In custom lightsabers, windows in the emitter are cut out areas near the top of the lightsaber hilt that allows some light from the LED to shine through. They are primarily a cosmetic feature that adds some pizzazz to the hilt.
The style, size and shape of the windows can vary from saber hilt to saber hilt. For example, windows in the emitter on an Ultrasabers Apprentice V4 have a horizontal, oval shape. By contrast, the windows are vertical and longer on an Ultrasabers Initiate V4.
You do have to consider a few extra things with lightsabers with windows in the emitter that you wouldn’t have to worry about with sabers with no windows. First, you need to ensure you use the prop length of a blade plug. If the blade plug is too short, then the end of the blade plug might be visible through the windows. In a hilt without windows, only the top part of the blade plug is visible.
Similarly, if you plan on using a non-standard blade like a flat acrylic blade, light whip, or any other blade attachment with a stem (or tang), you need to ensure the stem is long enough to cover the windows.
You can also use blade plugs featuring a design on the side of the blade plug, since part of the design would be visible through the windows. In addition, some people will use a crystal chamber blade plug, so that part of a kyber crystal is visible through the windows.
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What do you like or dislike about windows in the emitter?