WO2020012516A1

WO2020012516A1 - Colour management in an led screen with rgbw pixels to minimize consumption.

Info

Publication number: WO2020012516A1
Application number: PCT/IT2019/050163
Authority: WO
Inventors: Stefano GRASSO; Francesco PUNIS
Original assignee: Macropix S.R.L.; Temporapix
Priority date: 2018-07-10
Filing date: 2019-07-09
Publication date: 2020-01-16

Abstract

The present invention object relates to a method which can conveniently be employed to increase the energy efficiency of maxi LED screens and in particular of those screens which include a white LED known as RGBW screens. In this type of screens, the addition of white LEDs (W) in the composition of the pixel can be exploited to reduce the consumption of a screen since the efficiency of a white LED with respect to the efficiency of three equivalent RGB LEDs (red, green and blue) is better, A method has been developed to adjust the activation of the white LED, maximizing its exploitation and limiting the use of RGB energy-intensive LEDs which consists essentially of: - Over-increasing the brightness of the display up to twice as much as the RGB part alone, accepting to de saturate the colour. Efficiency, under maximum brightness conditions, will therefore be equal to 3/2 compared to a pure RGB display. - Lighting on the white LEDs to replace the coloured LEDs where the three basic colours are all at least a little lit. The resulting energy saving obviously depends on the content of the image and the maximum value is on an all-white image: on all the pixels it will be possible to completely replace the basic colours with white.

Description

Colour management in an LED screen with RGBW pixels to minimize consumption

Technical field

The present invention relates to a method which can conveniently be employed to increase the energy efficiency of large LED screens and in particular of those screens wherein the pixels include a white LED, and which are known as RGBW screens.

Background art

The pixels of the LED screens are normally composed of LEDs of the three basic colours: red, green, blue from the combination of which it is possible to produce a wide range of colours. In these systems commercially known as RGB the white colour is obtained employing the same three LEDs with full brightness. Over time LED screens have been introduced with RGBW pixels i.e. characterized by the addition of a white LED. The RGBW pixel LED systems are characterized by different control algorithms necessary to map and adjust the activation of the additional white LED.

Although some applications of the aforementioned RGBW structure are known in the field of LED LCD screens, as for example in patents US20170039920A1, US20160267828A1, US2017116914A1,

US2014071189A1, there are no applications of the same technology in the so-called maxi LED screens. In this context, RGBW-type structures have never been used since the use of the additional LED makes the system significantly more complicated and consequently expensive. Furthermore, there are no techniques for reducing energy consumption for

LED screens with RGBW pixels, although this would make the solution more competitive, especially in the aforementioned area of large screens where energy consumption is of considerably important.

Disclosure of the invention

The technical problem addressed by this invention is therefore to find a colour management method that allows to conveniently apply the RGBW pixel structure in the maxi screen field allowing to exploit its potential and at the same time minimize its energy consumption. The proposed solution is intended to overcome the drawbacks of the already known solutions and to reduce the energy consumption of an RGBW -type screen allowing advantageously its employment in large-scale applications such as for example the maxi screens or the large decorative/advertising panels; while maintaining good performance in terms of colour quality and brightness of the image represented.

Proceeding with an energetic comparison of the LEDs with RGB pixels with those of the RGBW type, it is observed that the addition of a white LED or W LED in the composition of the pixel could advantageously be employed to reduce the consumption of a screen thanks to the fact that the efficiency expressed in cd/mA ("luminous intensity 7” current absorbed") of a white LED with respect to the efficiency of three red, green, blue RGB LEDs is approximately doubled (in the hypothesis of choosing a white colour temperature of about 7000K, as is usually done on LED screens). To fully exploit this intrinsic potential of the hardware it is necessary to develop a colour management system and in particular a control algorithm useful for mapping and adjusting the activation of the white LED, maximizing its exploitation and limiting the use of the three energy- intensive RGB LEDs as much as possible, while maintaining good quality and image visibility.

