English: CIE xy 1931 chromaticity diagram including the Planckian Locus. The Planckian locus is the path that a black body color will take through the diagram as the black body temperature changes. Lines crossing the locus indicate lines of constant correlated color temperature. Monochromatic wavelengths are shown in blue in units of nanometers. Latest version (16 April 2005) uses 1931 CIE standard observer, since this is the most commonly used standard observer.