Lumen studies made using cameraless and sun-exposed silver paper techniques.
Lumen dates back to the earliest experiments of photography in the 19th century, with Henri Fox Talbot using objects on silver sensitised material and exposing these to sunlight. The sensitised paper or film becomes permanent through chemical fixing.
There is no camera involved. Instead, the light changes the colour of the black and white photographic paper. The silver salts turn pink, purple, or bronze depending on the medium, the intensity of the sun, or even the juices from the plant.
The image can be digitally scanned before fixing to retain these colours. Otherwise, chemical fixing alters the colour again, although the original will then be permanent.