Lightness
Chiaroscuro and tenebrism both take advantage of dramatic contrasts of value to heighten drama in art.
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.
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.
While HSL, HSV, and similar spaces serve well enough to choose or adjust a single color, they are not perceptually uniform.
It is roughly similar, but differs at high chroma, deviating most from an achromatic signal such as linear luminance
"[2] 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.
Typically, the relative luminance is normalized so that the "reference white" (say, magnesium oxide) has a tristimulus value of Y = 100.
They suggest a quintic parabola (relating the reflectance in terms of the value):[8] Using Table II of the OSA report, Parry Moon and Domina Spencer express the value in terms of the relative luminance:[9] Jason Saunderson and B.I.
Milner introduce a subtractive constant in the previous expression, for a better fit to the Munsell value.
[10] Later, Dorothea Jameson and Leo Hurvich claim that this corrects for simultaneous contrast effects.
[11][12] Ladd and Pinney of Eastman Kodak are interested in the Munsell value as a perceptually uniform lightness scale for use in television.
After considering one logarithmic and five power-law functions (per Stevens' power law), they relate value to reflectance by raising the reflectance to the power of 0.352:[13] Realizing this is quite close to the cube root, they simplify it to: Glasser et al. define the lightness as ten times the Munsell value (so that the lightness ranges from 0 to 100):[14] Günter Wyszecki simplifies this to:[15] This formula approximates the Munsell value function for 1% < Y < 98% (it is not applicable for Y < 1%) and is used for the CIE 1964 color space.
CIELAB uses the following formula: where Yn is the CIE XYZ Y tristimulus value of the reference white point (the subscript n suggests "normalized") and is subject to the restriction Y/Yn > 0.01.
As early as in 1967 a hyperbolic relationship between light intensity and cone cell responses was discovered in fish, in line with the Michaelis–Menten kinetics model of biochemical reactions.
[18] In the 70s the same relationship was found in a number of other vertebrates and in 1982, using microelectrodes to measure cone responses in living rhesus macaques, Valeton and Van Norren found the following relationship:[19] where V is the measured potential, I the light intensity and σ a constant.
In 1986 Seim and Valberg realised that this relationship might aid in the construction of a more uniform colour space.
When n = 1/5, cz = 1, representing the assumption that most scenes have an average relative luminance of 1/5 compared to bright white, and that therefore a sample in such a surround should be perceived at its proper lightness.
The quantity A models the achromatic cone response; it is colour dependent but for a grey sample under bright conditions it works out as: Here Y is the relative luminance compared to white on a scale of 0 to 1 and LA is the average luminance of the adapting visual field as a whole, measured in cd/m2.
Suggestions for a more comprehensive model, CIECAM97C, were also formulated, to take into account several effects at extremely dark or bright conditions, coloured lighting, as well as the Helmholtz–Kohlrausch effect, where highly chromatic samples appear lighter and brighter in comparison to a neutral grey.
To model the latter effect, in CIECAM97C the formula for J is adjusted as follows: where C is the chroma and h the hue angle Q is then calculated from JHK instead of from J.
The absolute sine term has a sharp V-shaped valley with a zero at yellow and a broad plateau in the deep blues.
Since the total noise term adds up to 3.05, this means that A and consequentially J and Q aren't zero for absolute black.
[24] Although CIECAM97s was a successful model to spur and direct colorimetric research, Fairchild felt that for practical applications some changes were necessary.
Those relevant for lightness calculations were to, rather than use several discrete values for the surround factor c, allow for linear interpolation of c and thereby allowing the model to be used under intermediate surround conditions, and to simplify z to remove the special case for large stimuli because he felt it was irrelevant for imaging applications.
[25] Based on experimental results, Hunt, Li, Juan and Luo proposed a number of improvements.
[26] Li and Luo found that a colour space based on such a modified CIECAM97s using lightness as one of the coordinates was more perceptually uniform than CIELAB.
[29][30] This subjective perception of luminance in a non-linear fashion is one thing that makes gamma compression of images worthwhile.
Though the CIELAB space and relatives do not account for this effect on lightness, it may be implied in the Munsell color model.
