surface absorbs or reflects a little of it, so a scope with many lenses and basic coatings can lose a great deal before the image arrives. High-quality lens coatings are the single biggest factor a maker controls, and good designs push transmission into the low to mid nineties.
This number matters most at dawn and dusk, when there is little light to spare and a brighter image lets you see detail others miss. A large objective lens feeds the system more light to begin with and supports a larger exit pupil, while the twilight factor attempts to capture the practical result for low-light resolving.