The sun likely served as Vermeer’s light source
There’s a simple argument that counters the idea that Johannes Vermeer used an optical projection device, as cited in the Feb. 19 Style article “There will never be another Vermeer show to match this one.” It is that the light source, the sun, constantly changes. That would cause critical illuminated and shadow masses in Vermeer’s scenes to change as well, posing substantial difficulties for a painter of his exacting technique. More interesting and pertinent to Dutch interior paintings of his era is the manipulation of the windows shutters to compose the portrayal of normally diffused interior light.
As for optical devices, there were two types available to Vermeer. One was the camera obscura, similar to a box camera but much larger. The other was the camera lucida, a more compact mirror and lens arrangement. Either one would reverse the scene: right becomes left, top becomes bottom. And both would have posed the problem of the room’s ambient light muddying the projected image, especially exacting lines. But what about the figures in his paintings? Could they have stood perfectly still while he laboriously traced their images?
All that aside, why would an artist of Vermeer’s stature need any optical device other than his own eyes?