Color in Photography


To write this exactly, still extract information from Wikipedia to do a short summary. I recommend this succinct page to understand some of notions better.

Luminance

Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted or reflected from a particular area, and falls within a given solid angle.

The luminance indicates how much luminous power will be detected by an eye looking at the surface from a particular angle of view. Luminance is thus an indicator of how bright the surface will appear. In this case, the solid angle of interest is the solid angle subtended by the eye’s pupil.

Luminance is invariant in geometric optics. For real, passive, optical systems, the output luminance is at most equal to the input.

Brightness

As a basic notion, Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target.

This is a subjective attribute/property of an object being observed and one of the color appearance parameters of color appearance models. Brightness refers to an absolute term and should not be confused with Lightness(tone).

A given target luminance can elicit different perceptions of brightness in different contexts; see, for example, White’s illusion.

In RGB color space, brightness can be thought of as the arithmetic mean of red, green and blue color coordinates (but inaccurate).

It is also a color coordinate in HSL color space.

Lightness

In colorimetry and color theory, lightness, also known as value or tone, is a representation of variation in the perception of a color or color space’s brightness.

Various color models have an explicit term for this property. The Munsell color model uses the term value, while the HSL color model, HCL color space and Lab color space use the term lightness.

Hue

Hue is one of the main color appearance parameters, defined technically in the CIECAM02 model, as the degree to which a stimulus can be described as similar to or different from stimuli that are described as red, green, blue, and yellow, the so-called unique hues.

Hue can typically be represented quantitatively by a single number, often corresponding to an angular position around a central or neutral point or axis on a colorspace coordinate diagram or color wheel, or by its dominant wavelength or that of its complementary color.

Colorfulness, Chroma and Saturation

Colorfulness, chroma and saturation are attributes of perceived color relating to chromatic intensity. They respectively describe three different aspects of chromatic intensity, but the terms are often used loosely and interchangeably in contexts where these aspects are not clearly distinguished.

  • Colorfulness is the “attribute of a visual perception according to which the perceived color of an area appears to be more or less chromatic”.
  • Chroma is the “colorfulness of an area judged as a proportion of the brightness of a similarly illuminated area that appears white or highly transmitting”.
  • Saturation is the “colorfulness of an area judged in proportion to its brightness”,which in effect is the percolorceived freedom from whitishness of the light coming from the area.

As colorfulness, chroma and saturation are defined as attributes of perception they can not be physically measured as such, but they can be quantified in relation to psychometric scales intended to be perceptually even, for example the chroma scales of the Munsell system. (e.g.) saturation in CIECAM02

defined as the square root of the colorfulness divided by the brightness.

Color Model, Color Space and Gamut

A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers (e.g. triples in RGB and Lab, quadruples in CMYK). However, a color model with no associated mapping function to an absolute color space is a more or less arbitrary color system with no connection to any globally understood system of color interpretation. Once we say “color space which is not absolute”, it refers to color model, exactly, not a color space.

  • RGB: Red, Green and Blue
  • CMYK: Cyan, Magenta, Yellow and Key(Black)
  • Lab: Lightness and color components green–red/blue–yellow
  • HSB vs. HSL: Hue, Saturation and Brightness/Lightness
  • YUV: luma component Y and the chrominance components (U,V)
  • XYZ: encompasses all color sensations that are visible to a person

A color space is a specific organization of colors. In combination with physical device profiling, it allows for reproducible representations of color, in both analog and digital representations. There can be different color spaces under a color model, and they have different color gamuts depending on the conditions of the arrangement(e.g. Adobe RGB, sRGB and Apple RGB are all in RGB model).

Color Gamuts

The gamut or color gamut, is a certain complete subset of colors. The mot common usage refers to the subset of colors which can be accurately represented in a given circumstance, such as within a given color space or by a certain output device.

CIE 1931 Color Space

The human eye with normal vision has three kinds of cone cells that sense light, having peaks of spectral sensitivity in short (“S”, 420 nm – 440 nm), middle (“M”, 530 nm – 540 nm), and long (“L”, 560 nm – 580 nm) wavelengths.

