SAFETY LOCKING SYSTEM FOR LIGHTING SYSTEMS

Abstract

Lighting system (200, 400), consisting of: a lighting device (202); an optical element (206) arranged to receive light from the illumination device (202); a layer (210) of a transparent material disposed on the optical element (206) and positioned to receive light from the illumination device (202); a locking circuit (220) configured to measure a resistivity of the layer (210) of transparent material and to control the operation of the illumination device (202) based on the measured resistivity; wherein the locking circuit (220) is configured to measure the resistivity of the layer (210) of transparent material synchronously with a modulation frequency of the illumination device (202), perform lock-in demodulation, and turn off the illumination device (202) when the measured resistivity is not synchronized with the modulation frequency of the illumination device (202).

Description

background

[0001] Three-dimensional (3D) sensing can be used for a variety of applications, including facial recognition. 3D sensor systems can incorporate powerful illumination devices such as lasers.

[0002] Various lighting systems and their controls are known from: CN 107 608 167 A; US 2016 / 0 290 856 A1; CN 108 646 326 A; WO 2020 / 038 066 A1; or US 2018 / 0 180 238 A1. Summary

[0003] A lighting system is provided having the features of claim 1. Further developments are set out in the dependent claims.

[0004] The embodiments may include one or more of the following features.

[0005] The layer of transparent material consists of a material whose absorption is not zero at a wavelength of the light coming from the illumination device.

[0006] The layer of transparent material comprises a film of the transparent material disposed on a surface of the optical element.

[0007] The interlock circuit is configured to turn off the lighting device if the measured resistivity does not meet a safety criterion.

[0008] The locking circuit is configured to measure the resistivity of the transparent material layer synchronously with a modulation frequency of the illumination device. The locking circuit is configured to perform lock-in demodulation. The locking circuit is configured to turn off the illumination device if the measured resistivity is not synchronous with the modulation frequency of the illumination device.

[0009] The layer of transparent material has a lateral extent sufficient to cover the entire illumination field of the lighting device. The layer of transparent material has a substantially uniform thickness across the entire illumination field of the lighting device.

[0010] The illumination system includes a reference layer of the transparent material positioned so as not to receive light from the illumination device, and a reference circuit configured to measure a resistivity of the reference layer of the transparent material.

[0011] The transparent material consists of a conductive material or a doped semiconductor material.

[0012] The layer of conductive material consists of indium tin oxide (ITO).

[0013] The illumination device comprises a laser, e.g. a vertical cavity surface emitting laser (VCSEL).

[0014] The optical element consists of a lens.

[0015] The optical element consists of a diffuser.

[0016] In one aspect, a mobile communication device comprises a lighting system including one or more of the above features.

[0017] A method for operating a lighting device with the features according to claim 15 and according to claim 21 is further provided.

[0018] The embodiments may include one or more of the following features.

[0019] Controlling the operation of the lighting device includes shutting down the lighting device if the measured resistivity does not meet a safety criterion. Controlling the operation of the lighting device includes shutting down the lighting device if the measured resistivity is outside a permissible range.

[0020] Measuring the resistivity of the layer of transparent material involves measuring the resistivity of the layer of transparent material synchronously with a modulation frequency of the illumination device. Measuring the resistivity of the layer of transparent material synchronously with a modulation frequency of the illumination device involves performing lock-in demodulation. Controlling the operation of the illumination device involves shutting off the illumination device if the measured resistivity is not synchronous with the modulation frequency of the illumination device.

[0021] The method comprises measuring the resistivity of a reference layer of the transparent material, wherein the reference layer is not illuminated by the illumination device. The method comprises controlling the operation of the illumination device based on a comparison between the measured resistivity of the layer of transparent material and the measured resistivity of the reference layer. The method comprises controlling the operation of the illumination device based on the measured resistivity of either the layer of transparent material or the reference layer that does not meet a safety criterion.

[0022] The approaches described here can have one or more of the following advantages. The lighting systems incorporate a locking function that can mitigate the risk of eye damage to system users. The locking circuits described here are robust to noise and have high sensitivity to AC and DC signals, enabling safety monitoring of both the modulated lighting sources and the structural integrity of the lighting systems' optical elements. The locking circuits are based on sensor layers that cover the entire illumination field. These layers have minimal optical impact and can generally be manufactured easily and inexpensively.

