The light sensor of interest here is the human eye, with characteristics quite divergent from electronic detection. Asserting that any DC power supply has ripple amplitude high enough to meaningfully modulate LED output without upsetting other circuitry strains credulity. Coming from the other direction, even if you drove an LED with full wave rectified AC without any benefit of filtering whatsoever, the 120 Hz flash rate is too fast to be perceived as flicker. Try it.
http://webvision.med.utah.edu/temporal.html has a reasonable explanation of the science behind this. The eye itself provides smoothing because its minimum integration time is not faster than the 10 to 15 msec range. Under low illumination conditions it can approach 100 msec, which is why we can comfortably watch movies at 24 frames/sec in a darkened theater. The dividing line between ability to see flicker and only perceiving steady light is in the 75 to 85 Hz range for bright light. The dividing line between always seeing flicker and never perceiving steady light is nominally 12 Hz or so for very dim light. The xenon strobes sometimes used on stage and at dances to created jerky motion effects run slower than that. 150 Hz is sufficiently high that not only is flicker perception essentially zilch, but one suspects an intention to suppress motion artifacts as well. An example of this phenomenon can be seen at night when an auto with strobed LED tail lights crosses your visual field at a high rate (e.g. the vehicle can even be stationary, but you'll see a trail of dots if you flick your eyes past it). This has no relation with flicker perception as such. Even a 1000 Hz strobe rate is easily detected if the source flies by at 1000 feet/sec. But, looking around a room at normal rates is probably slow enough that 150 Hz strobing is not apparent. But to return to the regulatory question; did the new regs unthinkingly ban DC drive? If so, the responsible person(s) need(s) to be bopped over the head with an LED flashlight. Orin LEDs are more apt to cause perceptive flicker because they are so fast responding! Incandescents do intensity smoothing because of the glowing filament but are not immune to flickering either albeit at much lower ability. Of course that assumes that they are powered with varying dc hence the minimum 150 Hz requirement. (This is considered a reasonable threshold of minimum flicker perception by 90-something percent of the populace; unless you drink coffee all day!) A very cheap power supply would have minimal post-rectification capacitance and thus its DC is very rippley. This ripple, if large enough, will cause perceptive flicker that is more noticeable from LEDs than incandescents. Hans Mellberg From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Tuesday, December 15, 2009 2:53 PM To: [email protected] Subject: Fw: Re: [PSES] Energy Star Requirements for LED Luminaries Ordinary LEDs respond somewhere in the 50 nanosecond time range. With that kind of bandwidth there is no danger of optical smoothing below 10 MHz, thus the choice between measuring electrons or photons at 150 Hz is moot. Is there an exception to the 150 Hz lower limit, or is the use of 0 Hz now banned? Meanwhile, someone at the DOE is probably writing a proposal to form a committee to commission a study to propose a recommendation for the answer to your question. Orin Laney - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
