The penetration depth of electromagnetic radiation in a painting © INTK

A diagram schematically depicts the structure of a painting layer, starting from the canvas, via pre-glue, undercoat, imprimitura, underpainting, main colour and glaze layer. Arrows coming from above go to different depths into these layers and show how different electromagnetic rays penetrate the painting layer (UV: up to the varnish, visible rays up to the glaze layers/main colour, IR rays up to the imprint/underdrawing, X-rays up to the canvas).