Abstract:
A system and method for color navigation through a CIELAB color space with coordinates hue (H), lightness (L), chroma (C), with intervals ΔH, ΔL, ΔC such that the perceptual color difference AE1Nv, defined according to the formula: AZW =^{AL)2+{fc(H)−ACf+[AH)2, is constant for steps in direction C and L and for steps in direction H along boundary of color space. fc(H) is a proportionality factor between 0 and 1 depending on hue. For hue values between red (H 1 ) and green (H 2 ), fc(H) =1. For hue values between green (H 2 ) and blue (H 3 ), the proportionality factor fc(H) is set to a constant value foB such that Nκ,GB(fGB) differs less than 50% from NH,RG—For hue values between blue (H 3 ) and red (H 1 ), the proportionality factor fc(H) is set to a constant value fβR such that N^BR^BR) differs less than 50% from NH,RG—.

Description:
This application is a national stage application under 35 U.S.C. §371 of International Application No. PCT/IB2007/052430 filed on Jun. 22, 2007, and published in the English language on Jan. 3, 2008, as International Publication No. WO/2008/001290, which claims priority to European Application No. 06116110.5, filed on Jun. 27, 2006, incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates in general to the field of lighting. More particularly, the present invention relates to a color navigation system for generating light with a variable color. 
     BACKGROUND OF THE INVENTION 
     Illumination systems for illuminating a space with a variable color are generally known. Generally, such systems comprise a plurality of light sources, each light source emitting light with a specific color, the respective colors of the different light sources being mutually different. The overall light generated by the system as a whole is then a mixture of the light emitted by the several light sources. By changing the relative intensities of the different light sources, the color of the overall light mixture can be changed. 
     It is noted that the light sources can be of different type, such as for instance TL lamp, halogen lamp, LED, etc. In the following, simply the word “lamp” will be used, but this is not intended to exclude LEDs. 
     By way of an example of a variable color illumination system, an illumination system in a theatre is mentioned. During a show, it may be desirable to change the color of the lighting. However, also in the case of homes, shops, restaurants, hotels, schools, hospitals, etc., it may be desirable to be able to change the color of the lighting. In the case of a theatre or the like, the colors are typically changed with a view to enhance dramatic effects, but in other situations it may be more desirable to have smooth and slow transitions. 
     As should be clear to a person skilled in the art, the color of light can be represented by coordinates of a color point in a color space. In such representation, changing a color corresponds to a displacement from one color point to another color point in the color space, or a displacement of the setting of the color point of the system. Further, a sequence of colors corresponds to a collection of color points in the color space, which collection will be indicated as a path. Dynamically changing the colors can then be indicated as “traveling” such path. More in general, dynamically changing the colors of lighting will be indicated as “navigating” through the color space. 
     Typically, an illumination system comprises three lamps. Usually, these lamps are close-to-red (R), close-to-green (G), close-to-blue (B), and the system is indicated as an RGB system. For each lamp, the light intensity can be represented as a number from 0 (no light) to 1 (maximum intensity). A color point can be represented by three-dimensional coordinates (ξ 1 , ξ 2 , ξ 3 ), each coordinate in a range from 0 to 1 corresponding in a linear manner to the relative intensity of one of the lamps. The color points of the individual lamps can be represented as (1,0,0), (0,1,0), (0,0,1), respectively. These points describe a triangle in the color space. All colors within this triangle can be generated by the system. 
     It is desirable to have a color navigation system, allowing a user to navigate through the color space in a comfortable and intuitive way. Specifically, it is desirable that a color navigation system allows the user to take such steps in the color space such that the perceived color change is constant. 
     In theory, the color space can be considered as being a continuum. This will allow a user to select every possible color within the above-mentioned triangle. However, calculating the stepsize in a certain step direction such that the perceived color change has a certain value requires the use of rather powerful and costly microprocessors. Therefore, it is more advantageous for a color navigation system to have a color table, comprising predefined color points. Navigating through color space then translates to making steps from one color point in the table to the next color point in the table. Navigation can then be performed easily under the control of a simple user interface. An example of such simple user interface comprises six buttons, two buttons (step-up and step-down, respectively) for each color coordinate. Actuating one of these buttons will result in a step along the corresponding color coordinate axis, the step resulting in a predefined color perception difference. 
     A problem in this respect is that the RGB color space is not a linear space, so that, when taking a discrete step of a certain size along one of the color intensity coordinate axes, the amount of color change perceived by the user is not constant but depends on the actual position within the color space. 
     In order to solve this problem, different representations of the color space have been proposed, such as the CIELAB color space, where the independent variables are hue (H), saturation (S; in CIELAB calculated with S=Chroma/Lightness), brightness (B; in CIELAB calculated from Lightness). Because of the perceptual uniformity of Lightness (i.e. a linear change of Lightness level is also perceived as a linear change of light intensity level by the user), it is advantageous to use this parameter instead of Brightness. However, to generalize the description the parameter “Brightness” will be used in the explanation next, which values are also described with a perceptual uniform distribution (e.g. in u′V′Y space, with “Y” describing intensity, perceptual uniform Brightness distribution is logarithm(Y)). The CIELAB color space can be seen as a three-dimensional space of discrete points (3D grid). Each point in this space can be represented by coordinates m, n, p, and in each point the hue (H), saturation (S), Brightness (B) have specific values H(m,n,p), S(m,n,p), B(m,n,p), respectively. A user can take a discrete step along any of the three coordinate axes, resulting in predefined and constant changes in hue, saturation or Brightness, respectively, as long as the color is inside the outer boundary of the color space (color gamut). In principle, the variables hue, saturation and Brightness are independent from each other. 
     A problem is now to define a good navigation algorithm, defining navigation steps such that the perceived color change is constant, in order to build a suitable table or in order to step through an existing table. According to the CIELAB theory, when a step is made from a first color point with Chroma value C 1  and hue angle h 1  to a second color point with Chroma value C 2  and hue angle h 2 , the color difference ΔE is defined as
 
