What could explain the dark bands seen in these pictures?

Photo with dark bands artifact This photo was taken at the Harvard Museum of Natural History, using a Pixel phone. The picture is shot looking through a glass panel into a geology exhibit that has white illumination inside. There is also white room light outside the glass panel. The bands were aligned vertical relative to the phone, and decreased in contrast when taking the photo from farther away.

Here is another picture into the neighboring glass-covered exhibit, showing the vertical bands in different position and with different contrast to illustrate the effect: Photo showing mild dark banding

Pictures from more than 1 meter away from the glass panel didn't show the dark bands: Photo without dark bands


Some light sources that reflect in the glass are visible at about the same brightness even where the dark bands are - that almost excludes a camera problem.

Effects like that are not uncommon with certain brands and models of LED and/or CFL lights, either because PWM dimming is used or because the actual light source is fed rectified but unfiltered mains electricity. It seems that there is a mix of light sources with and without such problems in that venue.

Fully-electronic (rolling) shutter modes tend to exacerbate this kind of problem...

  • 1
    I'm guessing the differences in each of the frames has more to do with different exposure times. Longer exposures allow a full cycle of the light's flicker, shorter exposures do not. The darkest (shortest) exposure also demonstrates the most effect from the flickering lights.
    – Michael C
    Jan 26 '20 at 1:47
  • 1
    Pulse width modulation (PWM), or pulse-duration modulation (PDM), is a method of reducing the average power delivered by an electrical signal, by effectively chopping it up into discrete parts wikipedia. I didn't know LED's were dimmed by effectively turning them on and off very quickly. Reminds me of struggling to shoot CRT monitors with film camera's before decent colour printers were widely available.
    – dmkonlinux
    Jan 26 '20 at 6:10
  • Any other practical way to dim them would either require the added weight and cost of inductors, or wasting energy in a series resistor of some description... Jan 26 '20 at 18:58

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