GB2355841A

GB2355841A - Display device with brightness control

Info

Publication number: GB2355841A
Application number: GB0101988A
Authority: GB
Inventors: Taisuke Nakamura
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 1999-01-20
Filing date: 2000-01-20
Publication date: 2001-05-02
Anticipated expiration: 2020-01-20
Also published as: GB0101988D0; GB2355841B

Abstract

A display device is controlled such that the brightness of an image displayed thereon is maintained at a constant level when the colour of the image is changed. The device may comprise a plurality of light emitters 1, 2, 3, each of which is a different colour. A current controller 14 may be used to ensure that the sum of currents flowing through the light emitters is maintained at a constant level. The light emitters may be light emitting diodes.

Description

2355841 DISPLAY DEVICE, PORTABLE ELECTRONIC DEVICE AND METHOD OF

CONTROLLING DISPLAY DEVICE BACKGRO= OF THE INVENTION

1. Field of the Invention

The present invention relates to a display devi ce, a portable electronic device and a method of" controlling a display device, in particular to a back-light display device, a portable electronic device and a method of controlling a display device capable of developing a desired color for display. 2. Description of the Related Art

A conventional red (R), green (G) and blue (B) back-light display is disclosed in Japanese Patent Laid-open No.. 7-281647.

This conventional RGB back-light display intends to avoid degradation of image quality resulting from a color fringe or after-image. The color fringe or after-image, for example, may arise when red (R) and green (G) colors intermingle with each other by failing to smoothly switch to a green image from a red image on the same pixel - In this conventional RGB back-light display, color data written in advance in a memory and having a duration (for example 16 ms) equal to- one frame are compressed so as to have a duration. (for example 2.6 ms) short of one frame duration and then the compressed data is transferred to individual pixel areas. Light sources corresponding with images conveyed by the compressed data are turned on in synchrony with the compressed data reading time (for example 2.6 ms) as the blanking time. The remaining hold time (for example 13.4 ms) is used as the effective illumination time for display.

With this method, although an image depicted on the display device is turned of f via a s pecified signal during the blanking time (2.6 ms), the dot state of this image just turned of f is still held during the holding.time (that is, 13-.4 ms) on the display with declination of its intensity by a memorizing activity of liquid crystal. In the next frame, when depiction of an image of a different color is required, the foregoing dot state of liquid crystal declines sufficiently to allow the smooth switching to the image of the different color. This enables the smooth and swift swi-tching of multiple color images. Moreover, the intensity of the image brightness is not controlled in this conventional RGB back-light display.

SUMMARY OF THE INVENTION

It is therefore an object of the preferred embodiment of the present invention to provide a back-light display device, a portable electronic device and a method of 2 controlling a display device capable of maintaining intensity of brightness regardless of a displayed colour.

A method of controlling a display device according to the present invention comprises displaying an image on the display device, the image having a brightness, changing a colour of the image displayed on the display device, and maintaining the brightness of the image at a predetermined value even if the colour is changed.

In this method, the display device may have a plurality of light emitters, and the step of maintaining the brightness of the image may comprise maintaining a sum of currents flowing through the light emitters at a predetermined current value. At least one of the light emitters may he a light-emission diode, which preferably emits one of red- colour light, green-colour light and blue-colour light.

The predetermined brightness value may be changed by changing voltage supplied to the light emitters.

In the method, the display device may have a first light emitter for emitting a first-colour light, a second light emitter for emitting a second-colour light which is different from the first-colour light, and a third light emitter for emitting a third-colour light which is different from the first-colour light and from the second-colour light, whereby images with a fourth colour are adapted to be displayed in 3 cooperation with the first light emitter and second light emitter, wherein:

said step of changing a colour of the image displayed on the display device comprises changing the fourth colour by controlling the current flowing through at least one of the first, second and third light emitters; and, said step of maintaining the brightness of the image at a predetermined value when the colour is changed comprises maintaining a sum of the currents flowing through the first, second and third light emitters at a predetermined current value.

The invention is also a display device configured to display an image having a brightness on the display device, to change a colour of the image displayed on the display device, and to maintain the brightness of the image at a predetermined value when the colour is changed.

4 - 1.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:FIG. 1 is a circuit diagram of an embodiment of the related RGB back-light display device for the portable telephone.