Δ E =√{square root over (Δ L   2   +ΔC   2   +ΔH   2 )}
 
in which ΔH is the metric Hue difference, defined as ΔH=  C ·Δh
 
with  C  being the average of C 1  and C 2  and with Δh=h 2 −h 1 .
 
     Although the above formula is precise according to theory, it appears not to be precise in people&#39;s actual perception. Thus, there is a need for an improved color difference formula that is better adapted to personal perception. 
     In the article “Comparative Analysis of the Quantization of Color Spaces on the Basis of the CIELAB Color-difference Formula” in ACM transactions on Graphics, Vol. 16, nr.2, April 1997, p. 109-154, B. Hill et al describe an improved color difference formula: 
               Δ   ⁢           ⁢     E   94   ⋆       =           (       Δ   ⁢           ⁢     L   ⋆           k   L     ⁢     S   L         )     2     +       (       Δ   ⁢           ⁢     C   ab   ⋆           k   C     ⁢     S   C         )     2     +       (       Δ   ⁢           ⁢     H   ab   ⋆           k   H     ⁢     S   H         )     2               
where ΔL*, ΔC* ab , and ΔH* ab  are the CIELAB 1976 color differences of lightness, chroma, and hue, respectively;
 
where k L , k C , and k H  are factors to match the perception of background conditions;
 
and where S L , S C , and S H  are linear functions of C* ab .
 
     According to the article, standard values have been assumed as follows:
 
k L =k C =k H =1
 
S L =1
 
 S   C =1+0.045 ·C*   ab  
 
 S   H =1+0.015 ·C*   ab  
 
     However, experiments of the present inventor have shown that the above formula does not give satisfying results. Thus, a first objective of the present invention is to provide a color difference formula giving more satisfying results in practical experiments. 
     A further aspect of the problem relates to the fact that the boundary of the color space is usually not a circle but has the general shape of a triangle with curved sides. In the case of saturated LED primaries, the blue primary creates a long elongated corner in the triangle, and the RED-GREEN side is relatively short. When a user navigates along this boundary in the hue direction, taking steps such that the perceived color difference ΔE in accordance with the above-mentioned formula is maintained constant, he will find that there are far more steps in the cyan-blue-magenta region than in the red-green region. This is not an attractive situation. This situation is caused by the fact that, when navigating along the boundary of the color space, it is not possible to keep both the lightness and the chroma constant, and differences in lightness and chroma are considered to contribute to perceived color differences. When navigating close to blue, differences in chroma are relatively large so that, when the perceived color difference is maintained constant, the steps in hue are relatively small. 
     Thus, it is a further objective of the present invention to provide a solution for this problem as well. 
     More particularly, the present invention aims to provide a method for calculating color steps resulting in a good compromise between the desire to have the steps being equidistant in perceived color differences on the one hand, and the desire to have substantially comparable numbers of color steps along the boundary of the color space on the other hand. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, a color difference formula is defined as follows:
 
Δ E   INV =√{square root over ((Δ L ) 2 +( f   C ( H )·Δ C ) 2 +(Δ H ) 2 )}{square root over ((Δ L ) 2 +( f   C ( H )·Δ C ) 2 +(Δ H ) 2 )}{square root over ((Δ L ) 2 +( f   C ( H )·Δ C ) 2 +(Δ H ) 2 )}
 
wherein ΔE INV  indicates the color difference according to the present invention,
 
and wherein f C (H) is a factor between 0 and 1 which only depends on hue H. This factor is suitably chosen such as to mitigate the effect of chroma on the perceived color difference.
 
