I read all comments with great interest and my impression is that there may be some misunderstanding about the historical developments.
In 1941 Prof. Joos, scientist and Zeiss coworker published a paper in which he described the phase problems of roof prisms in detail. Knowing the phenomenon does not mean that you can solve it directly and scientific and technological developments made it only in 1988 possible to solve the problem with phase correction coatings.
(Surprisingly Weyrauch and Dörband do not even mention Joosts work in their paper from 1988 in which they describe the problems and use of phase-corrected roof prisms and also König and Köhler do not mention his paper, (it is unknown to me why they seem to have forgotten it, Prof Joos was a pretty important scientist for Zeiss in the 1940's).
Between 1897 and 1988 we had to deal with these phase problems, since Hensoldt started production of roof prism binoculars in 1897. Zeiss had started in 1894 with the production of porro prism binoculars and Hensoldt and Zeiss were in a tough competition, since the roof prism binoculars were also very attractive to the public.
In 1928 Zeiss acquired Hensoldt, but roof prism production was continued using Hensoldt's name for many years.
Now with regard to the dulness of roof prism binoculars without phase correction some measured data (and these are data from porro's as well as roofs), which we measured ourselves.
- Voigtländer Krimstechter 4x55 (few lenses only from 1866: lihgt transmission 67,5%
- Leitz Binodal 6x30 porro from 1908: light transmission 51%
- Leitz Fernglas 08 Holland glass (or Galilei f you want) 5x40 from 1917: light transmission 69,6%
- Leitz Binux 8x30 porro from 1927: light transmission 54%
- Leitz Camparit porro from 1950 (coatings applied): light transmission 73%
- Leitz Trinovid Uppendahl roof from 1965: light transmission 75%
- Leica Trinovid 8x32 Schmidt-Pechan roof from 1992: light transmission 88,5% (phase corrected)
- Leica-Kern porro 8x30 from 2000: light transmission 68%
We have a lot of similar data from other binocular producers and these show the same pattern and it is certainly not straightforward to conclude that porro's are much brighter than roofs. As far as I could see phase coating does have a great impact on image resolution, but not so much on image brightness.
Gijs van Ginkel