As the world of lighting moves more and more into LED sources, Pulse Width Modulation (PWM) has become the most common means for controlling LEDs. What is PWM exactly and, more importantly, how does it relate to lighting applicationis it and how does it affect your lighting design?
Pulse Width Modulation — Defined
“Pulse Width Modulation” refers to a technique where a microcontroller rapidly turns the power to a device (in our example an LED chip) on and off at a given frequency and with a certain “pulse width.” By controlling the frequency (how fast the on/off pulse cycle occurs) we can control whether or not the human eye, high-speed cameras, or other image capture devices can visibly detect the occurrence of the pulses. This is extremely important because, if the pulses are produced slow enough to be perceived, “flickering” is visible.
By varying the width of the pulse, more or less power is delivered to the LED chip, resulting in changes in the average light intensity and to the brightness perceived by your eye. For example, if the width of the pulse is 50 percent of the total Period (the available time allotted to the potential full length of a pulse), the power delivered to the LED will be 50 percent of the total possible power level. The term “duty cycle” is often used to describe the percentage of the time the power is “on” versus the time the power level is “off”. Image A highlights the difference between a 50 percent and 25 percent duty cycle at the same frequency.
Another factor that should be considered is PWM resolution, which reflects how thin the slices of each on/off pulse can be. PWM resolution is important because it affects how smoothly an LED can be controlled (dimmed) throughout all intensity levels and also dictates the minimum light output achievable before an LED is turned off completely (0 percent duty cycle). The more slices that can be achieved will yield better dimming and less visible “steps” between each light output level — even at the same frequency. When executed well, steps between light levels can be effectively eliminated. (See Image B)
The Importance of PWM
The factors associated with PWM can have a substantial effect on the performance of an LED fixture and should be considered prior to its use in different applications. Two critical applications which require consideration are:
Video
Thanks to the advent of DSLR and compact HD video cameras (including those integrated into mobile devices) — video capture happens everywhere today. Even though your eyes won’t always perceive the on/off PWM cycle, standard/HD video cameras can capture this phenomenon. Fixtures with low PWM rates can generate unwanted striping and/or random flickering — two lighting effects that will ruin a video. Imagine capturing cell phone video on opening day and, after posting it on your client’s Facebook page, all of the lighting in the video is flickering on and off. Not good.
Theater and Live-Event Production
As more cameras broadcast live events (either to large screens inside the venue — or to the world via the internet), considerations related to poor PWM rates have become more important when designing stage lighting too. Theatrical and live event production applications also require smooth and controllable dimming, which is essential for setting light levels, creating light movement, shifts/blends in color or color temperature and setting a mood on stage. Smooth dimming can only be achieved using LEDs controlled via PWM and with a large number of potential slices for each pulse width (high-resolution). Low-resolution PWM will result in the audience seeing steppy, stuttery transitions as the LED lights dim or change color.
The Proper PWM
How do you know if an LED product has the proper PWM specifications for your application? As a general rule of thumb, if the published PWM rate of the fixture is less than 3 kHz, then proceed with caution. If you’re not able to find a published PWM rate for your fixture, contact the manufacturer directly and ask for the fixture’s PWM rate. Then ask if they have video samples or examples of the fixtures installed in applications similar to your own.
Finally, try to get a demo unit and test it as closely as you can in the environment(s) the lights will be used in. During your tests, look to see if you can perceive steps/stutters as the fixture dims or changes color/color-temperature. If steps are perceived, this may mean that the PWM resolution may be too coarse for your application.
Be sure to shoot test footage while dynamically changing variables, including: varying the output to different intensities, dimming from zero to full and, changing the color or color-temperature at slow and fast rates.
If you want to test for flicker and you don’t have a camera, turn the fixture on at one-half power or less (50 percent duty cycle). Then using your peripheral (side) vision, scan your eyes from side-to-side past the fixture, or shake your head back and forth while looking directly at the LED sources within the fixture. If you perceive any flickering, you can conclude that the fixtures have an extremely slow PWM rate and you should be wary of using them.
Now that you know more about PWM, how it affects applications and how to do a quick test of a fixture – get out there and do amazing work!
Mike Johnson is vice president of engineering at The Black Tank (theblacktank.com), a product development and engineering design company based in Haverhill, MA.
