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.

Materials of Construction

Common materials of construction for optical windows include BK7 glass, crown glass, calcium fluoride, germanium, photonics DIAMOND, sapphire, thermoset ADC, UV grade fused silica, and zinc selenide.  Some windows are coated to increase transmission of light at the lens-air interface. Coatings for optical windows include none, antireflective, and conductive.  Surface quality ratings for optical windows include 10-5 scratch / dig, 20-10 scratch / dig, 40-20 scratch / dig, 60-40 scratch / dig, and 80-50 scratch / dig.  A dig is a defect on a polished optical surface that is nearly equal in terms of its length and width. A scratch is a defect on a polished optical surface whose length is many times its width.  10 / 5 indicates the average diameter of the digs to be .05 mm and the average length of a scratch is .10 mm.  20 / 10 indicates the average diameter of the digs to be .10 mm and the average length of a scratch is .20 mm.  40 / 20 indicates the average diameter of the digs to be .20 mm and the average length of a scratch is .40 mm.  60 / 40 indicates the average diameter of the digs to be .40 mm and the average length of a scratch is .60 mm.  80 / 50 indicates the average diameter of the digs to be .50 mm and the average length of a scratch is .80 mm.  Choices for surface flatness for optical windows include λ/2, λ/4, λ/5, λ/8, λ/10, and λ/20.