# Strange lighting phenomenon



## sorcerertd (Mar 6, 2022)

The fluorescent bulbs in my softboxes were seeming dim and I had this super bright LED sitting around.  Why not give it a try, right?  These were taken in a pop up photo box.  The stripes in the pictures were actually moving in a scrolling motion on my screen.  These were taken with my phone, a Samsung note 20.  The lines changed direction along with the rotation of the "camera".  The brightness was great, but wow.  Anybody know what this is or why it happens?


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## KMCloonan (Mar 6, 2022)

I believe (and I am not at all technically inclined) that the LEDs strobe to the household frequency. If you set your shutter speed on your camera to a multiplier of 60 (as in 60hz), so like a shutter speed of 240 or 300 you can sync the shutter with the LED Strobe. It's a bit of trial & error to get it to exactly sync.

I am sure there are others on this forum who know a lot more about this subject than me.

Here's a video that helps explain it:


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## monophoto (Mar 6, 2022)

I suspect that the light output from an LED source is not truly continuous.  You are in North Carolina where the mains power operates at 60 Hz - so I would expect that the output from LEDs would fluctuate at 120 cycles per second.  This happens because LEDs are DC devices that are powered from a full-wave rectifier that produces an output ripple at 120Hz.  There is usually a smoothing capacitor in the LED ddriver to minimize this ripple, but it doesn't go away entirely.   As a result, the actual light output from the LEDs fliclkers, but normally this is slight enough, and occurs quickly enough, that it is not perceptable to the human eye.

But a camera is different.  I don't understand how the shutter in a cell phone camera works, but in the old days of focal-plane shutters on film-type SLR cameras, the shutter was actually a slit aperture that moved across the film plane during the exposure, with the width of that aperture determining the exposure that the film received.  So if the camera was used to make a photograph in a situation where the light was actually flickering, it was possible for the actual exposure on the film to show variations like this.  You may recall that one of the quirks of those cameras is that images of TV screens often displayed diagonal stripes that came about if the chosen exposure speed happened to synchronize with the scan rate of the TV cathode ray tube.


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## jrista (Mar 6, 2022)

I would bet its the CFL, not the LEDs, that is the problem. The CFL would oscillate at the 60Hz of the mains. I've had this problem with CFL lighting myself, when my shutter is in sync.

You can either change the shutter speed...or, many cameras these days are able to detect this phenomena, and have a setting you can enable for the camera to automatically adjust itself to compensate.

BTW, the CFL is the light in the foreground here:


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## sorcerertd (Mar 6, 2022)

jrista said:


> I would bet its the CFL, not the LEDs, that is the problem. The CFL would oscillate at the 60Hz of the mains. I've had this problem with CFL lighting myself, when my shutter is in sync.
> 
> You can either change the shutter speed...or, many cameras these days are able to detect this phenomena, and have a setting you can enable for the camera to automatically adjust itself to compensate.
> 
> BTW, the CFL is the light in the foreground here:


I've never had a problem with the CFL bulbs.  This happened after I replaced the CFL with that monster LED.   It was an experiment.  Interesting to note that, while I could see the lines on the SLR screen, they didn't show in the pictures.  That's just a Canon T100 I recently picked up to use for Etsy shop.  Still getting a feel for it as my last SLR was film.


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## mgoetzke (Mar 6, 2022)

Never seen that with a cell phone pic but my DSLR warns me when I shoot with LED lighting.


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## jrista (Mar 7, 2022)

sorcerertd said:


> I've never had a problem with the CFL bulbs.  This happened after I replaced the CFL with that monster LED.   It was an experiment.  Interesting to note that, while I could see the lines on the SLR screen, they didn't show in the pictures.  That's just a Canon T100 I recently picked up to use for Etsy shop.  Still getting a feel for it as my last SLR was film.


