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The viewing cone refers to the effective viewing directions of an LCD display, as seen from the eye. This collection of angles resembles a cone. The concept has been introduced as an international standard ISO 13406-2, which defines it as the range of viewing directions that can safely be used for the intended task without "reduced visual performance". This standard describes a complex procedure which evaluates the viewing cone from measurements of luminance and chromaticity versus direction of observation. ISO 13406-2 introduces 4 viewing direction range classes, from a wide viewing cone, for many simultaneous observers, to the so-called "privacy display", with a severely limited viewing cone. Compliance routes for different display applications can now be found in the successor standard ISO 9241-300.