WO2013064959A1

WO2013064959A1 - Device and method for multi-spectral illumination

Info

Publication number: WO2013064959A1
Application number: PCT/IB2012/055969
Authority: WO
Inventors: Tommaso Gritti; Jelte Peter Vink; Ruud Vlutters
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2011-11-04
Filing date: 2012-10-29
Publication date: 2013-05-10
Anticipated expiration: 2014-05-04

Abstract

The invention proposes a lighting device. The lighting device comprises a light source including a plurality of lighting units, each lighting unit being capable of generating a sub-beam; a controller configured to control, at each of a plurality of time periods, the on/off state of each lighting unit such that the light source is capable of generating a plurality of desired light beams having different spectra at the plurality of time periods respectively, each desired light beam corresponding to one time period and having at least one sub-beam; wherein the duration of the on state or off state of each lighting unit is less than a first predetermined value. By using the proposed lighting device for multi-spectral illumination, no flicker will be perceived, and thus user experience will be improved.

Description

DEVICE AND METHOD FOR MULTI-SPECTRAL ILLUMINATION

FIELD OF THE INVENTION

The invention relates to illumination, in particular to a device and a method for multi-spectral illumination.

BACKGROUND OF THE INVENTION

As disclosed in PCT Patent application No. PCT/CN2010/ 075872 under the title "Method and apparatus for recognizing an interesting object", filed by the current applicant on 11 Aug. 2010, an apparatus for recognizing an object through multi-spectral imaging is proposed.

The proposed apparatus for recognizing an interesting object comprises: a light source configured to generate a plurality of light beams, each light beam comprising at least one sub-beam having a predefined spectrum, and different light beams having at least one different sub-beam; a light sensor configured to capture a plurality of images of a target object being illuminated by the light source, each image corresponding to one light beam; a processor configured to compare at least two of the plurality of images captured by the light sensor to extract at least one difference feature between every two captured images; wherein the processor is further configured to identify the interesting object from the target object, based on at least one difference feature.

OBJECT AND SUMMARY OF THE INVENTION

The present invention aims to improve the light source and the corresponding controlling system capable of being used in the application as mentioned above. More specifically, the present invention aims at reducing or eliminating the flicker perceived by a user due to alternating colors of light, without reducing the time available for a sensor to capture the multispectral data. In one aspect, an embodiment of the present invention provides a lighting device comprising: a light source including a plurality of lighting units, each lighting unit being capable of generating a sub-beam; a controller configured to control, at each of a plurality of time periods, the on/off state of each lighting unit such that the light source is capable of generating a plurality of desired light beams having different spectra at the plurality of time periods respectively, each desired light beam corresponding to one time period and having at least one sub-beam; wherein the duration of the on state or off state of each lighting unit is less than a first predetermined value.

The first predetermined value may depend on various factors, such as the distance of a target object from the light source, the size of the illuminated area, the intensity of the illumination, and the intra-person variations, for example. In one embodiment, the first predetermined value is 20ms.

When a target object is illuminated by the proposed lighting device, the duration of the on state or off state of each lighting unit is controlled to be less than a first predetermined value, for example, 20ms, so that the luminance of each lighting unit will stay constant to the human eye, and thus the temporal illumination variation, i.e. flicker, will be invisible for the user, which leads to a better user experience. Or, the proposed lighting device may be used in security systems that require unobtrusive monitoring.

Advantageously, the lighting device may further comprise a sensor configured to capture, within each time period, an image of a target object being illuminated by the light source so as to obtain a plurality of images, wherein the length of each time period is greater than a second predetermined value.

The second predetermined value may also be related to various conditions, such as the intensity of illumination, the distance from the target object to the light source, the distance from the target object to the sensor, the reflectivity of the target object, for example. Advantageously, the second predetermined value may be the exposure time of the sensor. In this case, the exposure time of the sensor is set such that no clipping in the analogue to digital conversion takes place, and the noise level is below a maximum level, specific to each application.

