Lightness is a visual perception of the luminance ( L ) {\displaystyle (L)} of an object. It is often judged relative to a similarly lit object. In colorimetry and color appearance models, lightness is a prediction of how an illuminated color will appear to a standard observer. While luminance is a linear measurement of light, lightness is a linear prediction of the human perception of that light. This distinction is meaningful because human vision's lightness perception is non-linear relative to light. Doubling the quantity of light does not result in a doubling in perceived lightness, only a modest increase. The symbol for perceptual lightness is usually either J {\displaystyle J} as used in CIECAM02 or L ∗ {\displaystyle L^{*}} as used in CIELAB and CIELUV. L ∗ {\displaystyle L^{*}} ("Lstar") is not to be confused with L {\displaystyle L} as used for luminance. In some color ordering systems such as Munsell, Lightness is referenced as value. Chiaroscuro and tenebrism both take advantage of dramatic contrasts of value to heighten drama in art. Artists may also employ shading, subtle manipulation of value.
Lightness in different colorspaces
In some colorspaces or color systems such as Munsell, HCL, and CIELAB, the lightness (value) achromatically constrains the maximum and minimum limits, and operates independently of the hue and chroma. For example Munsell value 0 is pure black, and value 10 is pure white. Colors with a discernible hue must therefore have values in between these extremes. In a subtractive color model (e.g. paint, dye, or ink) lightness changes to a color through various tints, shades, or tones can be achieved by adding white, black, or grey respectively. This also reduces saturation. In HSL and HSV, the displayed luminance is relative to the hue and chroma for a given lightness value, in other words the selected lightness value does not predict the actual displayed luminance nor the perception thereof. Both systems use coordinate triples, where many triples can map onto the same color. In HSV, all triples with value 0 are pure black. If the hue and saturation are held constant, then increasing the value increases the luminance, such that a value of 1 is the lightest color with the given hue and saturation. HSL is similar, except that all triples with lightness 1 are pure white. In both models, all pure saturated colors indicate the same lightness or value, but this does not relate to the displayed luminance which is determined by the hue. I.e. yellow is higher luminance than blue, even if the lightness value is set at a given number. While HSL, HSV, and similar spaces serve well enough to choose or adjust a single color, they are not perceptually uniform. They trade off accuracy for computational simplicity, as they were created in an era where computer technology was restricted in performance. If we take an image and extract the hue, saturation, and lightness or value components for a given color space, we will see that they may differ substantially from a different color space or model. For example, examine the following images of a fire breather (fig. 1). The original is in the sRGB color space. CIELAB L ∗ {\displaystyle L^{*}} is a perceptually uniform lightness prediction that is derived from luminance Y {\displaystyle Y} , but discards the X {\displaystyle X} and Z {\displaystyle Z} , of the CIE XYZ color space. Notice this appears similar in perceived lightness to the original color image. Luma ( Y ′ ) {\displaystyle (Y^{\prime })} is a gamma-encoded luminance component of some video encoding systems such as ( Y ′ I Q ) {\displaystyle (Y^{\prime }IQ)} and ( Y ′ U V ) {\displaystyle (Y^{\prime }UV)} . It is roughly similar, but differs at high chroma, deviating most from an achromatic signal such as linear luminance Y {\displaystyle Y} or non-linear lightness L ∗ {\displaystyle L^{*}} . HSL L {\displaystyle L} and HSV V {\displaystyle V} are neither perceptually uniform, nor uniform as to luminance.
Relationship to value and relative luminance The Munsell value has long been used as a perceptually uniform lightness scale. A question of interest is the relationship between the Munsell value scale and the relative luminance. Aware of the Weber–Fechner law, Albert Munsell remarked "Should we use a logarithmic curve or curve of squares?" Neither option turned out to be quite correct; scientists eventually converged on a roughly cube-root curve, consistent with the Stevens's power law for brightness perception, reflecting the fact that lightness is proportional to the number of nerve impulses per nerve fiber per unit time. The remainder of this section is a chronology of lightness models, leading to CIECAM02. Note. – Munsell's V runs from 0 to 10, while Y typically runs from 0 to 100 (often interpreted as a percentage). Typically, the relative luminance is normalized so that the "reference white" (say, magnesium oxide) has a tristimulus value of Y = 100. Since the reflectance of magnesium oxide (MgO) relative to the perfect reflecting diffuser is 97.5%, V = 10 corresponds to Y = 100/97.5% ≈ 102.6 if MgO is used as the reference.
1920 Irwin Priest, Kasson Gibson, and Harry McNicholas provide a basic estimate of the Munsell value (with Y running from 0 to 1 in this case):
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