LED illumination system having an intensity monitoring system

[0001] The present invention relates to light sources.

[0002] Light emitting diodes (LEDs) are attractive candidates for replacing conventional light sources such as incandescent lamps and fluorescent light sources. The LEDs have higher light conversion efficiencies and longer lifetimes. Unfortunately, LEDs produce light in a relatively narrow spectral band. Hence, to produce a light source having an arbitrary color, a compound light source having multiple LEDs is typically utilized. For example, an LED-based light source that provides an emission that is perceived as matching a particular color can be constructed by combining light from red, blue, and green emitting LEDs. The ratios of the intensities of the various colors sets the color of the light as perceived by a human observer.

[0003] Unfortunately, the output of the individual LEDs vary with temperature, drive current, and aging. In addition, the characteristics of the LEDs vary from production lot to production lot in the manufacturing process and are different for different color LEDs. Hence, a light source that provides the desired color under one set of conditions will exhibit a color shift when the conditions change or the device ages. To avoid these shifts, some form of feedback system must be incorporated in the light source to vary the driving conditions of the individual LEDs such that the output spectrum remains at the design value in spite of the variability in the component LEDs used in the light source.

[0004] White light sources based on LEDs are in backlights for displays and projectors. If the size of the display is relatively small, a single set of LEDs can be used to illuminate the display. The feedback photodetectors in this case are located in a position that collects light from the entire display after the light from the individual LEDs is mixed.

[0005] As the size of the display increases, an array of LED light sources is needed to provide uniform illumination over the entire array. Such an array complicates the feedback system. If the photodetectors are positioned in the mixing cavity, light from the entire display is collected and analyzed. Hence, only the overall light intensity level of each color can be adjusted by the feedback system. Thus, if a particular LED is performing differently from the others that supply light in that color, the feedback system cannot adjust just that LED.

[0006] The present invention includes a light source and method for controlling the same. The light source includes a first component light source that includes N LEDs, a photo-detector, and a collector, where N>1. Each LED has a light emitting chip in a package. The light emitting chip emits light in a forward direction and light in a side direction. The light generated in the forward direction is determined by a drive signal coupled to that LED. A portion of the light in the side direction leaves the package. The collector is positioned such that a portion of the light in the side direction that leaves the package of each of the LEDs is directed onto the photo-detector. The photo-detector generates N intensity signals, each intensity signal having an amplitude related to the intensity of the light emitted in the side direction by a corresponding one of the LEDs. The intensity of light in the side direction is a fixed fraction of the intensity of light in the forward direction. In one embodiment, each of the LEDs emits light at a wavelength that is different from the wavelength at which the others of the LEDs emit light. In one embodiment, the collector is cylindrical, the LEDs being arranged along a line parallel to an axis of the collector. In another embodiment, the photo-detector includes N photodiodes for measuring light received through N wavelength filters, each wavelength filter passing light from one of the LEDs. In another embodiment, two of these component light sources are connected to a bus connected to a feedback controller. In this embodiment, each component light source also includes an interface circuit that controls N signals, each signal determining a light intensity to be generated in the forward direction by a corresponding one of the LEDs. The interface circuit also couples the N intensity signals to the bus in response to a control signal identifying the first interface. The feedback controller utilizes the intensity signals of each of the component light sources to control the drive signals so as to maintain the intensity signals at predetermined target values.

[0007] FIG. 1A is a top view of a prior art display system.

[0008] FIG. 1B is an end view of the display system shown in FIG. 1A.

[0009] FIG. 2 is a top view of a component light source.

[0010] FIG. 3 is a cross-sectional view of the light source shown in FIG. 2 through line 3-3.

[0011] FIG. 4 is a top view of an extended light source according to one embodiment of the present invention.

[0012] FIG. 5 is a top view of a component light source.

[0013] FIG. 6 is a cross-sectional view of the component light source shown in FIG. 5 through line 6-6.

[0014] FIG. 7 is a top view of an extended component light source.

[0015] The manner in which the present invention provides its advantages can be more easily understood with reference to FIGS. 1A and 1B. FIG. 1A is a top view of a prior art display system 100. FIG. 1B is an end view of display system 100. Display system 100 utilizes an LED source 130 having red, blue, and green LEDs to illuminate a display device 170 from a location behind display device 170. For example, display device 170 may include an imaging array constructed from an array of transmissive pixels. Light from LED source 130 is “mixed” in a cavity 160 behind display device 170 to provide uniform illumination of display device 170. The walls of this cavity are typically reflective. A photo-detector 110 measures the intensity of light in cavity 160 at three wavelengths corresponding to the LEDs in LED source 130. A controller 120 uses these measurements in a servo loop to adjust the drive currents of each of the LEDs in LED source 130 to maintain the desired illumination spectrum.