For multi-spectral imaging, images of the target object illuminated under different lighting conditions are captured, wherein each image corresponds to one of the plurality of desired light beams. Each image should be captured by the sensor within one time period during which the corresponding light beam is maintained constant or at least substantially constant. With the length of each time period being greater than the second predetermined value, for example greater than the exposure time of the sensor, there would be enough time for the sensor to capture a sufficiently illuminated image within one time period. In other words, there is no need to adopt a quite expensive sensor whose hardware allows exposing multiple times without flushing the accumulated charges to capture a sufficiently illuminated image. Therefore, the cost of the sensor may be reduced.

In another aspect, one embodiment of the present invention provides a method of illuminating a target object by using a light source, the light source including a plurality of lighting units, each lighting unit being capable of generating a sub-beam, the method comprising the step of: controlling the on/off state of each lighting unit at each of a plurality of time periods to generate a plurality of desired light beams having different spectra at the plurality of time periods respectively, each desired light beam corresponding to one time period and having at least one sub-beam; wherein the duration of the on state or off state of each lighting unit is less than a first predetermined value.

Advantageously, the method further comprises the step of: capturing, within each time period, an image of the target object being illuminated by the light source by using a sensor so as to obtain a plurality of images, wherein the length of each time period is greater than a second predetermined value.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention will become more apparent from the following detailed description considered in connection with the accompanying drawings, in which:

FIG. 1 illustrates a block diagram of a lighting device according to an embodiment of the present invention;

FIG. 2 illustrates a block diagram of a lighting device according to another embodiment of the present invention;

FIG. 3 illustrates an exemplary schematic representation of one binary sequence used to control the on/off state of one lighting unit in

FIG. 1 and FIG .2;

FIG. 4 illustrates an exemplary schematic representation of a

combination of binary sequences used to control the on/off state of the lighting units in FIG. 1 and FIG .2;

FIG. 5 illustrates another exemplary schematic representation of one binary sequence used to control the on/off state of one lighting unit in FIG. 1 and FIG .2;

FIG. 6 illustrates another exemplary schematic representation of the combination of binary sequences used to control the on/off state of the lighting units in FIG. 1 and FIG .2;

FIG. 7 illustrates a flow chart of a method of illuminating a target

object.

Throughout the above drawings, like reference numerals will be understood to refer to like, similar or corresponding features or functions.

DETAILED DESCRIPTION

Reference will now be made to embodiments of the invention, one or more examples of which are illustrated in the figures. The embodiments are provided by way of explanation of the invention, and are not meant as a limitation of the invention. For example, features illustrated or described as part of one embodiment may be used with another embodiment to yield a still further embodiment. The invention is intended to encompass these and other modifications and variations as come within the scope and spirit of the invention.

FIG.1 illustrates a block diagram of a lighting device according to an embodiment of the present invention.

Referring to FIG. 1, the lighting device 10 comprises a light source 20 which includes three lighting units 21, 22 and 23, for example. In this embodiment, the lighting units 21, 22 and 23 are light emitting diodes (LED) capable of generating sub-beams of red, green and blue color respectively, namely, the lighting unit 21 is a red LED capable of generating a sub-beam of red color; the lighting unit 22 is a green LED capable of generating a sub-beam of green color; the lighting unit 23 is a blue LED capable of generating a sub-beam of blue color.

Those skilled in the art may appreciate that the lighting units 21, 22 and 23 may also be illuminating units other than LEDs, which could achieve the same purpose.

Those skilled in the art may also appreciate that the three lighting units 21, 22 and 23 included in the light source 20 are only illustrative examples; however, in other examples, the light source 20 may include two lighting units or more than three lighting units, each of which is capable of generating a sub-beam; and light beams of different spectra are generated by different combinations of the sub-beams. For example, the light source 20 may include seven LEDs which are capable of generating sub-beams of royal blue, blue, cyan, green, amber, red-orange and red color, respectively. Additional LEDs whose spectrum is outside the visible range may be included. As an example, Near Infra-Red LEDs with different wavelengths are available, and a common CCD (Charge-coupled Device), a CMOS (Complementary Metal-Oxide-Semiconductor) image sensor used for standard RGB image capturing are known to be sensitive to wavelengths up to 900 nm.

