This process has been used before and notably in wonderful art works by Susan Boafo, Edgar LIssel and Roy Amiss. It utilizes the phototactic response of photosynthetic microorganisms, that is, their ability to deliberately move towards a source of light so that photosynthesis become possible.
Here the phototactic protozoan Euglena gracilis has been exposed to the lit silhouette of a bicycle so that it might function as living algal emulsion. Over the course an hour the microscopic migrate en masse from the dark to the lit sections of the projected image. The process of the movement of the microorganisms bares similarities with the exposure of photographic paper with the areas which are stuck by light becoming dark, and those which receive little light remaining clear.