CIE 1931 CIE 1976
|
|
Objectively communicating a particular color to someone without some type of standard is difficult.
How would you describe the color of this surface? Would you say it’s yellow, sort of lemon yellow or maybe a bright canary yellow?
The solution is a measuring instrument that explicitly identifies a color. That is, an instrument that differentiates a color from all others and assigns it a numeric value.
Colorimeters are tristimulus (three-filtered) devices that make use of red, green, and blue filters that emulate the response of the human eye to light and color.
Each color has its own distinct appearance, based on three elements: hue, chroma, and value (lightness).
Hue is how we perceive an object’s color— red, orange, green, blue, etc.
Chroma describes the vividness or dullness of a color, for example, the appearance of a tomato and a radish.
Lightness is the luminous intensity of a color, for example the red of the tomato appears to be much lighter. In contrast, the radish has a darker red value.
Color Maps
The rainbow color map is the most dominate in scientific visualization. Based on the colors of light at different wavelengths, the rainbow color map’s design has nothing to do with how humans perceive color. This results in multiple problems when humans try to do the reverse mapping from colors back to numbers.
Color Spaces
All color spaces are based on the tristimulus theory, which states that any perceived color can be uniquely represented by a 3-tuple.
Colorimeters are tristimulus (three-filtered) devices that make use of red, green, and blue filters that emulate the response of the human eye to light and color.
We take the reflectance curve and multiply the data by a CIE standard illuminant. The illuminant is a graphical representation of the light source under which the samples are viewed. Each light source has a power distribution that affects how we see color. Examples of different illuminants are: incandescent and D65 – daylight.
We multiply the result of this calculation by the CIE standard observer. The CIE commissioned work in 1931 and 1964 to derive the concept of a standard observer, which is based on the average human response to wavelengths of light.
How would you describe the color of this surface? Would you say it’s yellow, sort of lemon yellow or maybe a bright canary yellow?
The solution is a measuring instrument that explicitly identifies a color. That is, an instrument that differentiates a color from all others and assigns it a numeric value.
Colorimeters are tristimulus (three-filtered) devices that make use of red, green, and blue filters that emulate the response of the human eye to light and color.
Each color has its own distinct appearance, based on three elements: hue, chroma, and value (lightness).
Hue is how we perceive an object’s color— red, orange, green, blue, etc.
Chroma describes the vividness or dullness of a color, for example, the appearance of a tomato and a radish.
Lightness is the luminous intensity of a color, for example the red of the tomato appears to be much lighter. In contrast, the radish has a darker red value.
Color Maps
The rainbow color map is the most dominate in scientific visualization. Based on the colors of light at different wavelengths, the rainbow color map’s design has nothing to do with how humans perceive color. This results in multiple problems when humans try to do the reverse mapping from colors back to numbers.
Color Spaces
All color spaces are based on the tristimulus theory, which states that any perceived color can be uniquely represented by a 3-tuple.
Colorimeters are tristimulus (three-filtered) devices that make use of red, green, and blue filters that emulate the response of the human eye to light and color.
We take the reflectance curve and multiply the data by a CIE standard illuminant. The illuminant is a graphical representation of the light source under which the samples are viewed. Each light source has a power distribution that affects how we see color. Examples of different illuminants are: incandescent and D65 – daylight.
We multiply the result of this calculation by the CIE standard observer. The CIE commissioned work in 1931 and 1964 to derive the concept of a standard observer, which is based on the average human response to wavelengths of light.
The CIE 1931 Chromaticity Diagram represents the whole gamut (color range) of human color perception.
It was developed based on the standard observer, who represents how an average person sees colors across the visible spectrum. Once the values were calculated, the data were converted into the XYZ tristimulus values. These values can now identify a color numerically. But XYZ had limited use as color specifications because they did not correlate well with visual attributes, even though Y relates to LIGHTNESS, X and Z do not correlate to HUE and CHROMA. Thus CIE recommended a different formula for chromaticity values and xyz were defined. These values can now identify a color numerically.
To overcome the limitations of chromaticity diagrams, after the 1931 CIE standard observer was established, CIE recommended a uniform color scale: CIE 1976 (L*a*b*) or CIELAB.
CIELAB (strictly CIE 1976 L*a*b*) derived from the "master" space CIE 1931 XYZ color space, to produce a color space that is more perceptually linear.
There is a nonlinear relationship between light intensity and color perception. When defining a color map, we are more interested in how a color is perceived than how it is formed. In these cases, it is better to use a color map based on how humans perceive color. CIELAB and CIELUV are two common spaces. The choice between the two is fairly arbitrary; this paper uses CIELAB.