F 1G. 2 is a circuit diagram of another embodiment of the related RGB back-light" display device for the portable telephone.

FIG., 3 is a circuit diagram of an embodiment of an RGB back-light display device for a poxtable electronic device according to the present invention.

FIG. 4A. is a timing chart showing control signals output from pulse width modulation circuits described in FIG. 3 when all light emitting diodes are activated.

FIG. 4B is a timing chart showing control- signals output from pulse width modulat-ion circuits described in FIG. 3 when two light emitting diodes are activated and one light emitting diode is inactivated.

FIG. 4C is a timing chart showing control signals output from pulse width modulation circuits described in FIG. 3 when one light emitting diode S is activated and two light emitting diodes are inactivated.

FIG. 5 is a-circuit diagram showing another embodiment of an RGB backlight -display device for a 5 portable dev ice according to the present invention.

DETAILED DESCRIPTi6N OF THE PREFERRED EMBODIMENTS Now, related back-light.display devices will be described with respect to FIGS. I and 2, before preferred embodiments according to the p. resent invention will be explained. The related back-light. display devices are disclosed in Japanese Patent Laid-open No. 11-266295, which has been filed by the assignee of the present invention. The related back-light display devices are provided with a portable electronic device such as a portable telephone.

Referring to FIG. 1, the related back-light display dev ice has a redcolor light-emitting diode (LED) 101, an LED-driving transistor 108 to drive the red-color LED 101, and a pulse-width modulator circuit (PWM) 105 installed in a central processing unit (CPU) 104 to control the LEDdriving transistor 108. Further, the device has a green-color light emitting diode (LED) 102, an LED-driving transistor log to drive the green-color. LED 102,. and a pulsewidth modulator circuit (PWM). 106 installed in the 6 CPU 104 to control the LED-driving transistor 109.

The device includes a blue-color light-emitting diode (LED) 103, an LED-driving transistor 110 to drive the blue-color LED 103, and a pulse-width modulator circuit (PWM) 107 installed in the CPU 104 to control the LED-dkiving transistor 110.

In FIG. 1, the PWM 105 controls the LED-driving transistor 108 to switch on the red-color LED -101:

The PWM 106 controls the LED-driving transistor 109 to -switch on the green-color LED 102. The PWM 107 controls the LED-driving transistor 110 to switch on the bl-ue-color LED 103. The red-color, green-color and blue-color LEDs 101, 102 and 103 generate colors corresponding to three primary colors (RGB), respectively.

The PWM 105 is connected to the base terminal of the LED-driving transistor 108. The PWM. 106 is connected to the base terminal of the LEDdriving transistor 109. The PWM 107 is connected to the base terminal of the LED-driving transistor 110. Then, it is possible to change average currents flowing through the red-color, green-color and blue-color LEDs 101, 102 and 103 by means of the LED-driving transistors 108, 109 and 110, respectively, by individually adjusting duty cycle or outputs from the PWMs 105, 106 and 107.

7 Through this arrangement, the related backlight display device can control currents flowing through the red-color, green-color and bluecolor LEDs 101, 102 and 103, and thus choose any color according to the user's liking as a back-light color of the portable telephone.

Next, another embodiment: of the related backlight display device will be described with reference to FIG. 2. According to FIG. 2,' the CPU 204 achieves an ONIOFF control of an LED-driving transistor 111 via:output from an output port 129. Similarly, the CPU 204 executes an ONIOFF -control of an LED-driving transistor 112 'via output from an output port 130, and an ONIOFF control of an LED- driving transistor 113 via output from an output port 131. The CPU 204 switches on any one of the LED-driving transistors 111,.112 and 113. Current restricting resistances 120, 121 and 122 are chosen in such a way that their resistance values R120, R121 and R122 are set R120 > R121 > R122. When the CPU 204 turns on the LED-driving transistor 111, the.

brightness of the red-color LED 101 becomes the darkest state. When the CPU 204 switches on the LED driving transistor 113, the brightness of the red- color LED 101 becomes the lightest state. When the CPU 204 turns on the LE,D-driving transistor 112, the 8 brightness of the red-color LED 101 becomes an intermediate state between the above cases.

