EYE-SAFE LASER-BASED LIGHTING

Abstract

A laser-based light source includes a laser device configured to generate laser light of a predetermined laser wavelength and emit this laser light as a laser beam. A light-conversion device is configured to convert at least part of the laser light into converted light and a laser-output sensor is configured to determine a laser-output signal proportional to the output of laser light emitted by the laser device. Further, a converted-light sensor is configured to determine a converted-light signal proportional to the output of converted light emitted by the light-conversion device. A controller is configured to receive the laser-output signal and the converted-light signal, to determine a safe-to-operate parameter, based on the laser-output signal and the converted-light signal, and to control the operation of the laser-based light source based on a comparison of the safe-to-operate parameter with a at least one predefined threshold.

Claims

1. A laser-based light source, comprising: a laser device configured to generate laser light of a predetermined laser wavelength and emit the laser light as a laser beam; a phosphor light-conversion device configured to convert at least part of the laser light into phosphor converted light; a laser-output sensor configured to determine a laser-output signal being correlated with an output of the laser light emitted by the laser device; a phosphor converted-light sensor configured to determine a converted-light signal being correlated with an output of the phosphor converted light emitted by the phosphor light-conversion device; and a controller configured to receive the laser-output signal and the phosphor converted-light signal and determine a safe-to-operate parameter of the laser device based on a ratio of the laser-output signal and the phosphor converted-light signal or based on phase delay between the laser-output signal and the phosphor converted-light signal, and control operation of the laser-based light source based on a comparison between the safe-to-operate parameter and at least one predefined eye-safety threshold or legislation.

2. The laser-based light source of claim 1, wherein the laser-output sensor is formed by a laser driving current monitor configured to monitor the laser driving current.

3. The laser-based light source of claim 1, further comprising at least one optical fiber configured to guide the laser light to the light-conversion device.

4. The laser-based light source of claim 3, wherein the phosphor converted-light sensor is arranged at an end of the optical fiber which is remote from the phosphor light-conversion device.

5. The laser-based light source of claim 3, wherein the phosphor converted-light sensor is arranged at a position along the optical fiber, and further comprising a decoupling device configured to couple the phosphor converted light out of the optical fiber and into the phosphor converted-light sensor.

6. The laser-based light source of claim 3, wherein the optical fiber is provided with an electrical wiring for detecting damage to the optical fiber.

7. A method of operating a laser-based light source, comprising acts of: generating laser light of a predetermined laser wavelength and emitting this laser light as a laser beam; converting at least part of the laser light into converted light; determining a laser-output signal being correlated with the output of emitted laser light; determining a converted-light signal being correlated with the output of converted light; and determining a safe-to-operate parameter based on the laser output signal and the converted light signal; comparing the safe-to-operate parameter with a at least one predefined threshold; and controlling the operation of the laser-based light source based on the comparison of the safe-to-operate parameter with the predefined threshold.

8. The method according to claim 7, comprising an act of comparing the safe-to-operate parameter with a second predefined threshold.

9. The method according to claim 7, comprising an act of determining the safe-to-operate parameter based on a ratio between the laser-output signal and the converted-light signal.

10. The method according to claim 7, comprising an act of determining the safe-to-operate parameter based on the derivative with respect to time of the ratio between the laser-output signal and the converted-light signal.

11. The method according to claim 7, comprising acts of modulating the emitted laser light in time; and determining the safe-to-operate parameter based on a ratio between the laser-output signal and the converted-light signal at different points in time.

13. The method according to claim 7, wherein the act of controlling the operation of the laser-based light source based on the comparison of the safe-to-operate parameter with the predefined threshold comprises shutting down laser emission or switching it to a safe intermediate level or/and generating a warning.

13. The laser-based light source of claim 1, wherein the safe-to-operate parameter of the laser device is further based signal strengths of the converted-light signal at different points in time, relative to a strength of the laser output.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

[0038] In the drawings:

[0039] FIG. 1 schematically depicts an embodiment according to the invention;

[0040] FIG. 2 schematically depicts another embodiment of the invention;

[0041] FIG. 3 depicts modulation of the laser output and the corresponding converted light according to another embodiment of the invention;

[0042] FIG. 4 schematically depicts another embodiment of the invention; and

[0043] FIG. 5 schematically depicts a block diagram according to another embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0044] A first embodiment according to the invention is schematically depicted in FIG. 1. Laser light is generated by a laser device 1, i.e. a diode laser, e.g. in an edge-emitting geometry or a vertical-cavity surface-emitting geometry (VCSEL). This laser light is directed via optics 3 to a volume of phosphorous material of a light-conversion device 2 which is in a powderous or crystalline, preferably polycrystalline, state. The laser device 1 emits blue light, preferably around 445 nm. The light-conversion device 2 converts the blue laser light into broadband yellow light. The combination of this yellow light and the remainder of the blue light results in white light. The white light generated in this way is shaped into a beam by means of collimating optics 4.

