The present invention relates to illumination systems, and in particular to control systems for red-green-blue (RGB) light emitting diode (LED) illumination systems that allow the light temperature and intensity generating by the illumination systems to be dynamically calibrated.
LEDs are semiconductors that convert an electrical energy into light. Since LEDs generate relatively little heat compared to other common forms of lighting, such as incandescent lights, the energy conversion process performed by LEDs is quite efficient. This is a highly desirable trait in lighting systems to be used for illumination, since excessive heat production not only wastes electricity, but may also require extensive heat dissipation efforts and may even raise safety concerns depending upon the fixture installation. Some of the other advantages that make LEDs desirable for illumination applications include their small size; their relatively high radiance (that is, they emit a large quantity of light per unit area); their very long life, leading to increased reliability; and their capacity to be switched (that is, turned on and off) at very high speeds.
While visible light LEDs have been applied in a number of fields since their invention in 1960, they have been used for illumination applications only relatively recently. One of the primary limitations in the use of LEDs in this field has been the difficulty of producing white light. White light consists of a mixture of light wavelengths across the visible light spectrum. Traditional LEDs cannot produce white light; instead, each LED can produce only light in one very narrow frequency band. It is well known that the combination of light in the three primary colors of red, green, and blue will produce white light. In fact, any color of light may be produced by the appropriate combination of light in these three colors. While red and green LEDs have been commercially available for decades, the blue LED was not developed until 1993, when it was introduced by the Nichia Corporation of Japan. By combining these traditional red, green, and blue LEDs in a tightly coupled pattern, a crude form of white light could then be produced. By varying the relative intensity of the light emitted by the red, green, and blue LEDs, one could alter the color of light produced, thereby providing a light source that will generate light of any color desired.
An alternative method of producing white light, developed by the Nichia Corporation in 1996, is the coating of a blue LED with a white phosphor. The blue LED stimulates the phosphor to generate a broad band of visible light emissions, thereby producing white light. This method suffers from the limitation that the frequency band of light produced is fixed, and cannot be altered to produce different lighting effects from the same LED. This method is therefore inappropriate for applications where different colors of light or lighting effects may be desired.
In addition to the problem with producing white light, the other primary limitation on the use of LEDs for illumination applications has been their brightness, which historically was far below that of typical incandescent and fluorescent light sources. By 1997, however, the Nichia Corporation, along with Texas Instruments Incorporated of Dallas, Tex., were producing LEDs of sufficient brightness for many illumination applications. It thus became possible to provide complete illumination solutions using only LEDs in certain applications, such as relatively small, indoor areas.
As already explained, a very simple system for producing white light with LEDs could involve the application of a pre-set current to a combination of red, green, and blue LEDs. It would be possible with such a system to emulate, for example, the color of light produced by daylight or by a typical incandescent bulb. Such a simple system would not, however, allow the user to take advantage of the many opportunities for temperature variance made possible by the use of an LED illumination system. (It should be noted that light color is often referred to as its “color temperature” or simply “temperature,” corresponding to the temperature of a black body that would produce light of that color measured in degrees Kelvin.) Since both temperature and intensity of the light produced by an LED illumination system may be varied simply by varying the amount of electrical current applied to the red, green, and blue LEDs in the system, many desirable illumination effects become possible that would not be available with incandescent lights. For example, an illumination system might include settings to emulate ambient lighting conditions at different times of day. Or the system might allow for variance in the light temperature depending upon application, such as applying a “cold” blue-tinged light for reading purposes, while allowing a “warm” red-tinged light setting to be chosen at meal times. Far more subtle and complex effects are possible. In order to take advantage of such flexibility offered by an LED illumination system, however, some form of electronic control system is required.
The use of electronic control systems for the purpose of mixing light from red, green, and blue LEDs to produce lighting effects is known. For example, U.S. Pat. No. 5,420,482, issued to Phares, teaches a controlled lighting system that includes a set of light elements each having a control unit. The control units are individually addressable along a data bus. Information packets may be sent to each control unit by addressing each packet to match the address of the control unit. The data packets may contain information necessary to manipulate the output level of each of the light elements controlled by a particular control unit. In this way, the temperature and intensity of the light produced by each of the light elements may be manipulated by the use of digital information packets sent along a control bus. The system can thus produce an overall light output of varying temperature and intensity in response to digital signal inputs.
