• Pathway Lighting Products and Flicker

    08/31/2016 06:46 pm

    Pathway lighting fixtures use state-of-the-art constant current drivers to control the illumination of their LED lighting products. That being said, all constant current drivers have a certain level of current variation that can cause flicker. Depending on the level of dimming required, two mechanisms of current control are used; 1) constant current reduction (CCR) and 2) pulse width modulation (PWM). Products that dim to 10% typically use the CCR method, while products that dim to 1% and below typically use the PWM method. For either case, the drivers we specify have been thoroughly tested to ensure that flicker is undetectable.

    Pathway characterizes flicker by using the % flicker metric defined by:

    % Flicker = (Max L – Min L) / (Max L + Min L) x 100%

    Where:

    Max L = Maximum light intensity

    Min L = Minimum light intensity

    Since the light intensity is directly proportional to the current driving the LED, the flicker equation can be rewritten in terms of drive current as follows:

    % Flicker = (Max Current – Min Current) / (Max Current + Min Current) x 100%

    Pathway accurately measures the maximum and minimum currents with a digital storage oscilloscope and precision AC/DC current probe to determine the max and min current levels.

    The level of flicker perception varies from person to person, but a general industry rule of thumb is that using a CCR drive method, a flicker percentage of less than 50% at a frequency of 120 Hz or more will not be visible to the average observer. Even if flicker is not quite visible, it has been shown that flicker can cause health related issues such as fatigue, headaches, eye strain and even seizures. To ensure our lighting products are truly free from flicker related issues, the CCR drivers we use are characterized such that the flicker percentage is less than 22%, more than twice as good as the generally accepted rule of thumb. This 22% limit is achieved by ensuring that the peak–to–peak ripple current measured on the constant current supplied to the LED is no more than 35% of the peak current value. Due to the design of the constant current driver, the frequency is always 120 Hz or greater and thus in the acceptable range.

    For the PWM drive method, the rule of thumb is that a refresh frequency of greater than 200 Hz is required to make flicker not visible to the average observer. The flicker percentage metric cannot be used with the PWM drive method because the flicker % will always be 100%. However, even though the LED actually turns on and off with the PWM technique the frequency at which this turn on/turn off occurs is faster than the human eye can detect. Once again, to ensure that there will be no flicker related issues, Pathway products using the PWM technique use refresh frequencies greater than 260 Hz at all times. The only time the refresh frequency is as low as 260 Hz is when dimming levels are at the 1% and sub 1% levels. At dimming levels above 3% the refresh frequency is above 500 HZ and at typical brightness levels the refresh frequency is between 1000 to 3000 Hz.

    In summary, for lighting products using either the CCR or PWM drive methods, flicker related issues will be non-existent with Pathway lighting products. Through a rigorous in-house testing program, great care is taken to ensure that all Pathway products meet or exceed our performance specifications.

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    Author

    Russell Budzilek is Director of Engineering at Pathway Lighting Products and is responsible for advancing driver technologies, and developing wireless and next generation digital DMX controls. He joined Pathway in 2013, bringing 32 years of experience in R&D of LED display systems and LED lighting. Russell attained his MSEE and BSEE degrees from University of Bridgeport, both Summa Cum Laude. He is also an adjunct Professor of Electrical Engineering at Bridgeport University, College of Engineering, teaching graduate-level courses in Microelectronics and Power Electronics analysis and design.