Chromatic luminance carries both wavelength and radiance of light, is the source of all psychophysical dimensions and all color attributes, and is a key to understand their relations. It has long attracted research, hindered in the past by flawed definitions of colorimetric purity (pc), remedied in a recent article. This article investigates relations between luminous reflectance Y (i.e., total luminance, chromatic plus achromatic), chromatic luminance (calculated from Y per pc), and chroma/colorfulness. Relations are clarified, illustrated by formulas and graphs including three-dimensional schemas of luminance Y, chromatic luminance, and color solids. A useful new term, relative chromatic luminance, is introduced. Munsell chroma is shown to resemble inverted log luminance much more closely than inverted log colorimetric purity as claimed by previous researchers. The relationship is used in Part II to model chroma, colorfulness, and brightness.