Similarly, the CPU 204 achieves an ONIOFF control of an LED-driving transistor 114 via output from an output port 132. The CPU 204 also executes an ON/OFF control of an LED-driving transistor 115 via out put f rom an output port 133, and an ONIOFF control-of an LED-driving transistor 116 via output from an output port 134. The CPU 204 switches on any one of the LED-driving transistors 114, 115 and 116. Current restricting resistances 123, 124 and are chosen in such a way that. their resistance values R123, R124 and R125 are set R123 > R124 > R125. When the CPU 204 turns on the LED-driving transistor 114, the brightness of the green-color LED 102 becomes the darkest state. When the CPU 204 switches on the LED-driving transistor 115, the brightness of the green-color LED 102 becomes the lightest state. When the CPU 204 turns on the LED- driving transistor 116, the brightness of the green- color LED 102 becomes an intermediate state between the above cases.

The CPU 204 achieves an ON/OFF control of an LED-driving transistor 117 via output from an output port 135. Similarly, the CPU 204 executes an ONIOFF control of an LED driving transistor 118 via 9 41 output f rom an output port 136, and an ONIOFF control of an LED-driving transistor 119 via output from an output port 137. The CPU 204 switches on any one of the LED-driving transistors 117, 118 and 119. Current restricting resistances 126, 127 and 128 are chosen in such a way that their resistance values R126, R127 and R128 are set R126 > R127 > R128. When the CPU 204 turns on the LED-driving transistor 116, the brightness of the blue-color LED 103 becomes the darkest state. When the CPU 204 switches on the LED-driving transistor 118, the brightness of the blue-color LED 103 becomes the lightest state. When the CPU 204 turns on the LED dri-ving transistor 119, the brightness of the blue- color.LED 103 becomes an intermediate state -between the above cases.

Through these operations, in this embodiment of the back-light display. device, the brightness of the red-color, green-color and blue-color LEDs 101, 102 and 103 are appropriately controlled for selecting a desired back-light color of a portable telephone.

Next, referring to the drawings, a preferred embodiment according to the present invention will be described in detail below. The present invention is capable of maintaining intensity of brightness regardless of a displayed color as well as displaying a desired color.

A preferred embodiment of an RGB back-light display device for a portable electronic device such as a portable telephone according to the present invention is illustrated in FIG. 3. According to FIG. 3, current flowing. through a red-color light emitting diode (LED) 1 is controlled by a resistance (Ri) 4- The LED 1 is driven by a field-effect transistor (FET) switch 10. The FET switch 10 is controlled by a pulse-width modulator (PWM) 7. Similarly, current flowing through a green-color light-emitting diode (LED) 2 is controlled by a resistance (R2) 5. The LED 2 is driven by a fieldef f ect transistor (FET) switch 11. The FET. switch

15. 11 is controlled by a pulse-width modulator (PWM). 8.. Current flowing through a blue-color light-emitting diode (LED) 3 is controlled by a resistance. (R3) 6. The LED 3 is driven by a field-effect transistor (FET) switch 12. The FET switch- 12 is controlled by a pulse-width modulator. (PWM) 9. A control section 14 controls the timing of signals delivered by the PWMs 7, 8 and 9 to switch the FETs 10, 11 and 12 respectively. The duty cycle of rectangular waves of the signals generated f rom the PWMs 7, 8 and 9 is also controlled by the control section 14. A direct-current (DC) source 13 supplies power to the LEDs 1, 2 and 3.

The state under which all the LEDs 1, 2 and 3 are turned off will be described with reference to FIG. 3. The PWM 7.delivers a control signal at a low level to the FET 10. In this case, the drain terminal of the FET 10 is open, and thus current from the DC source 13 will not flow through resistance 4 and the LED 1. It causes the LED 1 to be inactivated. Similarly, the PWM 8 delivers 'a control signal at a low level to the FET 11. In this case, the drain terminal of the FET 11 is open, and thus current from the DC source 13 will not flow through resistance 5 and the LED 2. It causes the LED 2 to be inactivated. Further, the PWM 9 delivers a control signal at a low level to the FET 12. In this case, the drain terminal of the FET 12 is open, and thus current from the DC source 13 will not f low through resistance 6 and the LED 3. it causes the LED 3 to be inactivated. 20 Next, the state under which the LEDs 1, 2 and 3 are switched on will be described with reference to FIG, 3. The PWM 7 delivers a control signal at a high level to the FET 10. In this case, the drain terminal of the FET 10 is shunted, and thus current from the DC source 13 will flow through resistance 4 and the LED.1 It causes the LED 1 to be activated.