CIELAB is an approximation of how humans perceive light. The Euclidean distance between two points is the approximate perceived difference between the two colors. This Euclidean distance in CIELAB space is known as ΔE and makes a reasonable metric for comparing color differences. The notation ΔE {c1,c2} denotes the ΔE for the pair of colors c1 and c2.
Perceptual uniformity means that all pairs of adjacent colors will look equally different from each other. That is, the ΔE for each adjacent pair is (roughly) the same.
CIELAB is the most complete color model used to describe all the colors visible to the human eye.
The three parameters in the model represent for a particular color:
L: lightness of the color (L*, L*=0 yields black and L*=100 indicates white)
a*: its position between magenta and green (-a* indicate green ; +a* indicate magenta)
b*: its position between yellow and blue (-b* indicate blue ; +b* indicate yellow)
CIELAB color space is used in: Adobe Photoshop, image editing; ICC Profiles; TIFF files; PDF documents.
RGB and CMYK conversions
The color space most frequently used in computer applications is the RGB color space. The three values in the RGB color space refer to the intensity output of each of the three light colors used in a monitor, television, or projector.
It is impossible for all colors viewed on a monitor to be identically matched in a print from a desktop printer. A printer operates in a CMYK color space, and a monitor operates in an RGB color space. Their gamuts (the subset of colors which can be accurately represented in a given color space) are different. Even though it is impossible to perfectly match all colors on different devices, you can use color management to ensure that most colors are the same or similar enough so they appear consistent.
RGB and CMYK are not absolute color spaces, so to convert between RGB and CIELAB (L*a*b*) it is necessary to assume an absolute color space for the RGB data, such as sRGB.
We assume the RGB space conforms to the canonical monitor defined by the sRGB specification, a standard of the International Electro technical Commission (IEC61966-2-1).
It was developed based on the standard observer, who represents how an average person sees colors across the visible spectrum. Once the values were calculated, the data were converted into the XYZ tristimulus values. These values can now identify a color numerically. But XYZ had limited use as color specifications because they did not correlate well with visual attributes, even though Y relates to LIGHTNESS, X and Z do not correlate to HUE and CHROMA. Thus CIE recommended a different formula for chromaticity values and xyz were defined. These values can now identify a color numerically.
To overcome the limitations of chromaticity diagrams, after the 1931 CIE standard observer was established, CIE recommended a uniform color scale: CIE 1976 (L*a*b*) or CIELAB.
CIELAB (strictly CIE 1976 L*a*b*) derived from the "master" space CIE 1931 XYZ color space, to produce a color space that is more perceptually linear.
There is a nonlinear relationship between light intensity and color perception. When defining a color map, we are more interested in how a color is perceived than how it is formed. In these cases, it is better to use a color map based on how humans perceive color. CIELAB and CIELUV are two common spaces. The choice between the two is fairly arbitrary; this paper uses CIELAB.
CIELAB is an approximation of how humans perceive light. The Euclidean distance between two points is the approximate perceived difference between the two colors. This Euclidean distance in CIELAB space is known as ΔE and makes a reasonable metric for comparing color differences. The notation ΔE {c1,c2} denotes the ΔE for the pair of colors c1 and c2.
Perceptual uniformity means that all pairs of adjacent colors will look equally different from each other. That is, the ΔE for each adjacent pair is (roughly) the same.
CIELAB is the most complete color model used to describe all the colors visible to the human eye.
The three parameters in the model represent for a particular color:
L: lightness of the color (L*, L*=0 yields black and L*=100 indicates white)
a*: its position between magenta and green (-a* indicate green ; +a* indicate magenta)
b*: its position between yellow and blue (-b* indicate blue ; +b* indicate yellow)
CIELAB color space is used in: Adobe Photoshop, image editing; ICC Profiles; TIFF files; PDF documents.
RGB and CMYK conversions
The color space most frequently used in computer applications is the RGB color space. The three values in the RGB color space refer to the intensity output of each of the three light colors used in a monitor, television, or projector.
It is impossible for all colors viewed on a monitor to be identically matched in a print from a desktop printer. A printer operates in a CMYK color space, and a monitor operates in an RGB color space. Their gamuts (the subset of colors which can be accurately represented in a given color space) are different. Even though it is impossible to perfectly match all colors on different devices, you can use color management to ensure that most colors are the same or similar enough so they appear consistent.
RGB and CMYK are not absolute color spaces, so to convert between RGB and CIELAB (L*a*b*) it is necessary to assume an absolute color space for the RGB data, such as sRGB.
We assume the RGB space conforms to the canonical monitor defined by the sRGB specification, a standard of the International Electro technical Commission (IEC61966-2-1).