EP1618550A1

EP1618550A1 - Automatic photo-colorimetric parameter control device for light boxes with colour leds

Info

Publication number: EP1618550A1
Application number: EP04724024A
Authority: EP
Inventors: Guy Thales Intellectual Property SCHOU
Original assignee: Thales SA
Current assignee: Thales SA
Priority date: 2003-04-25
Filing date: 2004-03-29
Publication date: 2006-01-25
Also published as: JP2006526886A; WO2004097784A1; US7804478B2; US20060256049A1; FR2854252B1; FR2854252A1

Abstract

The invention relates to light boxes which are used to illuminate light valve display screens, for example LCD (Liquid Crystal Display) matrices. At present, the light from light boxes is provided by electroluminescent diodes emitting in different spectral bands such as to reconstruct a white light. For a certain number of applications, such as aeronautical applications, it is necessary to retain the photometric and colorimetric characteristics of said light, independently of environmental conditions or component ageing. For said purpose, the invention relates to an electronic automatic control device which can be used to maintain the photometric and colorimetric characteristics of the light at given set values without disruptive optoelectronic devices being introduced into the light box. The invention also relates to several possible technical solutions.

Description

DEVICE FOR CONTROLLING PHOTOCOLORIMETRIC PARAMETERS FOR LIGHT BOX WITH COLORED LEDS

The field of the invention is that of light boxes (acronym. BAL) used for lighting display screens with optical valve, in particular matrix liquid crystal screens called LCD (English acronym for Liquid Crystal Display). relates more particularly to polychrome display screens having light boxes emitting white light

The invention relates to the coloπmétπque and photometric control of the light emitted by said light boxes

The field of application is more particularly that of displays on board aircraft, but can be used for all applications requiring displays with an optical valve having precise color or photometric tolerances (computer monitors, laptop screens,)

The visualizations on board aircraft have particularly severe characteristics and specificities. These are in particular: • High luminance (typically of the order of several thousand cdm ² )

• A large luminance dynamic (called dimming in English terminology) so that it can be used day and night • Precise colorimetric characteristics independent of the aging of the components

• Long service life and high reliability

• Great robustness to specific aeronautical environments (extreme temperatures, depressurization, humidity, salt spray, vibrations and shocks,)

• Low weight and volume Until recently, the only light sources that lit the optical valves were fluorescent tubes. There are two main types.

• The tubes called HCFL (acronym for Hot Cathode Fluorescent Lamp) which have internal preheating and operate at average excitation voltages

• The so-called CCFL tubes (acronym for Cold Cathode Fluorescent Lamp) have the advantage of not having internal preheating. They require higher excitation voltages but allow tubes of very small diameter (a few millimeters) to be produced. and have an improved lifespan They are generally preferred to HCFL

The use of so-called CCFL tubes, however, has many disadvantages

• They require a high supply voltage which can reach 2000 volts in alternating current and which has the main consequences

• Use of unreliable, bulky, heavy, specific and expensive wound components

• The use of printed circuits and specific wiring techniques which increase costs and lead times

• The use of complex assembly and finishing technologies, necessary to ensure correct operation even in the event of depressurization, high humidity or thermal shock

• The risk of electric arcs (with smoke generation) in the event of component failure

• The emission of significant electromagnetic radiation which is difficult to control insofar as by nature they are emitted on the front of the displays • They have a limited luminance dynamic. Indeed, the "dimming" is conventionally obtained by a temporal modulation of the luminance emitted. Below a certain lighting time, the fluorescent tube has an erratic behavior.

