English: Ray-tracing through a K-mirror at nominal zero angle, the upright position. The K-mirror is the combination of three flat mirrors with surfaces shown in purple. If viewed from the left, the three surfaces appear like the shape of the letter K faced down. Three rays enter from the right, are each reflected by the three mirrors, and arrive at the green screen where their positions are recorded. The reference ray (in blue) is at the center. The brown ray starts to the right of it and arrives to the right of it on the green screen. The yellow ray starts above the blue ray and ends below the blue ray at the green screen. The three reflections have the combined effect of flipping beams and images upside-down.