IMAGE PROJECTION DEVICE WITH SYNCHRONIZED LED

Description

TECHNICAL FIELD

[0001] The technical field of the invention is that of image projection devices and more particularly those equipping motor vehicles and enabling the projection of images onto the ground. TECHNOLOGICAL BACKGROUND

[0002] Today there are such devices, sometimes called "dynamic carpet projectors" in English (modular floor projectors), which allow images to be projected onto the floor when a door is opened or unlocked, or when approaching it, when the driver brings a key or a vehicle opening badge close to it.

[0003] Such a device uses, for example, a disc on which images are placed. This device includes a light source, usually an LED, which produces a beam of light. This beam is shaped by an illumination or collimation lens located downstream of the source, between the source and the disc. To project a particular image from the disc, an electric motor rotates the disc, bringing the image to be projected into the path of the light beam (in practice, in front of the illumination lens). The device also includes an optical projection system, located opposite the source, on the other side of the disc. This system includes at least one lens and projects the image placed in the light beam onto the ground. An example of such a device is given in document WO 2021 / 228369 A1.

[0004] In such a device, the LED (according to the Anglo-Saxon acronym for "Light Emitting Diode", i.e., light-emitting diode or LED) is generally powered by an electric current that varies over time, with a square wave variation.

[0005] It is desirable that this square wave variation meet certain criteria. In particular, its frequency of variation should be quite high, for example, around 50 Hz or more, to prevent flickering or brightness fluctuations from being perceived by the user. Indeed, from an electrical standpoint, the response time of an LED is generally very short, so the LED stops emitting light as soon as it is no longer powered, between two square wave pulses. If the frequency of the square wave signal is low, this repeated switching off of the LED can be perceived by the user, due to the limited persistence of vision in the human eye, and give rise to an undesirable visual impression of flickering.

[0006] It is also generally desirable to use a moderate duty cycle, so that between two square pulses there is sufficient time to switch from one image to the other (by rotating the disk). while the LED is off. This allows you to switch from one image to another without blurring or overlapping between images, or masking parts of the image.

[0007] The duty cycle is then chosen to achieve a compromise between: on the one hand, a sufficiently long extinction interval between two pulses to allow for image change, and on the other hand, an extinction interval that is not too long so as not to be perceptible to the eye, thus avoiding a flickering sensation in the projected images. This compromise most often results in a fairly low duty cycle, on the order of 10 to 20%, for example.

[0008] This type of projection device improves visibility in the vehicle's access area, creates visual effects in its vicinity, and communicates information to vehicle users in an easily understandable way as they approach the vehicle. Existing projection devices are generally satisfactory in terms of size, robustness, and the visual quality of the projected images.

[0009] Nevertheless, in a continuous effort to reduce energy consumption, particularly for vehicles, it is desirable to decrease the electrical energy consumed by such a projection device, while maintaining its performance in terms of display, i.e., in terms of visual rendering. SUMMARY

[0010] In this context, the present technology relates to an image projection device comprising: at least one LED, which emits a beam of light when powered by an electric current, an image-changing module comprising different images to be projected mounted or engraved on a moving element coupled to an electromechanical actuator, configured to selectively place one or the other of the images to be projected onto the path of the light beam, on command, and an electrical control circuit, connected to the LED and to the actuator of the image-changing module, the electrical control circuit being configured such that: in an image display mode: the image present on the path of the light beam is held in position and the electric current that powers the LED varies over time with a first duty cycle and a first peak current,and in an image-changing mode: the electric current powering the LED varies over time with a second duty cycle lower than the first duty cycle, and with a second peak current higher than the first peak current, and the actuator sets the moving element in motion to replace the image present in the beam path with another of said images.

[0011] In image-changing mode, the LED is therefore powered with the second duty cycle, which is low (lower than the first duty cycle). The time interval during which the LED is off is thus significant, allowing the image to change while the LED is off. To maintain a comparable (or even identical) average brightness for the user, the peak electrical current is, however, higher than in image display mode.

[0012] Conversely, in image display mode, the LED is powered with a high first duty cycle (which isn't a problem for image changes, since the projected image isn't changed in this mode). Increasing the duty cycle in this way allows, for the same average brightness, a corresponding reduction in the peak current (the first peak current is actually lower than the second peak current used in image change mode).