These cone cells underlie human color perception in conditions of medium and high brightness; in very dim light color vision diminishes, and the low-brightness, monochromatic “night vision” receptors, denominated “rod cells”, become effective. Thus, three parameters corresponding to levels of stimulus of the three kinds of cone cells, in principle describe any human color sensation. Weighting a total light power spectrum by the individual spectral sensitivities of the three kinds of cone cells renders three effective values of stimulus; these three values compose a tristimulus specification of the objective color of the light spectrum.

The CIE model define Y as luminance. Z is quasi-equal to blue stimulation, or the S cone response, and X is a mix (a linear combination) of cone response curves chosen to be nonnegative. Defining Y as luminance has the useful result that for any given Y value, the XZ plane will contain all possible chromaticities at that luminance.

Color appearance model

A color appearance model(CAM) is a mathematical model that seeks to describe the perceptual aspects of human color vision. i.e. viewing conditions under which the appearance of a color does not tally with the corresponding physical measurement of the stimulus source. (In contrast, a color model defines a coordinate space to describe colors, such as the RGB and CMYK color models.)

CIECAM02

CIECAM02 is the color appearance model published in 2002. Its equations for calculating mathematical correlates for the six technically defined dimensions of color appearance: brightness, lightness, colorfulness, chroma, saturation, and hue.

Contrast Ratio

The contrast ratio is a property of a display system, defined as the ratio of the luminance of the brightest color (white) to that of the darkest color (black) that the system is capable of producing. A high contrast ratio is a desired aspect of any display. It has similarities with dynamic range.

Static contrast ratio is the luminosity ratio comparing the brightest and darkest color the system is capable of producing simultaneously at any instant of time, while dynamic contrast ratio is the luminosity ratio comparing the brightest and darkest color the system is capable of producing over time (while the picture is moving). Moving from a system that displays a static motionless image to a system that displays a dynamic, changing picture slightly complicates the definition of the contrast ratio, due to the need to take into account the extra temporal dimension to the measuring process.

Color balance and Color temperature

In photography and image processing, color balance is the global adjustment of the intensities of the colors. An important goal of this adjustment is to render specific colors – particularly neutral colors – correctly. Hence, the general method is sometimes called gray balance, neutral balance, or white balance.

Color balance changes the overall mixture of colors in an image and is used for color correction. Generalized versions of color balance are used to correct colors other than neutrals or to deliberately change them for effect.

Color temperature is a characteristic of visible light. In practice, color temperature is meaningful only for light sources that do in fact correspond somewhat closely to the radiation of some black body, i.e., those on a line from reddish/orange via yellow and more or less white to blueish white. It does not make sense to speak of the color temperature of, e.g., a green or a purple light.

In digital photography, the term color temperature is sometimes used interchangeably with white balance, which allow a remapping of color values to simulate variations in ambient color temperature.

Gamma Correction

Gamma correction, or often simply gamma. Gamma encoding of images is used to optimize the usage of bits when encoding an image, or bandwidth used to transport an image, by taking advantage of the non-linear manner in which humans perceive light and color.

The human perception of brightness, under common illumination conditions (not pitch black nor blindingly bright), follows an approximate power function, with greater sensitivity to relative differences between darker tones than between lighter ones. If images are not gamma-encoded, they allocate too many bits or too much bandwidth to highlights that humans cannot differentiate, and too few bits or too little bandwidth to shadow values that humans are sensitive to and would require more bits/bandwidth to maintain the same visual quality.

The concept of gamma can be applied to any nonlinear relationship.

For the power-law relationship:

the curve on a log–log plot is a straight line, with slope everywhere equal to gamma.

That is, gamma can be visualized as the slope of the input–output curve when plotted on logarithmic axes.

Appendix: Noun table in chinese

You know, the use of these nouns in English is chaotic in many different systems(e.g. the noun “Saturation” has different meaning between HSL and HSV/HSB). It is much more worse in other languages. Hence the national standard table can be a good reference.

  • Hue: 色相
  • Luma: 辉度
  • Saturation: 饱和度
  • Chromaticity: 色品
  • Chroma: 色彩浓度/纯度/色度
  • Brightness: 明度
  • Lightness: 亮度
  • Color model: 色彩模型
  • Color space: 色彩空间
  • Color Gamut: 色域
  • Color Wheel: 色轮
  • Colorfulness: (色)彩度
  • Color temperature: 色温
  • Color balance: 白平衡
  • Color appearance: 色貌
  • Color appearance model: 色貌模型
  • Color stimulus: 色刺激