WO2004073316A1

WO2004073316A1 - White point setting

Info

Publication number: WO2004073316A1
Application number: PCT/IB2004/050073
Authority: WO
Inventors: Vedran Kovacevic
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-02-17
Filing date: 2004-02-04
Publication date: 2004-08-26

Abstract

A white color adjustment circuit in a display apparatus (8) comprises a calculation unit (1) which calculates a desired gain (Gd) of a video amplifier (A) based on: information (IDW) on a desired white color (DW), information (IRW) on a reference white color (RW) and a reference gain (Gr) of the video amplifier (A) required for obtaining the reference white color (RW). The information (IRW) on the reference white color (RW) which may comprise chromaticity coordinates or the color temperature of the reference white color (RW), and the reference gain (Gr) may be stored in a non-volatile memory during the manufacturing of the display apparatus (8). The desired white color is preferably inputted by a user.

Description

White point setting

The invention relates to a white point adjustment circuit for setting a white point of a display device, a display apparatus comprising such a white point adjustment circuit, and a method of white point adjustment for setting a white point of a display device.

US 5,512,961 discloses a system and a method to achieve and maintain an accurate white point setting of a CRT display in a computer system. The CRT is initially calibrated by individually driving the individual color cathodes and by measuring tristimulus values and cathode beam current for each of the three primary colors. The tristimulus values are normalized by dividing each value by the beam current producing it, and the normalized values are then stored in a calibration memory contained in the display unit. A table of gamma values representing beam current as a function of video drive voltage is measured and also added to the calibration memory. Calibration of the display is implemented by driving the display controller with video drive voltages corresponding to a particular white point, and by calculating the cathode beam currents required to obtain the particular white point from the stored tristimulus values. Once the beam currents are calculated for the desired particular white point, a video signal is generated which has a defined luminance. The individual beam currents used to drive the Red, Green, and Blue cathodes of the CRT are sensed by current samplers and digitized by an analog to digital converter. The gain of the video amplifiers is adjusted by a control signal to achieve the beam current values calculated. When the desired beam currents are obtained, the desired tristimulus values (brightness) for each primary cathode (Red, Green, and Blue) will be obtained and the particular white point is displayed. It is a drawback of the prior art that the beam current has to be sampled.

It is an object of the invention to provide a white point adjustment in a display apparatus in which a desired white point is set without requiring sampling the beam current.

A first aspect of the invention provides a white point adjustment circuit for setting a white point of a display device as claimed in claim 1. A second aspect of the invention provides a display apparatus comprising such a white point adjustment circuit as claimed in claim 15. A third aspect of the invention provides a method of white point adjustment for setting a white point of a display device as claimed in claim 17. Advantageous embodiments of the invention are defined in the dependent claims. The white point adjustment circuit in accordance with the first aspect of the invention comprises a calculation unit which calculates the desired gain of the video amplifier based on information on the desired white point, and information on the reference white point and the reference gain of the video amplifier required for obtaining the reference white point. The white points are also referred to as white colors. In an embodiment in accordance with the invention as defined in claim 2, the information on the desired white color may comprise the chromaticity coordinates of the desired white color, and the information on the reference white color may comprise the chromaticity coordinates of the reference white color. The information on the desired white color may also comprise the color temperature of the desired white color, the chromaticity coordinates can be calculated from this color temperature. In the same manner, the information on the reference white color may comprise the color temperature of the reference white color. Instead of the chromaticity coordinates it is also possible to use a linear transformation on it. For example, a scaling with respect to one of the primary colors (for example Green) may be used, in particular when the gain of this one of the primary colors is kept constant.

The chromaticity coordinates or the color temperature of the reference white color, and the reference gain may be stored in a non- volatile memory during the manufacturing of the display apparatus.

Usually, the display apparatus has three video amplifier channels to drive the three primary colors (Red, Green, and Blue) of the display device. At least two of the three video amplifier channels have a controllable gain, the third video amplifier channel may have a fixed gain.

The color temperature of the reference white point and the reference gains for the two or three video amplifier channels are known. The reference gains are the gains required to obtain the reference white color. The desired gains which are required to obtain the desired white color can be directly calculated from the difference of the color temperature of the desired white color and the color temperature of the reference white color, because it is known what the drive signals should be for the reference white color and for the desired white color. Instead of the color temperatures it is also possible to use the color coordinates which are either available or can be calculated from the color temperatures.

In an embodiment of the invention as defined in claim 3, the reference gain is stored in a memory of the calculation unit. Preferably, this reference gain is detennined during the manufacturing of the display apparatus. For example, the white color of the light emitted by the display device is measured while the gains of the video amplifier channels are controlled until the required reference white point (or white color) is reached. The gains found are the reference gains of the reference white color for the different primary colors of the display. In an embodiment of the invention as defined in claim 16, the display apparatus is a color cathode ray tube (further referred to as a color CRT). In a color CRT, three beam currents corresponding to three primary colored phosphors are controlled with corresponding electron guns. The gain of all the three video amplifier channels (one for controlling a corresponding one of the beam currents) may be controlled, or the gain of one of the three video amplifiers is fixed and the gain of the two other video amplifiers is controlled. Usually, the beam currents generated by the electron guns are controlled at control electrodes of the CRT which usually are the cathodes. But, it is as well possible to control the beam current of the guns of the CRT at another suitable electrode such as the first or second grid. In the CRT, the drive signals which define the white points are the beam currents.