5 We then perceive the periods of extinction of the tube in the form of flicker (in English terminology - "fhcker")

• Their optical characteristics vary over time The degradation of the performance of fluorescent lamps is due to the following phenomena

10 • Depletion of vaporized gas (mercury)

• Degradation of the emissive power of the electrodes

• Opacification of the glass of the fluorescent tube

• Loss of yield of phosphors lining the inside of the tube which evolve differently and

15 change the color of the light emitted

• Their photometric performance at low temperature is poor and cold starts reduce their lifespan

• The start-ups of fluorescent tubes after a long downtime are ineffective (first appearance of

20 delayed light FOLLOWING chaotic operation)

• The ends of fluorescent tubes that do not emit light have a significant length, often greater than one centimeter

• Their relative fragility due to their material (glass tube) 25 associated with a small diameter (of the order of 2 millimeters) and a great length which may exceed 200 millimeters

• Their delicate fixing which must ensure mechanical maintenance and electrical insulation

• Their poor thermal control due to a very poorly drained thermal dissipation by conduction to the structure, thermal evacuation being carried out only by natural convection

• The risk of obsolescence of these very specific components which are difficult to replace Also, for a few years, it has been envisaged to replace these light sources by light-emitting diodes also called

LEDS (acronym for Light Electroluminescent Diode) Light-emitting diodes have many advantages "They are semiconductor components that can be easily integrated into printed circuits

• They require low supply voltages to operate

• The emission spectra make it possible to cover the entire visible spectrum

• They have a very large bandwidth which allows a great dynamic of luminance by using the temporal modulation of their control voltage

• They have a high reliability and a long lifespan. It should also be noted that a LED light box requires a greater number of components than a fluorescent tube box, therefore the death of an LED can lead to a less significant drop in luminance than switching off a fluorescent tube

There are two main types of light boxes In a first embodiment, the optical valve is lit by a matrix of LEDs arranged in a plane located under the optical valve In a second embodiment, the LEDs are arranged on the periphery of the optical valve, along the edge of a light guide which returns the light from the LEDs to the imager

However, until recently, their use was limited to the extent that the photometric efficiency of LEDs, that is to say the percentage of electrical energy effectively transformed into light energy, remained fairly poor and much lower than that of tubes. fluorescent

Recent progress has made it possible to produce LEDs having yields close to those of fluorescent tubes. To obtain white light, different solutions are then possible. It is possible to use • LEDs initially emitting in blue, covered with a yellow luminophore converting part of the blue light emitted into yellow radiation, the final shade of the emitted light, mixture of blue and yellow, being white • LEDS initially emitting in blue and covered with three phosphors emitting in three different spectral bands (classically red - green - blue), the final shade of the light emitted, mixture of blue, green and red, being white • Monolithic components integrating on a chip single three LEDs emitting in three different spectral bands The mixing of colors is obtained by a common encapsulation optic

• Components hybπdant three LEDs emitting in three different spectral bands

• Three different types of LEDs emitting in three different spectral bands In this case, the light box achieves the mixing of the different colored lights so as to obtain a uniform white tint

The use of blue LEDs covered with a yellow phosphor has several drawbacks On the one hand, the photometric yields of around 25 lumens / Watt for the best LEDs are still lower than those of fluorescent tubes which are around of 50 lumens / watt On the other hand, the emitted luminance decreases sharply with the operating time The emitted luminance can thus lose half of its value after 10,000 operating hours Then, the red component of the emitted light is generally quite weak Finally, the light output of the yellow phosphor varies as a function of the temperature, the operating time and the manufacturing conditions. These variations in yield cause variations in coloπmétπe which are difficult to control.

The use of LEDs emitting initially in blue and covered with three phosphors emitting in three different spectral bands makes it possible to partially solve the problems of blue diodes with yellow phosphor Indeed, the coloπmétπe obtained is more satisfactory and its variations with the more limited operating time However, the light yields are not satisfactory and this type of component remains marginal on the LED market, which eventually poses supply or obsolescence problems Monolithic or hybrid components theoretically allow to achieve better colorimetric yields However, these complex technologies to implement remain marginal

The most promising solution in the medium term therefore consists in using three different types of LEDs emitting in three different spectral bands. Indeed, this solution makes it possible to obtain high efficiencies, insofar as the light emitted by the LEDs is no longer attenuated by the installation of phosphors or conversion phosphors The LEDS used are simple components to manufacture and to implement In this case, the light box achieves the mixing of the different colored lights coming from each type of LED so to obtain a uniform white shade To achieve a satisfactory mixture of colors, it suffices, for example, that the light box has a sufficient depth. The technological process of manufacturing the different types of LEDs does not, however, guarantee perfect reproducibility of the characteristics. photometric and colorimetric This point can be easily resolved using separate and independent electrical control networks for each type of LED To obtain the desired coloπmétπe, it suffices to increase or decrease the respective intensities in each network

However, this solution has a significant drawback. In fact, the photometric and colorimetric characteristics of the LEDs will evolve, depending on their operating time or on the temperature, in a different way, thus modifying the coloπmetry and the intensity of the white light emitted.

