Information on the CIE LUV color space

The Applet:

General information on all the color space applets:

All the applets have a button at the top of the applet that says "Click here for more information." Clicking the mouse on that button will take you to a different document that is like this one, that will give more information on that particular color space and that particular applet.
All the applets also display some pictures that try to give the user an idea of where in the color space they are, and what the particular color space looks like.
They all have three slider bars near the bottom that control the three parameters for the color space, though some also have other controls to manipulate their parameters. To the right of each slider bar is a label telling the name of the parameter (such as R for Red in the RGB space, or L for Luminance in the CIE Luv space) as well as showing the current value of this parameter.
There is a rectangle at the very bottom of the applet that shows the color the current parameter values for the color space define.
Some applets also have an area for a message below this rectangle, which will print out a message if the user generates a color in this color space that is undisplayable on the monitor, because the RGB translation of the current parameters gives one or more values outside the proper RGB range [0,1].
If you choose to have more than 1 applet running (by picking more than 1 color space from the initial screen), then when one applet is modified, the other applets modify themselves to show the same color. With this, you can see better how different color spaces relate to one another.
I think it's very informative when having more than one applet running, to put one or two of a color space's parameters at 0, and manipulate the other to see how that specific parameter moves you around in this space and whatever other space(s) you have running. For example, when running the RGB and CIE LUV color space applets, moving the R, G, or B scroller of the RGB applet around, with the other two set at 0 seems to give a decent impression of the shape of the CIE LUV color space.

Specific information on this color space's applet:

The L component has the range [0,100], the U component has the range [-134,220], and the V component has the range [-140,122].

The two pictures shown in the applet are pictures of the RGB cube when converted into the CIE LUV color space. The topmost picture in the applet is a picture of the RGB cube as seen from the U-axis, with the L-axis pointing upwards and the V-axis pointing towards the right (this picture is shown to the left here). The picture below that (shown to the right here) is a picture of the RGB cube as seen from the V-axis, with the L-axis pointing upwards, and the U-axis pointing to the left (to conform to a right-handed coordinate space). I produced these pictures with a simple raycaster-type program I wrote that will display the RGB color cube in different color spaces.

As the L, U, and V values are changed via the scrollbars (or another color space applet), a white dot with a blue outline will move to indicate the current position in this color space.

There is a list of different monitor choices to choose from (that appears as to the left). This is because the CIE LUV space is device-independent, and colors with the same LUV components are to look the same on all monitors. The technically correct way to run a model of the CIE LUV color space is to run a calibration test on the monitor the viewer is using, and then transform the chosen LUV coordinates into RGB space dependent on the results of that calibration test. However, most monitors conform to some standard, so I chose what appear to be the most popular 3 computer monitor standards, and allow the user to choose which one his/her monitor most closely matches. These different standards were developed by the CCIR (Comite Consultatif International des Radiocommunications). They are:
  • CCIR Rec [for Recommendation] 601-1: This is the old NTSC (National Television System Committee) standard. Its white point is called "C Illuminant", with CIE (xn;yn) values of (0.310063; 0.316158).
  • CCIR Rec [for Recommendation] 709: Its white point is D65, with CIE (xn;yn) values of (0.312713; 0.329016).
  • ITU D65: CCIR and CCITT were absorbed into their parent body, the International Telecommunications Union (ITU), which made its new recommendation also with a white point of D65 and CIE (xn;yn) values of (0.312713; 0.329016). This is probably the monitor standard that most monitors adhere to, so it is the default one chosen.

    Some interesting things to notice:
  • The L parameter corresponds roughly to illuminance or brightness. If both U and V are set to 0, then varying L will take you through the gray-scale.
  • The U parameter seems to mimic mostly shifts from green to red (with increasing U), and the V parameter seems to mimic mostly blue and purple type colors. This is just a rough idea only, that I've acquired through running these applets.

    The CIE LUV color space:

    More Information:

    There is too much information about CIE color spaces to go into here, so for more information, try some of the links further below or a good book like Computer Graphics by Foley, van Dam, Feiner, and Hughes, published by Addison-Wesley, 1990. I'll try to give just the most basic aspects here.

    The CIE, which stands for International Commission on Illumination (or Commision Internationale de l'Eclairage), in 1931, defined three standard primaries (X, Y, and Z) to replace red, green, and blue, because all visible colors could not be specified with positive values of red, green and blue components. (In other words, not all visible colors can be specified in the RGB color space! -- at least not with positive coefficients.) With this newly created X, Y, and Z primaries, all visible colors could be specified with only positive values of the primaries. The primary Y was intentionally defined to match closely to the quality of luminance of a color. Note that arbitrarily combining X, Y, and Z values can easily lead to a "color" outside the visible color spectrum. The picture to the left is taken from Computer Graphics, mentioned above, and shows the cone of visible colors, as well as the plane X+Y+Z=1 (discussed below).

    Lowercase x, y, and z refer to the normalized X, Y, and Z values [normalized such that x + y + z = 1, so x = X/(X+Y+Z), y = Y/(X+Y+Z), and z=Z/(X+Y+Z)]. Thus, x, y and z are on the (X + Y + Z = 1) plane of the solid of visible colors in X,Y,Z space. These lowercase letters are called chromaticity values, and a projection onto the (X,Y) plane of the (X+Y+Z=1) plane of the figure to the left is called the chromaticity diagram. On this diagram, all perceivable colors with the same chromaticity but different luminances (brightness) map into the same point. This chromaticity diagram is shown to the right (it is also taken from Computer Graphics), with the dot marking the "C-illuminant" (or white point), and the numbers along the boundary correspond to wavelength in nanometers.

    The CIE LUV color space is a perpetually uniform derivation of this standard CIE XYZ space. (Percpetually uniform means that two colors that are equally distant in the color space [specified by the standard cartesian distance function of the square root of the squares] are equally distant perceptually.)



    Links:

    The Color Science & Technology page in the U.K., with links to organizations and resources on color science.
    An abstract on using different color spaces to choose colors.
    Here is a link to a color space FAQ.



    Back to the main color space applet page.