The light-transmission is a measure of the permeability of light waves through a medium used in a lighting system.
Reflection, transmission and absorption of a lightwave through a medium B
Transmittance Τ (λ) of a 1 cm thick synthetic ruby in the optical range
The narrow absorption band at 694 nm is the wavelength of the ruby laser.
If a wave moving in the medium A (e.g., air) hits a medium B having a finite thickness (e.g., a lens or a wall), then depending on the material properties of the obstacle it becomes completely or partially reflected at the interface. Any part of the wave traversing the medium can be partially absorbed and the remaining part is transmitted through and exits on the opposite side of the medium B.