It is known practice to use servo-control systems which, depending on photometric and colorimetric measurements carried out in the light box, modify the electrical controls of the light-emitting diodes so as to find set values for the photometric parameters. However, the measure disturb necessarily the proper functioning of the BAL Indeed, or these devices are located in the useful area of the lighting and introduce defects in uniformity of luminance or coloπmétπe, or these devices are located outside the useful area of the lighting and in this case, the lighting is larger than that of the light valve, thereby increasing the final size of the display. The object of the invention is to overcome these drawbacks by providing photometric or coloπmétπque measurement devices which can be located outside the light box More specifically, the subject of the invention is an electronic device for controlling photometric or colorimetric characteristics for a light box for lighting imagers with an optical valve, in particular with matrix crystal crystal screens, said box comprising at least a first and a second network of light-emitting diodes, said networks being comma supplied by control electronics, the first array consisting of a first type of diodes emitting light in a first spectral band, the second array consisting of a second type of diodes emitting light in a second spectral band, said electronic servo device comprising an electronic processing and calculation unit making it possible to control the control electronics of the networks of light-emitting diodes and optoelectronic devices for measuring the photometric and colorimetric characteristics of the light-emitting diodes connected to said electronic processing unit and of calculation, said optoelectronic devices being characterized in that they comprise at least a first optoelectronic assembly consisting of a light-emitting diode of the type identical to one of the types of the diodes of the light box and of a photosensitive sensor placed opposite the di the light-emitting diode, said diode being controlled by the control electronics of the diode arrays of the light box controlling this type of diode, said assembly comprising means or a structure making it possible to isolate it from outside light and said assembly being placed in an environment close to that of the light box The invention will be better understood and other advantages will appear on reading the description which follows given without limitation and thanks to the appended figures among which

• Figure 1 shows a general diagram of the light box and the servo device according to the invention

• Figure 2 shows a variant of the device of Figure 1

• Figure 3 shows a first variant of the electronic processing and calculation unit of the servo device according to the invention • Figure 4 shows a second variant of the electronic processing and calculation unit of the servo device according to the invention

• Figure 5 shows a third variant of the electronic processing and calculation unit of the servo device according to the invention

FIG. 1 represents a general diagram of an electronic assembly comprising the control device according to the invention The assembly comprises three parts the light box 2, the control device 1 and an assembly comprising the control electronics 3 of the light emitting diode networks Each control electronics comprises several control modules 31 Each electronic module 31 controls a type of diodes

The light box comprises several arrays 22 of diodes as shown in FIG. 1 Each array comprises light-emitting diodes of the same type Each array 22 is formed by several branches 221 connected to an electronic control module 31 by electrical links 21, each branch 221 includes LEDs 222 of the same type arranged in series Of course, other arrangements are possible (matrix arrangements of LEDs in particular) The light-emitting diodes 222 of the different networks 22 emit in different spectral bands Conventionally, to obtain white light, it it is necessary to make sub-assemblies comprising three different types of diodes emitting in red, green and blue (hatched arrows in the figure) However, the devices according to the invention can operate with other arrangements of LEDs For the sake of clarity, the optical devices making it possible to ensure the mixing of the colored lights coming from the LEDs to obtain the illumination of the imager (thick white arrows) are not shown These devices are known 5 skilled in the art

Each branch 221 of a type of LED is controlled by an independent control electronics 31 Generally, the control of the light-emitting diodes is carried out in electrical intensity, the

10 photometric properties of diodes directly dependent on this electrical intensity

The electronic servo device 1 surrounded by a dotted line in FIG. 1 essentially comprises an electronic unit

