The device is a white screen with a multitude of little holes on all its surface. Each hole is filled with a pin that can slide back and forth and the screen is lit by the two vertical sides, allowing the pins to cast their shadow on the screen. If all the pins are pushed in the screen, they can't cast any shadow and the screen appears entirely white. Conversely, if all the pins are pushed out of the screen, it's total darkness. Depending of how far the pins jut out of the front of the screen, their shadows are shorter or longer, allowing the artist to create images in different tones of gray.