     According to a second aspect of the present invention, the color space is divided in three sectors RED-GREEN, GREEN-BLUE, and BLUE-RED, and values for f C (H) are defined such that the numbers of color points along the color space boundary in the hue direction for said three sectors are mutually equal, or at least do not differ more than 50%. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects, features and advantages of the present invention will be further explained by the following description with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which: 
         FIG. 1  schematically shows a chromaticity diagram; 
         FIG. 2  schematically shows a CIELAB plot; 
         FIG. 3  is a block diagram schematically illustrating a color navigation system. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  schematically shows a CIE(xy) chromaticity diagram. This diagram is well-known, therefore an explanation will be kept to a minimum. Points (1,0), (0,0), and (0,1) indicate ideal red, blue and green, respectively, which are virtual colors. The curved line  1  represents the pure spectral colors. Wavelengths are indicated in nanometers (nm). A dashed line  2  connects the ends of the curved line  1 . The area  3  enclosed by the curved line  1  and dashed line  2  contains all visible colors; in contrast to the pure spectral colors of the curved line  1 , the colors of the area  3  are mixed colors, which can be obtained by mixing two or more pure spectral colors. Conversely, each visible color can be represented by coordinates in the chromaticity diagram; a point in the chromaticity diagram will be indicated as a “color point”. 
     It is noted that a different graphical color representation, for instance the RGB diagram, may also be used, as should be clear to a person skilled in this art. However, the distribution of colors in the RGB space is completely device-dependent (e.g. a certain RGB value will in general give different perceived colors with different lamps that each have different RGB primary colors). 
     Preferably, the colors are represented in a device-independent color space, like the CIELAB color space, also referred to as the L*a*b* color space.  FIG. 2  is a diagram illustrating the CIELAB space. The CIELAB space is preferred due to its perceptual uniformity; however, as mentioned above, it can clearly be seen that the CIELAB curve is distorted in comparison to the CIE(xy) diagram, with quite a large blue “tail”. 
     Since the color definitions associated with these color spaces are known to persons skilled in this art, an extensive explanation will be omitted here. It suffices to mention that these spaces have hue (this will be explained hereinafter), saturation (this will be explained hereinafter), and brightness (a measure for the overall light intensity) as independent variables, and that a color representation in RGB space can be converted to a color representation in CIELAB color space, or vice versa, via one-to-one matrix transformations. 
     The basic concepts of Hue, Saturation and Brightness are most easily explained in the CIE 1931 (x,y) color space, although in other color space other definitions can be obtained. For simplicity, we use CIE 1931 (x,y) color space next. 
     When two pure spectral colors are mixed, the color point of the resulting mixed color is located on a line connecting the color points of the two pure colors, the exact location of the resulting color point depending on the mixing ratio (intensity ratio). For instance, when violet and red are mixed, the color point of the resulting mixed color purple is located on the dashed line  2 . Two colors are called “complementary colors” if they can mix to produce white light. For instance,  FIG. 1  shows a line  4  connecting blue (480 nm) and yellow (580 nm), which line crosses a white point, indicating that a correct intensity ratio of blue light and yellow light will be perceived as white light. The same would apply for any other set of complementary colors: in the case of the corresponding correct intensity ratio, the light mixture will be perceived as white light. It is noted that the light mixture actually still contains two spectral contributions at different wavelengths. 
     If the light intensity of two complementary colors (lamps) is indicated as I 1  and I 2 , respectively, the overall intensity Itot of the mixed light will be defined by I 1 +I 2 , while the resulting color will be defined by the ratio I 1 /I 2 . For instance, assume that the first color is blue at intensity I 1  and the second color is yellow at intensity I 2 . If I 2 =0, the resulting color is pure blue, and the resulting color point is located on the curved line  1 . If I 2  is increased, the color point travels the line  4  towards the white point. As long as the color point is located between pure blue and white, the corresponding color is still perceived as blue-ish, but closer to the white point the resulting color would be paler. 
     In the following, the word “color” will be used for the actual color in the area  3 , in association with the phrase “color point”. The “impression” of a color will be indicated by the word “hue”; in the above example, the hue would be blue. It is noted that the hue is associated with the spectral colors of the curved line  1 ; for each color point, the corresponding hue can be found by projecting this color point onto the curved line  1  along a line crossing the white point. 
     Further, the fact whether a color is a more or less pale hue will be expressed by the phrase “saturation”. If a color point is located on the curve  1 , the corresponding color is a pure spectral color, also indicated as a fully saturated hue (saturation=1). As the color point travels towards the white point, the saturation decreases (less saturated hue or paler hue); in the white point, the saturation is zero, per definition. 
     It is noted that many visible colors can be obtained by mixing two colors, but this does not apply for all colors, as can easily be seen from  FIG. 1 . In order to be able to produce light having any desired color, three lamps producing three different colors are needed. More lamps may be used, but that is not necessary. 
       FIG. 3  schematically shows a block diagram of an illumination system  10 , comprising a lamp assembly  14 . The lamp assembly  14  comprises a plurality (here: three) of lamps  12 A,  12 B,  12 C, each with an associated lamp driver  13 A,  13 B,  13 C, respectively, controlled by a common controller  15 . A user input device is indicated at  19 . The three lamps  12 A,  12 B,  12 C generate light  16 A,  16 B,  16 C, respectively, with mutually different light colors; typical colors used are red (R), green (G), blue (B). Instead of pure red, green and blue, the lamps will typically emit light close-to-red, close-to-green and close-to-blue, as indicated by three exemplary color points C 1 , C 2 , C 3  in  FIG. 1 , respectively. The overall light emitted by the lamp assembly  14  is indicated at  17 ; this overall light  17 , which is a mixture of individual lights  16 A,  16 B,  16 C, has a color point within the triangle defined by corner points C 1 , C 2 , C 3 . With the system  10 , it is possible to set the mixture color of the output light mixture  17  at any desired location within said triangle, if it is possible to change the light intensities of the individual lamps  12 A,  12 B,  12 C continuously. Typically, however, the controller  15  is a digital controller, and the light intensities of the individual lamps  12 A,  12 B,  12 C can only be changed with discrete steps. In such case, the attainable color points are located along a grid in the color space. As regards color representation, CIELAB color space is preferred, because the distance between two neighboring grid points corresponds to substantially equal differences in perceived color over the entire CIELAB color space. 
     In the CIELAB color space, the hue, saturation and brightness can be varied independently from each other, as long as the color is inside the color space boundary. In the present invention, we use linear axes for Hue, Saturation and Lightness; these linear axes span the color space with cylindrical coordinates. Further, each axis is discretized, i.e. it is only possible to take discrete steps along each axis, respectively. Those steps are chosen such that corresponding steps in perceived color ΔE INV  are uniform. Here, the perceived color step is defined according to the following formula:
 
