What is a backlit cel?
A backlit cel is one where light was shining through from the back during filming. It’s a kind of special effect. Often this means the cel had no visible background stock (but not always). So that the light doesn’t shine through the paint, the back of the cel paint is often painted over in all one color (most often black). I’ve also seen paper (black construction paper, card stock) taped to the back of cels to achieve the same effect.
