Reflection prism
A reflection prism is an optical prism that corresponds in its effect to one or a combination of several flat mirrors. The light to be deflected by reflection mainly enters and exits vertically through a prism surface. This largely avoids its wavelength-dependent refraction , which is the main purpose of a dispersion prism , and minimizes reflection losses. The reflection usually takes place from one or more boundary surfaces back into the interior of the prism. There the light is usually inclined enough so that total reflection takes place. In other cases, the reflective boundary surfaces are mirrored , that is, coated with metal.
A reflection prism is used in optical systems to change the direction of light beams as a deflecting prism or to change the position and / or for axis reflection of a transmitted image as an inverting prism .
The division of the reflection prisms into deflecting prisms and erecting prisms is primarily dependent on the application. A prism of a certain shape can be both, depending on whether the focus is on the deflection of a bundle of rays or the often simultaneous change in the image position or an axis reflection of the image.
Deflection prism
right: 180 ° deflecting prism (double reflection, non-mirrored image)
total reflection in both prisms
A deflecting prism is used to deflect a light beam into another direction by reflecting it. It thus fulfills a function similar to that of a deflecting prism , which deflects a beam by refraction. The simplest deflecting prism has an isosceles, right-angled triangle (90 ° prism, half-cube prism, see adjacent picture) and usually deflects 90 °. In this case, the light enters and exits vertically through the two surfaces that are at right angles to one another. The large surface serves as a totally reflective deflection surface.
The half-cube prism is also used for 180 ° deflection. Entry and exit occur through the hypotenuse surface, and each of the two short surfaces is reflected once. The beam path is "reversed", which is why it is sometimes referred to as an inverting prism with the risk of confusion .
Deflection prisms are used in optical instruments to guide light in place of mirrors, for example for the low-loss deflection of a laser beam in FTIR spectrometers . Thanks to the total reflection that is effective in them, they have lower losses than mirrors reflecting on the air side.
Inverting prism
The term erecting prism is not generally defined in the literature and is therefore used differently. In principle, however, two different functions can be assigned to an erecting prism:
- "Image reversal" through an axis mirroring,
- "Image reversal" in the form of an image rotation by 180 ° around the image vertical.
Prisms, which only cause an axis mirroring, exchange two opposite sides of an image for one another. The image is reversed in only one direction, that is, according to height or side. Prisms with such a function are sometimes referred to as "turning prisms". Examples are the Dove prism (also known as Dovean erecting prism) and its generalizations with other prism angles.
The second form of inverting prisms causes the image to be rotated by 180 °, which is also referred to as "completely" inverting the image. A typical area of application is the erection of an upside-down image in telescopes; In this context one can also find the colloquial term “erecting prism”, which is rarely used. The best known erecting prism for this purpose is the Porro prism used in binoculars . The requirement that at least two reflections must take place in different main sections is achieved by adding a second half-cube prism that is turned sideways towards the first. The beam path is offset in parallel. The parallel offset can be avoided with a prism in which one surface is folded into two roof edges. An example of this is the Abbe-König prism .
Advantages and disadvantages of reflection prisms over mirrors
The function of reflection prisms can in principle also be taken over by ordinary mirrors. The advantage of prisms, however, lies in the unchangeable angle allocation of the surfaces during operation and a smaller space requirement. Compared to mirrored prisms, totally reflecting reflection prisms have reduced reflection losses and do not lead to polarization effects .
The disadvantage of reflection prisms is that, like other optical elements, they introduce imaging errors into the optical system. Of the light rays involved in image generation , only the central one hits the entry and exit surfaces perpendicularly. The others are subject to the wavelength-dependent refraction and lead to color errors. Astigmatism , an axial beam displacement or change in focus can also occur and must be taken into account when designing the optical system and corrected if necessary. The higher weight of the prisms and the relatively long paths in the glass, which can be contaminated and interfere with the propagation of light, are also disadvantageous.
literature
- Dietrich Kühlke: Optics: Basics and Applications . Harri Deutsch Verlag, 2004, ISBN 978-3-8171-1741-3 , pp. 127-130 .
- Heinz Haferkorn: Optics - physical-technical basics and applications . Barth, Leipzig, 1994, ISBN 3-335-00363-2 , pp. 466-487.
- Fritz Hodam: Technical optics . VEB Verlag Technik, Berlin, 1967, pp. 251-256.
References and comments
- ↑ Fritz Hodam: Technical Optics , VEB Verlag Technik Berlin, 1967, S. 251st
- ↑ Dietrich Kühlke: Optics: Basics and Applications . Harri Deutsch Verlag, 2004, ISBN 978-3-8171-1741-3 , pp. 126-130 .
- ↑ Dietrich Kühlke: Optics . Harri Deutsch Verlag, 2004, ISBN 978-3-8171-1741-3 , pp. 123 .
- ↑ a b Fritz Hodam: Technical optics . VEB Verlag Technik, Berlin 1967, p. 256.
- ↑ Heinz Haferkorn: Optics - physical-technical basics and applications . Barth, Leipzig 1994, ISBN 3-335-00363-2 , p. 475
- ↑ Heinz Haferkorn: Optics - physical-technical basics and applications . Barth, Leipzig 1994, ISBN 3-335-00363-2 , p. 474.
- ↑ a b Fritz Hodam: Technical optics . VEB Verlag Technik, Berlin 1967, p. 254.