Chlorophyll prints are created by using an opaque or semi-translucent object to block light from a still-living or just-cut leaf. The areas of the leaf that are protected from light remain green, while the sunlight breaks down the chlorophyll in any un-protected areas, turning the leaf pale. When the exposure is complete, a positive image is left. While photograms are the easiest, it's very possible to create images using digitally printed positives.