Lamp and corresponding control method

Abstract

Lamp and corresponding control method. The invention relates to a lamp (1), in particular a portable one such as a headlamp, comprising: - one or more light sources (2), intended to emit light generally parallel to the direction of the user's gaze, - one or more light sensors (6), - a control circuit (8). The control circuit (8) is configured to increase the emitted light intensity (IE) to a first intensity value when: - the detected light intensity (IC) is below a predetermined threshold after having been greater than or equal to said predetermined threshold for a duration greater than or equal to a first predetermined duration, and - the emitted light intensity (IE) is below said first value. Figure for the abstract: Fig. 2.

Description

Title of the invention: Lamp and corresponding control method technical field

[0001] The present invention relates to the field of sports equipment and in particular to the field of personal lighting. More specifically, the present invention relates to portable lamps, in particular headlamps. Previous technique

[0002] There already exist headlamps comprising, in addition to lighting means, a light sensor and means for controlling the lighting means, allowing the brightness emitted by the lighting means to be adjusted according to the brightness received by the sensor.

[0003] US patent 6,966,668 discloses a lamp with a sensor, in which the lamp's power is reduced proportionally when the intensity of the light captured by the sensor increases. The purpose of such a lamp is to limit energy consumption by reducing the lamp's power when external conditions permit.

[0004] However, such a lamp does not always provide satisfactory results, particularly when the ambient light illuminates the user of the lamp more than the area they are looking at. In such a case, not only is the user dazzled by the ambient brightness, but the area illuminated by the lamp is also less brightly lit, making it even less visible to the user.

[0005] In document FR 2 930 706, the lamp measures, via a sensor, the reflection of the light emitted by an object more or less close to the user, and adapts accordingly the power of the lamp so that the intensity of the light reflected towards the user corresponds to the level of lighting desired by the user, without the user needing to change the lighting mode of the lamp.

[0006] Such a lamp makes it possible in particular to avoid self-glare of the user when he finds himself facing a reflective object such as a map.

[0007] Document FR 2 792 594 relates to a lamp comprising in particular a control module with image sensor: an image of illuminated area is generated in order to determine the appropriate control signal of the lamp.

[0008] However, such lamps still have limited autonomy, as the reduction in light output is restricted to situations where the user is facing an object reflecting the lamp's light back towards the sensor. Description of the invention

[0009] The present invention aims to solve the various technical problems stated above. In particular, the present invention aims to provide a lamp, especially a headlamp, that offers improved battery life while maintaining the same level of user comfort. The present invention also aims to provide a lamp, especially a headlamp, that takes into account the user's physiology to increase its battery life.

[0010] Thus, according to one aspect, a lamp is proposed, in particular a portable one such as a headlamp, comprising: - one or more light sources, preferably in the visible spectrum, for example LEDs, and designed to emit light generally parallel to the direction of the user's gaze, - one or more light sensors to detect the light arriving on said sensor(s), the lamp also includes a control circuit configured to modify the intensity of the light emitted by the light source(s) according to the intensity of the light detected by the sensor(s).

[0011] The control circuit is configured to increase the intensity of the emitted light to a first intensity value: - when the detected light intensity is below a predetermined threshold after having been greater than or equal to said predetermined threshold for a period greater than or equal to a first predetermined period, for example greater than or equal to 0.5 seconds, preferably 1 second and more preferably 5 seconds, And - when the intensity of the emitted light is less than said first value.

[0012] The lamp according to the present invention takes into account the eye's adaptation to brightness in order to modify the intensity of the light sources. More specifically, the lamp according to the present invention is configured to identify glare experienced by the user, and then to increase the light intensity of the light sources once this glare has subsided. The lamp's intensity thus adapts to the eye's sensitivity, particularly when the eye has just been dazzled and is therefore less sensitive to light: the lamp's intensity is then increased at the end of the glare to compensate for this decrease in the eye's sensitivity. However, in order to limit the lamp's energy consumption, this increase in lamp intensity is only provided for at the end of the user's glare, and for a limited duration corresponding to the user's visual adaptation time (or retinal habituation).This limits the lamp's energy consumption without reducing user comfort.

