Multi color light source

FIELD OF THE INVENTION

The invention relates to a multi color light source, at least comprising several light emitting elements, in particular LED's (LED: light emitting diode), emitting light beams of different colors, a beam combining optics arranged for combining said light beams to a combined light beam, said beam combining optics comprising at least a first wavelength selective element transmitting light of a first of said light emitting elements in a first wavelength region and reflecting light of a second of said light emitting elements in a second wavelength region, and a light sensing arrangement measuring fractions of light emitted by said light emitting elements in order to allow a control of the intensity and color output of the combined light beam. Multi color LED based light sources can be used in applications in which highly concentrated, full spectrum light is required. Examples of such applications are spot lighting and digital projection. To deliver a light source for such applications, the emission of several different colored LED's is combined by a beam combining optics, for example an arrangement of dichroic mirrors or cubes. The light spectrum emitted by such a light source is controlled by the ratio of power of the separate LED's. BACKGROUND OF THE INVENTION

The use of LED's in light sources for illumination or projection applications not only has advantages. One major disadvantage of LED's is their temperature dependent emission spectrum. Therefore, not only the efficiency but also the wavelength of emission of a LED changes with temperature, drive current and component age. This requires a complex sensing of the light emitted by the different LED's in order to achieve a constant color output of the combined light beam.

US 2006/0215122 Al describes an image projection apparatus for adjusting white balance in consideration of the level of light emitted from the LED's. This image projection apparatus comprises a light source unit for sequentially emitting light generated by a red, a green and a blue light emitting element. A beam combining optics combines the light beams of the different light emitting elements to a combined beam. A light sensor arranged at the light path of the combined beam measures the levels of light generated by the different light emitting elements. A controller controls the emission of the light emitting elements based on the measurement of the levels of light measured by the light sensor to adjust the white balance of the image projected from the image projection apparatus.

The measurement of spectral shift of the light emitted by the light emitting elements in an image projection apparatus requires appropriate filters for the red, green and blue wavelength range. In order to avoid a change of filters during operation, it is known to use a three channel light sensor for this measuring task. Such a three channel light sensor is a quite expensive component. Still the filters used for such sensors are often of limited quality. Aging and temperature behavior of such sensors and filters is largely unknown or worse than required. Filter characteristics also depend on the angle of incidence of the light, which requires fine tuning when using the sensor in the light path.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a multi color light source based on light emitting elements of different color, in which a sensing arrangement for controlling the spectral output of the light source provides a high reliability of measurement and can be realized at low costs.

The object is achieved with the multi color light source according to claim 1. Advantageous embodiments of this light source are subject matter of the dependent claims or are described in the subsequent portion of the description. The proposed multicolor light source at least comprises several light emitting elements, in particular LED's, emitting light beams of different colors, a beam

combining optics arranged for combining said light beams to a combined light beam and at least two light sensors arranged at separate locations to measure fractions of light emitted by said light emitting elements. The beam combining optics comprises at least a first wavelength selective element transmitting light of a first of said light emitting elements in a first wavelength region and reflecting light of a second of said light emitting elements in a second wavelength region. A first of said sensors is arranged to measure light of said first light emitting element reflected at said first wavelength selective element and/or to measure light of said second light emitting element transmitted through said first wavelength selective element. With such an arrangement of at least one of the sensors the wavelength selective properties of the wavelength selective element are also used for the sensor. This avoids the requirement of any additional wavelength selective filter in front of the sensor. By appropriately arranging the different sensors in this multi color light source, all of the information required for controlling the light emitting elements to achieve a desired spectral characteristic of the output of the light source can be achieved with simple and cheap sensors without any filters. The number of sensors required is dependent on the number and properties of the light emitting elements in this light source. For example, if one or several of these light emitting elements do not show any wavelength shift with temperature, less sensors have to be provided since only the intensity of these one or several light emitting elements has to be measured. Furthermore, when sequentially operating the different light emitting elements, which is the case in common projection applications, also less sensors are required compared to a non-sequential operation of the light emitting elements, i.e. a continuous operation of these elements, in which all elements are operated at the same time. In such a continuous operation, however, it is also preferable to include measurement periods in which the light emitting elements are sequentially switched. Such an operation, therefore, also requires less sensors.

