I had a dark apartment last year & I utilized whatever light there was by putting up decorative mirrors everywhere. I have never had so many mirrors in my life...Check to see how the light comes in and arrange your mirrors to bounce the light into the room...
Be sure to leave curtains or window coverings OFF that window. Nobody can see you -- it's under the deck! Also I saw on an HGTV show a little window box under the inside of the window. It it are hidden lights which shine up (not super bright) and give the iillusion of more daylight.
Also I saw in a Pottery Barn catalog a mirror that was actually many mirrors in a square grid, sectioned off as if they were window panes. In fact, in the catalog photo it looks like a window. Pottery Barn is expensive, but if you are creative, I bet you could make something similar.
Good luck, basements are hard. I know, I'm always trying to get more light into mine.
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