If the display device is not a CRT, the control electrode of the electron gun is replaced by another suitable control electrode or sets of drive electrodes. For example, for a liquid crystal display (further referred to as LCD) the control electrodes may comprise column electrodes on which the data in accordance with the picture to be displayed is provided to the pixels of the LCD.

In an embodiment of the invention as defined in claim 4, the customer inputs the desired white point to the calculation unit, preferably via a graphical interface.

In an embodiment of the invention as defined in claim 5, the calculation unit comprises a processor unit and a memory. The processor unit processes a computer program which is stored in the memory. The program comprises an algorithm which calculates the desired value of the gain from: the information on the desired white point, the information on the reference white point, and the reference gain of the video amplifier required to obtain the reference white point. The calculation unit retrieves the reference value of the gain from a memory. The reference value was stored in the memory during the manufacturing of the display apparatus.

In an embodiment of the invention as defined in claim 6, the calculation unit receives the desired white color via an input. Usually, the user provides the desired white color to the display apparatus. The desired white color may be inputted directly to the display apparatus or via a computer if the display apparatus is a computer monitor.

In an embodiment as defined in claim 7, the desired gain is calculated as

Gd = (Jd/Jr)^1/γGr wherein Gd is the desired gain of one of the video amplifier channels which controls the amount of light of one of the primary colors of the display, Jd is the desired chromaticity coordinate for this one of the primary colors, Jr is the reference chromaticity coordinate for this primary color corresponding to the reference white color, γ the gamma of the electron gun, and Gr the reference gain required to obtain the correct output signal to display the reference white color. The term Jd/Jr may also indicate a scaled chromaticity coordinate. For example, if the gain of the primary color Green is kept constant, Jd/Jr may be read as Jd/Jr * Gr/Gd, wherein Gr is the reference green chromaticity coordinate and Gd is the desired green chromaticity coordinate.

In an embodiment of the invention as defined in claim 8, the video amplifier comprises a register to store a gain value. The gain value in the register determines the gain of the video amplifier, or said differently, the gain of the video amplifier is a function of the gain value in the register. For example, for the video preamplifier IC (integrated circuit) TDA4887 of Philips Semiconductors, this function is defined by:

G = f (GV) = (80 GV) /255 + 20 wherein G is the gain, ranging from 20 to 100, GV is the gain value which is an integer value in the range from 0 to 255, and f is the function. The gain value is determined by the inverse function:

GV = f^nv (G) wherein ^nv is the inverse function of the function f. In an embodiment of the invention as defined in claim 9, the desired gain value for the desired white point is calculated as:

GVd = f^nv ( f (GVr) (Jd/Jr) ^{1 γ} ) wherein GVd is the desired gain value, GVr is the reference gain value for the reference white point, Jd is the desired chromaticity coordinate for the primary color for which the video amplifier is operating, Jr the reference chromaticity coordinate for this primary color corresponding to the reference white color, and γ the gamma of the electron gun.

In the embodiments in accordance with the invention as defined in claims 7 to 9, a single reference gain value of a single reference white point suffices to directly calculate the correct gain or gain value required to obtain the desired white color.

However, the desired white color is only calculated accurately if the black level of the video signal is correctly aligned with respect to the cut-off level of the display device. Such an alignment may be performed during the manufacturing of the display apparatus. The black level of the video signal is the drive voltage which corresponds to video information for which no light should be produced by the display device. The cut-off level of the display device is the actual drive voltage required such that just no light is produced by the display device. The calculated desired white color will deviate from the intended desired white color if the black level of the video drive signal and the cut-off level of the display device are different. In an embodiment of the invention as defined in claim 10, the calculation unit firstly determines an offset between a black level of the video drive signal and a cut-off level of the display and secondly the desired value of the gain taking this offset into account.

The offset between the black level of the video drive signal and the cut-off level of the display device is calculated based on: information on a first reference white point and a first reference value of the gain required to obtain the first reference white point, and information on a second reference white point and a second reference value of the gain required to obtain the second reference white point. By using the two reference white points and their associated reference gains, it is possible to determine the offset for the corresponding video amplifier channel. Once the offset is determined, the desired value of the gain is calculated based on: the information defining the desired white point, the information on either: the first white point and the first reference value, or the second reference white point and second reference value, and the offset.

As now first the offset is determined, and then the desired white point is calculated taking the offset into account, the correct desired white point will be calculated even if an offset exists between the black level and the cut-off level.

In an embodiment in accordance with the invention as claimed in claim 11, the calculation unit determines the offset from either the equation: Grl±dV = (Jrl/Jr2)^{1 γ}(Gr2±dV) or Gr2±dV = (Jr2/Jrl)^{1 γ}(Grl±dV) wherein Jrl is a first reference chromaticity coordinate for a primary color corresponding to the video drive signal, and corresponding to the first reference white point, Jr2 is a second reference chromaticity coordinate corresponding to the second reference white color, γ is a gamma of the display device, Grl is the first reference value of the gain, Gr2 is the second reference value of the gain, and dV is the offset which is defined as the cut-off level minus the black level (the - sign) or the black level minus the cut-off level (the + sign).

