One can call a lumen print a solar photograph. The process involves the laying a light sensitive paper in the sun and placing organic objects on top of it. It is important that they are fresh so that the juices of the objects can increase the strength of the print.
It is important to come up with a composition before exposing the paper to the sun. The paper is exposed to light with the objects in place. The composition is left undisturbed in the sun for a minimum of 30 mins up to a few hrs. The longer one leaves it in the sun, the stronger the detail of the print. The results depends on the time of year, the strength of the sun, the location, the humidity. Ultimately it is up to the artist to monitor the situation and obtain the best result possible.
The image is then taken back to the darkroom to be fixed. No developing takes place.. The colour changes from a pink hue to a brown.
This image was left in the sun for an hour. If I had the time, I would have kept the print out longer for a stronger contrast.
Artists who have used this technique are Paul Sisson, Steven H Silburg and Denis Roussel.