12 Similarly, the PWM 8 delivers a control signal at a high level to the FET 11. In this case, the drain terminal of the FET 11 is shunted, and thus current from the DC source 13 will flow through resistance 5 5 and the LED 2. It causes the LED 2 to be activated. Further, the PWM 9 delivers a control signal at a high level to the FET 12. In this case, the drain terminal of the FET 12 is shunted, and thus current from the DC source 13 will flow through resistance 6 and the LED 3. It causes the LED 3 to be activated. When the PWM 7 delivers the control signal at a high level to the FET 10, and if supply voltage of DC source 13 is represented by "E", the right-direction voltage drop across the LED 1 is represented by "Vl", and the resistance value of resistance 4 is represented by 11R111, current flowing thorough the LED 1, wherein it is represented by "IR", is shown as the following equation (1) IR = (E - V1) /R1... Equation (1) When the PWM 8 delivers the control signal at a high level to the FET 11, and if supply voltage of DC source 13 is represented by -"E", the right- direction voltage drop across the LED 2 is represented by "V2", and the resistance value of resistance 5 is represented by "R21', current flowing thorough the LED 2, wherein it is represented by "IG", is shown 13 as the following equation (2) IG = (E - V2) /R2... Equation (2). When the PWM 9 delivers the control signal at a high level to the FET 12, and if supply voltage of DC source 13 is represented by "E", the right-direction voltage drop across the LED 3 is represented by "V311, and the resistance value of resistance 6 is represented by 'IR3", current flowing thorough the. LED 3, wherein it is represented by 11IB11; - is shown io as the following equation (3) ID = (E V3). /R3... Equation (3).

If, in this embodiment, V1 = V2 = V3 VO (Equation (4) or right-direction voltage drops across the LEDs 1, 2 and 3, respectively, are all equal to "VO", currents flowing through the. LEDs 1, 2 and 3 are shown based on equations (1), (2), (3) and (4) as follows:

IR = E - VO)/Rl Equation (5) IG = (E - VO)/R2 Equation (6) IB = (E VO)/R3 Equation (7) For rendering intensity of light emitted by the LEDs 1, 2 and 3 substantially equal (a constant value), - it Is necessary to render values of - the current following through the LEDs 1, 2 and 3 substantially equal, or IR = IG = IB. If R1 = R2 R3 = RO. (Equation (8) wherein "RO" is a constant 14 value, currents flowing through the LEDs 1, 2 and 3 are shown based on equations (5), (6) (7) and (8) as follows:

IR = (E - VO) /RO Equation (9) IG = (E - VO) /RO Equation (10) IB = (E VO) /R6.. Equation (11) Thus, the intensities of light emitted by the LEDs 1,_ 2 and 3 are substantially equal to each other.

If the duty values of control signals delivered by the PWM 7,. 8 and 9 are "all, ""b" and "c" respectively, the sum of average current "I" flowing through each LED is derived from Equations (9), (10) and (11) as follows:

1 = IR x a + IG x b + 113 x c =(a + b + c) x (E - VO) /RO... Equation (12) In this embodiment, the resistance values 11R11', "R211 and "R3", the right-direction voltage drop "Vill, 11V211 and %%V311 and the duty values "'a", "b" and 11c" are set or controlled such that the sum of the average current "I" becomes a constant value, or maintained at a predetermined value.

FIGS. 4A-, 4B and 4C give time charts of control signals delivered by the PWMs 7, 8 and 9 to switch the FETs 10, 11 and 12, respectively. The control section 14 controls the PWMs 7, 8 and 9 according to operation by a user for displaying color image desired by the user.

FIG. 4A gives a timing chart of control signals delivered by respective PWMs 7, 8 and 9, when all 5 the LEDs 1, 2 and 3 are activated.

It is assumed here that the pulse periods of control signals delivered by. the PWMs 7, 8 and 9 are equal to 'IT", and every control signal has a pulse width of 'IT/3", or each duty value is equal to 33% of rectangular wave indicative of every control signal It is further assumed that the rising time.