Specifically, the proposed solution allows to have two different usages that result into two process phases and two employment advantages:

• Over-increase the brightness of the display up to twice as much as the RGB part alone, accepting to desaturate the colour. Efficiency, under maximum brightness conditions, will therefore be equal to 3/2 compared to a pure RGB display.

• Light on the white LEDs to replace the coloured LEDs where the three basic colours are all at least a little lit. The resulting energy saving obviously depends on the content of the image and the maximum value is on an all-white image: on all the pixels it will be possible to completely replace the basic colours with white. It should be noted that this operation must be reduced in case the aforementioned brightness over-increase is active. Brief description of the attached drawings

Further features and advantages of the proposed technical solution will appear more evident and better understood by every expert in the art in the following description of a preferred but not exclusive embodiment shown by way of not limiting example in the accompanying drawing, wherein: • Fig. l shows a diagram of the algorithm relating to the proposed solution underlying the input signals, of the processing performed and of the output signals. Best mode for carrying out the invention

According to a preferred but not exclusive embodiment, the solution method can be employed to reduce the energy consumption of an RGBW LED screen through the following steps:

• Over-increasing screen brightness.

· Lighting on white to replace the basic colours.

• Calibrating the four RGBW LEDs to be driven depending on the physical characteristics of the LEDs employed.

With reference to the accompanying drawings, and particularly to Fig. 1 the parameters and variables employed by the method are represented which can be substantially divided into:

Input variables:

• Vector RGB IN [R_IN, G IN, B IN]: vector relative to the input signal whose three components are normalized i.e. comprised between 0 and 1 which expresses the value of the individual colours of a standard RGB pixel. Concretely, said vector may for example be derived from the RGB output signals by a graphic card and subsequently normalized, these values being comprised in the range 0-255. • Parameter vector [AR, AG, AB] : vector whose components are fixed and express the weight with which an individual colour R or G or B contributes to the over-increase of white and therefore contributes to the overall brightness of all screen pixels. The components of this vector are characterized by the following normalization constraint: (AR+AG+AB= 1 ).

• Additional brightness parameter, (EXBR): RGB screens are normally characterized by brightness values adjustable from 0 to 1. The additional brightness adjustable parameter (EXBR) is a normalized value (comprised between 0 and 1) which is set by the user and adjusts an over-increase of said brightness on all the pixels of the screen which can therefore have overall brightness values comprised between 0 and 2.

• A calibration parameter vector [CR, CG, CB, CW]: whose components represent fixed parameters for the final calibration of the

RGBW LEDs to be driven. Said parameters [CR, CG, CB, CW] must be set during the construction of the maxi screen since they are linked to the intrinsic technological characteristics of the LEDs used.

Output variables:

• RGB OUT vector [R OUT, G OUT, B OUT]: RGB output vector with normalized components.

• RGB LED vector [R LED, G LED, B LED, W LED] : vector corresponding to the output signal i.e. to the lighting values of the 4 physical RGBW LEDs to be driven. According to the proposed solution, the colour management of a LED screen with RGBW pixels consists of three conceptual phases:

PHASE A: Over-increasing brightness.

"Brightness setting" of the display refers to the common brightness adjustment that can be made by the user in real time. When said "brightness setting" has reached its maximum and a further increase is desired, the user can over-increase it acting on the additional brightness parameter (EXBR) or a parameter comprised between 0 and 1. When said additional brightness parameter (EXBR) is at its maximum value i.e. 1, the total brightness of the display is therefore equal to 2, since the effect of the basic colour LEDs and the white LEDs are added.

The values of the vector relative to the input signal RGB IN [R_IN, G IN, B IN] are scalarly multiplied by a parameter vector [AR, AG, AB] according to the following overall formula that defines an activation contribution to the white LED (WSL) said contribution being calculable with the formula:

WSL = EXBR x (R_IN x AR + G IN x AG + B IN x AB)

wherein the components of said parameter vector [AR, AG, AB] are normalized with respect to the equation AR + AG + AB = 1.

