Can a xenon flash work with a CMOS rolling shutter sensor?
Asked 1/13/2011
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I'm using a CMOS image sensor with an electronic rolling shutter and currently reading out RAW frames at 15 fps. Because the sensor exposes rows sequentially rather than the whole frame at once, I'm trying to understand how flash synchronization would work.
With a xenon flash, when should the flash be triggered so the entire frame gets even exposure? Is a normal xenon flash practical with a rolling-shutter CMOS sensor, or would I need some kind of extended or pulsed flash output instead?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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I'm afraid this simply isn't going to work unless your flash duration is longer than 1/15th of a second (very unlikely unless you're discharging a truly giant capacitor) , or you can pulse the flash over the 1/15th of a second period.
The reason for this is simply that when your flash duration is shorter than the length of time it takes to sample the image then the flash output will have ceased by the time part of the image is sampled and thus it will appear dark.
The same thing occurs when using a mechanical shutter once you go above the sync speed (usually 1/250s). Beyond this point the shutter starts to close before it is fully open so there is no point in time where the entire sensor is exposed to the light flash so some of the image will be dark.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
15y ago
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A standard short xenon flash generally will not evenly expose a rolling-shutter sensor. With rolling shutter, different rows are exposed at different times, so a brief flash only illuminates the rows that are active during that instant; other rows will be darker.
To get even exposure, the flash must remain on for the entire time the exposure window travels across the sensor. At 15 fps, that would usually mean a much longer light output than a normal xenon burst provides. In practice, that means either:
- a very long flash duration, or
- pulsing/continuous flash output across the full sensor scan time.
This is analogous to focal-plane shutter flash sync: above the camera’s sync speed, the shutter forms a moving slit, so normal flash causes partial exposure. High-speed sync solves this by pulsing rapidly while the slit travels.
So yes, flash is feasible with CMOS rolling shutter, but not as a single conventional short xenon pop unless the sensor has a period where the whole frame is exposed at once. Otherwise you need a high-speed-sync-style pulsed light source or another illumination method that stays on during the full readout/exposure sweep.
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