[0023] The details of one or more embodiments are illustrated in the accompanying drawings and the following description. Further features and advantages will become apparent from the description and drawings, as well as from the claims. Brief description of the drawings • Fig. 1 is a diagram of a mobile device. • Fig. 2A and Fig. 2B are diagrams of a lighting system. • Fig. 3 is a block diagram of the lighting system of Fig. 2. • Fig. Figure 4 is a schematic diagram of a lighting system. • Fig. 5 is a block diagram of the lighting system of Fig. 4. • Fig. 6 is a circuit concept of the lighting system of Fig. 4. • Fig. 7 is a state diagram of the lighting system of Fig. 4. • Fig. 8 is a flowchart. Detailed description

[0024] Here, we describe interlock systems for lighting systems, such as lighting systems for 3D sensor applications or augmented reality systems. The interlock systems are capable of detecting indicators of malfunctions or structural damage in the lighting systems and shutting down the lighting devices in response to such detection, thereby reducing the risk of eye damage that could otherwise result from such malfunctions or structural damage.

[0025] As in Fig. 1, a mobile device 100 may include a 3D sensing system 110, e.g., for facial recognition of a user 104 of the device. The 3D sensing system 110 uses an illuminator 102, which may be a high-power illuminator such as a laser. In some cases, illuminating the face of a user 104 of the mobile device 100 with an illuminator 102 emitting high-power illuminator, such as a laser beam, may pose a safety risk, e.g., to the user's eyes. For example, a malfunction of the illuminator or a structural defect in a module enclosing the illuminator, e.g., an optical element such as a diffuser, may cause the illuminator to pose a safety risk.

[0026] To reduce the risk of damage, e.g., to the eyes, the 3D sensing system 110 includes an interlock system 112. The interlock system 112 can detect, in real time, indicators of a malfunction of the illumination device 102 and indicators of structural damage to optical elements in the illumination path. In response to detecting such an indication, the interlock system 112 can deactivate the illumination device 102, thus reducing the risk of eye damage due to malfunctions or damage to the illumination device 102 or its associated optical elements.

[0027] In the example of Fig. 1, the illuminator 102 is a forward-facing illuminator that illuminates toward the front of the mobile device 100 (e.g., the side with a screen 106). In some examples, the illuminator may be a world-facing illuminator that illuminates toward the back of the mobile device 100 (e.g., the side opposite the front).

[0028] As in Fig. 2A, an example illumination system 200 includes an illumination device 202, such as a laser diode, a vertical cavity surface emitting laser (VSCEL), another type of laser, or another type of illumination device. In the example of Fig. 2, the illumination device 202 is arranged on a substrate 204, e.g., a printed circuit board. In some examples, the illumination device 202 may be integrated into the substrate, e.g., into a substrate for integrated circuits. The illumination device 202 may be a modulated illumination device that emits light at a modulation frequency, e.g., for three-dimensional imaging applications such as time-of-flight imaging.

[0029] The illumination system 200 includes an optical element 206 positioned to receive light from the illumination device 202, e.g., in the path of a light beam 208 emitted by the illumination device 202. The optical element 206 may, for example, be a lens, a diffractive element, or another type of optical element. A sensor layer 210 is disposed on the optical element 206 and positioned to receive light from the illumination device 202, e.g., in the path of the light beam 208. See also Fig. 2B, the sensor layer 210 is a continuous, unpatterned film formed on the surface of the optical element 206. The lateral extent of the sensor layer 210 is sufficient to partially or completely cover the area of the optical element 206 that falls within the optical path of the light beam 208 (we sometimes refer to this area as the illumination field). The sensor layer 210 may have a substantially uniform thickness across the entire illumination field, e.g., a thickness that varies by less than about 25%, less than about 20%, less than about 10%, less than about 5%, or less than about 1% across the illumination field. By covering the entire illumination field, the optical impact of the layer 210 on the light beam 208 is minimal, e.g.,The homogeneous thin film of the layer contains no patterns or shapes that cause horizontal changes in the refractive index, which would introduce complexities into the path of the light beam. A homogeneous layer 210 is also generally easy to manufacture, e.g., without requiring patterning and lithographic processing.

