windows 　　Optical windows are used to isolate different physical environments while allowing light to pass relatively unimpeded.  Window types available are plane windows or Brewster windows.  Plane windows can come wedged or parallel and are designed to have the incident light come perpendicular to the window surface.  Brewster windows are used in optical systems at Brewster's Angle; the angle at which p-polarized light suffers no reflection loss.  Optical windows can either come round, square, or rectangular.  Important dimensions to consider include the diameter if a round window, side if a square window, and length and width if a rectangular window.  The thickness is also important to consider.  A window can come either wedged or parallel.  A problem with parallel substrates is that the second surface reflection can lead to unwanted spectral channeling. Reflections from the surface of the wedge are not parallel and also slightly separated.  The parallelism or wedge angle is important to consider, this unit is specified in arc minutes for both.