[0013] Glare is defined, in the present invention, as light detected on exceeding a predetermined threshold for a predetermined duration—that is, a temporary or momentary increase in detected light. The threshold and duration are chosen to closely match a stimulus that triggers a physiological adaptation of the user's eye, such as a pupillary response. During the glare period, i.e., at the beginning of the eye's adaptation and after it has fully adapted to the glare, the lamp's control can remain unchanged. Only when the control circuit detects the end of the glare—specifically, when the detected brightness falls below the predetermined threshold again—is the lamp's intensity adjusted and increased to compensate for the loss of sensitivity due to the glare.

[0014] Furthermore, adapting the lamp intensity to glare also implies that the lamp intensity is not at its maximum value, so as to allow for an increase in lamp intensity. Thus, the lamp is designed so that the intensity of the light emitted before or during glare is lower than the value emitted after glare. In other words, the control circuit is configured to increase the intensity of the emitted light to a first intensity value, particularly when the emitted light intensity was lower than said first value before and / or during the increase in light intensity detected above said predetermined threshold.

[0015] Preferably, the control circuit is also configured to decrease the intensity of the emitted light from said first value to a second value when, for at least a second specified duration, for example greater than or equal to 15 seconds, preferably 20 seconds and more preferably 30 seconds, the detected light intensity is: - below the specified threshold, - or greater than or equal to said determined threshold for a continuous period shorter than said first determined period.

[0016] As previously stated, the increase in lamp intensity is only planned after the glare has subsided, and for a period corresponding approximately to the user's visual adaptation period. Indeed, once the glare has ended, the user's eye will adapt again to the decrease in brightness, for example through pupillary response, to become more sensitive to light once more. The control circuit is thus configured to decrease the intensity emitted by the lamp when the detected light has remained below the predetermined threshold for a second predetermined period, or when the brightness peaks occurring during the second predetermined period have remained sufficiently short so as not to trigger a corresponding reaction from the user's eye.

[0017] Reducing the lamp intensity thus leads to better battery life the battery, while remaining barely perceptible to the user whose eye sensitivity increases as the lamp intensity decreases.

[0018] Preferably, the control circuit is configured to decrease the intensity of light emitted from said first value to said second value, maintaining said intensity at a third intensity value between said first value and said second value, for a third determined duration.

[0019] It is possible to specify a particular lighting intensity in case of glare, for example, a maximum intensity. In this case, when the light intensity decreases after the second predetermined time, the lamp intensity can initially be reduced to an initial value, for example, the user's setpoint, the intensity emitted before glare, or the intensity emitted before the increase. Then, after a third predetermined time, the emitted intensity can be reduced again, below this initial value, to continue adapting to the user's visual sensitivity and to save energy.

[0020] Preferably, the control circuit is configured to decrease the intensity of light emitted from said first value to a second value, for a fourth determined duration, for example less than or equal to 10 minutes, preferably 5 minutes and more preferably less than 3 minutes.

[0021] The decrease in intensity emitted by the lamp can be provided to take place over a fourth period, more or less long, corresponding substantially to the simultaneous increase in the visual sensitivity of the user.

[0022] Preferably, the control circuit is also configured to maintain the intensity of emitted light at said second value, when the detected light intensity is: - below the specified threshold, - or greater than or equal to said determined threshold for a continuous period shorter than said first determined period.

[0023] Here again, and in order to limit the lamp's energy consumption, the increase in emitted intensity is not triggered systematically as soon as the detected brightness exceeds a threshold, but only when the detected brightness exceeds a threshold for a predetermined duration. It is indeed pointless to increase the lamp's brightness if the user's eyes have not had time to adapt to a very brief period of intense brightness. The control circuit thus adjusts the lamp's intensity based on the user's physiological response.

