Correlated colour temperature is a one-dimensional description of the colour of near-white light sources. The concept relates the numerical value to a visual observation of smallest colour difference between the chromaticity, of the test source and of a point lying on the Planckian locus. In practice, chromaticity difference on a uniform chromaticity scale diagram is used. Since the introduction of the CIELUV and CIELAB spaces, it has been questioned whether the calculations should be done on the traditional u,v diagram, or by using a more equidistant colour space. Based on our visual experiments, we can state that the u,v diagram-based calculation is adequate to describe the correlated colour temperature of a source. The individual scatter of observations is so large that a visual definition has no practical reality. Thus, we recommend changing the definition of correlated colour temperature from a visual definition to a mathematical one and dropping any reference to visual investigations.