AC line frequency and shutter speed

The light output of many types of lamps varies at twice the frequency of mains AC line frequency. Alternating power follows a sinusoidal shape, alternating between positive and negative half cycles. This causes the doubling of the frequency, for each half cycle in the power supply there is pulse in the light output. Two different standard frequencies are used for household electric supply in different regions of the world: e.g. 50 hz in Europe and 60 Hz in USA. To ensure evenly exposed frames one needs to choose a suitable shutter speed: 1/100 s, or a multiple such as 1/50 s in Europe, and 1/120 s or A multiple such as 1/60 s in USA. The figure shows measurements done with an oscilloscope and light sensor, with three different lamps as examples (the LED is not the same as used in later examples, and LED lamps vary quite widely on how stable the light output is).

scope_fig_lamp

As can be seen in the figure above, incandescent bulbs, fluorescent lamps driven with electromagnetic ballasts, and even many modern LED-based light bulbs designed to be used as replacements of such bulbs, emit light that is pulsed. How deep are the valleys compared to peaks depends on the type of bulb.

Taking a burst of photographs at f/11, 1/200 s, or 0.5 light cycles per exposure we obtain successive photographs with different exposure. The histograms from nine consecutive frames showing variation in exposure by almost 1 EV.

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Taking a burst of photographs at f/13, 1/100 s, or 1 light cycle per exposure we obtain successive photographs with identical exposure. The explanation is that if the shutter speed matches the length of one whole cycle, or an exact number of multiple cycles, the exact moment when the exposure starts does not alter the amount of light reaching the sensor. The histograms from nine consecutive frames showing almost no variation in exposure.

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The phenomenon exemplified above with a digital camera also affects film cameras.

What I describe below, affects only digital cameras.


A further twist related to light sources which have a pulsed output, is the use of the electronic shutter built-in in some camera image sensors. The sensor can be enabled and disabled electronically to adjust the length of exposure while the sensor is continuously exposed to light . In contrast  to most modern mechanical shutters, which open and close extremely fast, this type of shutter does not enable and disable all pixels simultaneously. Consequently the whole image is not captured exactly at the same time. In this case, the fluctuation in the illumination results in banding: some rows of pixels receive more light than other rows during the time they are enabled. Olympus’s E-M1 (mk I) camera had this capability added through a firmware update (as “silent” shutter release modes). Sometimes it is said that this type of shutter cannot be used under artificial illumination. However, the solution to this problem is the same as discussed above: choosing a shutter speed that is a multiple of the frequency of the local AC mains line frequency ensures that all pixels are equally exposed. The next photo gallery shows a set of photographs taken under LED illumination (Osram 6 W, 220-240 V, 2700 K, E-14, driven by 230V AC at 50 Hz) using the “silent” shutter mode of the E-M1 (mk 1) camera. I took photographs of a white wall at each available shutter speed between 1/200 s and 1/8 s in shutter priority mode (edit settings used in Lightroom 6 to enhance visibility of banding: exposure +0.5, contrast +100, Black and White treatment).

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Least banding occurs at 1/100 s, 1/50 s, 1/20 s, 1/10 s. At shutter speeds slower than 1/20 s, there is slight banding in some cases but barely perceptible. The reason is that once the exposure comprises several whole light pulses, the contribution of an incomplete pulse to the whole exposure becomes smaller as the length of the exposure increases.


A caveat is that modern driver circuits for LEDs not always work at the AC line frequency. Constant current drivers and constant current dimming using well-regulated DC power supplies are alternative technologies that completely avoid light output fluctuations. Specially for dimming, in many cases use of frequencies higher than those of mains power, alleviate the problem. Frequencies can vary widely,  from about 150 Hz, which is enough to make the pulsing invisible to the human eye, to tens kHz in some LED light sources, which is fast enough for photography, except at extremely high shutter speeds.

In the case of fluorescent lamps, high frequency ballasts are becoming common and have in part substituted electromagnetic ballasts. These  ballasts use frequencies in the order of kHz, which combined with some delayed fluorescence of the “phosphors” can almost completely eliminate pulsing of the light output.


Nostalgia: You may have seen similar banding in photographs taken with film cameras of old TV screens based on CRT tubes. The explanation is that a moving electron beam in such displays also causes flicker, once again at 50 Hz or 60 hz depending the TV standard used in different countries. In addition, the mechanical shutter of some cameras was based on a curtain, and at high shutter speeds implemented as a narrow window or slit moving in front of the film frame being exposed.


Histograms were produced with RawDigger and are based on the actual raw data in .ORF raw files. RawDigger is available at https://www.rawdigger.com/

You may be surprised by the plots containing four histograms. Camera sensors do have pixels arranged in groups of four, with one red pixel, two green pixels and one blue pixel. Data for all of them are stored in raw files produced by digital cameras. The two separate green channels are not redundant, they contribute to the spacial resolution of the image.


All illustrations, text and measurements are of my own authorship, and copyrighted.

(c) 2017 Pedro J. Aphalo


 

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