[0030] The sensor layer 210 is a conductive material that is transparent to the wavelength of the light emitted by the illumination device 202, but has a low absorption at that wavelength, although not zero. By transparency at a particular wavelength, we mean a material that transmits at least a portion of the light at that wavelength, e.g., more than 50% of the light at that wavelength, e.g., more than 80%, more than 85%, more than 90% of the light, more than 95%, more than 98%, or more than 99%. The sensor layer 210 can, for example, be a highly doped semiconductor such as indium tin oxide (ITO). The absorption of ITO at energies below its band gap (about 4 eV) is close to, but not exactly zero. Due to the low, but non-zero absorption of the sensor layer 210, the specific resistance of the sensor layer 210 changes when the sensor layer 210 is irradiated, e.g.with light from the lighting device 202.

[0031] Since the resistivity of the sensor layer 210 changes with illumination, the sensor layer 210 can be used as an indicator of the functionality of the illumination device 202. The resistivity of the sensor layer 210 in the illuminated state is referred to as the illuminated resistivity of the sensor layer; the resistivity in the unilluminated state of the sensor layer 210 is referred to as the unilluminated resistivity. The illumination resistance of the sensor layer 210 can indicate the intensity of the illumination emitted by the illumination device 202. The frequency of the change in resistivity between the illuminated and unilluminated resistances (e.g., an AC signal) can indicate the modulation frequency of the illumination device 202. If the resistivity of the sensor layer 210 does not meet a safety criterion (described in more detail below), e.g.,If a safety criterion related to a value or frequency of the resistivity is exceeded, this may indicate a potential malfunction of the lighting device 202. To promote safety, e.g., to protect the eyes of a user of a device containing the lighting device 202, the lighting device 202 may be controlled based on the measured resistivity of the sensor layer 210. For example, the lighting device 202 may be turned off if the measured resistance of the sensor layer 210 does not meet a safety criterion.

[0032] A locking circuit 220 of the lighting system 200 is configured to measure the resistivity of the sensor layer 210 and control the operation of the lighting device 202 based on the measured resistivity. The locking circuit 220 can measure a change in the resistivity of the sensor layer 210 synchronously with the modulation frequency of the lighting device 202, e.g., using lock-in demodulation techniques. The locking circuit 220 includes electrical connections 222a, 222b that electrically connect the sensor layer 210 to corresponding electrical contacts 224a, 224b on the substrate 204. The electrical contacts 224a, 224b are, in turn, connected to circuitry, e.g., on or in the substrate 204 or elsewhere, to perform the locking function.The electrical contacts 224a, 224b may, for example, be pads formed on the surface of an integrated circuit.

[0033] The locking circuit 220 is a safety circuit that can shut down the lighting device 202 if the measured resistance of the sensor layer 210 does not meet a safety criterion. In some examples, the safety criterion may be a threshold resistance, such that the measured resistance does not meet the safety criterion if the measured resistance exceeds or falls below the threshold. In some examples, the safety criterion may be a resistance range, such that the measured resistance does not meet the safety criterion if the measured resistance is outside the resistance range.In some examples, the safety criterion may be the synchronization of the resistivity with the modulation frequency of the illumination device 202, such that the measured resistivity does not meet the safety criterion if the changes in the measured resistivity are asynchronous with the modulation frequency of the illumination device 202.

[0034] By switching off the illumination device 202 when the measured resistance of the sensor layer 210 does not meet a safety criterion, potential safety risks can be reduced. For example, if the measured resistance indicates a malfunction of the illumination device 202, e.g., the emission of light with excessive power or the continuous emission of high-power light, switching off the illumination device 202 can prevent this light from damaging the eyes of a user of a device with the illumination system 200.

[0035] The resistivity of the sensor layer 210 can indicate not only the functionality of the lighting device 202, but also the structural integrity of the optical element 206. For example, if the optical element 206 cracks or breaks, the sensor layer 210 also cracks or breaks, resulting in a change in the conductivity (and thus the resistivity) of the sensor layer. A crack or break in the optical element 206 can pose a hazard to the eyes. For example, in an optical element 206 that includes a diffuser, a crack or break can cause direct, strong light to escape from the lighting system 200. Such a malfunction can be detected as a constant change in the resistance of the sensor layer 210 (e.g., as a change in a DC signal). Shutting down the lighting device 202 if the measured resistance does not meet a safety criterion can, for example,prevent the escape of direct light from the lighting system 200 and thus eliminate the possibility of eye damage from this direct light.