The proposed multi color light source preferably also provides a controller for controlling the emission of the light emitting elements based on the measurement of the light sensors to achieve the desired intensity and color output of the combined beam. Nevertheless, it also possible to provide the proposed multi color light source without such a controller. In this case, the multi color light source must be

connected to an appropriate controller during operation, for example a controller of a digital imaging apparatus, in which the light source is mounted.

In the proposed multi color light source the sensors are arranged such that they profit from the wavelength selective properties of the already included wavelength selective elements of such a light source. By proper combination of sensor locations the emission properties, i.e. intensity and spectral output, of the light emitting elements can be calculated from the different measurements. This requires a calibration of the measuring system prior to operation. In such a calibration the measured values of the sensors are correlated to the intensity and wavelength characteristic measured separately at different operation temperatures of the light emitting elements. The result is a table or matrix in which the measured correlations are recorded. Such a calibration is preferably performed after the manufacturing of the light source and before its first use, but may also already be part of the design and measured on a prototype. Nevertheless, it is also possible to re-calibrate the multi color light source at any later time, if required. The controller for controlling the light output of the light source then controls the light emitting elements based on the measured values of the light sensors and the above matrix data. Since the sensors make use of the spectral characteristics of the high quality wavelength selective elements of the beam combining optics, a high quality of the measurements relating to the wavelength is assured. All light sensors can therefore be of the simplest type. Aging and temperature shift causes no significant problem because all sensors may be of the same type. Since the sensors are already part of the proposed multi color light source, the present invention provides a complete, ready to use illumination module.

The use of the wavelength selective element or wavelength selective elements of the beam combining optics for the above measurements is possible, since such a wavelength selective element, when designed for reflecting in a special wavelength region still allows a small transmission of light in this wavelength region and an even larger transmission at neighboring wavelength regions. Since LED's have a rather broad emission, an amount of light at the shoulders of this emission curve always is transmitted through the reflective element. The same applies to the reflection of portions of light emitted by a LED at wavelength selective transmissive elements. This

residual light is detected and measured by the light sensors according to the present invention. By combining the measurement of the different light sensors, the required information about a wavelength shift or intensity shift of the LED's can be derived. In a preferred embodiment, the desired information about a wavelength shift is achieved by forming a ratio of the measured values of at least two appropriately located sensors. In one embodiment of the light source comprising three light emitting elements of different colors, the beam combining optics comprises two wavelength selective elements for forming the combined beam. The first wavelength selective element transmits light of the first light emitting element in a first wavelength region and reflects light of the second light emitting element in a second wavelength region. The second wavelength selective element transmits light in the first and second wavelength regions, typically in a larger wavelength region including the first and second wavelength regions, and reflects light of the third light emitting element in a third wavelength region. Such a beam combining optics is known in the art, for example in RGB (RGB: red- green-blue) light sources. The wavelength selective elements of the beam combining optics in this and other embodiments of the proposed multi color light source are preferably dichroic mirrors. In this embodiment, three light sensors are arranged inside of the housing of the light source. The first light sensor is arranged to measure light of the first light emitting element reflected at the first wavelength selective element and to measure light of the second light emitting element transmitted through the first wavelength selective element. The second light sensor is arranged to measure light of the first and second light emitting elements reflected at the second wavelength selective element and at the same time light of the third light emitting element transmitted through said second wavelength selective element. The third sensor is arranged to measure light of the combined beam, for example by collecting light reflected at a beam forming optics arranged in the beam path of the combined beam. Such a reflecting surface may be the surface of a collimating lens, for example. Although these surfaces are intended to be non reflective, residual reflections provide sufficient signal for light measurement. Furthermore any other surface inside of the light source may be used, if a sufficient amount of light of the combined beam is reflected or scattered from this surface. When sequentially operating the three different light emitting elements, a complete information

for determining any intensity or wavelength shift of the three light emitting elements is achieved with the measurement values of the three sensors.