It should also be noted that said AR, AG, AB components can be freely chosen, but to have the best optical effect possible they must be chosen so as to be proportional to the colorimetric component y of the respective base colour according to the CIE 1931 model. This model was created by the International Commission on Illumination (CIE) in 1931 and the colour spaces defined by it represent quantitative links experimentally defined between the wavelength distributions in the visible electromagnetic spectrum and the colours physiologically perceived in the vision of human colours; they are still essential tools for colour management especially for the representation of colour through displays.

Following this model, the three normalized components of the parameter vector [AR, AG, AB] can then be set by the following formulas:

AR = Ry / (Ry + Gy + By),

AG = Gy / (Ry + Gy + By),

AB = By / (Ry + Gy + By)

Where Ry, Gy, By are the colorimetric components y of the respective base colour and are commonly obtained from the datasheet or alternatively measured for the three individual RGB physical LEDs that will be employed in the construction phase.

PHASE B: Lighting on white LEDs to replace the coloured LEDs.

The colour vector RGB IN [R_IN, G IN, B IN] can be expressed as the sum of two vectors, a so-called grey i.e. with equal components i.e. wherein R = G = B and the other with the remaining part with respect to said input vector. This component represents the so-called grey transfer (TT) and can be conveniently subtracted from the base colours and added to the white.

If there is also an over-increase in brightness or in the condition wherein EXBR> 0, the white LEDs or W LEDs will already be, at least partially, employed and then the grey vector transfer is limited to a value equal to the complement of the additional brightness parameter (EXBR) i.e. : 1 - EXBR. From the implementation point of view this corresponds to performing the following operations:

° Calculating the grey transfer component (TT) obtained as the minimum value between the three components of the input signal RGB IN [R_IN, G IN, B IN] and the residual value of the additional brightness parameter (EXBR) complying with the following equation:

TT = Min [R_IN, G IN, B IN, (1 - EXBR)]; ° Calculating an RGB output vector with normalized components RGB OUT [R OUT, G OUT, B OUT] obtained as a subtraction of the aforementioned grey transfer component (TT) from the input signal RGB IN [R_IN, G IN, B IN] complying with the following equations: R OUT = R_IN - TT,

G OUT = G IN - TT,

B OUT = B IN - TT;

° Calculating the sum of the grey transfer component (TT) and of the activation contribution of the white LED (WSL) complying with the following equation:

W OUT = WSL + TT;

PHASE C: LED calibration.

The final calibration has the purpose of adjusting the intrinsic brightness of the LEDs of the four colours in such a way that the fundamental conditions on which the algorithm of phases A and B is based and that is that there is equality of chromatic coordinates and luminous intensity between the sum of the basic colour LEDs and the white LEDs when all these LEDs are lighted up to the maximum. This condition is achieved primarily by determining the output signal RGBW LED [R LED, G LED, B LED, W LED] obtained by multiplying the components of the output vector with normalized components RGB OUT [R OUT, G OUT, B OUT,

W OUT] for a calibration parameter vector depending on the type of LED employed [CR, CG, CB, CW] complying with the following equations:

R LED = R OUT x CR,

G LED = G OUT x CG,

B LED = B OUT x CB,

W LED = W OUT x CW.

At the base of the determination of the components of the calibration parameter vector [CR, CG, CB, CW] there is the condition of equality of chromatic coordinates and luminous intensity between the sum of the basic colour LEDs and the white LEDs when all these LEDs are lighted on up to the maximum and this condition is described by the equations:

Cxy(R_LED+G_LED+B_LED) = Cxy(W LED)

I(R_LED+G_LED+B_LED) = I(W_LED) wherein said maximum lighting of the LEDs corresponds to the following conditions:

R OUT = 1,

G OUT = 1,

B OUT = 1,

W OUT = 1. Industrial Applicability

The invention has been described with reference to a preferred embodiment, but it is understood that equivalent modifications may be made without departing from the scope of protection granted to the present industrial property right application. The method according to the finding can be carried out and applied with technical and functional equivalents, with different physical devices or devices functionally equivalent to the mentioned RGBW LEDs and/or supplementary expedients suitable for the purpose and the application field. As an example, it should be noted that for example the range of the additional brightness parameter (EXBR) may be changed.

