Patent Application
20050135441
The present invention relates to light sources.
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.
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.
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.
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.
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.
The manner in which the present invention provides its advantages can be more easily understood with reference to
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.
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.
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
Referring to
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.
In the embodiment shown in
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.
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.
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
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.
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.
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”.
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
The embodiment shown in
The embodiments shown in
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.
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.
