Su.Ki. is correct. It's an unusual word, but I see this statement describes the Kunsthaus Bregenz (https://www.arch2o.com/kunsthaus-bregenz-peter-zumthor/) museum in Austria. According to the site: The minimalist structure stands as a light box that absorbs, reflects, and filters light across the façade and throughout the building. Those places in which it absorbs and filters the light throughout the building are the light plenum. Plenum (physics): 1. a space completely filled with matter, or the whole of space so regarded. (in this case, the matter is light) 2. an enclosed chamber where a treated substance collects for distribution, as heated or conditioned air through a ventilation system. (in this case, light collects for distribution)

This is a very unusual word. I know lots of words, but I had never heard of this one! I checked it in a dictionary and, among other definitions, found this: "an air-filled space in a structure". Here's another definition: "In building construction, a plenum (pronounced PLEH-nuhm, from Latin meaning full) is a separate space provided for air circulation for heating, ventilation, and air-conditioning (sometimes referred to as HVAC) and typically provided in the space between the structural ceiling and a drop-down ceiling" The light plenum seems to be a special architectural feature of this gallery. There's a space incorporated into the building, which catches and distributes the light.