The light sensors used in the proposed multi color light source can be, for example, simple photodiodes which do not require any additional wavelength selective filter on or in front of their photosensitive area.

A main application of the proposed multi color light source is digital projection, for example in form of pocket beamers, rear projection TV and similar applications. Furthermore, the proposed multi color light source may also be used in spot lighting applications, for example in theaters or for event lighting, in which high beam quality color controlled light is requested. For such applications, the multi color light source preferably contains a fully integrated controller to enable simple application of the light source for the customer.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described herein after. BRIEF DESCRIPTION OF THE DRAWINGS

The proposed multi color light source is described in the following by way of examples without limiting the scope of protection as defined by the claims. The figures show

Fig. 1 a schematic view of an example of the proposed multi color light source with three light sensors;

Fig. 2 a schematic view of a further example of the proposed multi color light source with two light sensors; Fig. 3 an example of the spectral output of three LED's of different colors;

Fig. 4 an example of the transmittance of two different dichroic mirrors; Fig. 5 an example of the spectral sensitivity of a standard silicon photodiode; and Fig. 6 an example of the power spectrum of a green LED at two different operation currents.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a schematic view of a first example of the proposed multi color light source. This light source comprises three LED's, a red LED 1, a green LED 2 and a blue LED 3. The light emitted by the different LED's is collimated by a beam collimating optics 4 arranged in front of each of the LED's in order to form a red 5, a green 6 and a blue light beam 7. These light beams are combined to a combined light beam 8 by a beam combining optics which comprises two dichroic mirrors 9, 10 in this example. The dichroic mirrors mix the light of the three LED's which may also be formed of known LED modules having several LED's of the same color side by side. While the first dielectric mirror 9 is coated such that it only reflects green light, for example in the wavelength region from 500 to 590 nm, the second mirror 10 has a coating for reflecting blue light, for example in the wavelength region from 400 to 500 nm. The first dielectric mirror 9 is designed to be mainly transparent in the wavelength region of the light emitted by the red LED 1. The second dielectric mirror 10 is designed to allow transmission of the red and green light emitted by the red 1 and green LED 2.

Since LED's have a rather broad transmission, a small amount of light of the green LED 2 is not reflected by the first dichroic mirror 9 and at the same time some amount of the light emitted by the red LED 1 is reflected at this dichroic mirror 9. This transmitted and reflected amounts of light have a strong dependency on the actual emission profile of the LED's 1, 2. If the dominant wavelength of the red LED 1 shifts to shorter wavelength due to temperature or changed power, the reflected light gets stronger. This is measured by appropriately placing a first light sensor 11 at a position gathering such reflected light.

Placing the three light sensors 11, 12 and 13 at the three separate positions indicated in Fig. 1, information about all three LED's can be measured. As there are always a lot of additional reflections and diffractions inside of the housing of such a multi color light source, the placement of the sensors is not critical. Due to the extreme high levels of light even outside of the main beams the light is sufficient for

measurement. It is even beneficial for the sensors life time to have only reduced light levels.

The second sensor 12 is arranged to gather light in the wavelength range below 480nm of the red LED 1 and the green LED 2 which is reflected at the second dichroic mirror 10. This second sensor 12 also gathers light emitted by the blue LED 3 and not reflected at the second dichroic mirror 10 in the wavelength range above 480nm.

The sensor signal of the second sensor 12, therefore, has a correlation to the "blue" emission of the red LED 1 and to the "blue" emission of the green LED 2, but additionally to a green and red (or yellow) component of the blue LED 3. The sensor signal of the first light sensor 11 has a correlation to the "green" (and probably "blue") emission of the red LED 1 with additional signals from the "red" (and probably "blue") component of the green LED 2.

