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The three primary lighting objectives when considering dimming are that it be smooth, deep, and has a low risk of visible distractions or physiological effects of flicker through the dimming range. Technically this requires a high degree of coordination between the various components in an LED system. Quality dimming is the result of engineering the right dimming methodology and ensuring performance compatibility among the system components.
There are two primary methodologies in use for dimming LEDs: Pulse Width Modulation (PWM) and Constant Current Reduction (CCR). Each has its own pros and cons as shown below.
Pulse Width Modulation
- Allows deep dimming.
- Higher flicker risk.
- Typically lower cost.
Constant Current Reduction
- Difficult to achieve deep dimming.
- Lower flicker risk.
- Better system efficacy in some cases.
- In PWM systems, the LED(s) are turned on and off very quickly. At full light output, the LED is on and never turns off.
- At 50% of light output, the LED is on half the time and off half the time. To dim the light to any given level, you simply change the ratio of time on and time off.
- If the speed of switching on and off (frequency) is high enough, the human eye won’t see the switching.
- In CCR systems, the current to the LED(s) is increased or decreased. Because LEDs are current driven, the light output changes. As current is decreased, the light output decreases.
- To dim the light to any given level, you simply reduce the current being applied to the LEDs.
- LEDs sources do not all behave the same, and therefore, at low dim levels performance can be inconsistent.
Xicato Intelligent Dimming Solutions
Xicato uses a hybrid approach to dimming in which it uses the attributes of both PWM and CCR to create a solution that is smoother, deeper (down to 0.1% for DALI systems and 1% for 1-10V systems), more responsive and has a low risk of flicker throughout the dimming range. PWM is used when lower light levels are needed and current dimming is used at higher levels of light output. Specific knowledge about the LEDs used allows Xicato to identify the optimum point at which to switch between the two dimming modes.
Quality dimming also requires that the LED source is matched with a quality LED driver. In many luminaire lighting systems (source+power supply), the LED source is made by one manufacturer, and the LED dimming driver by another manufacturer. This means that the dimming driver must be engineered to work with a wide variety of LED sources rather than being optimized for any one LED source. However, by integrating the dimming driver, micro-electronics and the LEDs in the same solution, the integrated dimming driver can use data and operating performance of the exact LEDs it is controlling to produce the best dimming effect and to ensure that the effect is consistent across an installation.
As LED solutions have grown in market share and made dimming a simpler and more valuable option for many applications, so has the concern about flicker. Managing flicker is particularly a challenge when light is being dimmed. The Institute of Electrical and Electronic Engineers (IEEE) is developing a standard to address the safe modulation of frequencies for High Brightness LEDs and recent research has begun to define the parameters of flicker. Both “low-risk” and “no-observable-effect” levels are defined. In the “low-risk” level, any distractions or negative biological effects caused from flicker are mitigated.
In a solution that uses PWM, there will always be the risk of flicker at dimmed levels as the LEDs are constantly cycling between on and off states. The key to mitigating the risk of flicker is to ensure that the frequency is high enough that flicker falls in the "low risk" or "no-observable-effect" zones and therefore is not visible to the human eye or to video equipment that might be in use. XIM was designed to reduce the risk of flicker, even at deep dim levels (see chart).
For more information on XIM, dimming, and flicker, please contact your Xicato Account Manager or review the materials below.