In the 1600's it was the camera obscura.  About the time of the War Between the States, 1860's, photographs were available and being used by artists.  Today a few painters work strictly from nature but most make some use of photographs to one degree or another. 

Neither Vermeer's painting nor this one is a true portrait.  They are both tronies, a term not in use today referring to heads or faces,  "an opportunity for the artist to demonstrate his ability in rendering some kind of exotic garment, lighting conditions or characteristic facial types that struck him in particular".  Rembrandt and his followers made a lot of these.  At the time they were not of much value.

In this image I carefully set the model up before a black background using only the light available from a west facing window in late afternoon with the blinds down, very soft light.  Looking through her dresser I found two interesting scarves.  The black one over the shoulders has special significance to her because it was actually touched by the Dali Lama.  To complete the outfit I added a large round earring similar to Vermeer's painting.

With a tripod mounted Fujifilm Finepix 3800 digital camera I made exposures of several poses at the highest setting, 3.2 megapixels.  The photos turned out very sharp.  In fact every pore and wrinkle was very apparent.  The black of the shawl blended into the background nicely after darkening the image and reducing the contrast in PaintShop Pro 8.  In that program the image was cropped, increased from 72 to 300 pixels per inch and reduced in size to 16 inches wide by 14+ high.

Switching to Painter 8, the entire picture was reworked carefully by hand a few pixels at a time.  Pores were removed and most wrinkles.  Areas of color were unified and simplified, some edges were blurred.  At the end highlights were reestablished as needed.  The overall intensity was increased and new layer of solid yellow ochre set at 22% transparency was added to unify and simulate the yellow tinted varnish Vermeer would have used.

The final result is a very interesting digital painting that the model was pleased with.  Or maybe it is just a touched up photograph.  You be the judge.  Vermeer's not talking, but he may have done something like this and then used it as a reference for a real oil painting.