The third light sensor 13 is positioned to gather light of the combined beam 8 reflected at the surface of a lens of the beam condensing optics 14 of the light source. The sensor signal of the third light sensor 13 is therefore closely related to the output light level of the light source, i.e. to the blue component of the blue LED 3, the green component of the green LED 2 and the red component of the red LED 1.

Measuring these responses after design or manufacturing, coefficients can be placed in a system matrix. With this system matrix the current characteristic of each LED can be measured. The shift of the wavelength of each LED is then for example detected as a change in the relation between the signals of the different light sensors which corresponds to a relation: light in neighbor color band / light in target color band. Although the spectral separation of the sensors is limited, the measurements allow a complete analysis of the emission characteristics of the LED's and will not change over system life.

As an example, when assuming for case of simplicity the following ideal conditions, the following calculations can be made . The LED's used might have the spectral properties shown in Figure 3. The dichroic mirrors 9 and 10 might have the transmission / reflexion properties (reflectance = 1 - transmittance) as shown in Figure 4. The spectral sensitivity S _re i = f(λ) of the sensors is depicted in Figure 5.

The signal on each of the sensors can be calculated by integration of sensor sensitivity, multiplied with all transmittances and reflectances and multiplied by output of the LED over the complete spectrum.

With: I = Intensity / Light flux Pr, Pb, Pg = Power spectrum of emission

Re = Reflectance (with dichroic filters normally equal to 1-Tr)

Tr = Transmittance

S = Relative sensitivity

As long as there are no changes in the spectrum of the LED's, the integral part of the equations can be handled as constants. In this case the system can be expressed as set of linear equations. With the three sensor currents the light flux of the three LED's can be calculated as the solution of these equations. Sequential operation of LED's is not required for this task.

The process flow is as follows: Design / Manufacturing Calculate or measure constants knm - Calibrate sensor signals

Calculate and save inverted system matrix Operation Operate LED's Measure all sensor signals - Calculate light flux of all LED's using stored matrix

If additionally also the spectral composition of the LED light has to be measured, it is required to make a measurement during a phase where only one LED is powered (with linear superposition and somewhat more complex calculation alternatively also measurements with two LED's (always one LED off) are possible). As example here the LED 2 is used. Figure 6 exemplary shows the power spectrum of a this LED at two different operation currents.

For example calculation, a silicon photodiode is used as sensor and the sensitivity is approximated for the wavelength range of 400nm to 700nm by a linear function. With λ in nm it is: S _Sensor (λ) = 0.002333λ - 0.8333 For mirror 9 and 10 ideal filter functions with one edge each are used:

- mirror 9 reflects 100% for wavelength < 600nm and transmits 100% above

- mirror 10 reflects 100% for wavelength <480nm and transmits 100% above

The optic 14 has a residual-reflection of 2.5%.

In the example the sensor signals, measured in application or calculated using the preceding equations, are:

The result can be evaluated by normalizing the measurement:

140OmA normalized to 13 100mA normalized to 13@1400mA 100mA normalized to 13

If now luminous intensity and color shift are to be evaluated, two signals have to be chosen. The best signals in this example are from sensors 12 and 13. The signal of sensor 13 is already close to the luminous flux. The ratio of 12 / 13 is strongly influenced by the shift in color; it changes from 0.879 at full current to 0.095 at low current. The proposed system first analyses the ratio of 12 / 13 and determines the current color point by the help of a look up table (which contains the correlation between color point and 12 / 13, which is calculated or measured during design or manufacturing of the system. Alternatively this can be described using a model function fitted to the behaviour). In a second step the luminous output is calculated based on the sensor signal of sensor 13, optional with a correction factor stored in the same look up table. In this example the ratio of 11 / 13 shows nearly no change. This is an indication for the smaller spectral width of the emission due to lower current. If also this

change in spectral width is changing, e.g. caused by different shifts induced by temperature and current, a two dimensional look up table and the signals 12 / 13 and 11 / 13 can be used to still measure the exact color point (general rule: one independent measure signal for each independent variable to analyse). The process flow is as follows:

