On the CEI diagram, draw a line from the White Point to the Chromacity Curve. On that line place a dot. That dot represents everything about the color over which it is placed. No other line from the White Point to the Chromacity Curve can have a dot in exactly the same place.

The dot divides the line into two segments unless it is placed at either end of the line, which is permissible. When this line represents "Chromacity" the segment between the dot and the White Point is termed the color's Excitation Purity. This same segment is termed the color's Luminous Excitance when the line represents "Luminosity." There are three terms that apply to the remaining segment: If the line represents "chromacity", the term for this segment is Saturation. If the line represents "luminosity" the segment may be termed either as Luminous Transmittance or as Luminous Reflectance, depending on the source of the colored light. If the source is direct, the former term applies. If the colored light is reflected light, it will have "picked up" some of the color of the surface it was reflected from and the term Luminous Reflectance indicates this fact.