In an embodiment in accordance with the invention as claimed in claim 12, the calculation unit determines the desired gain Gd as Gd = (Jd/Jrl)^{1 γ} Grl or

Gd = (Jd/Jr2)^1/γGr2 wherein Jd is a desired chromaticity coordinate for the desired white color for a primary color corresponding to the video drive signal.

In an embodiment in accordance with the invention as claimed in claim 13, or 14, instead of the gains, register values are used which determine the gains. Therefore, both the offset and the desired gains required for the desired white point are calculated by using the gain values in the registers and by introducing a function which defines the gain of the video amplifier channel as a funtion of the gain value in the register. The inverse function defines the gain value in the register as a function of the gain of the video amplifier channel. These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

In the drawings: Fig. 1 shows a prior art CRT video drive circuit with a white color adjustment circuit,

Fig. 2 shows the known CIE representation of the color space,

Fig. 3 shows a single video amplifier channel of an embodiment in accordance with the invention, Fig. 4 shows a full video amplifier circuit of an embodiment in accordance with the invention,

Fig. 5 shows a graph for elucidating a situation wherein the black level of the video drive signal and the cut-off level of the display device differ, and Fig. 6 shows an embodiment of an alignment set-up used during manufacturing of the display apparatus.

The same references in different Figs, refer to the same signals or to the same elements performing the same function.

Fig. 1 shows a prior art CRT video drive circuit with a white color adjustment circuit. The video processor 2 receives an input signal Vi and supplies three input video signals Ri, Gi, Bi to three amplifiers AR, AG, AB, respectively. The amplifiers AR, AG, AB supply the video drive signals Ro, Go, Bo, to three electron guns (not shown) of the CRT (cathode ray tube) 80, respectively. The video drive signals Ro, Go, Bo control the beam currents IR, IG, IB produced by the electron guns. Usually, the drive signals Ro, Go, Bo are supplied to cathodes of the electron guns. The CRT is constructed in the usual manner to direct the beam currents IR, IG, IB to the red, green, blue phosphors, respectively. A color reproduced by the display device depends on the ratio of the beam currents IR, IG, IB.

The prior art video drive circuit further comprises a black level stabilization circuit which comprises beam current measurement circuits 4 arranged between the outputs of the amplifiers AR, AG, AB and the electron guns, voltage samplers 5, an analog to digital converter 6, a CPU (central processing unit) 1, and a digital to analog converter 9. The CPU supplies a test signal TS to the video processor 2. This test signal TS will be supplied to the electron guns as the video drive signals Ro, Go, Bo. The voltage samplers 5 sample voltages which represent the measured beam currents IR, IG, IB occurring in response to the video drive signals Ro, Go, Bo caused by the test signal TS. The analog to digital converter 6 converts the analog sample voltages into digital sample values which are supplied to the CPU 1. The CPU 1 evaluates the digital sample values and controls the VG2 voltage at the input 10 of the CRT 80 and thus the electron guns via the digital to analog converter 9. In this manner, the black level of the video drive signals Ro, Go, Bo is controlled to substantially coincident with the cut-off level of the electron guns. Consequently, a level in the video drive signals Ro, Go, Bo which should be represented as black (the black level) on the CRT 80 is indeed represented as black, while a level of the video drive signals Ro, Go, Bo just above the black level will cause the CRT 80 to emit some light.

The prior art video drive circuit is able to achieve and maintain an accurate white point setting of a CRT display in a computer system. The CRT is during manufacturing calibrated by supplying the video drive signals Ro, Go, Bo to the cathodes to obtain the beam currents IR, IG, IB for the different primary colors, and by measuring both the tristimulus values (the brightness of the three primary colors on the display screen of the CRT 80, each separately) with a spectra-radiometer, and the beam current IR, IG, IB for each of the three primary colors. The tristimulus values are normalized by dividing each value by the corresponding beam current IR, IG, IB producing it, and the normalized values are then stored in a calibration memory (not shown, part of the CPU 1). A table of gamma values representing the beam current IR, IG, IB as a function of the video drive signals Ro, Go, Bo, respectively, is measured and also added to the calibration memory. A particular desired white color of the display apparatus is implemented by calculating the beam currents IR, IG, IB required to obtain the particular desired white color from the stored tristimulus values. Once the beam currents IR, IG, IB are calculated for the desired particular white color, the video drive signals Ro, Go, Bo are generated with a defined level. Then, the individual beam currents IR, IG, IB occurring at these video drive signals Ro, Go, Bo are sensed by the current samplers 4 and are digitized by the analog to digital converter 6. The CPU 1 adjusts the gain of the video amplifiers AR, AG, AB with a control signal CS. Again the beam currents IR, IG, IB are measured and compared with the desired values. The gain of the video amplifiers AR, AG, AB is adapted until the desired calculated beam current values IR, IG, IB are reached. When the desired beam currents IR, IG, IB are reached, the desired tristimulus values for each primary cathode (Red, Green, Blue) will be obtained and the pictures are displayed with the particular desired white color.

The lines from the voltage samplers 5 to the amplifiers AR, AG, AB are not essential, but may provide a feedback.

