Striped images with GM1 under LED lights

Discussion in 'Panasonic Cameras' started by vm666, Feb 11, 2015.

  1. vm666

    vm666 Mu-43 Regular

    132
    Jan 19, 2013
    Paris
    A friend tried to take pictures with a GM1 under a mix of daylight and LED lighting. She got dark and light stripes on her pictures.
    This kind of stroboscopic effect is known for electronic shutter under fluorescent lights. See http://m43photo.blogspot.fr/2014/03/gm1-shutters.html and http://m43photo.blogspot.fr/2013/03/gh3-electronic-shutter.html and http://m43photo.blogspot.fr/2014/06/gh4-shutters.html

    But LED appear to be worse than fluorescent lights.
    First, the stripes are much sharper and they are quite visible even when some daylight is mixed with the LEDs. I guess that the variator (?) switches the LED on and off and the intensity is controlled by the duty cycle. A square wave would give sharper stripes that the sinusoidal wave which powers fluorescent lights.
    And more important, the stripes also appear with the hybrid shutter (1st curtain = electronic, 2nd curtain = mechanical). The stripes disappear only with speeds below 1/8s. Does this mean that the GM1 hybrid shutter does not behave like a full mechanical shutter?
    LED lights are increasingly popular, this will be a problem.
    Has anybody seen similar effects? Is there any workaround?

    Edit: pictures here https://www.mu-43.com/gallery/showgallery.php?cat=2730
     
  2. Rudy

    Rudy Mu-43 Veteran

    449
    Jan 24, 2013
    Oakland, CA
    No workaround, but a solution.
    Global electronic shutter.
    Unfortunately, they are not yet available in "normal" cameras.
    Rudy
     
  3. vm666

    vm666 Mu-43 Regular

    132
    Jan 19, 2013
    Paris
    What's that? This thing which can be found in CCD? http://www.ptgrey.com/KB/10028

    Do you mean that a real mechanical shutter would immune to this stroboscopic effect under LED lighting?
    And if so, what's different in the "hybrid shutter" of the GM1? I thought that the first "electronic" curtain was just "powering the sensor on", at least for speeds below the flash sync. Am I wrong?
     
  4. Rudy

    Rudy Mu-43 Veteran

    449
    Jan 24, 2013
    Oakland, CA
    Some CCD sensors use a global shutter, but not all.
    A global electronic shutter requires that the charges accumulated at all photosite (pixel) can be transferred to separate charge storage capacitors simultaneously. The time from when all sites are reset to when their charges are transferred is the exposure time. After the transfer the capacitors are sequentially read out and presented to the ADC to convert them to digital values. On CCD sensors the ADC is usually off chip and on CMOS sensors the ADCs (there are several) are almost always integrated on the same chip. However sensors with global electronic shutter are more expensive and difficult to make. Also the circuitry takes up more space and usually affects the fill factor (percentage of the surface that is light sensitive) resulting in less sensitivity and gaps between the pixels.

    A mechanical shutter can show the wave pattern as well and is no different from a hybrid shutter.
    A hybrid shutter simply opens up the first curtain while holding all the photosites in reset.
    It then releases the reset for the first row of pixels which makes it start to collect light.
    It then proceeds to release the reset of the following row and so on.
    The second curtain of the mechanical shutter is then released to close the shutter.
    It trails behind the activation of the electronic shutter by a certain number of rows.
    The time it takes for the mechanical shutter to traverse those rows is your effective exposure time.
    The pixels are still light sensitive at that time, but won't see any light as they are obscured by the shutter.
    Therefore the sensor itself can read them out at leisure. However the different rows of pixels got their exposure at different points in time which will result in stripes if your illumination is strobed.
    A global electronic shutter also solves the flash sync speed problem as all pixels "see" light at the same time.
    Rudy
     
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  5. zensu

    zensu An Old Fool

    Aug 8, 2012
    Southeastern USA
    Bobby
    Yes, I have experienced this banding on the GM1 and, if it's any consolation, I'm getting banding on my E-P5 too. These LED lights are power saving but problematic.
     
    • Like Like x 1
  6. barry13

    barry13 Super Moderator; Photon Wrangler

    Mar 7, 2014
    Southern California
    Barry
    If you can, change the power supply. A good DC power supply will supply ("flat"?) DC (or close to it), not a square waveform.
    Or use a battery.

    I've got LED lights in my kitchen using an old 12V computer 'brick'-type power supply and a PWM dimmer and have not noticed any banding on the (limited) pictures I've taken.
    (I believe the PWM dimmer converts DC to a high-frequency square wave, but if the frequency is high enough it doesn't matter.)

    Barry
     
  7. vm666

    vm666 Mu-43 Regular

    132
    Jan 19, 2013
    Paris
    Because the curtains move in front of the sensors while the light blinks?

    That's no consolation: there is no good solution :mad:
     
  8. vm666

    vm666 Mu-43 Regular

    132
    Jan 19, 2013
    Paris
    Unfortunately, that is impossible. She was trying to take pictures in a restaurant (that's part of her job).
     