Δ E   INV =√{square root over ((Δ L ) 2 +( f   C ( H )·Δ C ) 2 +(Δ H ) 2 )}{square root over ((Δ L ) 2 +( f   C ( H )·Δ C ) 2 +(Δ H ) 2 )}{square root over ((Δ L ) 2 +( f   C ( H )·Δ C ) 2 +(Δ H ) 2 )}  (1)
 
wherein f C (H) is a proportionality factor between 0 and 1 which only depends on hue H, as will be described later. Thus, the color space is filled with a grid of color points, wherein the distance between neighboring color points in the L-direction is indicated as ΔL, the distance between neighboring color points in the C-direction is indicated as ΔC, and the distance between neighboring color points in the H-direction is indicated as ΔH. It is noted that C indicates chroma, defined as C=S·L, S indicating saturation and L indicating lightness.
 
     According to the invention proposal, the distribution of lightness levels is equidistant. Particularly, the lightness L can be varied from a minimum value Lmin (usually taken larger than zero) to a maximum value Lmax in equidistant steps. The number of possible lightness levels will be indicated by N L . Then, using a “lightness index” p, the N L  possible values of lightness L(p) can be expressed according to the following formula:
 
 L ( p )= L min+ p*ΔL   (2)
 
wherein index p is an integer from 0 to N L −1.
 