[0013] However, the energy efficiency of an LED is better when the peak current is lower. Indeed, as illustrated on the figure 5(which represents a typical LED current-voltage characteristic), an increase in the supply current I is accompanied by an increase in the supply voltage U (due to the non-zero internal resistance of the LED), and therefore an increase in the electrical power consumed. Thus, as an example, with a duty cycle R1 of 90% and a peak current of 11, the average electrical power consumed will be P1 = 0.9 × I1 × U(11). Whereas with a duty cycle R2 of 10% and a peak current I2=9×I1, the average light power produced will be the same (or almost the same), while the average electrical power consumed will be P2=0.1×I2×U(I2)=0.9×I1×U(9×I1), which is therefore significantly greater than P1, since U increases with I, for the LED (this decrease in efficiency, when increasing the peak current, is explained by the losses in the non-zero internal resistance of the LED).

[0014] It is therefore particularly advantageous to control the LED with different duty cycles in image display mode and image change mode, as this allows: in image change mode, to have sufficient off time to change the image, while driving the LED with a high duty cycle and low peak current, and therefore with high energy efficiency the rest of the time.

[0015] Compared to a device in which the LED would be controlled all the time with the same duty cycle, the present device therefore makes it possible to reduce energy consumption while maintaining the same characteristics in terms of display and visual rendering.

[0016] In addition to the characteristics mentioned above, the device just presented may have one or more of the following optional characteristics, considered individually or in all technically feasible combinations: the product of the first duty cycle by the first peak current is equal to the product of the second duty cycle by the second peak current, to within 20% or even 10%; in the image-changing mode: the electric current has a square wave variation, the current being equal to said second peak current during a supply interval and being substantially zero during a shutdown interval, and in which the actuator is controlled so as to replace the image present on the beam path during said shutdown interval;the electrical control circuit is configured so that, when switching between image display mode and image change mode, the device operates in a transition mode, during which the electrical current has a square wave time variation, with an intermediate duty cycle between the first and second duty cycles, and with an intermediate peak current between the first and second peak currents; in the transition mode, the electrical current varies in a square wave pattern, presenting several successive square waves whose duty cycle varies gradually and monotonically from one square wave to the next, and whose peak current varies gradually and monotonically from one square wave to the next;If the transition in question is a transition from image display mode to image change mode, then the gradual and monotonic variation of the duty cycle is a gradual decrease, from one time slot to the next, from the first duty cycle to the second duty cycle (while the peak current gradually increases); Conversely, if the transition in question is a transition from image change mode to image display mode, then the gradual and monotonic variation of the duty cycle is a gradual increase, from one time slot to the next, from the second duty cycle to the first duty cycle (while the peak current gradually decreases); in the transition mode, the product of the duty cycle and the peak current is constant, from one time slot to the next, to within 20% or even 10%; this product is, for example, equal to the product of the first duty cycle and the first peak current.In image change mode, the electrical current varies in pulses, presenting a single pulse before switching from image change mode to image display mode, or from image change mode to transition mode; the projection device further includes an optical projection system 21 arranged to project onto the ground, from a motor vehicle, the image placed in the path of the light beam; the electrical control circuit is configured to: receive a signal to open or unlock a vehicle opening, and in response to said signal, control a switch from image display mode to image change mode.

[0017] This technology also relates to a method for controlling an image projection device, the device comprising: at least one LED, which emits a beam of light when powered by an electric current, an image-changing module comprising different images to be projected mounted or engraved on a moving element coupled to an electromechanical actuator, configured to selectively place one or the other of the images to be projected in the path of the light beam, on command, and an electrical control circuit, connected to the LED and to the actuator of the image-changing module, a method in which: in an image display mode: the image present in the path of the light beam is held in position and the electric current supplying the LED varies over time with a first duty cycle and a first peak current, and in an image-changing mode: the electric current supplying the LED varies over time with a second duty cycle lower than the first duty cycle,and with a second peak current greater than the first peak current, and the actuator sets the moving element in motion to replace the image present on the beam path with another of said images.

[0018] The optional features presented above in terms of device can also be applied to the process that has just been described.