15 12 for processing and calculations and optoelectronic devices 1 1 for measuring the photometric and colorimetric characteristics of light-emitting diodes Each of said devices comprises a light-emitting diode of type identical to one of the types of diodes in the light box and a photosensitive sensor placed in look of said diode

20 light emitting, said diode being controlled by the electronic control 3 of the diode networks of the light box controlling this type of diode Said optoelectronic device is isolated from external light, in particular by a closed cover or simply because the distance separating the photosensitive sensor diode is low enough

25 to avoid any significant effect of stray light Said optoelectronic device is placed in a thermal environment close to that of the light box

This arrangement of the optoelectronic devices 1 1 is based on the very strong similarity in thermal behavior and drift in time of the light-emitting diodes which are purely semiconductor components. Consequently, subject to identical or similar conditions, their characteristics will vary from the same way It is therefore not necessary to measure the photometric or colorimetric characteristics directly on the diodes of the light box This measurement can be carried out on identical diodes outside the light box provided that they are controlled by identical electrical voltages and currents and that they undergo the same environment A possible installation of optoelectronic measurement devices is the rear side of the lighting card on which the LEDs of the light box are implemented In fact, these diodes are generally made in a SMD (Surface Mounted Component) box and therefore their temperature depends essentially on the circuit temperature which is identical on these two faces Another important advantage of this arrangement is that all the initial errors in the installation of electronic devices (dispersion of the levels of light emitted by the LEDs, misalignment of the photosensors, dispersion of sensitivity of said photosensors, dispersion in the control electronics of the res LED water,) has no impact on the quality of the control since the control always tends to bring the detected light levels back to their initial level

The electronic processing and calculation unit comprises at least • a storage unit for the set values of the photometric and colorimetric parameters 122,

A processing unit 121 for the data coming from the various photosensitive sensors connected to the optoelectronic measuring devices, an electronic comparator 123 comparing the data coming from the processing unit with the set values,

A control unit 124 connected on the one hand to the electronic comparator and on the other hand to the control electronics of the arrays of light-emitting diodes making it possible to maintain the set values of the photometric and colorimetric parameters

The operation of the entire device is described below The luminance of the screen must be adjustable insofar as the lighting conditions can vary in very large proportions between daylighting and nighttime lighting. This luminance instruction can be provided either by the user or by an additional system which measures the ambient brightness, a system not shown in the different figures The luminance control must therefore be integrated into the colorimetric parameter control device

The luminance setpoint is supplied to the storage unit for the setpoints of the photometric and colorimetric parameters 122 which already contains the setpoints of the colorimetric parameters

Preferably, these colorimetric reference values result from an initial adjustment carried out as follows For a given luminance reference, the intensities delivered in the various arrays of LEDs are adjusted until the mixing light is obtained desired We check this point for example with a photocoloπmeter or a spectrometer

When this light is obtained, the measurements delivered by the optoelectronic devices 1 1 are stored in the unit 122 This method makes it possible to eliminate all the inaccuracies of the system and does not require any prior calibration of said optoelectronic devices

This storage unit 122, through the electronic comparator 123, transmits the set values to the control unit 124 For the sake of clarity, the operation of the comparator will be explained below. Said unit has as main function to transform the values of photometric and colorimetric setpoints in electronic setpoints usable for control electronics 31 of light-emitting diode networks

The control electronics of the LED arrays generate from these electronic reference values the control intensities which are distributed in the various diode arrays 222 and in the optoelectronic devices 1 1 of measurement In order to generate identical intensities in the devices 1 1, preferably using so-called current mirror electronic devices. LEDs generate colored light (hatched arrows in Figure 1) The different colored lights mix to form a homogeneous light (thick white arrows), usually white, of the imager

Each photosensor receives a luminous flux coming from its associated LED (small hatched arrows of figure 1) This flux depends on two main parameters which are on the one hand the intensity of control of the LED and on the other hand, the possible variations due either to the aging of the LED, or to changes in these characteristics depending on the environment, and in particular the thermal environment The electrical signals from the sensors are transmitted to the processing unit 121

