Schmidt-Pechan prisms are the most widely used roof prism type in current binoculars. Compared to Abbe-König prisms they are more compact but have two disadvantages.
The first, which has been widely discussed here, is the need for one prism surface to provide total internal reflection and direct transmission. This demands a compromise in the anti-reflection coatings.
The second disadvantage is that the angle of incidence on one reflective surface is too acute for total internal reflection, requiring a mirror coating.
Up to the turn of the century this was usually provided by metallic coatings, either aluminium or silver. Recently these have been largely displaced by dielectric coatings.
Now dielectric coatings can, for a specific angle of incidence and a narrow wavelength band, provide reflectivity of up to 99,999% and allegedly around 99% for the visible spectrum. This is a few percent better than metallic coatings and anything with a numerically higher value has to be superior - right?
The transmission values quoted by the optics manufacturers are usually a peak value at a specific wavelength and this says nothing about the values at the extremes of the visible spectrum.
We know that one manufacturer, Meopta, has adhered to some form of silver coatings. They don't go into details but it's possible that they use a few dielectric layers between the glass surface and the silver coating to improve the peak value and maintain good transmission into the red.
Leica binoculars are often praised for their rendering of red tones and I have often suspected that they do something similar.
Take a look at the transmission curves for Leica and Meopta binoculars on Allbinos. They are still around 50-60% at 800 nm, comparable to some Zeiss binoculars with Abbe-König prisms, whereas recent Swarovskis start to plummet at 650 nm and are down to 60% at 700 nm.
Here is an interesting link: Baader BBHS ® reflective properties / Baader Planetarium Blog Posts. Astronomers would probably like to have correct images but roof edges generate spikes on bright light sources and even the 1 arcsecond (1/3600th of a degree!) tolerance on the 90° roof angle might not be adequate for high magnifications. Most are content with a prism or mirror diagonal giving an upright but reversed left/right image. Prisms which are not part of the original optical design, especially if used with modern "fast" focal ratio scopes can cause chromatic and spherical aberration, so mirrors are preferred and it seems that silver coatings can offer some advantages in colour rendering and lack of scatter over dielectrics. The only disadvantage is that they are less robust, but this is of no consequence in a sealed binocular.
Lastly, here is an iluustration from Wikipedia showing the function of a dielectric mirror: Dielectric mirror - Wikipedia. I have no idea if the lateral shift of reflections at different layers is of any consequence, but it does look rather disturbing.
John
The first, which has been widely discussed here, is the need for one prism surface to provide total internal reflection and direct transmission. This demands a compromise in the anti-reflection coatings.
The second disadvantage is that the angle of incidence on one reflective surface is too acute for total internal reflection, requiring a mirror coating.
Up to the turn of the century this was usually provided by metallic coatings, either aluminium or silver. Recently these have been largely displaced by dielectric coatings.
Now dielectric coatings can, for a specific angle of incidence and a narrow wavelength band, provide reflectivity of up to 99,999% and allegedly around 99% for the visible spectrum. This is a few percent better than metallic coatings and anything with a numerically higher value has to be superior - right?
The transmission values quoted by the optics manufacturers are usually a peak value at a specific wavelength and this says nothing about the values at the extremes of the visible spectrum.
We know that one manufacturer, Meopta, has adhered to some form of silver coatings. They don't go into details but it's possible that they use a few dielectric layers between the glass surface and the silver coating to improve the peak value and maintain good transmission into the red.
Leica binoculars are often praised for their rendering of red tones and I have often suspected that they do something similar.
Take a look at the transmission curves for Leica and Meopta binoculars on Allbinos. They are still around 50-60% at 800 nm, comparable to some Zeiss binoculars with Abbe-König prisms, whereas recent Swarovskis start to plummet at 650 nm and are down to 60% at 700 nm.
Here is an interesting link: Baader BBHS ® reflective properties / Baader Planetarium Blog Posts. Astronomers would probably like to have correct images but roof edges generate spikes on bright light sources and even the 1 arcsecond (1/3600th of a degree!) tolerance on the 90° roof angle might not be adequate for high magnifications. Most are content with a prism or mirror diagonal giving an upright but reversed left/right image. Prisms which are not part of the original optical design, especially if used with modern "fast" focal ratio scopes can cause chromatic and spherical aberration, so mirrors are preferred and it seems that silver coatings can offer some advantages in colour rendering and lack of scatter over dielectrics. The only disadvantage is that they are less robust, but this is of no consequence in a sealed binocular.
Lastly, here is an iluustration from Wikipedia showing the function of a dielectric mirror: Dielectric mirror - Wikipedia. I have no idea if the lateral shift of reflections at different layers is of any consequence, but it does look rather disturbing.
John