Its possible the LED lights use a low frequency PWM module... Dimmable LEDs usually rely on a PWM (pulse width modulation) to control brightness. Since LEDs need full voltage and current to deliver accurate color, instead of changing the voltage and current (which only works for a narrow range of brightnesses, and LEDs will usually stop emitting below voltage and current thresholds), pulse width modulation will pulse full voltage (and thus full current) for a certain duration. At 100% power, the voltage is full all the time, at 50% power the voltage is pulsed maximum for half as much time, 20% power its pulsed a fifth the time. Its a square wave, so the voltage oscillates on and off instantaneously.

If the pulse frequency is "low" then that could show up as the kind of banding you see in your photos if the shutter scanning is at a harmonic of the PWM frequency. In your case, it looks like the PWM is pulsing ~16x the rate of your shutter. So it might have a frequency around 1000Hz (assuming the shutter was around 1/60th).


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## duncsuss (Mar 7, 2022)

All my workshop lighting is LED (replaced the overhead fluorescent tubes with LED, and have a couple of bendy-neck magnetic mount lights stuck on the lathe).

When I shoot with my phone I get these stripes in stills, and a strobe effect in movies.

None of my lights have dimmers, unless the manufacturers are deliberately limiting the light output (and installing circuitry to do so) it has nothing to do with PWM.


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## sorcerertd (Mar 7, 2022)

Anyone have a bulb recommendation to fit in a standard E26 base with a high lumen output?  Preferred something that won't break the bank or be a fire hazard in a softbox?


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## sorcerertd (Mar 7, 2022)

duncsuss said:


> When I shoot with my phone I get these stripes in stills, and a strobe effect in movies.


Yes, this exactly what is happening.  It _is_ interesting that I don't notice it under the LED shop lights.  Must be more subtle with those, or the pulse timing in the fixtures is different enough to somewhat cancel out the effect?


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## monophoto (Mar 7, 2022)

With conventional cameras, the solution is to simply change the shutter speed.  That option doesn't exist on cell phones (at least not on my iPhone).  

I have an app on my phone called 'Neural cam' that might also help.  It take multiple exposures (you specify the number of exposures) and then blends them together while compressing the illumination range - and was designed for night photography where the illumination range is extreme.

Someone suggested that this could happen with CFLs.  Yes and no - yes because CFLs are also discontinuous sources, but no because they use electronic ballasts that increase the frequency applied to the discharge lamp, thereby increasing the flicker frequency to an essentially imperceptible level.


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## monophoto (Mar 7, 2022)

Regarding this flickering issue - fluorescent lighting became popular following WWII, but initially was mainly used in commercial applications - schools, stores, hallway lighting in commercial buildings, general area lighting, etc.  There were two applications that people tried to avoid.  One was task-lighting in machine shops.  The fear was that the flickering effect could have a stroboscopic effect on rotating or oscillating machinery and could therefore be dangerous - it could fool people into believing that a machine was off when in fact it was operating.  

The other situation was for lighting in office areas where people did intense work under relatively bright lighting - and specifically manual drafting.  The concern was that under those conditions, repeated light flicker could cause headaches.  I recall my first job in an engineering office that had several large drafting rooms, and where the area lighting was fluorescent.  But rather than supply those lamps with 60Hz power, they were fed at 400Hz through a set of motor-generator sets.  The first person to arrive in the morning had to first walk around to the utility room to turn on the MG set before trying to turn on the lights in the drafting rooms.


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## egnald (Mar 7, 2022)

Commonly but not always LED lighting uses high power LEDs that emit in the UV and blue area of the spectrum. To produce white light, these LEDs are used in conjunction with energized yellow phosphors .  These phosphors often store a certain amount of energy in the form of visible light for a short period of time. Because of the "storage" of light the phosphors continue to glow between power line cycles when the actual emitting LED is off. This has the effect of lessening the stroboscopic effect coming form the light. - Dave


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## farmer (Apr 15, 2022)

sorcerertd said:


> Anyone have a bulb recommendation to fit in a standard E26 base with a high lumen output?  Preferred something that won't break the bank or be a fire hazard in a softbox?


Flashlight , someone to hold the flashlight and one sheet of toilet paper over the end of the flashlight held on by a rubber band as a light diffuser.
Custom white balance .Get a 18% gray card
I use strobes, I have about 750.00 into each strobe lighting set up .


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