For context, this is likely the old testament prophet Isaiah, who in certain apocryphal sources died by being sawed in half on the orders of King Manasseh of Judah. The orientation of the saw, and whether Isaiah was magically hiding inside a tree trunk at the time, seem to have been left to the artist’s discretion. I can’t find this particular illumination after a bit of googling, but here are some more examples in this blog post (I have not vetted it and do not vouch for it, but the writer seems to be a professional researcher who identifies as having broken with fundamental Christianity in his youth).

As always, medieval illuminators are not particularly interested in historical accuracy, but rather familiarity with the cultural roles, hence a very European two-person saw and woodcutters in European garb, but Isaiah himself is in “Biblical” robes as they would have understood them. If there was any thought given to lighting, it’s perfectly sensible for it to be emanating from the prophet.