The wedge prism is a prism with a shallow angle between its input and output surfaces. This angle is usually 3 degrees or less. 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.
For a wedge prism in air, rays of light passing through the prism are deflected by the angle δ, which is approximately given by
where n is the index of refraction of the prism material, and α is the angle between the prism's surfaces.
The term "optical wedge" refers to any shallow angle between two plane surfaces of a window. This wedge may range from a few millionths of a degree of perfect parallelism to as much as three degrees of angle. Even though high-precision optics, such as optical flats, may be lapped and polished to extremely high levels of parallelism, nearly all optics with parallel faces have some slight wedge. This margin of error is usually listed in minutes or seconds of arc. Windows manufactured with an intentional wedge are often referred to as wedge prisms, and typically come with wedge angles of one, two, or three degrees. Many applications exist for wedge prisms, including laser-beam steering, rangefinding and variable focusing.
A pair of wedge prisms, called a Risley prism pair, can be used for beam steering. In this case, rotating one wedge in relation to the other will change the direction of the beam. When the wedges angle in the same direction, the angle of the refracted beam becomes greater. When the wedges are rotated to angle in opposite directions, they cancel each other out, and the beam is allowed to pass straight through.
Moving a wedge either closer or farther away from the laser can also be used to steer the beam. When the wedge is moved closer to the target (farther away from the laser), the refracted beam will move across the target. When two wedges in opposite directions slide relative to each other they can be used to provide variable focusing for cameras, allowing objects at vastly different distances to be photographed, in focus, at the same focal plane. This method is common in aerial or space launch-vehicle photography, when the distance to the object is changing very rapidly. [1] Wedges were sometimes used in rangefinding, by combining the image formed by one telescope with the image formed by another. [2]
The wedge prism is primarily used in a similar manner as an angle gauge in variable-radius plot sampling. In this type of sampling, the wedge prism is used to estimate basal area of a group of trees by counting trees which are "in" or "out" of a plot centered on a single point. Because the wedge prism refracts light to offset the object of interest (e.g. a tree), it can be used to determine whether or not the tree should be counted from a given point, based on the diameter at breast height of the tree and its distance from that point.
In this type of sampling, the prism is held a comfortable distance away from the eye with the bottom edge parallel to the ground, and trees are sighted through the prism approximately 4.5 ft. above the ground. A tree is an "in" tree if the offset section of the tree overlaps the bole as viewed without the prism (Figure 1). A tree where the offset section of the trunk is perfectly aligned with the original bole is a borderline tree (Figure 2) and DBH must be measured to determine if it should be counted (or, more commonly in practice, every other borderline tree is counted). A tree where the offset section of the tree does not overlap or touch the original bole is an "out" tree (Figure 3) and is not counted.
Basal area is estimated by multiplying the count of "in" trees at a given point by the 'factor' of the prism. Prism factor is based on the angle of the prism, and prisms are available in different factors, expressed in both square feet/acre (5, 10, 20 BAF are most common) and square meters/hectare (1-5 BAF are common). Prism size is chosen to yield a statistically valid estimate of basal area - 6-10 "in" trees per plot are required, which requires a prism of the proper factor depending on the size of the trees being cruised. Larger trees will be "in" from further away, and a larger factor prism (20 or 30 ft2/ac, 5–8 m2/ha) can be used. Smaller trees will be "out" in a larger factor prism unless they are very close, and consequently a smaller factor prism must be used.
Importantly, the bottom edge of the prism must be roughly parallel to the ground in order to provide an accurate estimate on sloped ground. 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, [3] 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.
A wedge prism can also be used with a target placed at plot center, to establish fixed radius plots. In this function, the size of the target is carefully calibrated to the desired plot size, and the plot is defined as all the area in which the target is "in" as viewed through the prism. [4]
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviolet, and infrared light. Light is a type of electromagnetic radiation, and other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.
In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. By incorporating index of refraction in its definition, NA has the property that it is constant for a beam as it goes from one material to another, provided there is no refractive power at the interface. The exact definition of the term varies slightly between different areas of optics. Numerical aperture is commonly used in microscopy to describe the acceptance cone of an objective, and in fiber optics, in which it describes the range of angles within which light that is incident on the fiber will be transmitted along it.
The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative focal length indicates that the system diverges light. A system with a shorter focal length bends the rays more sharply, bringing them to a focus in a shorter distance or diverging them more quickly. For the special case of a thin lens in air, a positive focal length is the distance over which initially collimated (parallel) rays are brought to a focus, or alternatively a negative focal length indicates how far in front of the lens a point source must be located to form a collimated beam. For more general optical systems, the focal length has no intuitive meaning; it is simply the inverse of the system's optical power.
Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes when viewing distant objects. Most binoculars are sized to be held using both hands, although sizes vary widely from opera glasses to large pedestal-mounted military models.
An optical prism is a transparent optical element with flat, polished surfaces that are designed to refract light. At least one surface must be angled — elements with two parallel surfaces are not prisms. The most familiar type of optical prism is the triangular prism, which has a triangular base and rectangular sides. Not all optical prisms are geometric prisms, and not all geometric prisms would count as an optical prism. Prisms can be made from any material that is transparent to the wavelengths for which they are designed. Typical materials include glass, acrylic and fluorite.
