Safety interlock system for illumination systems

1. An illumination system comprising:

an illumination device;

an optical element positioned to receive light from the illumination device;

a layer of a transparent material disposed on the optical element and positioned to receive light from the illumination device; and

an interlock circuit configured to measure a resistivity of the layer of transparent material and to control operation of the illumination device based on the measured resistivity,

wherein the interlock circuit is configured to measure the resistivity of the layer of transparent material synchronously with a modulation frequency of the illumination device.

2. The illumination system of claim 1 , in which the layer of transparent material comprises a material having a non-zero absorption at a wavelength of the light from the illumination device.

3. The illumination system of claim 1 , in which the layer of transparent material comprises a film of the transparent material disposed on a surface of the optical element.

4. The illumination system of claim 1 , in which the interlock circuit is configured to shut off the illumination device when the measured resistivity does not satisfy a safety criterion.

5. The illumination system of claim 1 , in which the interlock circuit is configured to implement lock-in demodulation.

6. The illumination system of claim 1 , in which the interlock circuit is configured to shut off the illumination device when the measured resistivity is not synchronous with the modulation frequency of the illumination device.

7. The illumination system of preceding claim 1 , in which the layer of transparent material has a lateral extent sufficient to encompass an entire field of illumination of the illumination device.

8. The illumination system of claim 7 , in which the layer of transparent material has a substantially uniform thickness across the entire field of illumination of the illumination device.

9. The illumination system of claim 1 , comprising:

a reference layer of the transparent material positioned to not receive light from the illumination device; and

a reference circuit configured to measure a resistivity of the reference layer of the transparent material.

10. The illumination system of claim 1 , in which the transparent material comprises a conductive material or a doped semiconductor material, optionally in which the layer of conductive material comprises indium tin oxide (ITO).

11. The illumination system of claim 1 , in which the illumination device comprises a laser and/or a vertical cavity surface emitting laser (VCSEL).

12. The illumination system of claim 1 , in which the optical element comprises a lens and/or a diffuser.

13. A mobile communications device comprising the illumination system of claim 1 .

14. A method comprising:

operating an illumination device to illuminate an optical element, including illuminating a layer of a transparent material disposed on the optical element; and

measuring a resistivity of the layer of transparent material by an interlock circuit; and

controlling operation of the illumination device based on the measured resistivity of the layer of transparent material,

wherein measuring a resistivity of the layer of transparent material comprises measuring the resistivity of the layer of transparent material synchronously with a modulation frequency of the illumination device, optionally in which measuring the resistivity of the layer of transparent material synchronously with a modulation frequency of the illumination device comprises implementing a lock-in demodulation, and/or wherein controlling operation of the illumination device comprises shutting off the illumination device when the measured resistivity is not synchronous with the modulation frequency of the illumination device.

15. The method of claim 14 , in which controlling operation of the illumination device comprises shutting off the illumination device when the measured resistivity does not satisfy a safety criterion, optionally which controlling operation of the illumination device comprises shutting off the illumination device when the measured resistivity falls outside of an allowable range of values.

16. The method of claim 14 , comprising measuring a resistivity of a reference layer of the transparent material, the reference layer not being illuminated by the illumination device.

17. The method of claim 16 , comprising controlling 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.

18. The method of claim 16 , comprising controlling operation of the illumination device based on the measured resistivity of either the layer of transparent material or the reference layer not satisfying a safety criterion.