[0024] Preferably, the second intensity is less than or equal to 40% of the first intensity of light emitted, preferably 30% and more preferably 20%.

[0025] The reduction in light intensity can represent 60 to 80% of the lamp's light intensity, compared to the user's initial setting. The energy savings are therefore significant for a relatively small impact on the user's daily use.

[0026] According to another aspect, a method for controlling a lamp, in particular a portable one such as a headlamp, is also proposed, comprising: - one or more light sources, preferably in the visible spectrum, for example LEDs, and designed to emit light generally parallel to the direction of the user's gaze, - one or more light sensors to detect the light arriving on said sensor(s), in which the intensity of the light emitted by the light source(s) is modified according to the intensity of the light detected by the sensor(s).

[0027] According to the process, the intensity of the emitted light is increased to a first intensity value: - when the detected light intensity is below a predetermined threshold after having been greater than or equal to said predetermined threshold for a period greater than or equal to a first predetermined period, for example greater than or equal to 0.5 seconds, preferably 1 second and more preferably 5 seconds, And - when the intensity of the emitted light is less than said first value.

[0028] In particular, the intensity of emitted light can be increased to a first intensity value when the intensity of emitted light is less than said first value before and / or during the increase in the intensity of light detected above said determined threshold.

[0029] Preferably, the intensity of emitted light is decreased from said first value to a second value when, for at least a second determined duration, for example greater than or equal to 15 seconds, preferably 20 seconds and more preferably 30 seconds, the detected light intensity is: - below the specified threshold, - or greater than or equal to said determined threshold for a continuous period shorter than said first determined period.

[0030] Preferably, the intensity of light emitted is reduced from said first value to a second value, for a fourth determined duration, for example less than or equal to 10 minutes, preferably to 5 minutes and more preferably less than 3 minutes.

[0031] Preferably, the intensity of emitted light is maintained at said second value, when the detected light intensity is: - below the specified threshold, - or greater than or equal to said determined threshold for a continuous period shorter than said first determined period.

[0032] According to another alternative or complementary aspect, a lamp, in particular a portable one such as a headlamp, comprising: - one or more light sources, preferably in the visible spectrum, for example LEDs, and designed to emit light generally parallel to the direction of the user's gaze, - one or more light sensors to detect the light arriving on said sensor(s), the lamp also includes a control circuit configured to modify the intensity of the light emitted by the light source(s) according to the intensity of the light detected by the sensor(s).

[0033] The control circuit is configured, particularly in the lamp's steady-state operating mode, to decrease the emitted light intensity to a second intensity value when, for at least a second predetermined period, for example greater than or equal to 15 seconds, preferably 20 seconds and more preferably 30 seconds, the detected light intensity is: - below a certain threshold, - or greater than or equal to said determined threshold for a continuous period shorter than said first determined period.

[0034] According to another alternative or complementary aspect, a method for controlling a lamp, in particular a portable one such as a headlamp, is also proposed, comprising: - one or more light sources, preferably in the visible spectrum, for example LEDs, and designed to emit light generally parallel to the direction of the user's gaze, - one or more light sensors to detect the light arriving on said sensor(s), in which the intensity of the light emitted by the light source(s) is modified according to the intensity of the light detected by the sensor(s).