Fig. 2 shows a representation of the CIE color space. According to Planck, the spectral power distribution radiated from a black body (a hot object) is a function of the temperature of the black body. Thus, said in other words, the color of the black body at a particular temperature can be unambiguously indicated by its color temperature.

Also, the color of the light radiated by a display device can unambiguously be characterized by its color temperature. This is in particular useful for the alignment of the white color displayed on the display device of the display apparatus. The white color of the display apparatus is the color which is displayed when a white object has to be reproduced. Some people prefer warmer white tints which are more reddish, other people prefer cooler white tints which are more bluish. That is why most display apparatus have an option to select the desired white color. For example, the display apparatus may be aligned during manufacturing to be able to display the video information with three white color temperatures of for example, 5500K, 6500K and 9300K, wherein K is degrees Kelvin. All the settings of the display apparatus to be able to display pictures with these three white color temperatures are stored in a memory of the display apparatus such that the required settings can be retrieved when the user requests a particular one of the three white colors. However, in such a setup, the available white colors are limited to three only.

In US 5,512,961 basic information is stored on the brightness of the primary colors at particular values of the beam current, and on the gamma of the CRT. If the user indicates that he wants a particular color to be the white color, the corresponding required beam currents at a predetermined test video signal are calculated and the gains of the video amplifiers are adapted as long as required to obtain the required beam currents. This is an iterative process.

Fig. 2 shows the y coordinate of the well known CIE color coordinate system along the vertical axis and the x coordinate of the CIE color coordinate system along the horizontal axis. The shaded area represents all natural colors. The display device 8 uses the primary colors R, G, B. For example, if the display device is a CRT these primary colors are defined by the colors of the light emitted by the corresponding phosphors. The primary colors are the corners of a triangle which is indicated by the white area within the gray area. This triangle and its inside area represent all the colors which can be represented by the display device 8 of Fig. 4. In the triangle area some white colors DW, RW1 or RW, RW2 are indicated. These white colors, also referred to as white points can be identified by unique color temperatures. The white points RW, RW1 and RW2 are reference white points which are pre-aligned and of which the relevant information is stored in the display apparatus. The white point DW is a desired white point as indicated by the user. The user may indicated the desired white point DW in any possible way, for example by inputting its color temperature, or by clicking in a graphical interface on the desired color. Dependent on the quality of the pre-alignment of the black level in the factory, one reference white point RW or two reference white points RW1 and RW2 are used to obtain the desired white point DW.

Fig. 3 shows a single video amplifier channel of an embodiment in accordance with the invention. An amplifier channel A comprises an amplifier AC and optionally a register RE. The amplifier AC receives an input video signal IV and supplies a video drive signal VD to a cathode CA of an electron gun GU of a CRT 80. The video drive signal VD is the input video signal IV amplified by the gain g of the amplifier AC. The electron gun GU further comprises several grids of which the first grid Gl is shown. The calculation unit 1, which may be a CPU, comprises a processor unit 11 and a memory 10. The reference information IRW defining the reference white point RW, and the value of the reference gain Gr are stored in the memory 10. The reference information IRW may be the color temperature of the reference white point RW, the CIE color coordinates (x and y of Fig. 2, plus z) of the reference white point RW, or normalized chromaticity coordinates. These normalized chromaticity coordinates have a value between zero and one. The CIE color coordinates and the normalized chromaticity coordinates are linked to each other via the color coordinates of the phosphors of the CRT. For example, if the phosphor colors of the CRT are defined by the CIE coordinates: phosphor color x y z red 0.62 0.35 0.03 green 0.29 0.61 0.1 blue 0.15 0.065 0.785 the normalized chromaticity coordinates and the CIE coordinates are linked by:

If in the claims is referred to chromaticity coordinates (for example, in claim 8 Jr), both the normalized chromaticity coordinates or the CIE coordinates may be used corrected by the CIE coordinates of the phosphors. The indices r and d indicate "reference" and "desired", respectively, while the primary colors of the phosphor are indicated by the indices re, gr and bl.

Because in Fig. 3 only one of the three video amplifier channels is shown, the elucidation of Fig. 3 is directed to one of video amplifier channels only. The operation of the other video amplifier channels is identical. In the explanation of Fig. 3, the indices re, gr, bl indicating the primary color of the phosphor to which the video amplifier channel is associated, are not mentioned, it is assumed that all items mentioned refer to the same primary color. The indices r of Gr indicates the reference value.

The reference information IRW on the reference white point RW may be stored in the memory 10 as reference normalized chromaticity coordinates or the CIE color coordinates, and the information IDW on the desired white point DW comprises the desired normalized chromaticity coordinates or the CIE color coordinates. It is also possible to store the color temperatures and to calculate the corresponding normalized chromaticity coordinates or the CIE color coordinates.

In an embodiment in accordance with the invention the register RE is not present and the gain g of the amplifier circuit AC is equal to the desired gain Gd calculated by the processor unit 11. The processor unit 11 calculates the desired gain Gd of the amplifier circuit AC required to obtain the desired white point DW based on the ratio of the desired normalized chromaticity coordinate Jd and the reference normalized chromaticity coordinates Jr multiplied by the stored reference gain Gr which was required to obtain the reference white color RW, wherein the ratio is corrected by the gamma γ of the CRT: Gd = (Jd/Jr)^{1 γ}Gr

Instead of the normalized chromaticity coordinates also the CIE color coordinates may be used.