Design / Manufacturing

Calculate or measure correlation between color changes and ration 12/13

(and 11/13 if required)

Calculate or measure correction factor for flux measurement 13 depending on color

Assemble look up table or design approximation function for color point and flux correction

Operation

Pulse only LED2 - Measure all (or only 12 and 13) sensor signals

Calculate quotient 11/13 and 12/13 (or only 12/13)

Determine color point and luminance correction factor from look up table or model function

Calculate light flux using signal 13 and correction - (Use light flux and color point result to correct light output to target light and color)

Fig. 1 shows an example in which the three LED's show a temperature dependent wavelength shift as well as an intensity shift during operation. When using one or two LED's without any temperature dependent shift, for example a special kind of phosphor converted LED's for red and green, even less sensors are necessary for measurement. This is shown in the schematic view of Fig. 2, in which the red LED 1 and the green LED 2 do not show any significant temperature dependent wavelength shift which would have to be compensated. For these LED's only the emitted intensity is of interest. Combining these LED's with a conventional blue LED, only sensors 12 and 13 are necessary to gather the required measurement data for appropriate control of the lights.

Fig. 1 and 2 also schematically show the controller 15 which is connected to the light sensors 11 to 13 and to the driver units of the LED's 1 to 3. The driver units are not separately depicted in these figures.

A control of the different LED's can also be achieved by combining for at least one color at least two LED's slightly deviating in color in the light source. For example, mixing the blue LED module 3 from different wavelength batches of blue LED's and providing separate drivers for these two LED's, allows a completely color stable illumination module. The desired color can always be achieved by appropriately balancing the output of the two LED's. With separate short pulses on each of the two different blue LED's and analysis of the sensor signals 12 and 13 (as explained previously) the color coordinates of both blue LED's can be determined. Depending on these color coordinates the balance between the two is set to achieve the required mixed color point. Finally the level of both LED's can be adjusted to the brightness required, using the sensor signal 13 and an optional correction factor (from look up table using signal 12/13) for the spectral shift of both LED's. This can greatly reduce the designing effort for all projector manufacturers. Alternatively the measurement can be used to communicate the light source properties for compensation done by downstream picture or light processing components.

Digital projection and spot lighting require high power low etendue light sources with precisely controlled color balance. The usually used optical solution is a color selective combination of multiple beams by dichroic reflections. By adding the light sensors in such an arrangement at selected locations according to the present invention, these sensors can obtain color and brightness information without having their own filters. Based on this, the design of a completely self contained illumination module with calibrated light output is possible. Expensive and unreliable color sensors are omitted.

While the invention has been illustrated and described in detail in the drawings and forgoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive, the invention is not limited to the disclosed embodiments. The different embodiments described above and in the claims can also be combined. 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. For example, also more than three LED's can be provided in the proposed multi color light source. Furthermore, the location of the sensors used for light measurement is not limited to the locations shown in the figures. These sensors must be arranged to use the wavelength selective property of at least one wavelength selective component of the beam combining optics and to allow to derive the required information from the different sensor values.

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 measures are recited in mutually different dependent claims does not indicate that a combination of these measures can not be used to advantage. Any reference signs in the claims should not be construed as limiting the scope of these claims.

LIST OF REFERENCE SIGNS

1 red LED

2 green LED

3 blue LED

4 beam collecting optics

5 red light beam

6 green light beam

7 blue light beam

8 combined light beam

9 first dichroic mirror

10 second dichroic mirror

11 first light sensor

12 second light sensor

13 third light sensor

14 beam condensing optics

15 controller