Editing Mirror (section)

===Transmissivity===
[[File:Laser dielectric output coupler centered @ 550nm.png|thumb|A dielectric, laser output-coupler that is 75–80% reflective between 500 and 600 nm, on a 3° [[wedge prism]] made of [[quartz glass]]. Left: The mirror is highly reflective to yellow and green but highly transmissive to red and blue. Right: The mirror transmits 25% of the 589 nm laser light. Because the smoke particles [[Diffraction|diffract]] more light than they reflect, the beam appears much brighter when reflecting back toward the observer.]]
Transmissivity is determined by the percentage of light transmitted per the incident light. Transmissivity is usually the same from both first and second surfaces. The combined transmitted and reflected light, subtracted from the incident light, measures the amount absorbed by both the coating and substrate. For transmissive mirrors, such as one-way mirrors, [[beam splitter]]s, or laser [[output coupler]]s, the transmissivity of the mirror is an important consideration. The transmissivity of metallic coatings are often determined by their thickness. For precision beam-splitters or output couplers, the thickness of the coating must be kept at very high tolerances to transmit the proper amount of light. For dielectric mirrors, the thickness of the coat must always be kept to high tolerances, but it is often more the number of individual coats that determine the transmissivity. For the substrate, the material used must also have good transmissivity to the chosen wavelengths. Glass is a suitable substrate for most visible-light applications, but other substrates such as [[zinc selenide]] or [[synthetic sapphire]] may be used for infrared or ultraviolet wavelengths.<ref name=greav1989/>{{rp|p.104–108}}