[0035] According to the alternative or complementary method, particularly in steady-state operation of the lamp, the intensity of emitted light is reduced to a second value when, for at least a second determined period, for example greater than or equal to 15 seconds, preferably 20 seconds and more preferably 30 seconds, the detected light intensity is: - below the specified threshold, - or greater than or equal to said threshold determined for a continuous period in- less than an initial specified period. Brief description of the drawings

[0036] [Fig.1] Fig.1 represents a headlamp according to the present invention;

[0037] [Fig.2] Fig.2 schematically represents the different means of a headlamp as illustrated in [Fig.1];

[0038] [Fig.3] Fig.3 represents an example of lamp control according to the present invention, depending on the outside brightness;

[0039] [Fig.4] Fig.4 represents a control flowchart for increasing the intensity emitted by the lamp according to the present invention, and

[0040] [Fig.5] Fig.5 represents a control flowchart for reducing the intensity emitted by the lamp according to the present invention. Description of the implementation methods

[0041] Figure 1 illustrates a portable lamp 1, in particular a headlamp, according to the present invention. The lamp 1 conventionally comprises one or more light sources 2, for example LEDs, as well as a carrying means 4, or fastening means, for example a tightening strap. In this case, the carrying means 4 allows the portable lamp 1 to be kept on the user, in this instance on the user's head.

[0042] The lamp 1 according to the present invention is preferably a single lamp, and the light source(s) 2 are intended to illuminate in the direction of the user's gaze. In other words, the light source(s) 2 are configured to emit light generally parallel to the direction of the user's gaze. The lamp 1 is thus intended to illuminate the space in front of the user, so that they can see it.

[0043] The lamp 1 also includes a light sensor 6 located near the light sources 2, in order to capture ambient light approaching the user, and more specifically, the user's gaze. The sensor 6 thus measures the light intensity Ic received by the user's eyes, and therefore detects any glare or, conversely, constant darkness in the surrounding environment. The sensor 6 is notably used to modify the control of the light sources 2, in order to adapt their intensity to the ambient brightness.

[0044] Thus, as illustrated in [Fig. 2], the lamp 1 includes a control circuit 8 which receives as input the value Ic of the light intensity detected by the sensor 6, and provides a value IE of the light intensity to be emitted by the light source(s) 2 of the lamp 1. In particular, the control of the light source(s) 2 by the control circuit 8 is determined according to the lighting conditions ambient measured by sensor 6.

[0045] As will be detailed below, the control circuit 8 is configured to determine the emitted light intensity command IE according to the user's visual adaptation (or retinal habituation). More specifically, the control circuit 8 is configured to decrease the light output of the light source(s) 2 when the user's visual sensitivity increases, for example after a prolonged period of exposure to low light intensity, and / or to increase the light output of the light source(s) 2 after the user is dazzled, for example after temporary exposure to high light intensity.

[0046] Such an adaptation of the control of the light source(s) 2 makes it possible, in particular, to provide the user with the light output they actually need, taking into account the ambient lighting, and specifically to have a high light output from the lamp 1 especially, or even only, when the user's visual sensitivity is low or reduced. Conversely, when the user's visual sensitivity is high, the light output of the lamp 1 is reduced in order to conserve its battery or batteries.

[0047] Fig. 3 illustrates an example of the control signal IE supplied by the control circuit 8 to the light source(s) 2, as a function of the light intensity Ic detected by the sensor 6.

[0048] In the curves of [Fig. 3], it is assumed that lamp 1 is switched on at time t0. At t0, the intensity of the light source(s) 2 is, for example, equal to IE0, and corresponds, for example, to a specific light intensity desired by the user, from among several predetermined values. The intensity IE0 can, for example, be selected by successive presses of a control button for lamp 1.

[0049] The start-up of lamp 1 corresponds to a transient operating period, or transient regime, of lamp 1 during which specific commands for the luminous intensity emitted by the light sources 2 can be provided, for example, for regulatory or standards reasons. Outside the start-up (or ignition) period of lamp 1, the lamp is considered to be in continuous operation, or steady state, that is to say, the control of the light sources 2 is determined independently of the constraints related to start-up but, for example, solely according to the user's commands and the external lighting conditions.