The main function of the processing unit is to transform this data into photometric and colorimetric parameters of the same nature as the set values supplied to the electronic comparator 123 The comparator 123 compares the set values from the electronic storage unit 122 and the values measured by the sensors coming from the unit 121 If these values are identical, the set values are transmitted to the control unit 124 without modifications If they are different, the comparator acts so as to increase the measured values if they are lower than the set values and to decrease them if they are higher than the set values according to the control techniques known to those skilled in the art

FIG. 2 shows a variant of the device of FIG. 1 An additional device 110 has been added This device essentially comprises at least one photosensitive sensor which captures the light directly inside the light box This sensor can be mounted, as for example, either inside the light box or outside, in this case, an opening in the light box allows the light flux to be transmitted to the sensor. This sensor is also connected to the unit of processing 121 This arrangement makes it possible to ensure redundancy in the measurements obtained by the devices described above and said sensors performing direct measurements. This thus secures the measurement. arrangement also makes it possible to separate the colorimetric measurement devices provided essentially by the devices 11 and the photometric measurement devices provided by the photosensitive sensor 110 which captures the light directly inside the light box

The control of the LED arrays is preferably carried out by the technique known as PWM (English acronym of Puise Width Modulation) This technique consists in periodically modulating the electric intensity delivered to the LEDs In a given period T ₀ of time, the maximum electrical intensity corresponding to a maximum luminous flux is delivered for a duration T proportional to the luminous flux that one wishes to obtain The intensity is zero during the remainder of the time period equal to T ₀ -T For example, if I we want a luminous flux which is half of the maximum flux, the intensity will be delivered half of the period of time FIG. 3 details the operating principle of the control unit 124 in a particular embodiment of the PWM In this arrangement, the unit 124 comprises as many electronic chains as there are types of LEDs. For example, if the light box comprises three types of LEDs as indicated in FIG. 3, in this case, the control unit will comprise three chains, each chain driving a control module 31 The control unit comprises a first electronic unit 1241 for formatting the set values This unit supplies the signals control initials for LED arrays Each initial signal is amplified by an amplifier device 1242, then filtered by a filtering device 1243, finally the signal is transformed into temporal amplitude modulation by the device 1244 The final signal thus set in form is delivered to the control module 31 concerned

FIG. 4 presents a first variant of this electronic configuration when the device comprises, as shown in FIG. 2, a sensor directly measuring the light flux in or in the vicinity of the light box. It is then possible to control the luminance setpoint separately. and the colorimetric instructions by two comparators 1231 and 1232 as indicated in FIG. 4 The processing device 121 then comprises two separate electronic modules 1211 and 1212, the first dedicated to the devices 1 1 and the second dedicated to the sensor 110 The storage unit 122 also includes two modules 1221 and 1222, the first dedicated to the colonometric setpoint values and the second to the photometric setpoint value We then have two control chains autonomous The first is used for the control of the colorimetric parameters It includes the optoelectronic devices 1 1, the module 121 1, the comparator 1232 and the control unit 124 The second is used for the control of the photometric parameters It essentially comprises the electronic module 1212 and the comparator 1231 In this case, the luminance setpoint is first controlled, then the colorimetric parameters as indicated in FIG. 4

FIG. 5 presents a second variant of this electronic configuration when the control device comprises a sensor directly measuring the light flux. In this configuration, the controls of the colorimetric and photometric parameters are separated up to the control electronics of the LEDS networks.

Thus, the luminance control chain comprises the following elements • the processing unit 1212

• reference memory 1222

• the comparator 1231

• The 1245 control module

The coloπmétπe control chain includes the following elements

• the processing unit 121 1

• the setpoint memory 1221

• the comparator 1232

• The 1246 control unit In this case, the LED control electronics are controlled by two different commands. The first output from the control module regulates the modulation time of the PWM delivered by the control modules 31 and thus makes it possible to obtain the desired luminance The second output from the control module 1245 controls the amplitudes of electrical intensity delivered by the control modules 31 The realization of the electronic control device 1 according to the invention can advantageously be made on a single electronic card grouping together the electronic processing and calculation unit 12 and the optoelectronic devices 11 and 110 This same electronic card can also have on its face opposite the light-emitting diodes of the light box, so the optoelectronic devices are necessarily in environmental conditions close to those of the diodes of the light box