In another embodiment in accordance with the invention, the register RE is present. The processor unit 11 now calculates a gain value GV which is stored in the register RE as the desired gain value GVd. Fig. 3 shows that the desired gain value GVd is read from the register RE. The gain g of the amplifier circuit AC is a function f of the gain value GV in the register. The processor unit 11 calculates the desired gain value GVd for the desired white point DW as:

GVd = f^nv ( f (GVr) (Jd/Jr) ^{1 γ} ) wherein f^nv is an inverse function of the function f of the gain value GVd, the inverse function f^nv determining the gain value GV as a function of the gain g, GVr is a reference gain value for the reference white point RW, Jd is a desired chromaticity coordinate, Jr is a reference chromaticity coordinate of the reference white color RW, and γ the gamma of the CRT 80. By way of example, if the video pre-amplifϊer integrated circuit TDA4887 of

Philips Semiconductors is used, the function f is defined by g = f (GV) = (80 GV) /255 + 20 wherein g is the gain, GV is the gain value, and f is the function.

The desired gain value GVd for the desired white point DW is then calculated as:

GVd = f^nv ( f (GVr) (Jd/Jr) ^{1 γ} ) = 255((80 GVr/255 + 20) (Jd/Jr) ^{1 γ}- 20V80

Thus, the processor unit 11 is able to exactly calculate the desired gain value GVd to be supplied to the register RE in one step. Only one reference white point RW and the corresponding information on the white color of the reference white point RW are required to directly determine the desired gain value GVd. It is not required to perform the iterative process of the prior art wherein the beam current is measured and compared with a desired value, while the gain g is controlled until the required beam current occurs. It is even not required to measure the beam current at all. Fig. 4 shows a full video amplifier circuit of an embodiment in accordance with the invention. The video processor 2 receives an input signal Vi and supplies three input video signals Ri, Gi, Bi to three amplifiers AR, AG, AB, respectively. The amplifiers AR, AG, AB supply the video drive signals Ro, Go, Bo, respectively to the display device 8. The display device 8, which for example is a plasma panel comprises the usual column drivers which receive the video drive signals Ro, Go, Bo. Usually, the plasma pixels comprise red, green and blue sub-pixels, which are controlled by the video drive signals Ro, Go, Bo, respectively. A color reproduced by the display device depends on the ratio of the video drive signals Ro, Go, Bo.

The gains gre, ggr, gbl of the amplifiers AR, AG, AB are controlled by the control signals or desired gains Gdre, Gdgr, Gdbl, respectively. The indices re, gr, bl refer to the primary colors red, green and blue. The index d refers to a desired value. The processor unit 11 of the calculation unit 1 calculates the control signals Gdre, Gdgr, Gbl based on the inputted information IDW of the desired white color DW, and stored information IRW1 on either one reference white point RWl and the corresponding reference gain values Grrel, Grgrl, Grbll, or stored infonnation IRW1, IRW2 on two reference white points RWl and RW2 and the corresponding reference gain values Grrel, Grgrl, Grbll and Grre2, Grgr2, Grbl2, respectively. Again the indices re, gr, bl refer to the primary colors red, green and blue, and the index r refers to a reference value. If one of the gains of the amplifiers AR, AG, AB is fixed only two reference gain values are stored for each one of the two reference white points RWl and RW2. Thus although in the following always three reference gain values are mentioned for each one of the two reference white points RWl and RW2, in a practical embodiment only two will be used and only two desired gains will be calculated because the third one is fixed.

In an embodiment in accordance with the invention, the processor unit 11 calculates the desired gains Gdbl, Gdgr, Gdre based on the inputted information IDW of the desired white color DW and on the stored information on the one reference white point RWl and the corresponding reference gain values Grrel, Grgrl, Grbll. The calculation of each one of the desired gains Gdbl, Gdgr; Gdre is performed in the same way as elucidated with respect to Fig. 3. The reference gain values Grrel, Grgrl, Grbll of a single reference white point RWl suffices to directly calculate the correct desired gains Gdbl, Gdgr, Gdre required to obtain the desired white color DW. Instead, as elucidated with respect to Fig. 3, it is also possible to calculate gain values if the amplifiers AR, AG, AB comprise registers (not shown in Fig. 4). However, the desired white color DW is only calculated accurately if the black level VB (see Fig. 5) of the video drive signals Ro, Go, Bo is correctly aligned with the cutoff level Vco (see Fig. 5) of the display device 8. The black level VB of the video drive signals Ro, Go, Bo is the drive voltage which corresponds to video information for which no light should be produced by the display device 8. The cut-off level Vco of the display device 8 is the actual level of the video drive signals Ro, Go, Bo at which just no light is produced by the display device 8. If the black level VB of the video drive signals Ro, Go, Bo and the cut-off level Vco of the display 8 device differ, the calculated desired gains Gdbl, Gdgr, Gdre and thus the desired white color DW will deviate from the intended desired white color DW. In another embodiment in accordance with the invention, the processor unit 11 calculates the desired gains Gdbl, Gdgr, Gdre based on the inputted information IDW on the desired white color DW and the two reference white points RWl and RW2 and the corresponding reference gain values Grrel, Grgrl, Grbll and Grre2, Grgr2, Grbl2, respectively. Again it is possible to select one of the gains fixed which has the advantage that only two of the desired gains Gdbl, Gdgr, Gdre need to be calculated and only two values of the reference gain values Grrel, Grgrl, Grbll and Grre2, Grgr2, Grbl2 need to be stored.