     It can easily be seen that ΔL=(Lmax−Lmin)/(N L −1). When stepping in the L-direction, ΔE INV =ΔL applies. It is noted that a linear increase of lightness is also perceived by human observers as a linear increase of brightness. 
     Further according to the invention proposal, the distribution of saturation levels is equidistant. Particularly, the saturation S can be varied from a minimum value Smin (usually equal to zero) to a maximum value Smax (usually equal to one) in equidistant steps. The number of possible saturation levels will be indicated by N S . Then, using a “saturation index” n, the N S  possible values of saturation S(n) can be expressed according to the following formula:
 
 S ( n )= S min+ n*ΔS   (3)
 
wherein index n is an integer from 0 to N S −1.
 
     It can easily be seen that ΔS=(Smax−Smin)/(N S −1). 
     In view of the fact that C=S·L, when stepping in the S-direction, ΔH=ΔL=0, therefore 
     ΔE INV =f C (H)·ΔC=f C (H)·ΔS·L applies. Thus, the perceived color differences are constant when stepping in the S-direction, but the size of these differences does depend on H and L. 
     Regarding the hue difference between two color points having chroma values C 1  and C 2 , respectively, and having hue angles h 1  and h 2 , respectively, it is noted that, in CIELAB, the metric Hue difference ΔH is used, defined along a hue circle around the color space boundary with:
 
Δ H=  C *Δh*  
 
with  C * being the arithmetic mean of C 1  and C 2  and with Δh=h 1 −h 2 . (H 2 −H 1 ) is the metric length of the hue circle segment along the color space boundary (which is calculated as the sum of all ΔH differences between sequential colors along the boundary).
 
     In prior art, the hue H is discretized over the entire color gamut from a suitably chosen minimum value Hmin to a suitably chosen maximum value Hmax in equidistant steps. As explained earlier, when navigating along the border of the color space taking steps of size ΔE, this results in a relatively large number of steps in the BLUE range (i.e. the difference in H between subsequent steps is relatively small in the BLUE range) and a relatively low number of steps in the range RED-GREEN. This is recognized by the present inventor as being caused by the fact that the chroma C is given the same weight over the entire color space. According to the present invention, discretization is done in a different way such as to reduce this problem. According to the present invention, the color space is divided in three sectors  44 ,  45 ,  46 , as illustrated in  FIG. 2 , and the weight of the chroma C is set separately in these three sectors  44 ,  45 ,  46 . In  FIG. 2 , a white point is indicated W. A line  41  of constant red hue connects white point W with a red color point; the corresponding hue is indicated as H 1 . A line  42  of constant green hue connects white point W with a green color point; the corresponding hue is indicated as H 2 . A line  43  of constant blue hue connects white point W with a blue color point; the corresponding hue is indicated as H 3 . A first color sector  44 , extending from RED to GREEN and therefore indicated as the RED/GREEN sector, is defined between lines  41  and  42 . A second color sector  45 , extending from GREEN to BLUE and therefore indicated as the GREEN/BLUE sector, is defined between lines  42  and  43 . A third color sector  46 , extending from BLUE to RED and therefore indicated as the BLUE/RED sector, is defined between lines  43  and  41 . The borderline of the color space is indicated as  47 . 
     To define the Hue distribution, the following method is used. The lightness L is set at a certain level, preferably the lightness value for which Cmax is found, with Cmax being the maximum value of chroma C; this lightness value will be indicated as L(Cmax). While keeping lightness L constant, i.e. ΔL=0, the borderline  47  is navigated with steps of size ΔE INV  being constant. It should be clear that, with each step, chroma and hue change according to the following formula derived from formula 1:
 
Δ E   INV =√{square root over (( f   C ( H )·Δ C ) 2 +(Δ H ) 2 )}{square root over (( f   C ( H )·Δ C ) 2 +(Δ H ) 2 )}  (4)
 