[0036] The synchronous lock-in detection of the resistivity of the measuring layer 210 enables the resistance signal to be read at the modulation frequency of the illumination light. This synchronization generally makes the detection method robust against noise, e.g., due to the reduced bandwidth of the lock-in detection method. For example, the illumination device 202 can be operated with large currents, such as currents up to 3 A, in very short pulses, e.g., at modulation frequencies of more than 100 MHz. In this environment, noise due to electromagnetic interference can occur. By synchronizing the detection scheme and the illumination modulation, the effect of this noise can be reduced or eliminated, enabling highly sensitive measurements of both AC and DC resistance signals.

[0037] In addition, the sensor layer 210 is only sensitive to the modulation frequency of the laser (ie, the wavelength of the illuminating light), which is the signal that is to be detected to ensure eye safety.

[0038] In some examples, the resistance of the sensor layer 210 may be monitored over time, e.g., to monitor drift or degradation of the lighting device 202. For example, small changes in the resistivity of the sensor layer 210 over time may accumulate to a large difference from the initial resistance 210, which may indicate developing drift, degradation, or malfunction of the lighting device 202.

[0039] Fig. Figure 3 is a block diagram of the operation of the illumination system 200. The illumination device 202 illuminates the passive optical element 206, e.g., a diffuser or lens, and the sensor layer 210 disposed on the optical element 206. Light 302 from the illumination device 202, passing through the optical element 206 and the sensor layer 210, illuminates a target 304, e.g., an optical system or free space, e.g., the exterior of a mobile device.

[0040] The latch circuit 220 receives a signal 308 from the sensor layer 210, processes this signal 308, and, based on the signal 308 from the sensor layer 210, outputs a control signal 309 to the illumination device 202. As described herein, the signal 308 is a resistance of the sensor layer 210. In some examples, other properties of the sensor layer 210 may also be used as the signal 308, such as a capacitance of the sensor layer 210 or a light sensitivity of the sensor layer 210.

[0041] The signal 308 from the sensor layer 210 is sampled by a lock-in sensing unit 310. For example, the lock-in sensing unit 310 may monitor the signal 308 synchronously with the modulated operation of the illumination device 202 via an electrical connection between the lock-in sensing unit 310 and a laser driver 312. A control unit 314 controls the operation of the lock-in sensing unit 310 and the laser driver 312 based on the sensed signal 308 from the sensing layer 210. For example, the control unit 314 determines whether the signal 308 meets a safety criterion and shuts down the operation of the illumination device 202 if the safety criterion is not met.

[0042] As in Fig. 4, an example lighting system 400 includes the sensor layer 210 and the locking circuit 220 of the Fig. 2A-2B, and a reference circuit 420. The reference circuit 420 is configured to measure the resistivity of a reference layer 410 formed on the optical element 206. The reference layer 410 is a conductive material that is transparent to the wavelength of the light emitted by the illumination device 202, but has a low, non-zero absorption at that wavelength. The reference layer 410 may, for example, be a highly doped semiconductor such as ITO. In some examples, the reference layer 410 is formed from the same material as the sensor layer 210.

[0043] The reference layer 410 is arranged on the optical element 206 such that the reference layer 410 is not located in the beam path of the light beam 208, ie the reference layer 410 is not illuminated even if the illumination device 202 emits light.

[0044] The reference circuit 420 includes electrical connections 422a, 422b that electrically connect the reference layer 410 to corresponding electrical contacts 424a, 424b. The electrical contacts 424a, 424b may be located on the same substrate 204 as the electrical contacts 224a, 224b of the locking circuit 220 (as shown in Fig. 4) or formed on another substrate. The electrical contacts 424a, 424b of the reference circuit 420 are connected to the latch circuit 220.

[0045] The reference circuit 420 can help compensate for temperature drift in the lighting system 400. For example, the temperature coefficient of resistivity of ITO is approximately 2E-4 per °C, which can lead to problems when reading the resistivity of the sensor layer 210 in the temperature range typical for consumer electronics, e.g., in the range from approximately -20 °C to approximately 80 °C. The presence of the non-illuminated reference layer 410 on the same optical element 206 can help account for temperature drift, e.g., by providing a stable, "dark" reference for resistance measurement. In addition, the reference circuit 420 provides redundancy, e.g., when verifying DC resistance measurements. The resistivity of the reference circuit 420 can, for example, be an indicator of the structural integrity of the optical element 206.