[0050] As can be seen in [Fig. 3], the light intensity Ic detected by the sensor 6 remains below a threshold value ICs during and after the lamp 1's ignition phase, so that the control circuit 8 can consider that the user's vision is adapting to the ambient darkness, and adjust accordingly. Consequently, the control of the light sources 2. Thus, after a predetermined time, the control circuit 8 provides the light sources 2 with a setpoint for the emitted light intensity IE, which decreases over time from the initial value IE0 to a value IE2 lower than the initial value IE0. Since the user's eyes have had time to adapt to the darkness during this decrease in the emitted light intensity IE, such a decrease remains practically imperceptible to the user, who retains the same level of comfort, while conserving the electrical energy of their lamp 1.

[0051] It is assumed that the lamp 1 is now in steady state.

[0052] At time tb, it is observed that the light intensity Ic detected by sensor 6 exceeds a threshold ICs determined by the control circuit 8. The ICs threshold corresponds to a light intensity at which the user could be dazzled. It is also observed that the light intensity Ic detected by sensor 6 remains above the ICs threshold for a continuous period corresponding to a first determined duration DSi. In this case, the control circuit 8 considers that the ambient light intensity Ic, for a duration DSi, is sufficient to cause dazzling of the user and, in particular, pupil constriction, thus reducing their visual sensitivity.

[0053] The first determined duration DSi can for example be chosen to be greater than or equal to 0.5 seconds, preferably 1 second and more preferably 5 seconds.

[0054] At time t2, the light intensity Ic detected by sensor 6 falls below the threshold Ies, meaning that the ambient light around the user becomes dim again, even though their visual sensitivity is reduced. The control circuit 8 then modifies the value of the emitted light intensity IE to compensate for this decrease in visual sensitivity that occurs at the end of the glare: the light intensity emitted by the light sources 2 is then increased, at time t2, to a value greater than IE2. For example, the emitted light intensity IE can be increased to the initial value IE0, or even to a first value IEi (see [Fig. 3]) greater than the initial value IE0. It is then easier for the user to see what is illuminated by lamp 1, despite the recent glare.

[0055] It is therefore understood that the control circuit 8 is configured to identify glare and determine its end, in order to then increase the intensity of the light emitted by the lamp 1 and thus compensate for the decrease in visual sensitivity of the user following the glare.

[0056] From t2, we can see on [Fig.3] that the light Ic detected by the sensor 6 remains below the threshold determined ICs- In this case, and as during the transient phase of the lighting of the lamp 1, the control circuit 8 can provide a procedure for progressively decreasing the light intensity emitted IE by the light sources 2, over time.

[0057] Thus, at time t3, after a second determined duration D2, the control circuit 8 can decrease the value IE of the emitted light intensity from the first determined value IE1. In the example illustrated in [Fig.3], the emitted light intensity IE is decreased from the first value IEi to a third determined value IE3 which can, for example, be equal to the initial value IE0 or which can be equal to the average of the first and second values ​​IEi and IE2.

[0058] The second determined duration D2 can, for example, be chosen to be greater than or equal to 15 seconds, preferably 20 seconds and more preferably 30 seconds,

[0059] At time t4, the value of the emitted light intensity IE is equal to the third determined intensity IE3, and the control circuit 8 can then maintain the emitted light intensity IE at said third determined value IE3 for a certain duration, for example for a third determined duration D3. The third value of emitted light intensity IE3 thus constitutes an intermediate step in the decrease of the emitted light intensity.

[0060] At time t5, after a third determined duration D3, the control circuit 8 can decrease the value IE of the emitted light intensity from the third determined value IE3. In the example illustrated in [Fig.3], the emitted light intensity IE is decreased from the third value IE3 to the second determined value IE2.

[0061] At time t6, the value of the emitted light intensity IE is equal to the second determined intensity IE2, and the control circuit 8 can then maintain the emitted light intensity IE at said second determined value IE2 until the next glare.

[0062] In the end, the control circuit 8 reduced the emitted light intensity IE from the first value IEi to the second value IE2 for a total duration corresponding to a fourth determined duration D4 extending between t3 and t6.