[0019] The present technology and its various applications will be better understood by reading the following description and examining the accompanying figures. BRIEF DESCRIPTION OF THE FIGURES

[0020] The figures are presented for illustrative purposes only and are not exhaustive. [ Fig. 1 [ ] schematically represents a vehicle equipped with an image projection device implementing this technology, viewed from the side. ] Fig. 2 [ ] schematically represents the image projection device, seen in perspective. ] Fig. 3] schematically represents a disk of the device of the figure 2 . [ Fig. 4 ] represents elements of the device of the figure 2 , in the form of functional blocks. Fig. 5 [ ] schematically represents a current-voltage characteristic of an LED in the device of the figure 2 . [ Fig. 6 [ ] schematically represents the operating points of the device, in a frequency-duty ratio plane. ] Fig. 7 ] schematically represents the evolution over time of an electric current powering the LED. DETAILED DESCRIPTION

[0021] There figure 1 represents a motor vehicle 1 equipped with an image projection device, 2, allowing the projection of an image 3 onto the ground. As will be seen below, the device 2 can be controlled by opening a front or rear door 10, or another opening such as the trunk 11.

[0022] Device 2 is placed on the undercarriage, for example directly beneath the vehicle where height is limited and the environment is harsh (water spray, risk of impact with road debris, etc.). It is therefore protected by a small enclosure (since the device is 20cm to 30cm from the ground), for example, 4 to 10cm on each side.

[0023] The image projection device 2 includes at least one LED 60 (possibly several) which serves as a light source. This LED emits a beam of light, F, when powered by an electric current, I. The device 2 also includes an illumination lens 20 (more generally, an optical system comprising at least one lens), which acts as a condenser. This lens shapes the light beam emitted by the LED, in particular by reducing its divergence. The illumination lens 20 is placed downstream of the LED 60, between the LED and the image to be projected (Im1, on the figure 2 The device 2 also includes an optical projection system 21 arranged to project the image to be projected from the motor vehicle 1 onto the ground. The optical projection system 21 is located in the path of the light beam F, downstream of the image to be projected. Here, it is situated opposite the LED 60, on the other side of a disk 5 that carries the image to be projected. The optical projection system 21 comprises one or more lenses or mirrors. It forms an image 3 of the image to be projected Im1 on the ground, with a very high magnification and, generally, a dilation effect. Indeed, the image 3 formed on the ground is at least 0.5 m on each side and can even occupy an area 1 m long by 1 m wide or more, and the ground is illuminated grazingly by the projection device.

[0024] The projection device 2 also includes an image-changing module 7, which contains various images Im1, Im2, Im3,... to be projected, mounted or engraved on a moving element, here a disk 5. The disk 5 is coupled to an electromechanical actuator, here an electric motor 4, by a shaft 40. It should be noted that a moving element other than a disk could be used to support and move the images, as an alternative. In any case, the image-changing module 7 is configured to selectively (i.e., one image at a time), on command, place one or the other of the images to be projected onto the path of the illumination beam F. To do this, the motor 4, typically a stepper motor, rotates the disk 5 to bring the image to be projected onto the ground within the light beam F, in front of the projection optical system 21.

[0025] The images Im1, Im2, Im3,... of the change module are each formed on an image support, for example, glass, which is essentially transparent (i.e., allowing most of the incident light power to pass through). These images can be engraved (possibly by laser), glued, or lithographed onto the image supports in question. The images Im1, Im2, Im3,... are arranged in a circle on disk 5 ( figure 3 ), here on the periphery of the disk, over its entire circumference, for example with a constant angular gap between images.

[0026] The projection device 2 also includes an electrical control circuit 8, connected to the LED 60 and the motor 4 ( figure 4 The electrical control circuit 8 can be implemented in whole or in part on a printed circuit board 6 located in the housing of the projection device (and to which the LED 60 is soldered). It includes, for example: a controllable current source, connected to the LED 60 to power it electrically, an amplifier stage, of the driver type, to control the motor 4 (unless this driver is included in the motor), and a control module, for example of the programmable circuit type, to control the current source of the LED and the motor driver.