[0046] Fig. 5 is a block diagram of the operation of the illumination system 400. In the illumination system 400, the illumination device 202 illuminates the passive optical element 206 and the sensor layer 210 arranged on the optical element 206. However, the illumination device 202 does not illuminate the reference layer 410, which is also arranged on the optical element 206.

[0047] The latch circuit 420 receives the signal 308 from the sensor layer 210 and a signal 508 from the reference layer 410, processes both signals 308, 508, and provides a control signal 509 to the illumination device 202 based on the signals 308, 508. As described here, the signals 308, 508 are the resistivity of the sensor layer 210 and the reference layer 410. In some examples, other properties of the sensor layer 210 and the reference layer 410 may also be used as signals 308, 508, such as a capacitance or a light sensitivity of the sensor layer 210 and the reference layer 410.

[0048] The signals 308, 508 from the sensor layer 210 and the reference layer 410 are sampled by a lock-in sampling unit 510. For example, the lock-in sampling unit 510 may monitor at least the signal 308 from the detection layer 210 synchronously with the modulated illumination of the illumination device 202 through an electrical connection between the lock-in sampling unit 510 and a laser driver 512. In some examples, the lock-in sampling unit 510 may also monitor the signal 508 from the reference layer 410 synchronously with the modulated illumination of the illumination device 202, e.g., to reduce bandwidth and improve noise robustness.

[0049] A control unit 514 controls the operation of the lock-in detection unit 510 and the laser driver 512 based on the detected signals 308, 508 from the detection layer 210 and the reference layer 410. For example, the control unit 514 determines whether one or both signals 308, 508 satisfy a safety criterion and disables the operation of the illumination device 202 if the safety criterion is not met. In some examples, the control unit 514 controls the operation of the illumination device 202 based on a synchronization between the signal 308 from the detection layer 210 and the modulated illumination of the illumination device 202, e.g., when the safety criterion is a synchronization between the signal 308 and the modulation frequency of the illumination.In some examples, the control unit 514 controls the operation of the lighting device 202 based on a comparison between the signal 308 from the sensing layer 210 and the signal 508 from the reference layer 410, e.g., using the signal 508 from the reference layer 410 to reduce the effects of drift or noise on the signal 308 from the sensing layer 210. For example, the control unit 514 may control the operation of the lighting device 202 based on the result of the comparison satisfying a safety criterion. In some examples, the control unit 514 controls the operation of the lighting device 202 based on either the signal 308 from the sensing layer 210 or the signal 508 from the reference layer 410 satisfying a safety criterion, e.g., using the reference layer 410 as a redundant sensing system.

[0050] Fig. 6 is a circuit diagram of the lighting system 400, wherein the reference circuit 420 serves to provide a stable reference, e.g., to account for drift. As shown in Fig. In Figure 4, the locking circuit 220, including the sensor layer 210, is represented by a box 602. The reference circuit 420, including the reference layer 410, is represented by a box 604. A resistance signal 606 is received from the locking circuit 220. Drift occurs in both the locking circuit 220 and the reference circuit 420, e.g., due to temperature fluctuations, aging, or other causes, and causes a change in the impedance 608, 610 of both circuits 220, 420. By using the change in the impedance 610 of the reference circuit 420 to cancel the effect of the change in the impedance 608 of the locking circuit 220, the useful signal (the resistance signal 606) can be isolated from noise and drift effects. For example, the resistivity of the measuring layer 210 may be compared with the resistivity of the reference layer 410 to eliminate the effect of drift.The result of the comparison, e.g., the resistivity of the sensing layer 210, which is indicative of the performance of the lighting device 202, can be used to control the operation of the lighting device 202.

[0051] Fig. 7 is a state diagram illustrating the role of the reference circuit 420 as a redundant indicator of the structural integrity of the optical element 206. With reference to Fig. 4, in a first state 700, both the DC resistance of the sensor layer 210, as sensed by the latch circuit 220, and the DC resistance of the reference layer 410, as sensed by the reference circuit 420, satisfy a safety criterion, e.g., they fall within a target range or satisfy a threshold. In this first state 700, the resistances of the sensing layer 210 and the reference layer 410 indicate that the optical element 206 is structurally sound, and the operation of the illumination device 400 is not altered.

[0052] In a second state 702, both the DC resistance of the sensor layer 210 and the DC resistance of the reference layer 410 fail to meet a safety criterion, i.e., they are outside a target range or do not meet a threshold. In this second state 702, both resistance values indicate a possible structural problem with the optical element 206, and operation of the illumination device is discontinued.

