By using the same principles by which the OSA Subcommittee on the Spacing of the Munsell Colors extended the definition of the ideal Munsell system beyond the gamut of the colors actually observed, this definition has been further extended to Munsell renotation values less than 1/. These principles conform to the logical requirement that the constant-chroma locus for any chroma, however small, must extend beyond the spectrum locus (and the straight line connecting its extremes) as Munsell renotation value approaches 0/ if the Munsell renotation chroma is to correlate with the saturation of the color perception of the specimen viewed against a neutral background of Munsell value greater than 5/.
DEANE B. JUDD and GÜNTER WYSZECKI, "Extension of the Munsell Renotation System to Very Dark Colors," J. Opt. Soc. Am. 46, 281-284 (1956)