Twenty years ago when the pair were setting up an art installation that covered a room with grass, they took note of the yellow imprint made on the grass by a ladder they'd accidentally left leaning against the a wall and which blocked the portion of grass in its shadow from receiving light.
Moved by this, the pair began to think about ways to effect the light hitting grass and within a year they were projecting light through an old 35mm Kodak projector onto a grass patch on the wall. Since then they have perfected the process, growing grass in a pitch black area except for their "sun" which is a 2500 watt projector. They always project onto walls vertically. Almost all the photos they project are their own. To read more about this amazing project and see more examples, check out the full article here.