There is set forth herein in one embodiment, a structure including a metallic grating having a grating pattern, the metallic grating including a critical dimension. The metallic grating can output a spectral profile when exposed to electromagnetic radiation, the spectral profile having a feature. The grating pattern can be configured so that a change of the critical dimension produces a shift in a value of the feature of the spectral profile. A method can include propagating input electromagnetic radiation onto a metallic grating having a two dimensional periodic grating pattern and measuring a critical dimension of the metallic grating using output electromagnetic radiation from the metallic grating.