on "power frequency" and avoiding the flicker from LED lights

[dv416]

I will be expanding my lighting package soon and am considering par cans and "DJ" lights as definite possibilties.

Many lights of this ilk are also LED lights, of which I have had varying levels of success. Some LEDs cause a flicker in the image, some don't.

A search of the forum has suggested to me that inadequate "power frequency" is the cause of this phenonemon.

My question is: how can one determine the power frequency of an LED light? I can't find that specification on the info on these lights, unless it is referred to as something else in electronics-speak? Also, I am correct in that this number should be equal to, or faster than my shutter speed?

 

[j1clark@ucsd.edu]

I will be expanding my lighting package soon and am considering par cans and "DJ" lights as definite possibilties.

Many lights of this ilk are also LED lights, of which I have had varying levels of success. Some LEDs cause a flicker in the image, some don't.

A search of the forum has suggested to me that inadequate "power frequency" is the cause of this phenonemon.

My question is: how can one determine the power frequency of an LED light? I can't find that specification on the info on these lights, unless it is referred to as something else in electronics-speak? Also, I am correct in that this number should be equal to, or faster than my shutter speed?

From other posters, I don't think you can count on much in the way of 'foreknowledge'... you will have to shoot with the LED lights in question and determine the effect, if any.

LED lights are often 'pulsed' with high current to get more light emitted per 'pulse', but over time, average is low. However, this pulsing, results in the digital camera's sensor picking acquiring the light at a different phase from the LED driver, resulting in flicker. The pulse frequency may or most likely is not tied to the 'power line' frequency of 50 or 60 Hz, of which ever is the standard in the location.

The human eye 'averages' the light, and so the human becomes unaware of the pulsations... you can sort of see the 'flicker' if you move your eyes quickly, and there's a 'strobing' effect.

I don't know if one could turn that into some sort of estimate of how much of a problem the lights could be...

At this point there may be a gadget that samples the light, and estimates the frequency and duty cycle of the light coming from the LED, that would be the only 'sure fire' way to find it out.

I did a quick check, didn't find anything, but that doesn't mean something isn't out there...

 

[nwstudios]

I did a quick check, didn't find anything, but that doesn't mean something isn't out there...

Photocell hooked to an oscilloscope should do the trick.

 

[j1clark@ucsd.edu]

Photocell hooked to an oscilloscope should do the trick.

Yeah, I always pack my o'scope when I go out and about... The Wife already yells at me for packing the camera around with me everywhere...

 

[Norman99]

Led flicker that is not noticeable to the eye but causes banding issues when video is played back is most likely caused by the led dimming method that is employed by many led light makers. The driver that powers the led(s) also has a dimming function that is designed as PWM or Pulse Width Modulation, which cycles the On/Off duration period of the led "pulses" dependent on the brightness required. This is a common and effective method for dimming leds and although good for general illumination, is not the best for video capture. At certain camera shutter speed(s) the frequency of the driver PWM interacts with the camera's speed and exhibits the "horizontal venetian blind effect" not unlike the old television sets scanning method, although much narrower. The horizontal lines look like they are marching vertically. The better led units designed for video use a different method of dimming by varying the direct current that flows through them, which is also incorporated in the driver electronics and is really constant DC current that is varied by amplitude rather than by frequency cycling. Of course the PWM would be the cheaper way to build them as PWM drivers come a dime a dozen. Another cause of noticeable flicker is caused by the driver's inability to cope with low voltage differential from the source voltage to the minimum driver input voltage, as an example of led units powered by AA batteries. When the batteries are low on juice, the driver goes into it's strobe mode like it is not deciding to shut down or stay on to extract the last drop of power. Quality units also may incorporate hybrid driver designs that a pwm signal is the controlling command to tell the linear ( DC driver) what constant current to feed the leds.
I doubt very much it has to do with AC line frequency, as all leds need DC to work, wether it is PWM or linear it's still DC that the leds need.

 

[nwstudios]

Yeah, I always pack my o'scope when I go out and about...

You need to trade that cold war relic in for a Fluke 97

 

[gonzo_entertainment]

Another reason to not like LEDs, imagine that.

 

[Cool Lights]

LED units designed for stage use may have this problem (and many do), but ones designed for film and television use generally don't have the problem. So if you're looking at the LED stage lighting / par cans, yes I think that's something to be concerned about and will have to be tested quite a bit before you add it to your every day use kit.

 

[Norman99]

Leds are really getting a bad name eh ? I wish people would understand that leds come in different flavors other than the cheap garbage that is sold cheap to compliment the quality. You really get what you pay for BUT there are stellar exceptions which I shall not name any that you pay far more than they are really worth. Not liking leds means one has not seen the good ones that are out there, albeit at a higher price of course. Leds do have a lot of advantages which I will not go in detail but aside from the fact that they run cool and consume far less power, they can be used in situations and locations that would not be possible any other way. For any reason not to like leds, I can find 10 reasons to that saved the day due to lack of powering, tight spaces, accent lighting, close proximity product lighting, salt water aquarium lighting that need specific blue wavelength light for live coral, background venue lighting that any filament bulb will trip the fuses, solar powered house lighting (try that with flos or filaments), operating room lighting ( see Steris led as an example) that will not make the surgeon sweat, etc., etc.