Inventor Fusion – Survey Prism

If you’ve ever seen a survey prism, you may find them hypnotic to look at.  You’ll always see the pupil of your eye in the dead center, no matter what angle you look into the prism from.  So, how does a survey prism work?  We can use Autodesk Inventor Fusion to help us create and visualize the geometry so that we can better understand.

Survey prisms are typically corner cube prisms.  The name comes from the fact that if you were to slice off the corner of a cube, the piece that you cut off would be the prism.

Typically, this shape is cut from a cylinder rather than a cube.  To model this on your own in Inventor Fusion, start by drawing three lines on the top of a cylinder, each 120 degrees apart.   To help draw the lines for the plane, press the tab key to set a value of either 0 or 120 degrees and press tab again to lock the angle in.Once you have the lines, you can create three orthogonal planes by using the Plane At Angle command.  Select the lines that you just drew and use a 45 degree angle.

Once the planes are created, you can cut the cylinder, creating three orthogonal faces.Since the three sides of the prism are orthogonal, any light that enters through the circular face of the prism is reflected directly back, parallel to the incoming rays of light that initially entered.  This is easier to see with a cross section through the prism.  If you trace the rays inside the prism, you’ll see that they all return parallel, no matter what the incoming angle is.You can download the Inventor Fusion dwg file of the survey prism here.