The wedge prism is a precise optical instrument developed to simplify the application of the variable-plot method. The prism is a wedge-shaped piece of glass that bends light rays, establishing a critical angle which can be projected easily and accurately.
In using the prism, the distance from the eye to the prism is not important so long as the tree detail can be seen clearly. The prism, and not the cruiser's body, must be held directly over the sample point, especially when determining whether a "borderline" tree is "in" or "out".
Accurate to =-1% of rated BAF. The efficient familiar design (30% ellipse curvature at top) for all general cruising. Base is parallel to horizontal axis for slope correction reference.