13500 pieces per inch (2.54 cm) 2-axis Reflex Grate 30cm x 15cm (1 x 6") Price is for 1 foot or 12 inches. Holographic Diffraction Lattice is a highly efficient embossed holographic optical element (H.O.E.) A refractor grate is used for direct vision and analysis of the spectrum from different gas pipes and other light sources. Our lattice produces a high quality spectrum to minimize visual noise that impede observation. Our grates are the main component of a protractor, and are used in learning experiments on light and color. Project the spectrum using an overhead projector or a 35mm sliding projector for spectrum observation. Use with colored film gel to demonstrate coloring/fading colors.