luadocs/content/lua.api.manual/darktable/darktable.collection.md

title	id	weight	draft	author
darktable.collection	darktable.collection	180	false	people

Allows to access the currently worked on images, i.e the ones selected by the collection lib. Filtering (rating etc) does not change that collection.

darktable.collection.#

types.dt_lua_image_t

Each image in the collection appears with a numerical index; you can iterate them using ipairs.