The most commonly used gratings are generally ruled or etched using either a process involving the mechanical scratching of the surface of a substrate, or exposure of a photoresist layer deposited on a substrate.where the interference pattern produced by two coherent light beams is used to expose the photoresist. The diffraction gratings produced with this process are often called holographic. Both of these processes are used to create a fine, periodic relief pattern on the surface of a blank substrate. Such a relief structure can predictably modify the amplitude and/or phase of the incident light. The superposition of the effects of many such grooves allows for the angular separation of the spectral components of the incident light. Depending on whether the light is reflected from the grooved surface, or transmitted through it, the grating is called either a reflection or transmission diffraction grating.