Still referring to FIG. 1, the lighting device 10 further comprises a controller 30 which controls the on/off state of each LED 21, 22, 23. By controlling, at each of a plurality of time periods, the on/off state of each LED 21, 22, 23, different combinations of the sub-beams can be achieved, and thus a plurality of desired light beams having different spectra can be generated respectively at the plurality of time periods, each desired light beam corresponding to one time period. In this embodiment, the desired light beams are of red color, green color and blue color, respectively. The desired light beam of red color may be generated at the first time period by controlling LED 21 to be "on", while controlling LEDs 22 and 23 to be "off'; the desired light beam of green color may be generated at the second time period by controlling LED 22 to be "on", while controlling LEDs 21 and LED 23 to be "off; the desired light beam of blue color may be generated at the third time period by controlling LED 23 to be "on", while controlling LEDs 21 and 22 to be "off. In other embodiments, light beams of other spectra may also be achieved. For example, the light beam of purple color may be generated by combining the sub-beams of red and blue color, namely by controlling LEDs 21 and 23 to be "on", while controlling LED 22 to be "off.

To make sure that the temporal illumination variation, i.e. flicker, of each LED 21, 22, 23 is invisible to the user, the duration of the on state or off state of each LED 21, 22, 23 should be controlled to be less than a first predetermined value.

The first predetermined value may depend on various conditions, such as the distance of the target object from the light source 20, the size of the illuminated area, the intensity of the illumination, and the intra-person variations, for example. In one example, the first predetermined value is 20ms. In other examples, the first predetermined value may be a different value that may reduce the perceivable flicker under various conditions. FIG. 2 shows a block diagram of a lighting device according to another embodiment of the present invention. Compared to the lighting device 10 of FIG. 1, the lighting device 10' of FIG. 2 further comprises a sensor 40 configured to capture, within each time period, an image of a target object being illuminated by the light source 20, so as to obtain a plurality of images. Each image corresponds to one of the plurality of light beams generated by the light source 20, that is to say, each image is captured under the illumination of one desired light beam.

The sensor 40 may be a CCD, a CMOS image sensor, or any other suitable image sensor. The image sensor may be integrated into a camera, for example.

In one embodiment, the controller 20 controls the sensor 40 to capture, within each time period, an image of a target object. In other embodiments, the sensor 40 may be controlled to capture images by a separate controller.

Advantageously, the length of each time period is controlled to be greater than a second predetermined value. The second predetermined value may also be related to various conditions, for example, the intensity of illumination, the distance from the target object to the light source, the distance form the target object to the sensor, the reflectivity of the object, etc.. In one example, the second predetermined value may be the exposure time of the sensor 40.

When the length of each time period is controlled to be greater than the second predetermined value, there is enough time for the sensor 40 to capture a sufficiently

illuminated image within each time period. Thus, there is no need to adopt a quite expensive sensor whose hardware allows exposing multiple times without flushing the accumulated charges so as to capture a sufficiently illuminated image. Therefore, the cost of the sensor may be reduced.

To be specific, the controller 30 of FIG. 1 and FIG. 2 may control the on/off state of each LED 21, 22, 23 in a variety of ways. In one example, the controller 30 may control the on/off state of each LED by a binary digit which represents the state of the LED. For example, binary digit "0" may be used to control the LED to be "on", and binary digit "1" may be used to control the LED to be "off. Alternatively, binary digit "1" may be used to control the LED to be "on", and binary digit "0" may be used to control the LED to be "off.

FIG. 3 shows an exemplary schematic representation of one binary sequence used to control the on/off state of one lighting unit, for example LED 21 in FIG.1 and FIG. 2, in which the lateral direction represents time and the longitudal direction represents the level of the binary digits. Referring to FIG. 3, each time interval between two adjacent dashed lines is referred to as one time slot, which represents the minimum time unit for LED 21 to maintain its state. In this embodiment, each time period T11, T12, T13 includes one time slot, namely, the time period Ti l corresponds to the first time slot; the time period T12

corresponds to the second time slot; the time period T13 corresponds to the third time slot. However, in other embodiments, the time period during which LED 21 generates a desired light beam may comprise two or more than two time slots, which will be described later in detail.

