The SAGA GIS tool "Topographic Openness" calculates the positive and negative topographic openness and is based on Yokoyama et al. (2002) who do not specifically say the unit of the final result. They refer to intermediate angles as > or < 90° (which are then added together and divided by the number of angles), but the maps I get in SAGA have values between roughly 0.6 and 1.7 which would equal 35 to 97 degrees if the unit were radians. So which is it? Or is it unitless?
Maybe the source code helps (I don't understand much of it, unfortunately...): https://sourceforge.net/p/saga-gis/code/ci/master/tree/saga-gis/src/tools/terrain_analysis/ta_lighting/topographic_openness.cpp
Yokoyama, R. / Shirasawa, M. / Pike, R.J. (2002): Visualizing topography by openness: A new application of image processing to digital elevation models. Photogrammetric Engineering and Remote Sensing, Vol.68, pp.251-266. (Link)