Sometimes one measures the thickness of a material in the number of mean free paths. Material with the thickness of one mean free path will attenuate to 37% (1/ e) of photons. This concept is closely related to half-value layer (HVL): a material with a thickness of one HVL will attenuate 50% of photons. A standard x-ray image is a transmission image, an image with negative logarithm of its intensities is sometimes called a number of mean free paths image.