[0045] Some of the light generated by the light-conversion device 2 as well as some of the laser light is redirected towards the laser device 1. A beam splitter 5 at an angle of 45 is used to direct a fraction of this light towards a converted-light sensor 6 which is formed by a photodiode. A small fraction of the blue laser light is directed towards a laser-output sensor 7 which is formed by another photodiode. The beam splitter 5 may be a simple thin piece of glass or a dichroic filter reflecting the yellow light generated by the light-conversion device 2 and transmitting the blue light of the laser device 1. Also the photodiodes may be equipped with colour filters in order to enhance the selectivity for either the laser-converted light or the phosphor-converted light.

[0046] By monitoring the signal generated by converted-light sensor 6 and laser-output sensor 7, preferably after further filtering and amplifying, it is determined whether the light-source is safe to operate or not. For example, as long as the ratio of the phosphor output and the laser output lies within a lower and an upper predefined threshold, respectively, the laser device 1 can be considered to be safe to operate. Similarly, when the modulus of the derivative in time of the ratio of the phosphor output and the laser output exceeds a predefined threshold, the laser device 1 might be considered to be unsafe to operate and is switched off.

[0047] Another embodiment according to the invention is schematically depicted in FIG. 2. In this case, the location of laser light generation is decoupled from the location of light conversion. An optical fiber 8 is used to transport the laser light towards the remote light-conversion device 2. Part of the light generated by the remote light-conversion device 2 is sent back through the fiber towards converted-light sensor 6. Alternatively, a separate return fiber (not shown) can be used. As shown in FIG. 2, the optical fiber is provided with electrical wiring 12 for detecting damage of the optical fiber. With respect to this, it is especially preferred that conducting wires 12 are wound around the optical fiber, and when a wire is cut or when the capacity changes, it is detected that the fiber might be broken. In this case, a warning can be given and/or the laser device and, thus, the complete light source are preferably shut down.

[0048] The principle of operation of a still other embodiment is explained in the following with reference to FIG. 3. The light output from the laser device 1 as measured by laser-output sensor 7 is modulated in time, which is shown by the dotted line (signal Sz in FIG. 3). The signal measured by the phosphor-output detector looks like the solid line (signal Si) in FIG. 3 since a phosphor has a finite response time, given by a decay time. The detected phosphor output is proportional to the phosphor-converted yellow light generated and might contain a fraction of the blue laser light. By monitoring for example the signal strengths a and b of the detected phosphor output at different points in time, relative to the strength c of the laser output, as depicted in FIG. 3, it can be ascertained whether the light source is safe to operate.

[0049] Alternatively, the laser output can be modulated in a periodic fashion. Due to the finite response time of the phosphor, the signal measured by the converted-light sensor 6 is delayed in phase relative to the signal measured by the laser-output sensor 7. Monitoring the modulation depth of the detected signals as well as the phase delay between them provides sufficient information to ascertain whether the light source is safe to operate.

[0050] Still another embodiment according to the invention is shown in FIG. 4. In this embodiment, only one photodiode is provided which forms the converted-light sensor 6. The role of the other photodiode in the embodiments described above is taken over by monitoring the driving current of the laser device 1. This current is proportional to the laser output, apart from a threshold current.

[0051] Finally, a general schematic block diagram of the laser-based light source according to an embodiment of the invention is shown in FIG. 5. Part of the light emitted by the laser device 1 and part of the light converted by the light-conversion device 2 is detected by a laser-output sensor 7 and a converted-light sensor 6, respectively, as has been described above. These signals are read by a controller 9, which controls a laser driver 10. The laser driver 10 sets the laser power and can switch the beam on and off, or can switch it to an eye-safe intermediate state. As shown in FIG. 5, the controller is directly connected to the converted-light sensor 6 and the laser-output sensor 7.

[0052] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

[0053] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word comprising does not exclude other elements or steps, and the indefinite article a or an does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.