- In a waveguide, fine parallel lines which filter certain types of waves. In a circular waveguide, for instance, radial wires serve to block transverse electric waves. Also called waveguide grating.
- A surface with many fine parallel lines, grooves, or slits, which are extremely close together, and which serve to disperse light so as to produce a spectrum. Such a surface may be made of metal or glass, and may have over 15,000 lines, grooves, or slits, per centimeter. Also called diffraction grating.