     With each such step, a color point is reached having a certain Hue value H(m), m being a hue index. These values H(m) will be used as the discretized values of the hue axis, i.e. the hue coordinates of the grid of color points. 
     In the RED/GREEN sector  44 , the proportionality factor f C (H) is set to be equal to 1, and the resulting number of steps from H 1  (red) to H 2  (green) will be indicated by N H,RG . 
     In the GREEN/BLUE sector  45 , the proportionality factor f C (H) is set to a constant value f GB , and the resulting number of steps from H 2  (green) to H 3  (blue) will be indicated by N H,GB (f GB ), indicating that this number of steps depends on f GB . 
     In the BLUE/RED sector  46 , the proportionality factor f C (H) is set to a constant value f BR , and the resulting number of steps from H 3  (blue) to H 1  (red) will be indicated by N H,BR (f BR ), indicating that this number of steps depends on f BR . 
     In the prior art, f GB  and f BR  are equal to 1, but this has the problem that it would result in large differences between N H,RG , N H,GB  and N H,BR . According to the invention, this problem is solved by selecting f GB  and f BR  smaller than 1, ideally such that N H,RG =N H,GB (f GB )=N H,BR (f BR ), but in any case such that N H,GB (f GB ) and N H,BR (f BR ) differ less than 50% from N H,RG . In a practical experiment, values f GB =f BR =0.5 proved to be satisfactory. 
     Thus, the complete trajectory of the borderline is divided into N H,RG +N H,GB (f GB )+N H,BR (f BR ) steps. In other words, the number of possible hue levels, indicated as N H , is equal to N H,RG +N H,GB (f GB )+N H,BR (f BR ). Those levels, which can be indicated by a hue index m, are not equidistant, but have mutual distances ΔH(H), i.e. the size of the steps ΔH depends on the actual value of the hue, in such a way that, when stepping from one hue level to the next along the borderline  47 , the above formula 4 is obeyed. 
     Thus, the invention describes a particularly advantageous way of discretizing the color space. Navigation through the colors that are generated by this discretization results in approximately perceptual equidistant color steps ΔE INV  along the axes of Saturation and Brightness as long as the colors are inside the color space boundary, and in approximately perceptual equidistant color steps ΔE INV  when stepping in the hue direction along the color space boundary. Thus, the distribution is not equidistant for hue within the color space boundary, but results in a better distribution along the color space boundary. A further advantage is that the overall number of steps in the hue direction can be smaller, thus the required memory space for the color table can be reduced. 
     It should be noted that, when using the CIELAB color space, a certain reference white point has to be chosen. The preferred choice, based on the experience of the inventor, is to choose a white point on the blackbody line (described with (x_ref,y_ref) in CIE1931 space) with the same correlated color temperature (CCT) as the RGB luminaire with RGB=[1 1 1], i.e. the luminaire at full light output. Alternatively, an arbitary white point with a CCT between 2500 [K] and 6500 [K] can be used. 
     From the above, it follows that points in the color space can be defined by indices m, n, p, and the color in those points can be considered as being a function of 3 independent parameters m, n, p.  FIG. 3  illustrates that the user input device  19  allows the user to independently select values for m, n and p; particularly, the user is allowed to take steps along the axes of Lightness, Saturation and Hue. The user input device  19  is shown as a combination of three independent input devices  19 H,  195  and  19 B, independently providing input values m, n, p for the controller  15 . On the basis of those input values m, n, p, the controller generates a set of control signals (ξ 1 , ξ 2 , ξ 3 ) for the drivers  13 A,  13 B,  13 C of the lamp assembly  14 . 
     The controller  15  is provided with a memory  18  which contains tables for hue, saturation and brightness, respectively. Using the user instructions received from user interface  19 , the controller  15  takes values for hue, saturation and brightness from said memory  18  and generates its control signals (ξ 1 , ξ 2 , ξ 3 ) on the basis of these values. 
     The present invention provides two possible implementations. In a first implementation, the tables in the memory  18  are filled using the above-described formulas. In that case, navigating through the color space on the basis of the tables from memory  18  always results in perceptually uniform color steps. Such implementation does not require any specific adaptation of a microprocessor: when taking a step from one color point to the next, the controller  15  simply takes the next information from the table. Thus, the invention is embodied in the contents of the table in the memory. 
     In a second implementation, the table in the memory  18  may be filled in an arbitrary way (but with Hue values increasing monotonically with increasing index ‘m’, Saturation values increasing monotonically with increasing index ‘n’, and Lightness values increasing monotonically with increasing index ‘p’), and the invention is embodied in the controller  15 . The user is allowed to enter a navigation command defining a certain color step of size ΔE INV  in a certain direction. The controller  15 , on receiving the navigation command from the user, calculates the corresponding values of ΔL, ΔC, ΔH, and then looks in the table for the color point closest to color point defined by the calculated values of ΔL, ΔC, ΔH. 
     It should be clear to a person skilled in the art that the present invention is not limited to the exemplary embodiments discussed above, but that several variations and modifications are possible within the protective scope of the invention as defined in the appending claims. 
     For instance, in the example the color table  18  is associated with the controller  15 . It is, however, also possible that the color table  18  is associated with the user interface  19 . 
     In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, where the function of such functional block is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such functional block is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, digital signal processor, etc.