The calculation unit 11 firstly determines an offset dV (see Fig. 5) between the black level VB and the cut-off level Vco, and secondly the values of the desired gains Gdbl, Gdgr, Gdre.

The offset dV between the black level VB and the cut-off level Vco is calculated for each video amplifier channel based on: the information IRW1 on the first reference white point RWl and a corresponding one of the first reference values Grrel, Grgrl, Grbllof the gain g required to obtain the first reference white point RWl, and information IRW2 on a second reference white point RW2 and a corresponding one of the second reference values Grre2, Grgr2, Grbl2 of the gain g required to obtain the second reference white point RW2. By using the two known reference white points RWl and RW2 and their associated known reference gains Grrel, Grgrl, Grbll and Grre2, Grgr2, Grbl2, it is possible to determine the offset dV which is present in the drive voltages for both the reference white points RWl and RW2. As will be elucidated in more detail, the same function which defines the relation between the desired gain and one of the reference gains can be used to determine the offset dV by substituting in this function both reference gains, both corrected with the offset dV.

The desired value of the gain Gdre, Gdgr, Gdbl is calculated based on: the information IDW defining the desired white point DW, the information IRW1 or IRW2 on either: the first reference white point RWl and the first reference gain value Grrel, Grgrl, Grbll, or the second reference white point RW2 and second reference gain value Grre2, Grgr2, Grbl2, and the offset dV.

As now first the offset dV is determined, and then the desired gains Gdre, Gdgr, Gdbl for the desired white point DW are calculated taking the offset dV into account, the correct desired white point DW will be calculated even when an offset dV exists between the black level VB and the cut-off level Vco.

In a preferred embodiment in accordance with the invention, the calculation unit 11 determines the offset dV from either the equation: Grl±dV = (Jrl/Jr2)^{1 γ}(Gr2± dV) or Gr2±dV = (Jr2/Jrl)^{1 γ} (Grl±dV) wherein Jrl is a first reference chromaticity coordinate for a primary color R, G, B corresponding to the control electrode of the display device 8 which receives the video drive signal Ro, Go, Bo, and corresponding to the first reference white point RWl, Jr2 is a second reference chromaticity coordinate corresponding to the second reference white color RW2, γ is a gamma of the display device 8, Grl is the first reference value of the gain which is the corresponding one of the first reference gain values Grrel, Grgrl, Grbll, Gr2 is the second reference value of the gain which is the corresponding one of the first reference gain values Grre2, Grgr2, Grbl2, and dV is the off-set which is defined as the cutoff level minus the black level (the - sign) or the black level minus the cutoff level (the + sign). Then, the calculation unit may determine the desired gain Gd as

Gd = (Jd/Jrl)^{1 γ} Grl or Gd = (Jd/Jr2)^1/γGr2 wherein Jd is a desired chromaticity coordinate for the one of the primary colors R, G, B which corresponds to the control electrode which receives the video drive signal Ro, Go, Bo, respectively. The desired chromaticity coordinates for the three primary colors R, G, B determine the desired white color DW.

Instead of the gains Gdre, Gdgr, Gdbl which directly determine the gains gre, ggr, gbl of the amplifiers AR, AG, AB, register values may be used which determine the gains gre, ggr, gbl. Therefore, the calculation unit 11 may calculate both the offset dV and the desired gains Gdre, Gdgr, Gdbl required for the desired white point DW by using the gain values in the registers (not shown in Fig. 4) and by introducing a function f which determines the gain gre, ggr, gbl of the video amplifiers AR, AG, AB based on the gain values in the registers. The inverse function f^nv determines the gain values in the registers from the gains gre, ggr, gbl of the video amplifiers AR, AG, AB.

Now, the off-set in the gain register domain (dVg) is calculated with either the equation:

GVr2±dVg = f^nv ( f (GVrl±dVg) (Jr2/Jrl) ^1/γ ) or

GVrl±dVg = ^v ( f (GVr2±dVg) (Jrl/Jr2) ^1/γ ) and the desired gain value (GVd) for the desired white point (DW) is calculated as:

GVd ±dVg = f^nv ( f (GVrl±dVg) (Jd/Jrl) ^{1 γ}) or

GVd ±dVg = ^nv ( f (GVr2±dVg) (Jd/Jr2) ^{1 γ}) wherein GVd is the desired gain value, f is the function of the gain value GV, f^nv is the inverse function of the function f of the gain value GV, GVrl is a reference gain value for the first mentioned reference white point RWl, GVr2 is a reference gain value for the further reference white point RW2, and dVg is the off-set dVg which is defined as the register value corresponding to the cutoff level Vco minus the black level VB as indicated by the minus signs, or the black level VB minus the cut-off level Vco as indicated by the plus signs. Fig. 5 shows a graph for elucidating a situation wherein the black level of the video drive signal and the cut-off level of the display device differ. The vertical axis depicts the brightness LO of one of the primary colors R, G, B or if the display device 8 is a CRT 80 the beam current lb flowing into one of the electron guns GU. The horizontal axis depicts the drive voltage VD. The curve shown indicates the brightness LO or beam current lb as a function of the drive voltage VD for one of the primary colors R, G, B. The brightness LO or the beam current lb starts to rise if the drive voltage VD becomes larger than the cutoff voltage Vco. The black level VB of the drive signal Ro, Go, Bo may differ from the cutoff voltage Vco.