Claims

1 Electronic servo device (1) for photometric or colorimetric characteristics for a light box (2) for lighting of imagers with optical valve, in particular with matrix liquid crystal screens, said box comprising at least a first and a second network ( 22) of light-emitting diodes (222), said arrays being controlled by control electronics (3), the first array consisting of a first type of diodes emitting light in a first spectral band, the second array consisting of a second type of diodes emitting light in a second spectral band, said electronic servo device (1) comprising an electronic processing and calculation unit (12) making it possible to control the control electronics (3) of light emitting diodes and optoelectronic devices (1 1, 1 10) for measuring photometric and colorimetric characteristics light emitting diodes (222) connected to said electronic processing and calculation unit (12), said optoelectronic devices being characterized in that they comprise at least a first optoelectronic assembly consisting of a light emitting diode of the type identical to one of the types diodes of the light box and a photosensitive sensor placed opposite said light-emitting diode, said diode being controlled by the control electronics (3) of the diodes networks of the light box controlling this type of diode, said assembly comprising means or a structure making it possible to isolate it from outside light and said assembly being placed in an environment close to that of the light box

2 electronic servo device (1) according to claim 1, characterized in that said electronic servo device (1) comprises as many different optoelectronic assemblies (1 1) as there are different types of light emitting diodes in the light box (2) 3 electronic servo device (1) according to claim 1, characterized in that the optoelectronic devices comprise at least a second optoelectronic assembly (1 10) consisting of at least one photosensitive sensor, said sensor being placed in the light box (2) or in the vicinity of said box so as to capture part of the light from the imaging light

4 electronic servo device (1) according to claim 1, characterized in that the electronic processing and calculation unit (12) comprises at least

A storage unit (122) for the set values of the photometric and colonmetric parameters,

A processing unit (121) for the data coming from the various photosensitive sensors connected to the optoelectronic measurement devices,

• an electronic comparator (123) comparing the data from the processing unit with the set values,

• a control unit (124) connected on the one hand to the electronic comparator (123) and on the other hand to the control electronics (3, 31) of the light-emitting diode arrays making it possible to maintain the set values of the photometric and colorimetric parameters

5 electronic servo device (1) according to claim 4, characterized in that the control unit (124) of the diode arrays controls the light emission of the diodes by at least a first electronic device for temporal modulation of their intensity electric says to PWM (acronym Anglo-Saxon of Puise Width Modulation)

6 electronic servo device (1) according to claim 4, characterized in that the control unit (124) of the diode networks control I emission of the diodes by, at least

• a first electronic device (1245) for temporal modulation of their electrical intensity known as PWM (acronym) Anglo-Saxon Puise Width Modulation), said device being connected to the various control electronics (3, 31) making it possible to obtain the set value of the photometric parameter, • a second electronic control device (1246), said device also being connected to the various control electronics (3, 31) and controlling the amplitude of the electric intensity of the light-emitting diodes, said amplitude modulation making it possible to obtain the set values of the colorimetric parameters

8 electronic servo device (1) according to one of the preceding claims, characterized in that it comprises a single electronic card grouping the electronic processing and calculation unit (12) and the optoelectronic devices (1 1, 1 10)

9 electronic card, characterized in that it comprises on one of these faces the electronic control device (1) according to one of the preceding claims and on its opposite face the light-emitting diodes of the light box (2)

1 0 Light box (2) connected to a servo device according to one of the preceding claims, characterized in that it comprises three types of light-emitting diodes (222), the diodes of the first type emitting light substantially in green, the diodes of the second type emitting light substantially in red, the diodes of the third type emitting light substantially in blue, the lighting obtained being substantially white

1 1 Lighting assembly characterized in that it comprises at least one light box (2), an electronic control unit (3) and a control device (1) according to one of the preceding claims

12. Visual valve display for aeronautical applications, characterized in that it comprises a servo device according to one of the preceding claims.