Reflectance is the ratio of the light flux returned from a surface to the amount of light flux incident on the surface. In the context of precision optical components, the returned light flux specifically refers to specular reflectance (energy contained within the usable returned wavefront), and does not include diffuse reflectance (scattered light).
Transmittance is the ratio of the light flux passed by an optic to the amount of light flux incident on the optic. Again, the definition of transmitted flux is not meant to include scattered light. Note that transmittance takes into account the effects of all the surfaces encountered by the light (typically just the entrance and exit faces), whereas reflectance of first surface mirrors only includes the characteristics of that first surface.
ARO measures both reflectance and transmittance of components using high precision spectrophotometers. These instruments are capable of performing measurements at multiple angles of incidence and over wide wavelength ranges, and can also measure s and p polarization values separately. In order to perform measurements at deep ultraviolet wavelengths at which atmospheric water absorption is significant, ARO maintains a dedicated spectrophotometer that is completely contained within a sealed, dry nitrogen purged enclosure.
Producing optics with very high transmittance and reflectance values (e.g. reflectance over 99.5%) requires careful control over a number of parameters. First, the purity of both substrate and coating materials must be maintained to a high degree. Next, scrupulous cleaning of substrates must be performed at several stages during the fabrication process in order to ensure maximum removal of all residual polishing compounds. In addition, for some deep ultraviolet applications, specialized polishing compounds that do not absorb in the ultraviolet may be used.
Additionally, steps may have to be taken to minimize subsurface damage (SSD), a phenomenon first identified by researchers at Lawrence Livermore National Laboratory (Berkeley, California). Subsurface damage refers to fractures and scratches that occur during the grinding and polishing process which become partially or totally covered by the polishing redeposition layer. This is a thin layer of material that flows while the substrate is being worked that covers the surface. Trace amounts of residual polishing compound may become incorporated into the redepostion layer, or trapped in microfractures and surface defects. Producing optics with minimum SSD involves using specialized tools, polishing compounds and fabrication procedures.