[0053] The third and fourth states 704, 706 illustrate the value of redundant measurement circuits. In the third state 704, the DC resistance of the sensor layer 210 does not meet the safety criterion, while the DC resistance of the reference layer 410 does meet the safety criterion. In the fourth state 706, the DC resistance of the sensor layer 210 meets the safety criterion, while the DC resistance of the reference layer 410 does not meet the safety criterion. In both the third and fourth states 704, 706, due to the redundancy, one failure is sufficient to indicate a possible structural problem with the optical element 206, and operation of the illumination device is discontinued.

[0054] Fig. 8 is a flowchart of a method for operating an illumination system. An illumination device is operated to illuminate a layer of transparent material disposed on an optical element (800). The resistivity of the layer of transparent material is measured by a locking circuit (802), e.g., synchronous with a modulation frequency of the illumination device. The resistivity may be measured, e.g., by lock-in demodulation. In some examples, the resistivity of a reference layer of the transparent layer is also measured by a reference locking circuit, wherein the reference layer is not illuminated by the illumination device (804).

[0055] The operation of the illumination device is controlled based on the measured resistivity of the layer of transparent material (806). In some examples, the illumination device may be turned off if the measured resistivity does not meet a safety criterion, e.g., if the measured resistivity is outside an acceptable range of values or above or below a threshold. In some examples, the illumination device may be turned off if the measured resistivity is not synchronized with the modulation frequency of the illumination device. In some examples, the operation of the illumination device may be controlled based on a comparison between the measured resistivity of the layer of transparent material and the measured resistivity of the reference layer.

[0056] The lighting systems described here can be used in mobile devices (e.g. as in Fig.1), e.g., for forward-facing or world-facing illumination. The illumination systems described here can be integrated, for example, into three-dimensional sensing systems, such as time-of-flight, pattern, or stereo systems, in mobile devices. The illumination systems described here can be integrated into augmented reality systems, e.g., for applications such as gaming, industrial applications, educational applications, or automotive applications (e.g., driver monitoring). The illumination systems described here can also be integrated into other systems, e.g., time-of-flight or infrared imaging systems, e.g., automotive-based systems, security systems, facial or gesture recognition systems, industrial control systems, robotic systems, agricultural systems, or various other types of systems.In general, the lighting systems described here can be integrated into systems that use high-power, direct lighting to increase eye safety in such systems.

[0057] A number of embodiments have been described. However, it should be understood that various changes may be made without departing from the spirit and scope of the invention. For example, some of the steps described above may be sequential and may therefore be performed in a different order than that described.

[0058] Other embodiments also fall within the scope of the following claims.

