The Principle and Application of Roof Prism

Two mutually perpendicular reflectors are called the ridge surface, and the prism with the ridge surface is called the ridge prism.



The ridge prism is small in size and can make the objective lens and the eyepiece on a straight line, so it is often used in extremely compact binoculars. Under the condition of not changing the optical axis direction and the imaging direction in the main section, a reflection is added, so that the total number of reflections of the system is changed from odd to even, so as to meet the requirements of object image similarity.


The key to the roof prism is the existence of the roof surface. The so-called roof surface is a roof-shaped reflective surface sandwiched by two reflective surfaces in the optical path. The edges of the two surfaces are in the middle of the optical path, so some roof prisms can see a dividing line in the middle. In fact, it can also be understood as dividing the beam into two halves and then combining them. The two mirrors form a right angle to form a ridge surface, one of the most commonly used Biehan prism principle to reflect 6 times. Correspondingly, there is also an abbe prism commonly used in modern Zeiss telescopes. It is also a roof prism, which is slightly larger in length, but only has four reflections and does not require a reflective layer, so the efficiency is higher than that of the Behan prism and is similar to that of the ordinary Paul prism.


In the early development of the telescope, there was a ridge prism, which can keep the output light and the incident light in a straight line. Its lens tube is straight, distance sense, stereo sense, size sense and so on are also close to the naked eye.


The main reason for the popularity of ridge mirrors among bird-watching enthusiasts is the difference in the size of the Paul telescope and the ridge telescope mentioned above. The bird will look larger in the ridge prism telescope, but it is not really larger. If an 8 times Paul is measured and an 8 times ridge is imaged, it will be found to be the same size. But it 's really hard for us to get our brains to accept that what we see is actually the same big object. I have a friend who has quantitatively studied this phenomenon, linking the size of the object he feels to the distance between the lens. In this regard, the reverse Paul prism telescope, that is, the telescope whose objective lens is closer than the distance between human eyes, is particularly prominent ).


There are some other aspects of the roof prism that are worth noting. In the most common roof prism : Schmidt special Han prism, there is an interface that cannot produce total reflection, and most of the light will be emitted rather than reflected. So we must plate this reflective surface into a mirror. A thin metal reflective layer allows light to be reflected. Start with silver until aluminum begins to be produced in large quantities ( the advantage of aluminum is that it does not oxidize as quickly as silver and reduces reflectivity ). Most advanced ridge prism telescopes return to silver coating because of its higher reflectivity. For nitrogen-filled sealed telescopes, silver is not easily oxidized.


Unfortunately, even the best silver reflective film does not have high total reflection efficiency, and some light is always lost. The light loss of the aluminized prism can reach 15 %. Until recently, the brightness of the roof prism telescope was still unable to compete with Paul 's in contrast.


In addition, when the light is reflected back from the mirror, its phase changes. We can regard the light wave as a wave that vibrates in all directions. When reflected back from the mirror, it will be partially polarized, and the wave energy that vibrates in the horizontal direction will be higher. Part of the energy ( brightness ) and part of the information ( resolution ) are lost. When two partially polarized beams meet and interfere with each other, this loss will be greater. If we do not take any measures, then the ridge prism telescope and the same grade Paul telescope than will be darker, imaging softer. As mentioned above, birdwatchers prefer to use the ridge prism telescope even if its imaging is slightly worse, because it has a better sense of perspective and operation. The ridge telescope can achieve better light transmittance and clarity through better lens and coating technology, so more expensive and complex ridge prism telescopes are designed.

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