Wedge prism

The wedge prism is a prism with a shallow angle between its input and output surfaces. Refraction at the surfaces causes the prism to deflect light by a fixed angle. When viewing a scene through such a prism, objects will appear to be offset by an amount that varies with their distance from the prism.

Applications

Beam steering

A pair of wedge prisms, called a Risley prism pair, can be used for beam steering.

Forestry

Figure 1. View through 10 factor wedge prism of an "IN" tree.

Figure 2. View through 10 factor wedge prism of a "Borderline" tree.

Figure 3. View through 10 factor wedge prism of an "OUT" tree.

The wedge prism is primarily used as an angle gauge in variable-radius plot sampling. The wedge prism can be used, “To rapidly establish fixed-radius plots in the field, common forest survey techniques can be used with a target object calibrated to the desired size of the plot…Wedge prisms used with a 10.2-cm PVC pipe or cylinder were accurate, precise, inexpensive, and easy to use”. In forestry, the wedge prism is used to record and calculate basal area of a stand via a timber cruise known as tree inventory. Because the wedge prism refracts light to offset the object of interest (ex. A tree), it can be used to determine whether or not the tree is inside the plot radius based on the diameter at breast height of the tree and its distance from the plot center. If the tree is inside the plot radius, then the offset section of the tree will overlap the original bole making it an "IN" tree (Figure 1); if the tree is on the border of your plot radius the offset section of the trunk will be aligned with the original bole which is referred to as a borderline tree(Figure 2); if the tree is outside your plot radius then the offset section of the tree will not overlap the original bole which is referred to as an "OUT" tree (Figure 3). Wedge prisms come in different factors (5, 10, 20 BAF) and the factor of the prism one chooses to use is multiplied by the number of trees tallied in each plot as an observation of the basal area in the stand on a per acre basis. Wedge prisms can be difficult to use in wet conditions due to the effect water droplets have on the optical properties of the glass. Wedge prisms come in different colors such as clear or amber. The amber provides the same function as the clear wedge prism, only it reduces glare and is easier to use on overcast or cloudy days. Operating a wedge prism is one technique used in forestry today because, the wedge prism is simple, relatively inexpensive, portable, and as accurate as other angle gauges when properly calibrated and used properly. One simply holds the prism directly over the plot center, and by focusing on a tree, the refracted light will offset the trunk of the tree. The wedge prism is used to take measurements in both land management and in timber procurement. Other tools often used to accompany the wedge prism in taking forest inventory are clinometers, Biltmore sticks, relascopes, and Diameter Tapes.