[0016] As the size of the display increases, the LEDs must be replaced by arrays of LEDs that have a spatial extent that is determined by the size of the display and the amount of light needed to illuminate the display. There is a practical limit to the amount of light that can be generated from a single LED. Hence, an illumination based on one set of RGB LEDs is limited to relatively small displays. To increase the available light beyond this limit, multiple sets of LEDs are required. Since the properties of the LEDs differ significantly from production batch to production batch, each set of LEDs must be separately controlled in a feedback loop to maintain the desired spectrum. Hence, a photo-detector array that samples light in the mixing cavity after the light from the various LEDs has been mixed together can only provide information about the overall performance of the array at each color. This information is insufficient to adjust the drive currents of the individual LEDs. The present invention overcomes this problem by providing an LED light source in which the light from each of the component LEDs is measured separately even when a number of LEDs of the same color are present in the mixing cavity.

[0017] The present invention utilizes the observation that a portion of the light generated in an LED is trapped in the active region of the LED and exits the LED through the sides of the chip. In general, an LED is constructed from a layered structure in which a light-generating region is sandwiched between n-type and p-type layers. The light that travels in a direction at about 90 degrees to the surface of the top or bottom layer is extracted and forms the output of the LED. The air/semiconductor boundary at the top of the LED and the semiconductor/substrate boundary under the LED are both boundaries between two regions having markedly different indices of refraction. Hence, light generated in the active region at angles greater than the critical will be internally reflected at these boundaries and remain trapped between the two boundaries until the light is either absorbed or reaches the edge of the LED chip. A significant fraction of this trapped light strikes the chip/air boundary at the edge of the chip at an angle that is less than the critical angle, and hence, escapes the chip.

[0018] The present invention utilizes this edge-emitted light to provide a monitoring signal. In general, the amount of light that exits the chip at the edge is a fixed fraction of the total light being generated in the LED. The precise fraction varies from chip to chip. Refer now to FIGS. 2 and 3, which illustrate a RGB component light source 200 according to one embodiment of the present invention. FIG. 2 is a top view of a component light source 200, and FIG. 3 is a cross-sectional view through line 3-3. Component light source 200 includes three LEDs 201-203 that emit red, green, and blue light, respectively. Each LED includes a chip that emits a fraction of the light generated therein through the side of the chip. The LED has a body which includes a transparent region that allows this light to exit in a direction that is different from that of the light that is emitted in a direction perpendicular to the chip surface. The chips in LEDs 201-203 are shown at 211-213, respectively.

[0019] Referring to FIG. 3, the light leaving the top of the chip is shown at 221, and the light leaving the side of the chip is shown at 222. To simplify the following discussion, the light leaving the top of the chip will be referred to as the “output light”, and the light leaving the side of the chip after one or more internal reflections at angles greater than the critical angle in the LED will be referred to as the side light. The present invention collects a portion of the side light using a collector 230. The light that is so collected will be referred to as the monitor light. The monitor light is directed onto a photo-detector 240 that measures the intensity of light in each of the three spectral regions of interest. In this case, photo-detector 240 measures light in the red, blue, and green spectral bands and generates the three signals shown at 241 whose amplitudes are a function of the measured intensities. The amplitude of these signals is, in turn, a measure of the output light. In the following discussion, these signals will be referred to as the monitor signals.

[0020] Photo-detector 240 can be constructed from 3 optical filters and 3 photodiodes for measuring the light transmitted by each filter. To simplify the drawing, the component photodiodes and optical filters have been emitted from the drawing.

[0021] In the embodiment shown in FIGS. 2 and 3, collector 230 is a circularly symmetric collector that has a surface 233 that reflects a portion of the side light leaving LED 201 in a downward direction. The collector can be constructed from a clear plastic. The reflectivity of the surface can be the result of the difference in the index of refraction of the plastic and air. Alternatively, the surface can be coated with a reflecting material such as aluminum.

[0022] In general, the ratio of the monitor light to the output light will vary from LED to LED. However, the precise value of this ratio does not need to be determined so long as it remains constant. As noted above, the monitor signals are used by a feedback controller to maintain the correct red, blue, and green light intensities to generate the desired spectrum. Each LED has a separate power line on which the LED receives a signal whose average current level determines the light output by that LED. The power line for LED 201 is shown at 251. The feedback controller adjusts the drive current to each LED until the monitor signals match target values stored in the feedback controller.

[0023] The target values can be determined experimentally by analyzing the light generated by the component light source as a function of the drive currents to the LEDs. When a satisfactory spectrum is achieved, the values of the monitor signals are recorded by the controller. The feedback controller then adjusts the drive currents to maintain the monitor signals at these recorded target values during the normal operation of the component light source. If, for example, one of the LEDs ages, and hence, produces less light, the monitor signal associated with that LED will be reduced in value. The feedback controller will then increase the drive current to that LED until the monitor signal once again matches the target value for that LED.