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.
In optics, a Porro prism, named for its inventor Ignazio Porro, is a type of reflection prism used in optical instruments to alter the orientation of an image.
A prism coupler is a prism designed to couple a substantial fraction of the power contained in a beam of light into a thin film to be used as a waveguide without the need for precision polishing of the edge of the film, without the need for sub-micrometer alignment precision of the beam and the edge of the film, and without the need for matching the numerical aperture of the beam to the film. Using a prism coupler, a beam coupled into a thin film can have a diameter hundreds of times the thickness of the film. Invention of the coupler contributed to the initiation of a field of study known as integrated optics.
An optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum. Common types of optical waveguides include optical fiber waveguides, transparent dielectric waveguides made of plastic and glass, liquid light guides, and liquid waveguides.
An autocollimator is an optical instrument for non-contact measurement of angles. They are typically used to align components and measure deflections in optical or mechanical systems. An autocollimator works by projecting an image onto a target mirror and measuring the deflection of the returned image against a scale, either visually or by means of an electronic detector. A visual autocollimator can measure angles as small as 1 arcsecond, while an electronic autocollimator can have up to 100 times more resolution.
A prism compressor is an optical device used to shorten the duration of a positively chirped ultrashort laser pulse by giving different wavelength components a different time delay. It typically consists of two prisms and a mirror. Figure 1 shows the construction of such a compressor. Although the dispersion of the prism material causes different wavelength components to travel along different paths, the compressor is built such that all wavelength components leave the compressor at different times, but in the same direction. If the different wavelength components of a laser pulse were already separated in time, the prism compressor can make them overlap with each other, thus causing a shorter pulse.
In optics, a dispersive prism is an optical prism that is used to disperse light, that is, to separate light into its spectral components. Different wavelengths (colors) of light will be deflected by the prism at different angles. This is a result of the prism material's index of refraction varying with wavelength (dispersion). Generally, longer wavelengths (red) undergo a smaller deviation than shorter wavelengths (blue). The dispersion of white light into colors by a prism led Sir Isaac Newton to conclude that white light consisted of a mixture of different colors.
A coincidence rangefinder or coincidence telemeter is a type of rangefinder that uses the principle of triangulation and an optical device to allow an operator to determine the distance to a visible object. There are subtypes split-image telemeter, inverted image, or double-image telemeter with different principles how two images in a single ocular are compared. Coincidence rangefinders were important elements of fire control systems for long-range naval guns and land-based coastal artillery circa 1890–1960. They were also used in rangefinder cameras.
A laser beam profiler captures, displays, and records the spatial intensity profile of a laser beam at a particular plane transverse to the beam propagation path. Since there are many types of lasers—ultraviolet, visible, infrared, continuous wave, pulsed, high-power, low-power—there is an assortment of instrumentation for measuring laser beam profiles. No single laser beam profiler can handle every power level, pulse duration, repetition rate, wavelength, and beam size.
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 similar to a wedge prism though it must be held a fixed distance from the eye to work properly. Unlike the wedge prism, which is held over the plot center, the surveyor's eye is kept over plot-center when using an angle gauge.
A depolarizer or depolariser is an optical device used to scramble the polarization of light. An ideal depolarizer would output randomly polarized light whatever its input, but all practical depolarizers produce pseudo-random output polarization.
The first description of multiple-prism arrays, and multiple-prism dispersion, was given by Newton in his book Opticks. Prism pair expanders were introduced by Brewster in 1813. A modern mathematical description of the single-prism dispersion was given by Born and Wolf in 1959. The generalized multiple-prism dispersion theory was introduced by Duarte and Piper in 1982.
The air-wedge shearing interferometer is probably the simplest type of interferometer designed to visualize the disturbance of the wavefront after propagation through a test object. This interferometer is based on utilizing a thin wedged air-gap between two optical glass surfaces and can be used with virtually any light source even with non-coherent white light.
The operation of a photon scanning tunneling microscope (PSTM) is analogous to the operation of an electron scanning tunneling microscope, with the primary distinction being that PSTM involves tunneling of photons instead of electrons from the sample surface to the probe tip. A beam of light is focused on a prism at an angle greater than the critical angle of the refractive medium in order to induce total internal reflection within the prism. Although the beam of light is not propagated through the surface of the refractive prism under total internal reflection, an evanescent field of light is still present at the surface.
A Perger prism or Perger–Porro prism system is a prism, that is used to invert an image. The special feature of this prism is that, like a traditional double Porro prism system, it manages this with only four beam deflections and has neither a roof edge with the accompanying phase correction problems, a mirrored surface or an air gap. However, in contrast to the traditional double Porro prism, it leads to a significantly reduced eyepiece/objective axis offset. The reduced beam offset allows for slimmer, more straight binocular housings usually found in roof prism binoculars. Complicating production requirements make high-quality roof prism binoculars relatively costly to produce compared to in optical quality equivalent Porro prism or "Perger–Porro prism system" binoculars.
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