[0027] The control module also includes receiving means, for example a wired or wireless communication interface, to receive a signal s , originating from another electronic unit of the vehicle, the electronic control circuit 8 being configured to control the LED and the motor according to the signal sreceived (as explained in detail later). Alternatively or in addition, the receiving means of the electronic control circuit could be configured to receive this signal directly from a remote control device of the vehicle, such as a mobile phone or a "flip" key.

[0028] The electrical control circuit 8 is configured to control the LED 60 and the motor 4 in a synchronized manner, in particular, to command the motor to change the image during a period of time during which the LED is kept off.

[0029] Remarkably, the electrical control circuit 8 is configured to command the projection device to operate alternately, according to: an image display mode, in which the image present on the path of the lighting beam is held in position, and an image change mode, the LED 60 being powered with different duty cycles, in the image display mode, and in the image change mode.

[0030] In this case, in the image display mode, which corresponds to the S1 phases, on the figures 6 And 7 The electric current I that powers LED 60 varies over time t with a first duty cycle R1 and a first peak current I1. While in image change mode (phase S2, on the figure 7The electric current I that powers the LED varies over time with a second duty cycle R2 lower than the first duty cycle R1 (for example, less than 0.8×R1, or even less than 0.5×R1), and with a second peak current I2 higher than the first peak current I1 R1 (for example, greater than 1.25×I1, or even greater than 2×I1). It is in the image-changing mode that the motor 4 sets the disk 5 in motion to replace the image (Im1, for example) present in the path of the light beam F with another image on the disk (Im2, for example).

[0031] As explained in more detail in the "summary" section, this provision allows: in image change mode, to have a sufficiently long LED off time to change the image without it being visually perceptible (thanks to a reduced duty cycle), while driving the LED with a high duty cycle R1 and a low peak current I1, and therefore with a high energy efficiency Eff in image display mode.

[0032] The electric current I that powers LED 60 has a square wave variation here. That is to say, it varies by alternately exhibiting: during a feeding time interval (for example, the interval t1, on the figure 7 ), a constant intensity, called peak current, then during an extinction interval (for example, the interval t2, on the figure 7 ), a zero or substantially zero intensity (i.e. less than one tenth of the peak current, or even less), then, again, a constant intensity, then zero, and so on.

[0033] The duty cycle of this square wave variation, denoted R, is equal to the duration of the on interval (t1) divided by the sum of the duration of the on interval (t1) and the duration of the off interval (t2). Thus, when R=10%, the LED is on for only 10% of the time period T of the square wave signal (it is then on for 9 times less time than it is off during this period).

[0034] Alternatively, the electric current could have a variation that is not exactly square (for example, with a slight variation in intensity during the power interval), but nevertheless, as before: a supply time interval, during which the current has a non-negligible intensity, for example greater at each instant than 1 / 10 of the maximum current intensity (again called peak current); then a shutdown time interval, during which the current has zero intensity or at least substantially zero (less than 1 / 10 of the peak current, or even less); then again a supply interval, then a shutdown interval, and so on...

[0035] In this case, the duty cycle remains defined as indicated above (duration of the supply interval t1, divided by the sum of the duration of the supply interval t1 and the duration of the shutdown interval t2).

[0036] There figure 6The graph shows, in the frequency-duty cycle plane, operating points corresponding respectively to the image display mode (S1) and the image change mode (S2). On this graph, the frequency f of the electrical current variation (square wave signal frequency) is represented on the x-axis, while the duty cycle R, expressed as a percentage, is represented on the y-axis.

[0037] On the figure 6Curve Cv1 shows the limit above which it is desirable to work to prevent an individual from perceiving flicker or fluctuations in the brightness of the image 3 formed on the ground. This curve illustrates that it is desirable for the frequency f to be sufficiently high and / or for the duty cycle R to be sufficiently high to avoid such flicker (indeed, a high frequency or duty cycle leads to a short, imperceptible extinction time between two current pulses). The hatched area below curve Cv1 thus corresponds to an area to be avoided in the f-R plane.

[0038] Curve Cv2 shows the threshold below which it is desirable to operate to ensure sufficient time for image switching during the LED's off interval between two current pulses. This curve illustrates that it is desirable for the frequency f to be relatively low and / or for the duty cycle R to be relatively low, in order to have a sufficiently long off time to allow for image switching while the LED is off. The area of ​​dashed hatching, located above curve Cv2, therefore corresponds to an area to be avoided, at least when image switching is planned.

