What range of L*-values are clearly perceived as jet-back by the human eye? It is in the 9.0's. Builders RN and RN-1 produce an astounding jet-black in polyester when added to the padding liquor in between the black disperse dyeing and the drying/curing stages.

<Deep-dyeing auxiliaries should be selected for their L*-value>

In deep black dyeing of polyester, the use of a deep-dyeing agent that improves the L*-value does yield a deeper black, however, according to visual perception assessments from apparel firms and consumers, the difference is unrecognizable to be casual observer. L*-values easily perceived as jet-black by the human eye are in the 9.0's. Builder RN maintains L*-values within that range, producing bluish black shades on polyester.

Numbers are more accurate than words at color communication

Example) Indication of the color of an apple in numerical figures: L*=42.83, a*=45.04, b*=9.52, Once you learn about hue, lightness and saturation in color, you might wonder if there are more accurate ways of describing colors other than using words. Quite so. These three attributes of color can be transposed into figures that indicate human perception. In this way, anyone can communicate colors through expressions common to all. First, let’s look at Fig. 1 in which L*, a* and b* coordinates are shown. When indicating a color by figures, lightness is represented by L*, and chromaticity (hue and saturation) by a* and b*. a* and b* each specifies the orientation of color, in terms of Red/Green and Yellow/Blue balances respectively.
The following coordinates indicate the color of an apple.

Fig. 1 L*a* b* color space

Now, let us find out what color these coordinates represent. Look at Fig. 2, where the chromaticity of the apple is indicated by point X, where a*(=45.04) and b*(=9.52) cross over. Equally, the tone of color can be found by assigning numerical values along the axes of lightness and saturation in Fig. 3. All of this can be summarized in words as 'a bright reddish shade'.

*Accurate figures for Saturation C* can be worked out using the following formula.

Fig. 2 L*a*b* color space: chromaticity diagram (hue and saturation)

Fig. 3 Tone (lightness and saturation)

Fig. 3 shows a cross section (from X) of the diagram in Fig. 2. The warp axis represents lightness. You may also refer to Fig. 1 for a better understanding.

*Hue angle H° can be worked out using the following formula.
H°=tan^-1(b*/a*)

Figures 1, 2, and 3, as well as the accompanying explanations were reproduced from Minolta Co.’s webpage, “Color Talk”