Abstract: To determine the appropriate length of the isotemperature lines on thePlanckian locus in the CIE 1960 UCS Diagram, 7 lights from 0--360μrd inequal intervals of 60 μrd in the Planckian locus were used as standards,with each standard light a set of 10--12 lights on each corresponding iso-temperature line was used as test lights. The standard and test lights werematched on a 2° bi-partite field in a visual colorimeter to evaluate theircolor difference. The 2 observers gave similar results. By determining 3levels of color difference from the Planckian locus along each isotemperatureline, it was possible to draw 2 zones along the Planckian locus. Lihgts fallingwithin these zones are either of the same color or of similar color as thecorrelated Planckian radiator.

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