1. Field of the Invention
The field of the invention is that of the back-lighting of passive viewing screens also called LCDs for “Liquid Crystal Displays”. These screens are light modulators and require an external lighting source in order to operate.
2. Description of the Prior Art
In a certain number of applications, in particular in the aeronautical field, these screens are used by day and at night. Consequently, the lighting source must possess a high luminance range so as to ensure both correct daytime contrast under strong sunshine and a faintly luminous night-time image so as not to hinder the pilot's nocturnal vision. Thus, luminance ranges of the order of 1000 to 10 000 may be specified.
Technically, to achieve these high ranges, use is made of control signals modulated in terms of duty ratio, also called “PWM” for “Pulse Width Modulation”. These periodic signals comprise, during each period, a variable activation time. However, the specified luminance range may be greater than the range of the PWM control signal provided. For example, the range of the PWM signal may be limited to 100 whereas the required range is of the order of 1000.
For certain lighting sources, control by duty ratio turns out to be sufficient. Mention will be made notably of fluorescent lamps of “HCFL” (“High Cathode Fluorescent Lamp”) or “CCFL” (“Cold Cathode Fluorescent Lamp”) type. Indeed, when the activation time is very small, having regard to the technical nature of these sources, the light emitted is not proportional to the activation time but is much smaller than the latter whereas, when the activation time is greater, the light emitted becomes proportional to the activation time. For example, for an activation time corresponding to 1% of the period of the PWM, the quantity of light emitted will be 0.1% of the possible maximum, whereas, for an activation time corresponding to 50% of the period of the PWM, the quantity of light emitted will be close to 50% of the possible maximum. Thus, naturally, the sought-after increased luminance range is obtained.
However, certain lighting sources like light-emitting diodes or LEDs have very low response times. Having regard to their performance in respect of dimensions, luminous efficiency and lifetime, LEDs are increasingly used to achieve lighting sources for display screens. In this case, the previous effect is no longer present. If the light-emitting diodes are solely controlled by the PWM signal, the luminance emitted is directly proportional to the activation time of the PWM, no longer making it possible to obtain the sought-after effect, that is to say a high brightness range.
To alleviate this drawback, the modulation of the luminance of the LEDs is achieved either by modulating the amplitude of the current which passes through them, or by modulating the activation time over a given period by a PWM signal, or by combining the two modulations to obtain a very high depth of modulation. Technically, to carry out this modulation of the amplitude/modulation of the activation time distribution, use is made of an arithmetical and logical calculation function which works on digital signals.
However, this technical solution may exhibit certain drawbacks. In the aeronautical context, even if these calculation resources are justified by other needs, the PWM/amplitude distribution calculation function is subject to the most constraining procedures of development and certification of RTCA/DO-254 type, entitled “Design Assurance Guidance For Airborne Electronic Hardware” or RTCA/DO-178 type, entitled “Software Considerations in Airborne Systems and Equipment Certification”.
Moreover, on account of problems of obsolescence related to the gradual disappearance of fluorescent lamps, equipment manufacturers are tending to replace back-lighting based on fluorescent lamps with lighting units based on LEDs. Now, as has been seen, fluorescent lamps are controlled by a simple PWM signal. In these cases, the equipment manufacturer or the aircraft manufacturer does not want to introduce modifications of the existing numerical calculation functions so as to avoid any re-certification of the viewing device or to add any, necessarily complex, digital circuit carrying out the PWM/amplitude distribution calculation.
The device according to the invention makes it possible to alleviate these various drawbacks. Indeed, it comprises analog electronic means making it possible to generate a control signal for the intensity of the electric current passing through the light-emitting diodes and which, combined with control by a conventional PWM signal, makes it possible to achieve high luminance ranges.
More precisely, the subject of the invention is a device for controlling luminance of a lighting device comprising light-emitting diodes, the said control device being driven by a cyclic input signal of determined period, each period comprising an activation time representative of a determined luminance level, the said cyclic input signal controlling the turning on of the light-emitting diodes during the said activation time, the said control device comprising analog electronic means generating a second control signal for the intensity of the electric current passing through the light-emitting diodes, characterized in that the amplitude of the second control signal is an increasing function of the activation time in such a way that the combination of the cyclic input signal and of the second signal applied to the light-emitting diodes gives a greater luminance range than the range of the cyclic input signal.
Advantageously, in a first embodiment, the analog electronic means comprise an integrator circuit, the second signal corresponds to the output signal of the said integrator circuit, the time constant of the said integrator circuit being greater than a predetermined minimum activation time.
Advantageously, in a second embodiment, the analog electronic means comprise an amplitude ramp generating circuit devised in such a way that the amplitude of the second signal is sawtooth-shaped, the period of the sawtooth being that of the cyclic signal.
The invention also relates to a viewing device comprising a display screen with light modulation, a lighting device comprising light-emitting diodes and a device for controlling the said lighting device such as defined hereinabove.
The invention will be better understood and other advantages will become apparent on reading the nonlimiting description which follows and by virtue of the appended figures among which:
By way of example,
The control device 11 is driven by a cyclic input signal denoted as previously SPWM. This signal has an insufficient range to cover the whole of the luminance range required for the diodes. For example, the range of the PWM signal is from 1 to 100 whereas the luminance range is from 1 to 1000.
The signal SPWM directly controls the turning on of the array of diodes. This control is symbolized by a switch I in
There exist various simple means making it possible to embody the electronic means 11. By way of first exemplary embodiment.
The simplest electronic circuit making it possible to carry out this function is an integrator circuit or RC circuit essentially comprising a resistor R and a capacitor C. This circuit is represented in
Each figure comprises three curves, dependent on the time t for about a period T of the PWM signal. The top curve represents the binary activity of the signal SPWM, the intermediate curve the amplitude variation of the signal SA-A applied to the diodes control circuit, the bottom curve the intensity of the current ILED which actually passes through the diodes and which is modulated both by the signal SPWM and the signal SA-A.
The activation time TA of
The activation time TA of
The activation time TA of
By way of second exemplary embodiment,
The electronic circuit of
In a certain number of cases, it is not possible to achieve a ramp extending temporally over the whole of the period of the PWM signal. Typically, the range of the PWM signal can be two to three decades whereas the range of the ramp extends only over a decade. In this case, the amplitude of the signal SA-A becomes an affine function of the activation time TA only when TA becomes greater than a certain value TA0:
This is what is illustrated in
When the duration of the ramp covers almost the entire period T, a second curve C2′ represented by the curve shown as a continuous bold line is also obtained. In this case, the luminance range is in this case much greater than the range of the signal SPWM.
When the duration of the ramp covers just a part of the entire period T, the third curve C3′ shown as a bold dotted line is obtained. In this case, the luminance range L is less than the previous.
As in the previous example, each figure comprises three curves, dependent on the time t for about a period of the PWM signal. The top curve represents the binary activity of the signal SPWM, the intermediate curve the amplitude variation of the signal SA-A applied to the diodes control circuit, the bottom curve the intensity of the current ILED which actually passes through the diodes and which is modulated both by the signal SPWM and the signal SA-A.
Of course, it is possible to embody numerous possible variants on the basis of these two exemplary embodiments. It is notably possible to alter the durations of return to the minimum level of the setting level of the current in the LEDs when the conduction in the LEDs is interrupted at the end of the activation time TA.
It should be noted that for each of the various possible embodiments, it is always possible to add a slaving device making it possible to adjust the duration of activation so as to obtain exactly the desired luminance.
The advantages of the control device according to the invention are as follows:
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