Claims

[1] Lighting system (200, 400), consisting of: a lighting device (202); an optical element (206) arranged to receive light from the illumination device (202); a layer (210) of a transparent material disposed on the optical element (206) and positioned to receive light from the illumination device (202); a locking circuit (220) configured to measure a resistivity of the layer (210) of transparent material and to control the operation of the illumination device (202) based on the measured resistivity; wherein the locking circuit (220) is configured to measure the resistivity of the layer (210) of transparent material synchronously with a modulation frequency of the illumination device (202), perform lock-in demodulation, and turn off the illumination device (202) when the measured resistivity is not synchronized with the modulation frequency of the illumination device (202). [2] The illumination system (200, 400) of claim 1, wherein the layer (210) of transparent material comprises a material having a non-zero absorption at a wavelength of the light from the illumination device (202). [3] The illumination system (200, 400) of claim 1 or 2, wherein the layer (210) of transparent material comprises a film of the transparent material disposed on a surface of the optical element (206). [4] The lighting system (200, 400) of any preceding claim, wherein the interlock circuit (220) is configured to deactivate the lighting device (202) if the measured resistivity does not meet a safety criterion. [5] Illumination system (200, 400) according to one of the preceding claims, wherein the layer (210) of transparent material has a sufficient lateral extent to cover the entire illumination field of the illumination device (202). [6] The illumination system (200, 400) of claim 5, wherein the layer (210) of transparent material has a substantially uniform thickness over the entire illumination field of the illumination device (202). [7] Lighting system (200, 400) according to one of the preceding claims, comprising: a reference layer (410) made of the transparent material, which is arranged so that it does not receive light from the illumination device (202); and a reference circuit (420) configured to measure a resistivity of the reference layer (410) of the transparent material. [8] Illumination system (200, 400) according to one of the preceding claims, wherein the transparent material of the layer (210) or the reference layer (410) comprises a conductive material or a doped semiconductor material. [9] Illumination system (200, 400) according to one of the preceding claims, wherein the layer (210) of transparent material or the reference layer (410) of conductive material comprises indium tin oxide (ITO). [10] Illumination system (200, 400) according to one of the preceding claims, wherein the illumination device (202) comprises a laser. [11] Illumination system (200, 400) according to one of the preceding claims, wherein the illumination device (202) comprises a vertical cavity surface emitting laser (VCSEL). [12] Illumination system (200, 400) according to one of the preceding claims, wherein the optical element (206) is a lens. [13] Illumination system (200, 400) according to one of the preceding claims, wherein the optical element (206) comprises a diffuser. [14] Mobile communication device with the lighting system (200, 400) according to one of the preceding claims. [15] Procedure comprising: Operating (800) an illumination device (202) to illuminate an optical element (206), including illuminating a layer (210) of a transparent material disposed on the optical element (206); and Measuring (802) the resistivity of the layer (210) of transparent material by a locking circuit (220); Controlling (806) the operation of the illumination device (202) based on the measured resistivity of the layer (210) of transparent material; wherein measuring the resistivity of the layer (210) of transparent material comprises measuring the resistivity of the layer (210) of transparent material synchronously with a modulation frequency of the illumination device (202), wherein the measuring comprises performing a lock-in demodulation and controlling the operation of the lighting device (202) comprises switching off the lighting device (202) if the measured resistivity is not synchronous with the modulation frequency of the lighting device (202). [16] The method of claim 15, wherein controlling (806) the operation of the lighting device (202) comprises turning off the lighting device (202) if the measured resistivity does not meet a safety criterion. [17] The method of claim 16, wherein the control (806) of the operation of the illumination device (202) comprises switching off the illumination device (202) when the measured resistivity is outside a permissible range of values. [18] Method according to one of claims 15 to 17, wherein the resistivity of a reference layer (410) of the transparent material is measured, wherein the reference layer (410) is not illuminated (804) by the illumination device (202). [19] The method of claim 18, comprising controlling (806) the operation of the illumination device (202) based on a comparison between the measured resistivity of the layer (210) of transparent material and the measured resistivity of the reference layer (410). [20] A method according to claim 18 or 19, comprising controlling (806) the operation of the illumination device (202) based on the measured resistivity of either the layer (210) of transparent material or the reference layer (410) that does not meet a safety criterion. [21] Procedure comprising: Operating (800) an illumination device (202) to illuminate an optical element (206), including illuminating a layer (210) of a transparent material disposed on the optical element (206); and Measuring (802) the resistivity of the layer (210) of transparent material by a locking circuit (220); Controlling (806) the operation of the illumination device (202) based on the measured resistivity of the layer (210) of transparent material; wherein the specific resistance of a reference layer (410) of the transparent material is measured, wherein the reference layer (410) is not illuminated (804) by the illumination device (202); wherein the control (806) of the operation of the illumination device (202) is based on a comparison between the measured resistivity of the layer (210) of transparent material and the measured resistivity of the reference layer (410), and the control of the operation of the illumination device (202) is based on the measured resistivity of either the layer (210) of transparent material or the reference layer (410) that does not meet a safety criterion. [22] The method of claim 21, wherein controlling (806) the operation of the lighting device (202) comprises turning off the lighting device (202) if the measured resistivity does not meet a safety criterion. [23] The method of claim 22, wherein controlling (806) the operation of the illumination device (202) comprises switching off the illumination device (202) when the measured resistivity is outside a permissible range of values. [24] The method of any one of claims 21 to 23, wherein measuring the resistivity of the layer (210) of transparent material comprises measuring the resistivity of the layer (210) of transparent material synchronously with a modulation frequency of the illumination device (202). [25] The method of claim 24, wherein the measurement (802) of the resistivity of the layer (210) of transparent material synchronously with the modulation frequency of the illumination device (202) comprises performing a lock-in demodulation. [26] The method of claim 24 or 25, wherein controlling (806) the operation of the illumination device (202) comprises turning off the illumination device (202) when the measured resistivity is not synchronous with the modulation frequency of the illumination device (202).

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