1,tn of a pulse representing the control signal delivered by the PWM 7 is taken as reference (t 0), the rising time 'It" of a pulse representing the control signal from the PWM- 8- is t = T/3, and the rising time "t" of a pulse representing the control signal from the PWM 9 is t = 2T/3. Then, the sum of the average current "I" flowing through each LED is derived from Equation (12) as follows:

1 (0.33 + 0.33 + 0.33) x (E - VO)/RO - E - VO)/RO.. Equation (A).

FIG. 4B is a timing chart of the control signals delivered by the PWMs 7, 8 and 9 when two LEDs are activated. It is assumed here as an illustration that the LEDs 1 and 3 are activated while the LED 2 is not activated, whereby images 16 with different color from that in the case shown in FIG. 4A are displayed. it is further assumed here that the pulse periods of control signals delivered by the PWMs 7 and 9 are equal to 'IT", and each of the control signals delivered by the PWMs 7 and 9 has a pulse width of T/2, or each duty value of the control signals delivered by the PWMs 7 and 9 is equal to 5016 of a rectangular wave indicative of 'each control signal, It is still further assumed that the rising time It" of a pulse representing the.control signal delivered by the PWM 7 is taken as reference (t 0), the rising time 'It" of a' pulse representing the control signal from the PWM 9 is t T/2, and the control signal delivered by the PWM always remains at a low level. Then, the sum of the average current "I" flowing through each LED- is derived from Equation (12) as follows:

I = (0.50 + 0.50) x (E - VO)/RO (E - VO)/RO. -Equation (B).

FIG. 4C gives a timing chart control signals delivered by the PWMs 7, 8 and 9 when a single LED is activated. It is assumed here as an illustration that the LED 1 is activated while the LEDs 2 and 3 are not activated, whereby images with different color from that, in the case shown in FIGS. 4A and 4B 17 are displayed.

It is further assumed here that the co ntrol signal delivered by the PWM 7 always remains at a high level. It is still further assumed that the control signals delivered by the PWMs 8 and 9 always remain at a low level. Then, the sum of the average currents flowing through each LED is derived from Equatio.n (12) as follows:

I = (1 + 0 + 0) x (E - VOURO (E - VO) /RO Equation (C).

As seen f rom above, the control section 14. controls the PWMs 7, 8 and 9 so that they may deliver control signals at the timing as depicted in FIG. 4A, 4B and 4C to change color of images to be displayed. Then, the sum of the average current "I" flowing through each LED becomes constant as indicated in Equations (A), (B) and (C), and hence the intensity of brightne ss in any given display color can be maintained at constant.

As described above, the back-light display device according to the present invention maintains a constant light intensity regardless of what color is displayed and the color presents high-quality images suitable to a viewer. Further, even if the displayed color is changed according to operation by a user, its light intensity is maintained at a - 18 predetermined value.

With reference to FIG. 5,. another embodiment of an back-light display device for an electronic device such as a portable telephone according to the 5 present invention will be discussed below.

The back-light display device shown in FIG. 5 is the same as that shown. in FIG. 3, except that the DC power source 13 is replaced with a D/D converter 16 and with a DC power source 15 for the D/D converter 16.

Hence, the back-light display device provided by this embodiment include's an LED 1, a resistance 4 to control current flowing through the LED 1, an FET 10 to drive the LED 1, and a PWM 7 to control the FET 10. Similarly, it has an LED 2, a resistance 5 to control current flowing through the LED 2, a FET 11 to drive the LED 2, a PWM 8 to control the FET 11, an LED 3, a resistance. 6 to control current f lowing through the LED 3, a FET 12 to drive the LED 3, and a control section 14 to control the timing of control signals delivered by the PWMs.7, 8. and 9 to the FETs 10, 11 and 12 and the duty values. of rectangular waves. In addition, the back-light display device has the DC power source 15 to supply power to the LEDs 1, 2 and 3, and the D/D converter 16 to.control the power supplied to the LEDs 1, 2 19 and 3.

In FIG. 5, the timing of control signals delivered by the PWM 7, 8 and. 9 to the FETs 10, 11 and 12 respectively is the same as shown in FIGS.