Still referring to FIG. 3, LED 21 is controlled to be "on" during the time period Ti l, controlled to be "off during the time period T12, and controlled to be "off during the time period T13. In the following time slots, the states of LED 21 may be repeated

periodically. It is to be noted that the length of Tl 1, T12 and T13 may be identical, or may be different from each other.

To make sure that the temporal illumination variation, i.e. flicker, of LED 21 is invisible to the user, the duration of the on state or off state of LED 21 should be less than the first predetermined value. In other words, the length of T12 plus T13 should be controlled to be less than the first predetermined value. In the situation that the first predetermined value is 20ms, the length of T12 plus T13 should be less than 20ms.

Advantageously, the length of each time period Ti l, T12, T13 may be controlled to be greater than the second predetermined value, for example greater than the exposure time of the sensor 40, in which case there would be enough time for the sensor 40 to capture a sufficiently illuminated image within each time period T11, T12, T13.

FIG. 4 shows an exemplary schematic representation of the combination of binary sequences used to control the on/off state of LEDs 21, 22 and 23 of FIG.1 and FIG. 2, in which the first binary sequence is used to control the on/off state of LED 21; the second binary sequence is used to control the on/off state of LED 22; the third binary sequence is used to control the on/off state of LED 23.

Referring to FIG. 4, within the first time slot, i.e. the time period Ti l, LED 21 is controlled to be "on", while LED 22 and LED 23 are controlled to be "off. That is to say, within the time period Ti l, the light source 20 generates a light beam which only includes one sub-beam of red color. Within the second time slot, i.e. the time period T12, LED 22 is controlled to be "on", while LED 21 and LED 23 are controlled to be "off. That is to say, within the time period T12, the light source 20 generates a light beam which only includes one sub-beam of green color. Within the third time slot, i.e. the time period T13, LED 23 is controlled to be "on", while LED 21 and LED 22 are controlled to be "off', which means that, within the time period T13, the light source 20 generates a light beam including only one sub-beam of blue color. It is to be noted that, in other embodiments, the light source 20 may also be configured to generate a light beam having more than one sub-beam.

The state of each LED 21, 22, 23 in the first three time slots may be repeated in the following time slots periodically. In this way, the light source 20 may generate light beams of red, green and blue color sequentially and periodically, and thus multi-spectral illumination can be achieved.

FIG. 5 shows another exemplary schematic representation of one binary sequence used to control the on/off state of one lighting unit, for example LED 21 in FIG. 1 and FIG. 2. Referring to FIG. 5, T21, T22 and T23 are time periods within which the lighting device 20 generates desired light beams. In this embodiment, each time period T21, T22, T23 includes two time slots. Any two adjacent time periods are separated from each other by a time interval that is used to interrupt the duration of the on state or off state of LED 21, in such a way that the duration of the on state or off state of LED 21 will not exceed the length of each time period T21, T22, T23.

In this embodiment, for the purpose of reducing flicker perception, the duration of the on state or off state of LED 21, i.e., each time period T21, T22, T23 is controlled to be less than the first predetermined value. Advantageously, the length of each time period Ti l, T12, T13 may also be controlled to be greater than the second predetermined value, for example greater than the exposure time of the sensor, in which case there would be enough time for the sensor 40 to capture a sufficiently illuminated image within each time period Ti l, T12, T13.

Still referring to FIG. 5, the time interval T31 between two adjacent time periods T21 and T22 may comprise two slots T41 and T42, wherein the time slot T41 is adjacent to the preceding time period T21, and the time slot T42 is adjacent to the following time period T22. Advantageously, the binary digit corresponding to the time slot T41 is configured to be opposite to the binary digit corresponding to the time period T21; the binary digit corresponding to the time slot T42 is configured to be opposite to the binary digit corresponding to the time period T22. In other words, one time slot of FIG. 3 is equal to four time slots in FIG. 5; for example, the first time slot of FIG. 3 corresponds to the first time slot set including four time slots in FIG. 5; the second time slot of FIG. 3 corresponds to the second time slot set including four time slots in FIG. 5; the third time slot of FIG. 3 corresponds to the third time slot set including four time slots in FIG. 5.

