It's a simple construction: There's a small hole with a lens on one side of the truck's lightproof interior. This lens projects an upside-down and backward image on the opposite wall. On that wall, Irving hangs 4-by-8-foot sheets of photo paper, which, when exposed to 5-30 second exposures, serve as giant negatives. He then takes his jumbo negatives to a darkroom, or just a room that's dark, and processes them. The whole process, he says, takes anywhere from 30 minutes to a few hours.
This is my favorite photo of the selection; I love how you can see the emulsion painted on the paper:
Moto del Cuerva, Spain