Putting into context several points made above . . .
1) In terms of the different optical surfaces in binoculars and their main coatings
a) Transmission surfaces on both lenses and prisms, benefit from anti-reflective (A-R) coating.
It minimises the loss of light due to unwanted reflections.
Originally a single layer was used (and usually not on all suitable surfaces). Now three layers are typically used
(and camera lenses often have many more layers).
b) Prism surfaces required to be reflective - that have Total Internal Reflection - are perfect mirrors.
They don’t require coating to aid reflection, and adding an A-R coating will degrade the reflection.
c) Prism surfaces required to be reflective - that lack TIR - need an external reflective coating to provide the internal mirror function.
Traditionally a single layer of stainless steel, aluminium or silver was used.
More recently multiple layers (70+) of different dielectric coatings are used, to cumulatively achieve reflection.
d) The two roofed surfaces of roof prisms have TIR, but the splitting/ dual reflection/ recombining of the light beam,
results in a degree of de-phasing (seen as a slight loss of sharpness).
So the two roof surfaces benefit from a special phase coating.
- - - -
2) And in relation to some common prism types
i) 'Non-roofed prisms' i.e. Porro Type I and II prisms, and the Perger prism version used by Leica
All have TIR, and only benefit from coating the two transmission surfaces with A-R coating.
ii) Abbe-Koenig prisms
Also have TIR. They benefit from coating the two transmission surfaces with A-R coating,
and also from phase coating the two roof surfaces.
iii) Uppendahl and Schmidt-Pechan prisms
Each has 6 reflective surfaces. Besides the 2 roofed ones, they also have:
• Uppendahl: 1 TIR; 1 non-TIR, and; 2 combined transmission/ reflection surfaces.
• S-P prisms: 1 non-TIR, and; 3 combined transmission/ reflection surfaces (plus 1 transmission only surface).
Necessarily there’s a contradiction between the optimal coatings needed for the transmission and reflection functions of the combined surfaces.
So there’s an optical performance tradeoff, for the compactness of the prisms that’s made possible by the use of 6 verses 4 reflections.
(Whether that's significant for a manufacturer or a user . . . ?)
In terms of the limits of what we currently know, see from post #310 on at:
Zeiss SFL 8x30?
The Uppendahl image is from Holger’s book (the blue dashed line indicates the mirrored surface, and the green one the two roof surfaces).
The S-P image is a well known one from Swarovski.
John
p.s. In terms of Perger prism optical performance, what we know is limited to the use in the 3rd generation Leica Geovid rangefinders.
One problem is that the additions needed to enable the RF functions, often degrade the optical transmission.
See Gijs’ work in posts #19 and 24 (specifically re the Gen 3 Geovid) at:
Loss Of Brightness With 8X42 RF (HT)