Have been surveying the machine translation of JP2013-218241 from Japanese into English as provided by JPO a bit...
In terms of prior art JP2013-218241 initially references JP2002-182268 which deals with a DSLR having a 45deg fixed half mirror. This patent does not deal with the half mirror as such but with the fact that a fixed half mirror does not exhibit a “mirror slab” and combined with a low noise shutter the photo taking can be made less intrusive. JP2013-218241 however uses said patent as an example of the inherent light sharing compromise between the OVF and the image sensor and states that a “modulated” mirror is a solution for this problem. As for an example of a modulated mirror JP2013-218241A refers to JP2007-102197 aka US75541551 which deals with electrochromic thin film/multilayer constructs, aimed at controlling heat transfer trough windows in houses and vehicles - in other words sun filters. By applying voltages of different polarity to the appropriate layers, the optical properties of the construct can be changed between a state with transmittance/reflectance of 0%/27% and another state with 48%/6% respectively - with one caveat though, the switch from low to high transmissivity takes about 40secs @ λ = 670nm (red/NIR) and the other from high to low transmissivity about 15secs.
JP2013-218241 in its embodiment goes on to state that since light measurement is made on the OVF path and to ensure adequate OVF brightness at low light levels the reflection towards the finder should not be less than 60% (40% left for phase detection on the image sensor). An interesting aspect of the patent app is that the modulated mirror not only removes the mirror slab but also replaces the shutter – thus constituting a virtually noise free DSLR. The patent mentions both a still and an “animated” mode with up to 15 frames/sec.
For those interested there is also the Canon patent app US20130002925 which also deals with a modulated mirror, or in this case referred to as “variable translucency” mirror. The technology used to achieve any level of translucency/reflectance between 100%/0% and 0%/100% respectively is based on so called “Micro blinds”, aimed at windows and automotive use as detailed in US7684105.
Surely, - only Canon knows what is going on in their “skunkworks”, but I see some substantial challenges for this type of modulated mirror technology that need to be overcome before a camera using this principle will materialize. For JP2013-218241 would include optical state switching speed, transmissivity, durability (i.e. lifetime), environmental sensitivity, and color casts and for US20130002925 the most obvious ones would include durability (i.e. lifetime), diffraction (even though this is considered US7684105), light incident angles when applied to a 45 deg mirror.
I personally would therfore not bet on seeing a camera based on either of these patents anytime soon, and view this mostly as a “possibly applications of technologies not initially intended for use in cameras and hence preventing others from doing it first” type of patent apps..
But as we all know however, prediction is a skilled art, especially when it comes to the future..