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The CIE 1924 photopic V(λ) luminosity function, [cie 3] which is included in the CIE 1931 color-matching functions as the y (λ) function, has long been acknowledged to underestimate the contribution of the blue end of the spectrum to perceived luminance.
Fig. 1. HSL (a–d) and HSV (e–h). Above (a, e): cut-away 3D models of each. Below: two-dimensional plots showing two of a model's three parameters at once, holding the other constant: cylindrical shells (b, f) of constant saturation, in this case the outside surface of each cylinder; horizontal cross-sections (c, g) of constant HSL lightness or HSV value, in this case the slices halfway ...
The two major parts of the model are its chromatic adaptation transform, CIECAT02, and its equations for calculating mathematical correlates for the six technically defined dimensions of color appearance: brightness , lightness, colorfulness, chroma, saturation, and hue. Brightness is the subjective appearance of how bright an object appears ...
Chromaticity consists of two independent parameters, often specified as hue (h) and colorfulness (s), where the latter is alternatively called saturation, chroma, intensity, [1] or excitation purity. [ 2 ] [ 3 ] This number of parameters follows from trichromacy of vision of most humans, which is assumed by most models in color science .
To model the latter effect, in CIECAM97C the formula for J is adjusted as follows: J HK = J + (100 – J) (C / 300) |sin( 1 / 2 h – 45°)|, where C is the chroma and h the hue angle Q is then calculated from J HK instead of from J. This formula has the effect of pulling up the lightness and brightness of coloured samples.
The difference between brightness and lightness is that the brightness is the intensity of the object independent of the light source. Lightness is the brightness of the object in respect to the light reflecting on it. This is important because the Helmholtz–Kohlrausch effect is a measure of the ratio between the two. [3]
The latter is often simplified as a single value measured at e.g. 50% saturation level, implying a linear approximation of the not perfectly linear photo response non-linearity (PRNL). [2] Often PRNU as defined above is subdivided in pure "(offset) FPN" which is the part not dependent on temperature and integration time, and the integration ...
Brightness, the subjective perception elicited by the luminance of a source; Luminance, the photometric equivalent of radiance (lm·sr −1 ·m −2) Photometry (optics), measurement of light, in terms of its perceived brightness to the human eye; Radiometry, measurement of light, in absolute power units; Luminosity