Response from Jonathan: Dave,

The smoked plexiglass is mostly used on older cocktail cabinets that have CRT (or Cathode Ray Tube) monitors in them. The smoked plexiglass is usually attached to a frame that mounts directly to the monitor brackets. The main reason for the smoked plexiglass is to cover up the effects of screen burn on the monitor. As you may know, when the same image is displayed on a CRT monitor for a long period of time it tends to leave an impression in the tube itself. This impression is known as screen burn. This problem was particularly prevalent in games such as Pac Man so the manufacturer would install the smoked plexiglass to darken the monitor area which would “hide” the screen burn on the tube. Usually the cocktail glass itself is clear but may have some artwork painted on it similar to a bezel in an upright arcade game. Hope that helps.