The binary digit at the first time slot of the first time slot set in FIG.5 is configured to be opposite to the binary digit at the first time slot of FIG. 3. The binary digits at the second and third time slots of the first time slot set in FIG.5 are configured to be the same as the binary digit at the first time slot of FIG. 3. The binary digit at the fourth time slot of the first time slot set in FIG.5 is configured to be opposite to the binary digit at the first time slot of FIG.3. The same principle is also applied to the second time slot set and the third time slot set of FIG. 5. Based thereon, binary digit "0" at the first time slot of FIG. 3 corresponds to four binary digits "1001" at the first time slot set in FIG. 5. Likewise, binary digit "1" at the second time slot of FIG. 3 corresponds to four binary digits "0110" at the second time slot set in FIG .5, and binary digit "1" at the third time slot of FIG. 3 also corresponds to four binary digits "0110" at the third time slot set in FIG .5. Therefore, the binary sequence used to control the on/off state of LED 21 is 100101100110.

Those skilled in the art may appreciate that each time period including two time slots and each time interval also including two time slots are only illustrative examples; however, in other embodiments, each time period may include more than two time slots and each time interval may also include more than two time slots.

Fig. 6 shows another exemplary schematic representation of the combination of binary sequences used to control the on/off state of the lighting units 21, 22 and 23 in FIG. 1 and FIG. 2, in which the first binary sequence is used to control the on/off state of LED 21; the second binary sequence is used to control the on/off state of LED 22; the third binary sequence is used to control the on/off state of LED 23.

Referring to FIG. 6, within the first and fourth time slots of the first time slot set, LED 21 is controlled to be "off', while LED 22 and LED 23 are controlled to be "on". Within the second and the third time slots, which constitute the time period Tl 1, of the first time slot set, LED 21 is controlled to be "on", while LED 22 and LED 23 are controlled to be "off". That is to say, within the time period Tl 1, the lighting source 20 generates a desired light beam which only includes one sub-beam of red color. Within the first and the fourth time slots of the second time slot set, LED 22 is controlled to be "off, while LED 21 and LED 23 are controlled to be "on". Within the second and the third time slots, which constitute the time period T12, of the second time slot set, LED

22 is controlled to be "on", while LED 21 and LED 23 are controlled to be "off. This means that within the time period T12, the light source 20 generates a desired light beam which only includes one sub-beam of green color.

Within the first and the fourth time slots of the third time slot set, LED 23 is controlled to be "off, while LED 21 and LED 22 are controlled to be "on". Within the second and the third time slots, which constitute the time period T13, of the third time slot set, LED

23 is controlled to be "on" while LED 21 and LED 22 are controlled to be "off. This means that within the time period T13, the light source 20 generates a desired light beam which only includes one sub-beam of blue color.

It is to be noted that, in other embodiments, the light source 20 may also be configured to generate a light beam having more than one sub-beam.

The first, second and third time slot sets are repeated in the following time duration periodically. In this way, the light source 20 may generate light beams of red, green and blue color sequentially and periodically, and thus multi-spectral illumination can be achieved.

With the binary sequences of FIG. 6, the maintaining time of each desired light beam may be twice as long as that of FIG. 4. Consequently, with the binary sequences of FIG. 6, the light source 20 is capable of generating desired light beams whose duration is long enough for the sensor 40 to capture sufficiently illuminated images, while not causing flicker.

In another aspect, one embodiment of the invention further provides a method of illuminating a target object by using a light source, the light source including a plurality of lighting units, each lighting unit being capable of generating a sub-beam. FIG. 7 shows a flow chart of a method of illuminating a target object.

The method comprises step S72 of controlling, at each of a plurality of time periods, the on/off state of each lighting unit to generate a plurality of desired light beams having different spectra at the plurality of time periods respectively, each desired light beam corresponding to one time period and having at least one sub-beam; wherein the duration of the on state or off state of each lighting unit is less than a first predetermined value. The first predetermined value may depend on various conditions, such as the distance from the target object to the light source, the size of the illuminated area, the intensity of the illumination, and the intra-person variations, etc.. In one embodiment, the first predetermined value is 20ms.

