Question about the pulsing lights of Europe

[Mark Andersen]

I just got through shooting in denmark and france, i was shooting 24FPS and noticed lots of lights, sodium vapor streetlights, and even traffic signals were pulsing or strobing. Is this because of the 50HZ current in Europe? is this why pal is 25 FPS? Any ideas?

[Martin Weiss]

You should be able to minimize the strobing by changing the shutter angle, no?

[Dan Hudgins]

I just got through shooting in denmark and france, i was shooting 24FPS and noticed lots of lights, sodium vapor streetlights, and even traffic signals were pulsing or strobing. Is this because of the 50HZ current in Europe? is this why pal is 25 FPS? Any ideas?

When TVs first came out they used "mains" line power x-former for the HV, so any ripple would move through the image, so US was 60Hz EIA video and Europe was 50Hz video (405 line at the start for BBC).

Later HV came from the Horizontal sweep so the ripple was not such an issue, so NTSC changed the VHz to 1000/1001*60Hz to avoid problems with the sound and color being added.

For shooting 24fps under 50Hz you may be able to adjust the shutter angle to 360 degrees, or 1/25th or 1/50th or 1/100th second, you will need to test to see if any of those work since the rolling shutter is a little different than a movie camera shutter. The crystal in the camera will not hold phase for more than a miinute, if that much, perhaps, so 1/100 can be iffy.

If you are shooting 23.976+ (NTSC) rather than 24fps, you may get a slow pulsation no matter what you set the shutter to since there is no close phase relationship between 23.976+ and 50Hz.

The light from discharge lamps is not a sine wave so when the phase drifts you get more or less exposure, and with the rolling shutter the amount of exposure can vary from the top to bottom of the frame, like "mains" ripple in an NTSC TV set, a slow band of bright moving up or down depening on the phase. If the phase error is large the rolling bright band can look like flicker.

It could be better to set the camera to 25fps and the shutter to 360 or 180 degrees, then drop a frame later every second or use some other frame rate adjustment, if the flicker is very bad.

[sbaechler]

Set the shutter to 172.8° tho shoot 24fps in Europe.

[Fredrik Callinggard]

Is this because of the 50HZ current in Europe? is this why pal is 25 FPS? Any ideas?

You said it!!!

You could have changed the shutter angle see link

http://www.cely.com/flickerfree.html

EDIT: Sorry that link only contained flicker free speeds, will try to find one with shutter angles.

[Dan Hudgins]

Set the shutter to 172.8° tho shoot 24fps in Europe.

When movie cameras used Sync motors powered by the AC mains, the camera would be in phase with the blinking lights.

This works better with 24fps and 60Hz since the lights flash exactly five times in 1/24 second, with the shutter set to 144 degrees you get two flashes.

Under 24fps and 50Hz there is a more complex relationship with the phase, so that the frame start is on different parts of the lights blink brightness. With 23.976+fps you get a rolling phase shift.

Some lights like high pressure sodium lights do not flash uniformly over the AC power cycle, so different parts of the image can get different amounts of light.

Other discharge lights use a phosphor that glows over the dark time of the discharge cycle, so you get bands of yellow and blue, yellow from the phosphor and blue from the mercury.

A crystal time base camera cannot lock phase to the power like a movie camera with a AC Sync motor, unless you somehow hook it to the AC with a time base like using pilot tone.

The crystal will drift phase through the bright and dim parts of the light blinking, so that can make the exposure drift up and down. If the camera can shoot an even "window" to capture an even number of cycles of the blinking light then the phase relationship may not matter since the pixels will intergrate the same amount of light, the irregularity will only show up if the subject moves or you pan/tilt the camera. If you only shoot 1/2 cycle or the blinking is not symmetric in the upper and lower AC cycles then you need phase lock more.

[sbaechler]

Under 24fps and 50Hz there is a more complex relationship with the phase, so that the frame start is on different parts of the lights blink brightness. With 23.976+fps you get a rolling phase shift.

If you shoot a TV screen, yes. But for fluorescent lights the 172.8° shutter works.
We shoot movies on 24fps as well here in Europe.

[Dan Hudgins]

If you shoot a TV screen, yes. But for fluorescent lights the 172.8° shutter works.
We shoot movies on 24fps as well here in Europe.

I did not say that setting the exposure to 1/50 second would not help, only that some residual dis-uniformity can arise, particularly with regard to parts of the image with motion blur and subject to camera motion. The light varies as the subject moves through the frame, so the shutter angle does not fix that problem, it only works for stationary subjects. Various discharge or AC powered LED lights are somewhere between DC tungsten and a strobe light with the flash they make. As I noted floresent lights vary in color and brightness while the shutter is open, so the motion blur will include that cycle. I do not know if floresent lights sold in Europe have a longer persistence phosphor to make up for the lower Hz?

With film cameras the color film stock has low contrast, so the light variations frame to frame may not be noticeable over the processing variations.