[0039] In prior art projection devices, the same constant duty cycle is used continuously. It is then chosen from the unhatched area between these two curves to satisfy the two criteria mentioned above. This leads to the continuous use of a rather low duty cycle (typically on the order of 10 to 20%), which is not very favorable from the point of view of the LED's energy efficiency (Eff).

[0040] Here, on the contrary, the LED is powered: with the second duty cycle R2, which corresponds to an operating point located in the unhatched area (and which is therefore compatible with the mechanical response time of the image change module), when the device operates in image change mode (phase S2), and with the first duty cycle R1, higher, and therefore more favorable in terms of energy efficiency (and which corresponds on the other hand to an operating point located outside the unhatched area) in image display mode, when the projected image is held fixed.

[0041] There figure 7represented the time evolution of the electric current I that powers the LED, for a sequence during which the device operates in display mode (phase S1), then in image change mode (phase S2), to change the projected image, and, between the two, in a transition mode (phase ST), with a gradual decrease in the duty cycle from the first duty cycle R1 to the second duty cycle R2.

[0042] As already mentioned, in image display mode (phase S1), the electrical current I varies in pulses, with the first duty cycle R1 being quite high, typically exceeding 30% and even reaching 60%. In the example shown, R1 is 90%. The pulse frequency f is, for example, between 50 and 150 Hz. This first phase S1 can, as here, comprise a large number N of periods (i.e., pulses; indeed, with such a projection system, the same image is often projected for quite some time, for example, 1 second or even a few seconds, before there is an image change).

[0043] A switchover to image-changing mode is then triggered at time t0, for example, following the reception of a signal s opening of opening 10, 11 by the electrical control circuit 8.

[0044] This switchover is performed gradually, passing through operation in the transition mode (ST phase), during which the duty cycle gradually decreases from one pulse C1, C2, C3 to the next, progressively moving from the first duty cycle R1 to the second duty cycle R2. Simultaneously, the peak current (IT,1, IT,2 for example) gradually increases from one pulse to the next, moving from the first peak current I1 to the second peak current I2.

[0045] This gradual variation of the LED's power parameters, and therefore of the projected image's lighting parameters, makes the transition from display mode to image change mode as undetectable as possible for a user from a visual point of view.

[0046] The electrical control circuit can be configured to maintain operation in transition mode for at least two, or even at least four, periods T of the square wave variation of the electrical current. In transition mode, the electrical current varies in square waves, displaying at least two, or even at least four, successive waves, which allows for a fairly gradual variation of the display parameters. In the example shown, in transition mode, the current I displays three successive waves C1, C2, C3. The total number of waves can, for example, be between 3 and 20, allowing for a gradual variation and, at the same time, a fairly rapid transition to image-changing mode to change the image.For example, at a frequency f of 66 Hz (i.e., T=15ms), with three transition pulses C1, C2, and C3, and one pulse for the image change itself (during phase S2), approximately 60 ms are required between receiving the door opening command signal and the actual replacement of one image with another (more suitable for the door opening context). This overall reaction time of the device is short, resulting in a near-instantaneous perception for the vehicle user. A longer total operating time in transition mode, for example 0.1 s or even 0.3 s (but preferably less than 0.3 s), would still be appropriate in terms of the device's reaction time.

[0047] In the transition mode, the successive duty cycle values, corresponding to the successive electrical current pulses C1, C2, C3, can, for example, be equally distributed between the first duty cycle R1 and the second duty cycle R2. In this case, if R1 = 90% and R2 = 10%, for example, and if the current has exactly three successive pulses in the transition mode (before switching to image change mode), the three successive duty cycle values ​​for these three pulses will be 30%, 50%, and finally 70%. Similarly, the successive peak current values, corresponding to the successive electrical current pulses in the transition mode, can, for example, be equally distributed between the first peak current I1 and the second peak current I2. Other distributions (non-equal distribution) are nevertheless possible.

