An increasing variety of lighting applications utilize electronic type emitters as light sources. Examples of such emitters include solid state light sources, such as light emitting diodes (LEDs) and organic light emitting diodes (OLEDs) as well as plasma type light emitters. For many lighting applications, it is desirable or even possibly required to effectively and efficiently dim the emitted light. Dimming of an electronic source, however, raises issues. Consider an LED type system by way of an example. An LED produces light output, when a voltage across two terminals thereof (e.g., anode and cathode) exceeds the LED's forward voltage so that forward current can flow through the LED. The intensity of light output from the LED is primarily governed by the amount of forward current flowing through the LED. Therefore, in order to dim a light emitted from the LED, the forward current flowing through the LED needs to be manipulated.
There are two commonly used methods for dimming lights from LEDs. One is Pulse Width Modulation (PWM) Dimming, and the other is Analog Dimming. Both methods result in changing the average current through the LEDs and hence provide a visual appearance of changing intensities of light output from the LEDs.
In its most common form, PWM Dimming turns an LED ON and OFF for variable amounts of time but at a frequency higher than the fusion frequency of a human's visual perception. This turning ON/OFF is normally performed at a fixed frequency. Because of the frequency, the light appears to be continuously ON. The pulse width of a duty cycle, i.e., how long the LED is ON or OFF, is varied to turn the LED ON and OFF for desired amounts of time. That is, a smaller duty cycle will result in smaller ‘ON TIME’ and hence lesser light per cycle. Thus, by changing the duty cycle and thereby varying the average current through the LED, PWM Dimming manipulates the average light output. In this method, however, the peak current is not varied, and the appearance of less or more light is achieved only by changing the duty cycle.
On the other hand, in Analog Dimming, the peak current flowing through the LED is directly manipulated to vary output light intensity. That is, current continuously flows through the LED, and dimming is achieved by changing the peak current (and hence the average) current flowing through the LED. Thus, when the current is lower, the light output will be lower.
Among these two dimming methods, PWM Dimming suffers from some drawbacks. PWM Dimming, or any of its varieties, requires the LEDs to frequently turn ON and OFF, which may lead to the visual perception of ‘flicker’ if the frequency is too low. Flicker effects increase at lower duty cycles. Such flicker can be very annoying to an observer, since such flicker indicates that the observer's eye detects LEDs turning ON and OFF. At low frequencies or even at high frequencies (several kilohertz) with low duty cycles, such flicker may be visible. Perception of flickers differs among people, and some people can see flickers at frequencies higher than other people may see. Flicker becomes even more of an issue when there is a relative motion between the observer and a source of light (for example, the LED). This flicker problem is more pronounced in multi-color LED systems in which it is possible that different colors from different LEDs are pulsing at different rates and phases. Hence, flicker in such multi-color LED systems may produce a very undesirable visual appearance. There also are many studies in progress, including Wilkins et al. “LED Lighting Flicker and Potential Health Concerns: IEEE Standard PAR1789 Update,” that suggest that flicker in LEDs can be a health hazard in humans.
Moreover, PWM Dimming is less efficient than Analog Dimming. FIG. 1 illustrates an example of an LED's relative luminous flux characteristic, i.e., a relationship between forward current and relative luminous flux. Consider an example of a pulse width modulating at 100 Hz at a maximum current of 1000 mA with a Green LED having the characteristic of FIG. 1. The graph of FIG. 1 shows a relative luminous flux with respect to the light output at 350 mA. Let X denote the light output at 350 mA. If the Green LED at 1000 mA was pulse width modulated at 100% duty cycle, which means ON for 100% time, then the average light output would be 2.1 times X lumens. In other words, to achieve X lumens, the Green LED would have to be pulsed at 47.6% duty cycle (1/2.1). In contrast, if the LED was dimmed using Analog Dimming, it would be continuously driven at 350 mA to get X lumens. In order to compare efficiency between the two dimming methods, referring to the graph in FIG. 2, power consumptions in the two dimming methods can be calculated as follows. In PWM Dimming, the forward voltage at 1000 mA is approximately 4.4 Volts. Thus, at 47.6% duty cycle, which is the duty cycle to achieve X lumens, the average power consumption is 2.094 W=(4.4V*1000 mA*0.476). On the other hand, in Analog Dimming, to achieve X lumens, the average power consumption is 1.225 W=(3.5V*350 mA). Therefore, in order to achieve the same average light output from the LED, PWM Dimming is less efficient than Analog Dimming.
For the above-noted reasons, there is an industry-wide consensus that Analog Dimming may be superior to PWM Dimming. However, Analog Dimming has a drawback of undesirable color variation. In a given LED, if the peak current is varied, the current density (or J) also varies. More particularly, in a Gallium Nitride (GaN) based LED system (for example, Blue and Green type LEDs), a varying current density may lead to not only a varying intensity output but also a varying chromaticity output. In other words, in GaN based materials, Analog Dimming may lead to both intensity and chromaticity variations. While the intensity variation is a desirable effect of dimming, the associated chromaticity variation may not be a desirable one. For example, referring to the graph in FIG. 3, with Analog Dimming in Green LEDs, the chromaticity ((x, y)-coordinates of five connected dots in FIG. 3) shifts due to different forward currents of the LEDs used to produce light (at the five connected dots in FIG. 3). Moreover, as shown in FIG. 4, this shift in chromaticity results in changing dominant wavelength.
Hence a need exists for techniques and equipment for color correction of a light emitted from a lighting system to correct for a color change with Analog Dimming of the light.
Additionally, almost all LEDs show a change in light output as the LEDs heat up or cool down. This change may be characterized in terms of the LEDs' color (chromaticity) or lumen output. Heat based change in LED output is more pronounced in AlNGaP based materials systems compared to GaN based materials. Recently developed closed loop color correction algorithms employ a color sensor in a feedback system. With the use of the color sensor, the changes in lumen output could be rapidly corrected. However, the color sensor does not detect and correct the changes in chromaticity as the LEDs heat up.
Furthermore, almost all LEDs show degradations in light output over time during the LEDs' lifetime. FIG. 5 illustrates an example of an LED's lifetime degradation characteristic (i.e., hours used vs. light output). More particularly, FIG. 5 shows that light output of the LED has degraded by 14 percent after ten thousand hours. There are various well-known methods for correcting for changes in LED output due to such lifetime degradations. For example, a recently developed method uses a color sensor for correcting for lifetime degradation.
Hence, when Analog Dimming current density correction is applied, there is still room for further improvement in correcting for changes in color or lumen output of LEDs, either due to temperature changes of the lighting system or due to the LEDs' lifetime degradation.
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