Advantageously, the method may further comprise the step of capturing, within each time period, an image of the target object being illuminated by the light source by using a sensor so as to obtain a plurality of images, wherein the length of each time period is greater than a second predetermined value.

The second predetermined value may be the exposure time of the sensor 40. The second predetermined value may also be related to a variety of conditions, for example, the intensity of illumination, the distance from the target object to the light source, the distance form the target object to the sensor, the reflectivity of the object, etc..

As the length of each time period is greater than a second predetermined value, there is enough time for the sensor to capture a sufficiently illuminated image within each time period. Thus, there is no need to adopt a quite expensive sensor whose hardware allows exposing multiple times without flushing the accumulated charges so as to capture a sufficiently illuminated image. Therefore, the cost of the sensor may be reduced.

Advantageously, two adjacent time periods are separated from each other by a time interval.

Advantageously, the time interval may comprise at least two time slots, and the step of controlling further comprises: controlling the on/off state of each lighting unit at the time slot adjacent to the preceding time period so as to be opposite to its state at the preceding time period, and controlling the on/off state of each lighting unit at the time slot adjacent to the following time period so as to be opposite to its state at the following time period.

Advantageously, the step of controlling may further comprise: controlling the on/off state of one lighting unit by using one binary sequence.

It should be noted that the above described embodiments are given for describing rather than limiting the invention, and it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art readily understand. Such modifications and variations are considered to be within the scope of the invention and the appended claims. The protective scope of the invention is defined by the accompanying claims. In addition, any of the reference numerals in the claims should not be interpreted as a limitation to the claims. Use of the verb

"comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The indefinite article "a" or "an" preceding an element or step does not exclude the presence of a plurality of such elements or steps.

Claims

CLAIMS:

1. A lighting device comprising:

- a light source including a plurality of lighting units, each lighting unit being capable of generating a sub-beam;

- a controller configured to control, at each of a plurality of time periods, the on/off state of each lighting unit such that the light source is capable of generating a plurality of desired light beams having different spectra at the plurality of time periods respectively, each desired light beam corresponding to one time period and having at least one sub-beam;

wherein the duration of the on state or off state of each lighting unit is less than a first predetermined value.

2. The lighting device of claim 1, further comprising a sensor configured to capture, within each time period, an image of a target object being illuminated by the light source so as to obtain a plurality of images, wherein the length of each time period is greater than a second predetermined value.

3. The lighting device of claim 1 or 2, wherein two adjacent time periods are separated from each other by a time interval.

4. The lighting device of claim 3, wherein the time interval comprises at least two time slots, and the controller is further configured to:

- control the on/off state of each lighting unit at the time slot adjacent to the preceding time period so as to be opposite to its state at the preceding time period, and control the on/off state of each lighting unit at the time slot adjacent to the following time period so as to be opposite to its state at the following time period.

5. The lighting device of claim 1, wherein the first predetermined value is 20ms.

6. The lighting device of claim 2, wherein the second predetermined value is the exposure time of the sensor.

7. A method of illuminating a target object by using a light source, the light source including a plurality of lighting units, each lighting unit being capable of generating a sub-beam, the method comprising the step of:

- controlling, at each of a plurality of time periods, the on/off state of each lighting unit to generate a plurality of desired light beams having different spectra at the plurality of time periods respectively, each desired light beam corresponding to one time period and having at least one sub-beam;

8. The method of claim 7, further comprising the step of:

- capturing, within each time period, an image of the target object being illuminated by the light source by using a sensor so as to obtain a plurality of images, wherein the length of each time period is greater than a second predetermined value.

9. The method of claim 7 or 8, wherein two adjacent time periods are separated from each other by a time interval.

10. The method of claim 9, wherein the time interval comprises at least two time slots, and the step of controlling further comprises:

- controlling the on/off state of each lighting unit at the time slot adjacent to the preceding time period so as to be opposite to its state at the preceding time period, and controlling the on/off state of each lighting unit at the time slot adjacent to the following time period so as to be opposite to its state at the following time period.

11. The method of claim 7, wherein the first predetermined value is 20ms.

12. The method of claim 8, wherein the second predetermined value is the exposure time of the sensor.