### Instruction: Given this paragraph about material, what is lithophane?
### Input: A lithophane is a thin plaque of translucent material, normally porcelain, which has been moulded to varying thickness, such that when lit from behind the different thicknesses show as different shades, forming an image. Only when lit from behind does the image display properly. They were invented in the 19th century and became very popular, typically for lampshades, nightlights, or to be hung on windows. They could also be given stands, to be placed in front of a light source. The longest side of a lithophane is typically between 6 and 10 inches.
### Response: A lithophane is a thin plaque of translucent material, normally porcelain, which has been moulded to varying thickness, such that when lit from behind the different thicknesses show as different shades, forming an image.