Lightness value

Influence of the brightness reference value on color perception :
A = 66 results in orange,
A = 30 results in brown.

The brightness reference value  A characterizes the brightness of a body color as assessed by the human eye in comparison to the brightness of another body color seen at the same time.

definition

The bright reference value is a relative luminance , which describes how bright a body color compared to a perfect white , to strength , direction and quality well-lit area. The photometric parameter for this is the spectral reflectance or the luminance factor :

${\displaystyle \beta _{v}={\frac {L}{L_{w}}}\leq 100\%}$

With

• the luminance L of the sampled and
• the luminance L _{w of} the (ideal) white surface.

The mutual relationship between body colors is therefore assessed, since the absolute luminance does not determine the color.

For the visual assessment in the CIE system , the green primary valence is adapted to the spectral eye sensitivity V (λ) of daytime vision .

Since the measuring principle is based on the Lambert-Beer law , the spectral reflectance must always be assumed, i.e. H. either measurement is made with monochromatic light or at least with a spectrophotometer on the split line. In the case of transparent colors, the measured variable is correspondingly the spectral transmittance .

In order to rule out disturbing factors such as the gloss of a smooth surface or the roughness of the structure, certain measurement conditions must be observed. Usually an integrating sphere is used for this , with which either the incident light or the emitted “measuring light” can be guided diffusely from the colored surface.

As a substitute for luminance, the luminous flux is measured (mainly in lighting technology) and the measurement result is referred to as the degree of reflection .

Wilhelm Ostwald was the first to recognize the importance of relative brightness . He therefore referred to body colors as related colors; The term “bound colors” is also used for this, as a contrast to free color. The free color is perceived when a colored surface is viewed through a screen in such a way that the structure of this surface is ignored.

Mathematical relationships

The light reference value A is the size of the light axis in the Lab color space and is related to the L * a * b * light value as follows:

${\displaystyle L^{*}=116\cdot \ {\sqrt[{3}]{\frac {Y}{Y_{n}}}}-16}$(results for ) ${\displaystyle L^{*}=100}$ ${\displaystyle Y=Y_{n}}$

The relationship A ≡ Y exists between the brightness reference value and the coordinate of the CIE color space. This is based on the aforementioned adaptation of the green coordinate to the spectral sensitivity curve for daytime vision. The third root corresponds to the widespread (roughly) logarithmic dependence between physical stimulus and physiological perception.

${\displaystyle \Rightarrow A=Y=Y_{n}\cdot \left({\frac {L^{*}+16}{116}}\right)^{3}}$

Ultimately, however, this relationship only applies to the 2 ° normal observer in the CIE color space system because of the adjustment of Y to daytime vision. The light reference value, on the other hand, occurs independently of the selected primary valences.

The exact relationship between photometric and colorimetric luminance is established by Abney's law :

${\displaystyle L=L_{R}\cdot R+L_{G}\cdot G+L_{B}\cdot B}$

With

• the luminance coefficients  L _k belonging to the respective primary valence.

For the CIE system, R = X, G = Y and B = Z apply. In addition, L _X  L _Z  = 0 was defined , which is why L _Y = 1, so the brightness reference value of the numerical value of the color valence Y = G is the same, even if there are differences in interpretation between the two sizes:

${\displaystyle {\begin{aligned}L_{CIE}&=L_{R}\cdot X+L_{G}\cdot Y+L_{B}\cdot Z\\&=L_{X}\cdot X+L_{Y}\cdot Y+L_{Z}\cdot Z\\&=0\cdot X+1\cdot Y+0\cdot Z\\&=Y\end{aligned}}}$

Individual evidence

1. ↑ The CIE Lab coordinates of the color example: Orange: {L * a * b * = 66, 45, 78}, brown: {L * a * b * = 30, 45, 78};
   in RGB coordinates: RGB _orange  = 222, 122, 0, RGB _brown  = 118, 31, 0
2. ^ Wilhelm Ostwald: Mathematical color theory (color theory volume 1). Unesema, Leipzig 1923

literature

• Manfred Richter: Introduction to colorimetry . Walter de Gruyter, Berlin New York 1976.

Web links

Wiktionary: black  - explanations of meanings, word origins, synonyms, translations

Wiktionary: gray  - explanations of meanings, word origins, synonyms, translations

• Example in signs

The colors shown in this article are not binding and may appear different on different display devices.
One possibility to approximately calibrate the display with purely visual means is offered by the adjacent test image (only with native display resolution and when the page is not displayed zoomed):

Set the display device to sRGB mode, if available. If a letter (“R” for red, “G” for green or “B” for blue) appears strongly on one or more of the three gray areas, the gamma correction of the corresponding color channel should be corrected. sRGB corresponds to a gamma value of 2.2, which is the common value for TVs and computers with Windows or macOS (from version 10.6).