The English painter David Hockney suspects in his book "Secret Knowledge" the use of mirror lenses (= concave mirrors) in painting from about 1430.

During the examination of paintings by an optician friend, optical refractive errors on paintings were discovered which can only be caused by the use of such lenses.

 The curved glass mirror is identical to today's shaving or cosmetic mirror.

The mirror lens is in principle a simple inwardly curved (= concave) mirror. The artist is sitting in a darkened room with a small window. The object or person to be drawn is positioned outside in front of the window.

Cosmetic mirror by AMAZON (Klick on image)

The light entering through the window is projected onto the drawing surface (easel) by the mirror mounted on a tripod inside the room, but upside down (but not inverted). Of course the whole thing only works in bright sunshine.

 
Secret Knowledge - Lost Techniques of the Old Masters, Rediscovered by David Hockney