  9. tkbslc

    tkbslc Mu-43 Hall of Famer

    I may be thinking of something else, but can't you set your shutter speed to an multiple of the AC frequency and take care of this issue? In the US, we are 60hz, so 1/60 or 1/120 or 1/240, etc. If you were in a 50hz area (i.e. Europe) you'd want to try 1/50, 1/100, 1/200 etc

    .
     
  10. brettmaxwell

    brettmaxwell Mu-43 Veteran

    350
    Dec 8, 2012
    The first major manufacturer to implement a no-compromise global electronic shutter CMOS will make a killing. Every flash photographer (unlimited flash sync) and video shooter (no jello effect) will kill for it. And plenty of other photographers will benefit with instances like this with flickering lights.
     
  11. OzRay

    OzRay Mu-43 Legend

    Jan 29, 2010
    South Gippsland, Australia
    Ray, not Oz
    I've been using LED downlights exclusively in some of my shots and experienced no banding. I also recently bought a Cree LED bicycle light to use as a portable light source and I get no banding in the shots; however, I do sometimes see banding in the viewfinder while shooting. LED studio lights are becoming very popular and if banding was an issue, I'm sure that we would have heard about it by now.

    Also, an electrician told me that if LED downlight transformers don't get sufficient draw ie 50W globes are replaced by 9W LEDs, then they can start flickering. The way to fix this is to connect more LED downlights to the one transformer.
     
  12. pellicle

    pellicle Mu-43 All-Pro

    Feb 10, 2010
    Southport, OzTrailEYa
    pellicle
    I guess noonr has stated the obvious: led lights are often not constant, they flicker too fast to register to your eye they are flickering, but flicker they often do.

    Your camera has captured that with the appropriate shutter speed.

    I've found this is common on stage lighting rigs :)
     
  13. vm666

    vm666 Mu-43 Regular

    132
    Jan 19, 2013
    Paris
    I suspect that the frequency used by the variator is not 50 or 60 Hz.
    Anyway, I played with a GM1 with the electronic shutter under a fluorescent light (which flickers at 2*50 Hz = 100 Hz in Europe) and I got stripes at any speed above 1/40s (if I remember correctly). Using a multiple of the light frequency or avoiding it did not help.
     
  14. vm666

    vm666 Mu-43 Regular

    132
    Jan 19, 2013
    Paris
    This depends on the power source. In all cases, there will be no banding at full power as the LEDs will constantly be on.
     
  15. OzRay

    OzRay Mu-43 Legend

    Jan 29, 2010
    South Gippsland, Australia
    Ray, not Oz
    I also wonder whether the type of LED will make a difference. All my experiences are with the Cree (SMD style) of LED, which are extremely bright and consistent.
     
  16. MadMarco

    MadMarco Mu-43 Veteran

    298
    Oct 30, 2014
    Guildford, England
    Were the LED lights dimmed or dimmable? In order to dim LED's they are pulsed at high frequency while varying the mark-space ratio of the signal. Also LED's switch on and off with a much sharper decay than either incandescent or florescent which might explain the banding being much more pronounced.

    I've got non-dimmable LED downlights in several areas in my home and I haven't seen any banding with any camera I've used.
     
  17. vm666

    vm666 Mu-43 Regular

    132
    Jan 19, 2013
    Paris
    Yes, that's the source of the problem, probably.
     
  18. barry13

    barry13 Super Moderator; Photon Wrangler

    Mar 7, 2014
    Southern California
    Barry
    One needs to use an equal shutter speed or slower, to capture full cycles of the light.

    If the light is on a 100Hz cycle (full cycle, not half), then a 1/100 shutter should work fine, as should a slower shutter (capture more full cycles) of 1/50, 1/25, 1/12, ...
    For 60Hz, use 1/60, 1/30, 1/15, ...

    See http://johnbdigital.com/lenses/fluorescent/fluorescent_lighting.php , but note that the half-cycle (1/120 for 60Hz or 1/200 if 100Hz) might not work for LEDs, depending on how the power supply works (a poor 1/2 wave rectifier or dimmer could conceivably output a square wave with 60Hz converted to 1/120 bursts of DC, and a 1/120 shutter would give inconsistent results).
    Shorter version: A half-cycle shutter (e.g. 1/120) works for fluorescent lighting, but may not work for LEDs. Faster than 1/2 cycle will not work reliably for any lights that flicker.

    Barry
     
    • Like Like x 1
  19. vm666

    vm666 Mu-43 Regular

    132
    Jan 19, 2013
    Paris
    This is not sufficient: there are stripes on pictures taken at 1/15s. They are less visible than on pictures at higher speed, but they are here nonetheless. One has to go down to 1/8s to remove the stripes.
    I'm quite sure that the LED cycle is quicker than 15 Hz, otherwise the human eye would see the lights flicker.
     
  20. MadMarco

    MadMarco Mu-43 Veteran

    298
    Oct 30, 2014
    Guildford, England
    I don't know how fast dimmable lights pulse, but I'd bet that it's considerably faster than mains frequency.