U.S. Pat. No. 6,016,038, issued to Mueller et al. and assigned to Color Kinetics, Inc. of Boston, Mass., teaches a method of controlling the intensity and temperature of an RGB LED system using pulse-width modulated (PWM) signals generated by a microcontroller. PWM is a well-known technique for controlling analog circuits with the output of a microprocessor or other digital signal source. A PWM signal is a square wave modulated to encode a specific analog signal level. In other words, the PWM signal is fixed frequency with varying width. The PWM signal is still a digital signal because, at any given instant of time, the full direct current (DC) supply current is either in the “on” or “off” state. The voltage or current source is thus supplied to the analog load by means of a repeating series of on and off pulses. The on-time is the time during which the DC supply is applied to the load, and the off-time is the period during which that supply is switched off. Given a sufficient bandwidth, PWM can be used to encode any analog value.
When the power to an LED is rapidly switched on and off, variance of the length of time during the on and off modes gives the effect of variance of the intensity of the light that is produced. As a result, a PWM signal can be used in place of a varying DC current to achieve intensity variance in an LED. PWM has numerous advantages over traditional analog control systems, including less heat production than analog circuits of similar precision, and significantly reduced noise sensitivity. Given the significant advantages that PWM control offers in communications and control systems applications, many microprocessors and microcontrollers produced today include built-in PWM signal generation units that may be directly applied to illumination control systems.
A significant limitation of the control system taught by the '038 patent, and of other prior art illumination control systems, is the inability to easily balance the spectral contributions of each LED source to permit each light module to be calibrated to match the color of a reference standard in a repeatable, standardized manner. The luminous and spectral content of commercially available LEDs varies significantly from unit to unit, in some cases by twenty percent or more. Because of this variability, some color balancing must take place in order to produce LED light fixtures of precisely consistent color temperature and intensity. While this level of precise light control is not necessary for many applications, such as the production of many lighting effects, this capability is critical in certain illumination applications, such as the illumination of small, interior spaces. In these applications, consistency in color and color temperature from lighting module to lighting module is very important. Where multiple LED lights or modules of LED lights are to be adjoined end to end or otherwise in close proximity to each other and are used to provide light over an area, the system must be balanced such that each LED light or module is producing light with the same temperature and intensity as other LED lights in the grouping. Otherwise, a visually noticeable variation in color and intensity of light output will be produced over the surface of the area to be illuminated. By allowing the light output of LED modules to be calibrated to match that of a reference standard, a control system could be configured to precisely produce light of a known temperature and intensity over the desired area.
The control system taught by the '038 patent does not allow for configuration or balancing of the light temperature and intensity between LEDs in a grouping or between LED groupings. In that controller, a simple current sink is used to drive the LED modules. The current sink is implemented using Darlington transistor pairs from a high current/voltage Darlington driver. The function of the current sink is to switch current between the LED module sets and system ground. The base of each Darlington pair is coupled to signal inputs. When a high-frequency square wave (PWM signal) is incident on a signal input, the Darlington transistor pair current sink switches current through a corresponding LED node with the identical frequency and duty cycle of the original PWM signal. This allows each color of LED to be varied in intensity independent of the other LED colors in a node. But the state of each signal output directly correlates with the opening and closing of the power circuit through the respective LED modules. The result is that the power to each LED set is controlled by the signal inputs, and the power circuit switching is performed at a frequency and duty cycle identical to that of the signal input.
The maximum current value that may be applied to each LED module as taught in the '038 patent is set by the use of static resistors added to each control circuit. The impedance value of these resistors can be altered only by replacing a resistor with one of a different impedance value. Because the current value applied to each of the LEDs is static when the LED is “on,” the controller lacks the functionality to electronically calibrate the temperature and intensity of LEDs with respect to each other or with respect to a known standard value.
Because the control system taught by the '038 patent does not effectively allow calibration across the lighting system, it is ineffective as a control system for an LED lighting illumination system that comprises a plurality of RGB groupings. What is desired then is a controller for an RGB LED-lighting system that allows for the calibration of the temperature and intensity of the light produced by LED arrays within the lighting system. Such a controller would ideally be capable of calibrating LED arrays with respect to other LED arrays in a single light source, such as when used to produce light of varying color in an RGB method, and capable of calibrating LED arrays with respect to a known reference standard source for purposes of matching of LED arrays within a grouping of such arrays. The present invention achieves these objectives as described below.