A simple definition of light transmission is: When light travels through a medium such as glass without being reflected absorbed or scattered.
When this happens light energy is not lost and can be considered 100% transmitted. However in all cases as light passes through a lens, losses come from three sources:
1) Reflections on all air to glass optical surfaces:
Optical surfaces are coated with various materials to reduce reflection losses. Each lens without coatings reflects approximately 4% on each air to glass surface. Coating effectiveness varies depending on the type. For example, a standard single layer coating of magnesium fluoride reduces the loss per surface from 4% to 1.5%. Multilayer coatings reduce reflections significantly and for visible light can reduce the loss per surface to less than 0.1 percent per surface. However, no coating can reduce the loss to zero.
2) Scattering of light by defects in the glass such as internal particles, dust, and scratches:
In the manufacturing and processing of a lens, there are always small imperfections such as digs, bubbles, coating pinholes, and scratches. In quality glass, the internal imperfections are quite small but are always present tot some degree. Scratches, digs, and coating pinholes are controlled with manufacturing tolerances. Losses from scattering in quality optics are quite small, in the order of less than 0.1%.
3) There is always some absorption for all glasses as light travels through a lens:
As light travels through glass some of it is absorbed and converted to heat energy. Absorption in the glass varies depending on both the glass type and the wavelength of light traveling through the glass. Absorption is proportional to the thickness of the glass. Absorption losses are very small for optical glasses used for quality optics, in the ballpark of 2% for an entire multi-lens optical system.
If an optical system uses a mirror, losses result from the mirrors coating efficiency and scattering from defects on the reflecting surface. A high-quality mirror surface will reflect 98 to 99%.