In the ideal situation the black level VB is adjusted during the manufacturing of the display apparatus to coincide with the cutoff voltage Vco. Consequently, the drive voltages Vdrl and Vdr2 of the reference white colors RWl and RW2 are referenced with respect to the cutoff voltage Vco and generate the reference beam currents Ibrefl and Ibref2, respectively. If however the black level VB does not coincident with the cutoff voltage Vco an offset voltage dV (or dVg in the register value domain) occurs. During the alignment in the factory, now the drive voltages Vdrln and Vdr2n will be required to obtain the correct beam currents Ibrefl and Ibref2. As in the equations, which define how to determine the desired gains or gain values dependent on the reference values of the single reference white color RW, the offset dVg is neglected, the offset dVg is compensated for by subtracting the offset dVg from the drive voltages Vdrln and Vdr2n. If these corrected drive voltages Vdrln and Vdr2n are substituted in the equation defining the relation between the desired gain and the reference gain, the offset will be the only unknown variable and thus can be determined from this equation directly, as is already elucidated with respect to Fig. 4.

Fig. 6 shows an embodiment of an alignment set-up used during manufacturing of the display apparatus. A brightness or light output measurement apparatus 200 (usually referred to as colorimeter) measures the brightness produced by each of the primary colors of the display apparatus 100 and provides this ratio or the color temperature of the light measured directly to a display 300. During manufacturing, for one or more reference white colors RWl, RW2, at a particular input signal (preferably white with equal RGB amplitudes) the gains of the video amplifiers AR, AG, AB are controlled until the correct reference gains Grrel, Grgrl, Grbll and Grre2, Grgr2, Grbl2 are found which provide the display with the correct reference white colors RWl and RW2, respectively. These reference gain values Grrel, Grgrl, Grbll and Grre2, Grgr2, Grbl2 are stored in a non- volatile memory. If one of the gains gre, ggr, gbl of the video amplifiers AR, AG, AB is fixed, for each reference white color RWl or RW2, only two of the reference gains Grrel, Grgrl, Grbll or Grre2, Grgr2, Grbl2 need to be adjusted and stored.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. A white point adjustment circuit for setting a white point of a display device (8), the white point adjustment circuit comprising: a video amplifier (A; AR, AG, AB) having a gain (g; gre, ggr, gbl) for amplifying an input video signal (IV; Ri, Gi, Bi) with the gain (g; gre, ggr, gbl) to obtain a video drive signal (VD; Ro, Go, Bo) to be supplied to the display device (8), a calculation unit (1) for calculating a desired value of the gain (Gd; Gdre, Gdgr, Gdbl) based on: information (IDW) defining a desired white point (DW), information (IRW; IRW1, IRW2) defining a reference white point (RW; RWl, RW2), and a reference value of the gain (Gr; Grbll, Grgrl, Grrel) required to obtain the reference white point (RW; RWl, RW2).

2. A white point adjustment circuit as claimed in claim 1, wherein the information (IDW) defining the desired white point (DW) comprises either an indication of a color temperature of the desired white point (DW), or chromaticity coordinates of the desired white point (DW), and the information (IRW) on the reference white point (RW) comprises either an indication of a color temperature of the reference white point (RW), or chromaticity coordinates of the reference white point (RW).

3. A white point adjustment circuit as claimed in claim 1, wherein the calculation unit (1) comprises a memory (10) for storing the reference value of the gain (Gr).

4. A white point adjustment circuit as claimed in claim 1, wherein the calculation unit (1) comprises an input for receiving the information (IDW) on the desired white point (DW).

5. A white point adjustment circuit as claimed in claim 1, wherein the calculation unit (1) comprises a processor unit (11) and a memory (10) for storing a program for calculating the desired value of the gain (Gd) and for storing the information on the reference white point (IRW) and the stored reference value of the gain (Gr), wherein the program comprises an algorithm for calculating the desired value of the gain (Gd) based on the stored information on the reference white point (IRW), the stored reference value of the gain (Gr), and the information on the desired white point (IDW).

6. A white point adjustment circuit as claimed in claim 5, wherein the calculation unit (1) comprises an input for receiving the information on the desired white point (IDW).

7. A white point adjustment circuit as claimed in claim 1, wherein the calculation unit (1) is arranged for calculating the desired gain (Gd) as

Gd = (Jd/Jr)^{1 γ}Gr wherein Jd is a desired chromaticity coordinate of a primary color corresponding to the video drive signal (VD), Jr is a reference chromaticity coordinate of the primary color corresponding to the reference white color, γ is a gamma of the display device (8), and Gr is the reference value of the gain (g).