[0048] After operation in transition mode (phase ST), the display device immediately switches to image change mode (phase S2). In this mode, the electrical current I has a single pulse, and operation then immediately switches back to image display mode, again with a transition phase S'T between the two. This transition phase corresponds once more to operation in transition mode.

[0049] In image-changing mode, the electrical current has a pulse during the power-on time interval t1, then zero or near-zero current during the previously mentioned power-off interval t2. During the power-off interval, the control circuit 8 commands the motor 4 to rotate the disc 5 to replace the image on the illumination beam with another image from the disc.

[0050] The duration of the extinction interval t2 is at least equal to an image replacement time of the image change module 7. This image replacement time, which is a kind of mechanical response time of this module, is equal to (or possibly slightly greater than) the time required for the motor to rotate the disk from a first angular position, in which one of the images Im1, Im2, Im3, ... is in the light beam F, to a second angular position in which another of these images is located, entirely, in the light beam (with immobilization in this second angular position).

[0051] During the second transition phase S'T, the electric current varies in square waves, and the current duty cycle gradually increases from one wave to the next, progressively moving from the second duty cycle R2 to the first duty cycle R1. Simultaneously, the peak current gradually decreases from one wave to the next, returning from the second peak current I2 to the first peak current I1. Here too, the number of waves is greater than or equal to 2, and can be, for example, between 3 and 20. The S'T transition phase is symmetrical to the ST transition phase (symmetrical with respect to time t). In other words, these two phases are identical, except that one corresponds to a gradual decrease in the duty cycle, while the other corresponds to a gradual increase in the duty cycle.

[0052] The electrical control circuit 8 is configured so that, in its different operating modes, and therefore during the different operating phases S1, ST, S2, S'T, S1 that have been presented: the product of the duty cycle and the peak current is constant (from one pulse to the next), to within at least 20%, or even 10%. This makes it possible to maintain a constant apparent brightness (average brightness – perceived by the human eye), or at least a substantially constant one, during these different phases. It is therefore assumed here, in particular, that I1×R1=I2×R2 (to within at least 20%, or even 10%). And also that IT,1 ×RT,1 = IT,2 ×RT,2 = I1×R1, among other things.

[0053] The switch from static display mode to image change mode could be triggered following the reception, directly by the electrical control circuit, of a door opening control signal emitted by a remote control device of the vehicle with transmitter, such as a mobile phone or a "plip" key.

[0054] Here, the switch from display mode to image change mode (and therefore the triggering of the sequence ST, then S2, then S'T, then S1) is however triggered by the reception, by the electrical control circuit 8, of a signal s transmitted by another electrical unit of vehicle 1 (for example, via a CAN network or equivalent), this other unit being responsible for the centralized control of the openings, or for the vehicle's wake-up command, for example. In this case, the signal sreceived by the electrical control circuit 8, and according to which a change in the projected image is triggered, is a signal to open the vehicle's opening 10, 11. This signal s This can be produced following the receipt, by the electronic unit in question, of an opening command from a remote key fob or equivalent. The signal s may be issued by the electronic unit in question (responsible for centralized opening) when it detects that an authorized user is near the vehicle, for example by detecting the proximity of a vehicle user identification badge (possibly a passive electronic badge), or a vehicle opening badge, an authorized mobile phone, or another identifiable electronic device.

[0055] As can be seen from the description above, the electrical control circuit 8 of the projection device is configured to control this device 2 according to a control method in which: in image display mode: the image (Im1 for example) present on the path of the lighting beam is held in position and the electric current I which supplies the LED 60 varies over time t with the first duty cycle R1 and the first peak current 11, and in image change mode: the electric current I which supplies the LED varies over time with the second duty cycle R2 lower than the first duty cycle R1, and with the second peak current I2 greater than the first peak current I1, and the actuator 4 of the image change module 7 sets in motion the moving element 5 to replace the image present on the path of the beam with another of said images (for example Im2).

[0056] This process includes the following steps: operation in image display mode, then, in response to signal reception s opening of the opening by the electrical control circuit 8, operation in the transition mode, then (once the transition has been completed) operation in the image change mode, with replacement of the projected image by another, then again, operation in the transition mode, to gradually return to the display mode, then resumption of operation in the display mode.