Claims

1. Method (100) to reduce the energy consumption of LED screens equipped with RGBW pixels, said method (100) capable of converting a generic vector input signal RGB IN [R_IN, G IN, B IN] into a corresponding vector output signal RGBW LED [R LED, G LED, B LED, W LED] and employed to control said RGBW pixels, said method comprising the following steps: (a) calculating the activation contribution to the white LED (WSL) by over-increasing the brightness of the RGB vector input signal [R_IN, G IN, B IN], said activation contribution to the white LED (WSL) being calculated as a scalar product between a parameter vector [AR, AG, AB] and the vector input signal RGB IN [R_IN, G IN, B IN], said scalar product further multiplied by an additional brightness adjustment parameter (EXBR) which can be set by the user, according to the following equation:

WSL = EXBR x (R_IN x AR + G IN x AG + B IN x AB), where said additional brightness adjustment parameter (EXBR) and the components of said parameter vector [AR, AG, AB] are subject to the following constraints:

AR + AG + AB = 1 ,

0 < EXBR < 1 ; (b)calculating the grey transfer component (TT), said component corresponding to the minimum value between the three components of the vector input signal RGB IN [R_IN, G IN, B IN] and the complement to one of the additional brightness adjustment parameter (EXBR) according to the following equation:

TT = Min [R_IN, G IN, B IN, (1 - EXBR)];

(c) calculating a RGB output vector with normalized components RGB OUT [R OUT, G OUT, B OUT], whose components

(R OUT), (G OUT), (B OUT) are obtained by subtracting said grey transfer component (TT) from the three components of the vector input signal RGB IN [R_IN, G IN, B IN] according to the following equations: R OUT = R_IN - TT,

G OUT = G IN - TT,

B OUT = B IN - TT;

(d) calculating the normalized component for activating the white output LED (W OUT), said component being obtained as the sum of the grey transfer component (TT) and the activation contribution of the white LED (WSL) and according to the following equation:

W OUT = WSL + TT; (e) calculating the vector output signal RGBW LED [R LED, G LED, B LED, W LED], said signal obtained from the components of said output vector with normalized components RGB OUT [R OUT, G OUT, B OUT] and from the normalized component of the white output LED (W OUT) said components being multiplied by a vector of corresponding calibration parameters [CR, CG, CB, CW]; said calibration parameter vector depending on the type of LED employed and the components of said vector output signal RGBW LED [R LED, G LED, B LED, W LED] calculated according to the following equations:

R LED = R OUT x CR,

G LED = G OUT x CG,

B LED = B OUT x CB,

W LED = W OUT x CW.

2. Method according to claim 1, wherein the components of said parameter vector [AR, AG, AB] represent the weight with which an individual component of R, G, B colour contributes to the over- increase of white and brightness on all screen pixels and said components are characterized by values proportional to the colorimetric component y of the respective base colour according to the CIE 1931 model and the following conditions:

AR = Ry / (Ry + Gy + By), AG = Gy / (Ry + Gy + By),

AB = By / (Ry + Gy + By)

AR+AG+AB=1.

3. Method according to claim 1, wherein said calibration parameter vector depending on the type of LED employed [CR, CG, CB, CW] is characterized by fixed values obtained by imposing equality of chromatic coordinates and luminous intensity between the sum of the basic RGB colour LEDs and white LEDs complying with the following equations:

Cxy(R_LED+G_LED+B_LED) = Cxy(W LED),

I(R_LED+G_LED+B_LED) = I(W_LED), where Cxy are the chromatic coordinates and I is the luminous intensity and the following maximum intensity conditions are respected:

R OUT = 1,

G OUT = 1,

B OUT = 1,

W OUT = l .