[0063] The fourth determined duration D4 can for example be chosen to be less than or equal to 10 minutes, preferably 5 minutes and more preferably less than 3 minutes.

[0064] Furthermore, the value of the second intensity IE2 can be chosen to be less than or equal to 40% of the value of the first intensity IEi of emitted light, preferably 30% and more preferably 20%.

[0065] It is thus understood that, in the absence of glare, the control circuit 8 is configured to progressively decrease the luminous intensity emitted by the lamp 1, according to characteristics corresponding to the user's visual adaptation mechanisms. Such a progressive decrease can be implemented when the lamp 1 is started, during the transient state, or after glare or a change in the lighting mode of the lamp 1 by the user, during the steady state.

[0066] As can be seen in [Fig. 3], the threshold determined ICs is not necessarily a constant value, but can instead be a value that is modified depending on the ambient light intensity. Indeed, it is understandable that a user's eyes can be more easily dazzled when the ambient light is very low, rather than when it is dim. In this case, the threshold determined ICs at very low ambient light intensity may be lower than the threshold determined ICs at dim ambient light. This is represented in [Fig. 3] by the line segments.

[0067] Figure 4 illustrates a first method 20 for controlling a lamp, in particular a lamp 1 as described above. In particular, the first control method 20 is configured to adjust the lamp's light intensity in response to glare.

[0068] In a first step 22, it is determined whether the intensity Ic of the light detected by the sensor 6 is greater than or equal to a predetermined threshold ICs, for a duration greater than or equal to the first predetermined duration DSi. Step 22 therefore consists of detecting glare.

[0069] If the conditions of step 22 are met, then during a second step 24, it is determined whether the intensity Ic of the light detected by the sensor 6 becomes lower than said threshold determined ICs- Step 24 therefore consists of detecting the end of the glare.

[0070] In a third step 26, for example simultaneous, it is determined whether the emitted intensity IE is less than a first value IEi. Such a step 26 makes it possible to determine whether it is possible to increase the emitted brightness IE to the value IEi or whether it is already equal to the value IEi and therefore cannot be increased further.

[0071] If the conditions of steps 24 and 26 are met, then in a step 28, the emitted intensity IE is increased to the first value IEi.

[0072] Figure 5 illustrates a second method 30 for controlling a lamp, in particular a lamp 1 as described above. In particular, the second control method 30 is configured to adjust the light intensity of the lamp in the absence of glare.

[0073] In a first step 32, it is determined whether the intensity IE emitted by the light sources 2 is greater than the second value IE2. Such a step 32 makes it possible to know whether it is possible to decrease the emitted brightness IE to the value IE2 or whether it is already equal to the value IE2 and therefore cannot be decreased any further.

[0074] In particular, the first step 32 can correspond to the last step 28 of the first method 20: when the emitted intensity IE is increased to the first value IEb it becomes in fact greater than the second value IE2, so that the second method 30 can be implemented in continuity with the first method 20, or independently of the first method 20, for example when the lamp is switched on or when the user changes the lamp's lighting mode.

[0075] In a second step 34, for example simultaneous, it is determined whether, during the second predetermined duration D2, the intensity Ic of the light detected by the sensor 6 is less than the predetermined threshold ICs, or greater than or equal to a predetermined threshold ICs for a continuous duration shorter than the first predetermined duration DSi. Step 34 therefore consists of detecting the absence of glare.

[0076] If the conditions of steps 32 and 34 are met, then in a step 36, the emitted intensity IE is decreased from the first value IEi to the second value IE2, possibly via a plateau to a third value IE3.

[0077] Thus, thanks to the lamp according to the present invention, it becomes possible to have lighting that adapts both to external conditions, but also and especially to the physiological variations of the user's visual sensitivity, in order to reduce the electrical consumption of the lamp while providing the same level of user comfort.