8. A white point adjustment circuit as claimed in claim 1, wherein the video amplifier (A) comprises a register (RE) for storing a gain value (GV), the gain (g) being a function (f) of the gain value (GV).

9. A white point adjustment circuit as claimed in claim 8, wherein the calculation unit (1) is adapted for calculating a desired gain value (GVd) for the desired white point (DW) as:

GVd = f^nv ( f (GVr) (Jd/Jr) ^{1 γ} ) wherein GVd is the desired gain value, f is the function of the gain value (GV), f^nv is an inverse function of the function of the gain value (f), the inverse function (f^nv) determining the gain value (GV) as a function of the gain (g), GVr is a reference gain value for the reference white point (RW), Jd is a desired chromaticity coordinate of a primary color corresponding to the video drive signal (VD), Jr is a reference chromaticity coordinate of the primary color corresponding to the reference white color (RW), and γ the gamma of the display device (8).

10. A white point adjustment circuit as claimed in claim 1, wherein the calculation unit (1) is adapted to successively determine: (i) an offset (dV) between a black level (VB) of the video drive signal (VD) and a cut-off level (Vco) of the display (8) based on: the information on the reference white point (IRW1) and a reference value of the gain (Grl) required to obtain the reference white point (RWl), and the information on a further reference white point (IRW2) and a further reference value of the gain (Gr2) required to obtain the further reference white point (RW2), and

(ii) the desired value of the gain (Gd) based on: the information defining the desired white point (IDW), the information on either the first mentioned or the further reference white point (IRW1, IRW2), and either the first mentioned reference value or the further reference value of the gain (Grl, Gr2) required to obtain either the first mentioned or the further reference white point (RWl, RW2).

11. A white point adjustment circuit as claimed in claim 10, wherein the calculation unit (1) is arranged for determining the off-set (dV) from either the equation:

Grl±dV = (Jrl/Jr2)^{1 γ} Gr2± dV or Gr2±dV = (Jr2/Jrl)^{1 γ} Grl±dV wherein Jr 1 is a first reference chromaticity coordinate of a primary color corresponding to the video drive signal (DR), and corresponding to the first mentioned reference white point (RWl), Jr2 is a second reference chromaticity coordinate corresponding to the further reference white color (RW2), γ is a gamma of the display device (8), Grl is the first mentioned reference value of the gain, Gr2 is the further reference value of the gain, and dV is the off-set (dV) which is defined as the cut-off level (Vco) minus the black level (VB) as indicated by the minus signs, or the black level (VB) minus the cut-off level (Vco) as indicated by the plus signs.

12. A white point adjustment circuit as claimed in claim 11, wherein the calculation unit (1) is arranged for determining the desired gain (Gd) as

Gd = (Jd/Jrl)^1/γ Grl or

Gd = (Jd/Jr2)^{1 γ}Gr2 wherein Jd is a desired chromaticity coordinate of a primary color corresponding to the video drive signal (VD).

13. A white point adjustment circuit as claimed in claim 12, wherein the video amplifier (A) comprises a register (RE) for storing a gain value (GV) determining the gain (g) being a function (f) of the gain value (GV).

14. A white point adjustment circuit as claimed in claim 13, wherein the calculation unit (1) is arranged for determining: the off-set (dV) from either the equation: GVr2±dV = f^nv ( f (GVrl±dV) (Jr2/Jrl) ^{1 γ} ) or GVrl±dV = f^nv ( f (GVr2±dV) (Jrl/Jr2) ^1/γ ) the desired gain value (GVd) for the desired white point (WD) from either the equation:

GVd ±dV = f^nv ( f (GVrl±dV) (Jd/Jr 1) ^1/γ) or GVd ±dV = f^nv ( f (GVr2±dV) (Jd/Jr2) ^{1 γ}) wherein GVd is the desired gain value, f is the function of the gain value (GV), ^nv is an inverse function of the function (f) of the gain value (GV), the inverse function (f^nv) determining tho gain value (GV) as a function of the gain (g), GVrl is a reference gain value for the first mentioned reference white point (RWl), GVr2 is a reference gain value for the further reference white point (RW2), and dV is the off-set (dV) which is defined as the cut- off level (Vco) minus the black level (VB) as indicated by the minus signs, or the black level (VB) minus the cut-off level (Vco) as indicated by the plus signs.

15. A display apparatus comprising a white point adjustment circuit for setting a white point of a display device (8) as claimed in claim 1.

16. A display apparatus as claimed in claim 15, wherein the display device (8) comprises a cathode ray tube (80) with an electron gun (GU) having a control electrode (CA; Gl) for receiving the video drive signal (VD).

17. A method of white point adjustment for setting a white point of a display device (8), the white point adjustment method comprises: controlling (1) a gain (g) of a video amplifier (A) for amplifying an input video signal (IV) with the gain (g) to obtain a video drive signal (VD) to be supplied to a control electrode (CA) of the display device (8), calculating (1) a desired value of the gain (Gd) based on: information on a desired white point (IDW), information on a reference white point (IRW), and a reference value of the gain (Gr) required to obtain the reference white point (RW).