[0024] The component light sources discussed above can be combined to construct extended light sources for illuminating a cavity in a manner analogous to that discussed above with reference to FIG. 1. Refer now to FIG. 4, which is a top view of an extended light source 300 according to one embodiment of the present invention. Light source 300 may be viewed as a linear light source having a constant light intensity along its length. Light source 300 is constructed from a plurality of component light sources of the type discussed above with reference to FIGS. 2 and 3. Exemplary component light sources are shown at 301-303.

[0025] Each component light source has six signal lines that may be viewed as a component bus 307. Component bus 307 includes the three lines that transmit the monitor signals and the three power lines that drive the individual LEDs within the component light source. The component bus is connected to a control bus 311 by an interface circuit. The interface circuits corresponding to component light sources 301-303 are shown at 304-306, respectively.

[0026] In this embodiment, each interface circuit provides two functions. First, the interface circuit selectively connects the monitor signals to a feedback controller 310 and receives signals specifying the drive currents to be applied to each of the LEDs in the component light source. The interface circuit includes an address that allows feedback controller 310 to selectively communicate with the interface circuit.

[0027] Second, the interface current includes the circuitry that maintains the drive current on each LED at the levels specified by the feedback controller when the component light source is not connected to bus 311. To carry out this function, the interface circuit includes three registers that hold values that determine the drive currents to each LED and the circuitry for converting these values into the actual drive currents. The drive currents may be set by varying the magnitude of a DC current through each LED or by varying the duty factor of an AC signal that switches the LED “on” and “off”.

[0028] The above-described embodiments of the present invention utilized a circularly symmetric light collector for collecting the side light from each LED and directing the light onto the photo-detector. However, other shapes of light collector can be utilized. Refer now to FIGS. 5 and 6, which illustrate a component light source that utilizes a cylindrically shaped light collector. FIG. 5 is a top view of component light source 400, and FIG. 6 is a cross-sectional view of component light source 400 through line 6-6. Component light source 400 has six LEDs 401-406. The side light from these LEDs is collected by a cylindrical light collector 410 that reflects a portion of the side light from each LED onto a photo-detector. The photo-detectors for LEDs 401-406 are shown at 411-416, respectively. Cylindrical light collector 410 includes a reflective surface 417 that can utilize total internal reflection or a reflective coating to provide the reflective function. Cylindrical light collector 410 can be constructed from a clear plastic extrusion to which an optional reflective coating is applied.

[0029] The embodiment shown in FIGS. 5 and 6 utilizes a separate photo-detector for each LED. The photo-detector is preferably a photodiode that is covered with an optical filter that prevents light from the surrounding LEDs from being measured. Embodiments in which a single photo-detector similar to photo-detector 240 discussed above can also be constructed by placing the photo-detector in the location occupied by photo-detectors 412 and 415 and eliminating the other photo-detectors. In such embodiments, cylindrical light collector 410 must act as a light pipe for moving the light from LEDs 401 and 403 to the detector. Such embodiments, however, are not preferred, as the efficiency with which the light from LEDs 401 and 403 is collected is less than the efficiency of the collection from LED 402. Hence, the signal-to-noise ratios for the monitor signals from LEDs 401 and 403 are less than the signal-to-noise ratio for the monitor signal from LED 402.

[0030] The embodiments shown in FIGS. 5 and 6 utilize one triplet of LEDs that generate red, blue, and green light on each side of the cylindrical light collector. However, embodiments in which the cylindrical collector is extended to accommodate additional LEDs and photo-detectors can also be constructed provided the light from one LED is not detected by the photo-detector associated with another LED. Such extended light sources are well adapted for applications that currently utilize a linear light source. Refer now to FIG. 7, which is a top view of an extended component light source 500. Component light source 500 includes 12 LEDs 501-512 that are arranged on the two sides of a cylindrical light collector 520. The LEDs on one side of cylindrical light collector 520 are offset relative to the LEDs on the other side of cylindrical light collector 520. This arrangement provides RGB triplets similar to those discussed above with reference to FIGS. 2 and 3. Each triplet involves one LED from one side and two LEDs from the other side.

[0031] The above-described embodiments have utilized component light sources that are constructed from red, green, and blue LEDs. However, embodiments of the present invention that utilize different numbers and colors of LEDs can also be constructed. For example, a light source that appears white to a human observer can be constructed by mixing light from a blue-emitting LED and a yellow-emitting LED. Hence, a white light source based on component light sources having two LEDs according to the present invention would be utilized to provide an extended white light source. Similarly, color schemes based on four colors are known to the printing arts. In such a color scheme, a component light source according to the present invention would have 4 LEDs.

[0032] Various modifications to the present invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.