Claims

Demands

1. Lamp (1), in particular portable such as a headlamp, comprising - one or more light sources (2), preferably in the visible spectrum, for example LEDs, and intended to emit light generally parallel to the direction of the user's gaze, - one or more light sensors (6) for detecting the light arriving on said sensor(s), the lamp (1) also comprising a control circuit (8) configured to modify the intensity of the light emitted (IE) by the light source(s) according to the intensity of the light detected (Ic) by the sensor(s) (6), characterized in that the control circuit (8) is configured to increase the intensity of the light emitted to a first intensity value (IEi),- when the detected light intensity (Ic) is less than a predetermined threshold (ICs) after having been greater than or equal to said predetermined threshold(s) for a duration greater than or equal to a first predetermined duration (DSi), for example greater than or equal to 0.5 seconds, preferably 1 second and more preferably 5 seconds, and - when the emitted light intensity is less than said first value (IE1).

2. Lamp (1) according to claim 1, wherein the control circuit (8) is also configured to decrease the intensity of light emitted from said first value (IE1) to a second value (IE2) when, for at least a second specified duration, for example greater than or equal to 15 seconds, preferably 20 seconds and more preferably 30 seconds, the detected light intensity is: - less than said specified threshold (ICs), - or greater than or equal to said specified threshold (ICs) for a continuous duration less than said first specified duration (DSi).

3. Lamp (1) according to the preceding claim, wherein the control circuit (8) is configured to decrease the intensity of the emitted light from said first value (IE1) to said second value (IE2), while maintaining said intensity at a third intensity value (IE3) between said first value and said second value, during a third fixed term (D3).

4. Lamp (1) according to the preceding 2 or 3, wherein the control circuit (8) is configured to decrease the intensity of light emitted from said first value (IE1) to a second value (IE2), for a fourth specified duration (D4), for example less than or equal to 10 minutes, preferably 5 minutes and more preferably less than 3 minutes.

5. Lamp (1) according to claim 1, wherein the control circuit (8) is also configured to maintain the emitted light intensity at said second value (IE2), when the detected light intensity is: - less than said determined threshold (the), - or greater than or equal to said determined threshold (ICs) for a continuous duration less than said first determined duration (DSi).

6. Lamp (1) according to any one of the preceding claims, wherein the second intensity (IE2) is less than or equal to 40% of the first intensity (IEi) of light emitted, preferably 30% and more preferably 20%.

7. A method for controlling a lamp (20, 30), in particular a portable lamp (1) such as a headlamp, comprising: - one or more light sources (2), preferably in the visible spectrum, for example LEDs, intended to emit light generally parallel to the direction of the user's gaze, - one or more light sensors (6) for detecting the light arriving on said sensor(s), wherein the intensity of the light emitted (IE) by the light source(s) is modified according to the intensity of the light detected (Ic) by the sensor(s), characterized in that the intensity of the emitted light is increased to a first intensity value: - when the detected light intensity is less than a predetermined threshold after having been greater than or equal to said predetermined threshold for a duration greater than or equal to a first predetermined duration, for example greater than or equal to 0.5 seconds,preferably at 1 second and more preferably at 5 seconds, and - when the intensity of emitted light is less than said first value.

8. A method (30) according to the preceding claim, wherein one decreases the intensity of light emitted from said first value to a second value when, for at least a second determined period, for example greater than or equal to 15 seconds, preferably 20 seconds and more preferably 30 seconds, the intensity of light detected is: - less than said determined threshold, - or greater than or equal to said determined threshold for a continuous period less than said first determined period.

9. A method (30) according to the preceding claim, wherein the intensity of light emitted is decreased from said first value to a second value, for a fourth specified duration, for example less than or equal to 10 minutes, preferably 5 minutes and more preferably less than 3 minutes.

10. A method (20, 30) according to any one of claims 7 to 9, wherein the intensity of emitted light is maintained at said second value, when the intensity of detected light is: - less than said determined threshold, - or greater than or equal to said determined threshold for a continuous duration less than said first determined duration.

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