 A particular application of bright field spectroscopy is the counting of the exact number of layers in layered materials such as (few layer) graphene, hexagonal boron nitride and some transition metal dichalcogenides.In photoemission electron microscopy (PEEM), upon exposure to electromagnetic radiation (photons), secondary electrons are excited from the surface and imaged.PEEM was first developed in the early 1930s, using ultraviolet (UV) light to induce photoemission of (secondary) electrons.