Claims

1. An image projection device (2) comprising: - at least one LED (60), which emits an illumination light beam (F) when it is powered by an electric current (I), - an image substitution module (7) comprising various images (Im1, Im2, Im3) to be projected that are mounted or etched on a movable element (5) coupled to an electromechanical actuator (4), configured to selectively place one (Im1) or the other (Im2) of the images to be projected on the path of the illumination light beam (F) on command; and - an electrical control circuit (8), connected to the LED and to the actuator (4) of the image substitution module, the device being characterized in that the electrical control circuit (8) is configured so that: - in an image display mode (S1): the image (Im1) present on the path of the illumination light beam is held in position and the electric current (I) that powers the LED (60) varies over time (t) with a first duty cycle (R1) and a first peak current (11); and - in an image substitution mode (S2): the electric current (I) that powers the LED varies over time with a second duty cycle (R2) lower than the first duty cycle (R1), and with a second peak current (I2) higher than the first peak current (11), and the actuator (4) moves the movable element (5) in order to replace the image (Im1) present on the path of the beam with another one of said images (Im2, Im3).

2. The device (2) as claimed in claim 1, wherein the product of the first duty cycle (R1) and the first peak current (11) is equal to the product of the second duty cycle (R2) and the second peak current (I2), to within 20 % or even 10 %.

3. The device (2) as claimed in any of the preceding claims, wherein, in the image substitution mode: the electric current (I) exhibits a square wave variation, with the current being equal to said second peak current (I2) during a power supply interval (t1) and being substantially zero during a turning-off interval (t2), and in which the actuator (4) is controlled so as to replace the image present on the path of the beam during said turning-off interval (t2).

4. The device (2) as claimed in any of the preceding claims, wherein the electrical control circuit (8) is configured to control the device to operate according to a transition mode (ST, S'T) when switching between the image display mode (S1) and the image substitution mode (S2), during which transition mode the electric current (I) exhibits a square wave time variation, with an intermediate duty cycle ranging between the first duty cycle (R1) and the second duty cycle (R2), and with an intermediate peak current ranging between the first peak current (11) and the second peak current (I2).

5. The device (2) as claimed in the preceding claim, wherein, in the transition mode (ST, S'T), the electric current (I) exhibits a square wave variation by having several successive square waves (C1, C2, C3) whose duty cycle (R) varies gradually and monotonously from one square wave to another and whose peak current (IT,1, IT,2) varies gradually and monotonously from one square wave to another.

6. The device (2) as claimed in claim 4 or 5, wherein, in the transition mode (ST, S'T), the product of the duty cycle and the peak intensity is constant from one square wave to another, to within 20 % or even 10 %.

7. The device (2) as claimed in any of the preceding claims, further comprising an optical projection system (21) arranged so that the image (Im1) placed on the path of the illumination light beam (F) is projected onto the ground from a motor vehicle (1).

8. The device (2) as claimed in the preceding claim, wherein the electrical control circuit (8) is configured for: - receiving a signal (s) for opening or unlocking a door (10, 11) of the vehicle (1); and for - controlling, in response to said signal, a switch from the image display mode (S1) to the image substitution mode (S2).

9. A method for controlling an image projection device (2), the device comprising: - at least one LED (60), which emits an illumination light beam (F) when it is powered by an electric current (I), - an image substitution module (7) comprising various images (Im1, Im2, Im3) to be projected that are mounted or etched on a movable element (5) coupled to an electromechanical actuator (4), configured to selectively place one or the other of the images to be projected on the path of the illumination light beam (F) on command; and - an electrical control circuit (8), connected to the LED (60) and to the actuator (4) of the image substitution module, characterized in that, during the method: - in an image display mode (S1): the image (Im1) present on the path of the illumination light beam is held in position and the electric current (I) that powers the LED (60) varies over time (t) with a first duty cycle (R1) and a first peak current (I1); and - in an image substitution mode (S2): the electric current (I) that powers the LED varies over time with a second duty cycle (R2) lower than the first duty cycle (R1), and with a second peak current (I2) higher than the first peak current (11), and the actuator (4) moves the movable element (5) in order to replace the image (Im1) present on the path of the beam with another one of said images (Im2, Im3).