Wedge prism
Encyclopedia
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.
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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 stick
s, relascope
s, and Diameter Tape
s.
Prism (optics)
In optics, a prism is a transparent optical element with flat, polished surfaces that refract light. The exact angles between the surfaces depend on the application. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, and in colloquial use...
with a shallow angle between its input and output surfaces. Refraction
Refraction
Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...
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.
Beam steering
A pair of wedge prisms, called a Risley prism pair, can be used for beam steeringBeam steering
Beam steering is about changing the direction of the main lobe of a radiation pattern.In radio systems, beam steering may be accomplished by switching antenna elements or by changing the relative phases of the RF signals driving the elements.In optical systems, beam steering may be accomplished by...
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Forestry
Angle gauge
An angle gauge is a tool used by foresters to determine which trees to measure when using a variable radius plot design in forest inventory. Using this tool a forester can quickly measure the trees that are in or out of the plot. An angle gauge is a very similar tool to a wedge prism though it...
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
Timber cruise
A timber cruise is a sample measurement of a stand used to estimate the amount of standing timber that the forest contains. These measurements are collected at sample locations called plots or quadrants. Each of these individual plots is one observation in a series of observations called a sample...
known as tree inventory
Tree inventory
A tree inventory is the gathering of accurate information on the health and diversity of a community forest. - Uses :Tree inventories focus on the attributes of individual trees, as compared to a Forest inventory which seek to assess timber attributes on forest stands.Information such as how many...
. 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
Diameter at breast height
Diameter at breast height, or DBH, is a standard method of expressing the diameter of the trunk or bole of a standing tree. DBH is one of the most common dendrometric measurements....
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
Borderline tree
Borderline tree is a term used in forestry.It is a concept that comes from variable radius plots, or point sampling. It happens when a tree cannot be easily determined as in or out when using a prism or angle gauge. Borderline trees occur only when the distance from the sampling point to the...
(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
Forestry
Forestry is the interdisciplinary profession embracing the science, art, and craft of creating, managing, using, and conserving forests and associated resources in a sustainable manner to meet desired goals, needs, and values for human benefit. Forestry is practiced in plantations and natural stands...
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
Forest inventory
Forest inventory is the systematic collection of data and forest information for assessment or analysis. It is also commonly known as timber cruising. It is important for owners to cruise the timber to get an estimate of the value and possible uses of the timber...
are clinometers, Biltmore stick
Biltmore stick
The Biltmore stick is a tool used to measure various tree dimensions, such as diameter at breast height and height. It looks much like an everyday yardstick. A Biltmore stick is considered to be only marginally accurate...
s, relascope
Relascope
The relascope, invented by Walter Bitterlich, is a multi-use instrument for forest inventory. It is primarily used to find height of a tree, basal area of a tree and diameter of a tree anywhere along the bole...
s, and Diameter Tape
Diameter Tape
A Diameter tape is a form of dendrometer that consists of a cloth or metal tape that is mainly used to measure diameter at breast height . DBH is measured at a fixed height of 4.5 feet above the ground, where it is most convenient to measure diameter. DBH is measured up high so to avoid measuring...
s.