4A, 4B and 4C. when a control signal from -at least one of the PWMs 7, 8 and 9 is at a high level, control section 14 can deliver a control signal to the D/D-converter 16 in order to raise output voltage "E" from the D/D converter 16. Then, the sum of the average current "I" becomes larger than that shown in the Equation (12) and the intensity of brightness of a displayed- color can be strengthened or brightened.

Likewise, when a control signal from at least 1 one of PWMs 7, 8 and 9 is at a high level, control section 14 can deliver a control signal to the D/D converter 16 in order to lower the output voltage "E" from the D/D converter 16. Then, the sum of the average current "I" becomes smaller than that shown in the Equation (12), and the intensity of brightness of a. displayed color is weakened or darkened.

According to this embodiment, it is possible for the control section to control supply voltage from the DC power source 15 to the LEDs 1, 2 and 3, thereby to adjust the sum of the average current "I" flowing through each LED. Thus, the level of light intensities of displayed color is adjusted, keeping highquality image suitable to the viewer.

The invention maybe embodied in other specific forms without departing from the essential characteristics thereof. The present invention embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing descripion and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Each feature disclosed in this specification (which term includes the claims) and/or shown in the drawings may be incorporated in the invention independently of other disclosed and/or illustrated features.

Statements in this specification of the "objects of the invention" relate to preferred embodiments of the invention, but not necessarily to all embodiments of the invention falling within the claims.

The text of the abstract filed herewith is repeated here as part of the specification.

Current flowing through a plurality of light-emission' diodes is individually controlled, whereby an image with a desired color is displayed according to color lights emitted from the light-emission diodes. A sum of current flowing through the light- emission diodes is maintained at a predetermined current, whereby brightness of the image is maintained at a predetermined brightness.

21

Claims

CLAIMS:

1. A method of controlling a display device, the method comprising: displaying an image on the display device, the image having a brightness; changing a colour of the image displayed on the display device; and, maintaining the brightness of the image at a predetermined value when the colour is changed.

2. The method of claim 1, wherein the display device has a plurality of light emitters, and wherein the step of maintaining the brightness of the image comprises maintaining a sum of currents flowing through the light emitters at a predetermined current value.

3. The method of claim 2, wherein at least one of the light emitters is a light-emission diode.

4. The method of claim 3, wherein the lightemission diode emits one of red-colour light, greencolour light and blue-colour light.

5. The method of claim 2, further comprising supplying voltage to the light emitters and changing a predetermined brightness value by changing the voltage - 22 supplied to the light emitters.

6. The method of claim 1, wherein said display device has a first light emitter for emitting a firstcolour light, a second light emitter for emitting a second-colour light which is different from the firstcolour light, and a third light emitter for emitting a third-colour light which is different from the firstcolour light and the second-colour light, whereby images with a fourth colour are adapted to be displayed in cooperation with the first light emitter and second light emitter, wherein: said step of changing a colour of the image displayed on the display device comprises changing the fourth colour by controlling the current flowing through at least one of the first, second and third light emitters; and, said step of maintaining the brightness of the image at a predetermined value when the colour is changed comprises maintaining a sum of the currents flowing through the first, second and third light emitters at a predetermined current value.

7. A display device configured to display an image having a brightness on the display device, to change a colour of the image displayed on the display device, and to maintain the brightness of the image at a predetermined value when the colour is changed.

- 23

8. A display device as in claim 7, wherein the display device has a plurality of light emitters, and the sum of currents flowing through the light emitters is maintained at a predetermined current value.

9. A display device as in claim 8, wherein at least one of the light emitters has a light-emission diode.

10- A display device as in claim 9, wherein the light-emission diode emits one of red-colour light, green-colour light and blue-colour light.

11. A display device as in claim 8, further comprising a voltage supply to the emitters, a change in the voltage supplied to the light emitters changing a predetermined brightness value.

12. A display device as in claim 7, wherein the display device comprises a first light emitter for emitting a first-colour light, a second light emitter for emitting a second-colour light which is different from the first-colour light, and a third light emitter for emitting a third-colour light which is different from the first-colour light and the secondcolour light, whereby images with a fourth colour are adapted to be displayed in cooperation with the first light emitter and second light emitter, wherein the fourth - 24 colour is changed by controlling the current flowing through at least one of the first, second and third light emitters, and wherein a sum of currents flowing through the first, second and third light emitters is maintained at a predetermined current value.

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