Canon patent for Optical Viewfinder with built-in image sensor

Interesting patent filed by Canon. If I got it right, this refers to an optical viewfinder with a built-in image sensor. I did not completely understand what this is for. The parts circled in red in the image above show where the image sensors are. The sensor at (24) is what we know from current DSLR designs. The sensor at (22) is placed immediately above the viewfinder. As usual with patents, I am glad for any additional interpretation.

• Patent Publication No. 2016-177038
  • Published 2016.10.6
  • Filing date 2015.3.19
• Canon patent
  • A finder optical system having a reduction optical system and the imaging device
  • Reduction optical system for forming an object image formed on the focusing plate an imaging element