Answers.Body: <p>You are seeing the scan line of the projector.  You need a camera with a screen sync feature in order to deal with the rolling shutter issue and depending on the technique used by the projector, it may not even be possible.  </p>
  
  <p>What you are actually seeing is what the projector actually does, it's just that it happens too fast for your eye to perceive it normally.  A video camera is however fast enough to catch it.</p>
  
  <p>This is regularly a problem even for professional gear and it is not at all uncommon for video screens to be shot with the device off and the video on the screen composited in after the fact.</p>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2013-04-24T04:51:38.400Z
Answers.Text: yeah, we're aware it's the scan lines; but there's glass distortion involved between screen and camera POV and compositing would have defeated the point.  We may be going with that anyway, if the T2i can't compensate, at any rate.
Answers.UserDisplayName: Junopsis
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2013-04-24T13:27:28.583Z
Answers.Text: @Junopsis can you post a frame grab of what you are getting?  Perhaps that will help in identifying it.  I'm still thinking it may be a side effect of the way the projector is operating.  Is it by any chance a DLP projector?
Answers.UserDisplayName: AJ Henderson
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2013-05-24T14:53:04.667Z
Answers.Text: Newton Rings or Polarizing artifacts? Some devices use polarization for 3d display. This could also be an aliasing problem: the sampling resolution needs to be approx 2x larger than the smallest frequency.
Answers.UserDisplayName: horatio
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2013-04-24T02:54:58.347Z
Answers.LastActivityDate: 2013-04-24T02:54:58.347Z
Answers.UserDisplayName: AJ Henderson
Answers.Body: <p>One way I have found that works is to change the ISO setting.</p>
  <p>For example, if I set my camera to 200 and look at an image being made by a projector, I can see a faint rainbow line go across the screen. If I set it higher, it gets bigger and at a point it looks like a continuous rainbow going across. Also zooming in seems to make it worse.</p>
  
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2021-05-06T09:00:28.233Z
Answers.Text: Slower shutter speed would reduce the rainbow a little.
Answers.UserDisplayName: kamae
Answers.ContentLicense: CC BY-SA 4.0
Answers.CreationDate: 2021-05-05T16:39:43.327Z
Answers.LastActivityDate: 2021-05-08T16:52:32.000Z
Answers.LastEditDate: 2021-05-08T16:52:32.000Z
Answers.UserDisplayName: kevin
Body: <p>We're doing rear-screen projection.  It looks great, but in tests with the canon t2i there's a scrolling rainbow in the video that changing shutter speed or frame rate won't get rid of.  Any advice?</p>
  
Tags: <video><projector><canon>
Title: Scrolling rainbow when projector screen is in frame
UserDisplayName: Junopsis
ThreadText: # Scrolling rainbow when projector screen is in frame
  
  (Asked by: username_0 on 2013-04-24)
  
  We're doing rear-screen projection. It looks great, but in tests with the canon t2i there's a scrolling rainbow in the video that changing shutter speed or frame rate won't get rid of. Any advice?
  
  **Tags:** video, projector, canon
  
  
  ---
  
  ## 2 answers
  
  ### Answer by username_1 on 2013-04-24. Score: 1
  
  You are seeing the scan line of the projector. You need a camera with a screen sync feature in order to deal with the rolling shutter issue and depending on the technique used by the projector, it may not even be possible.
  
  What you are actually seeing is what the projector actually does, it's just that it happens too fast for your eye to perceive it normally. A video camera is however fast enough to catch it.
  
  This is regularly a problem even for professional gear and it is not at all uncommon for video screens to be shot with the device off and the video on the screen composited in after the fact.
  
  > **Comments:**
  > **username_0**: yeah, we're aware it's the scan lines; but there's glass distortion involved between screen and camera POV and compositing would have defeated the point.  We may be going with that anyway, if the T2i can't compensate, at any rate.  
  > **username_1**: @username_0 can you post a frame grab of what you are getting?  Perhaps that will help in identifying it.  I'm still thinking it may be a side effect of the way the projector is operating.  Is it by any chance a DLP projector?  
  > **username_2**: Newton Rings or Polarizing artifacts? Some devices use polarization for 3d display. This could also be an aliasing problem: the sampling resolution needs to be approx 2x larger than the smallest frequency.  
  
  ---
  
  ### Answer by username_3 on 2021-05-05. Score: 0
  
  One way I have found that works is to change the ISO setting.
  
  For example, if I set my camera to 200 and look at an image being made by a projector, I can see a faint rainbow line go across the screen. If I set it higher, it gets bigger and at a point it looks like a continuous rainbow going across. Also zooming in seems to make it worse.
  
  > **Comments:**
  > **username_4**: Slower shutter speed would reduce the rainbow a little.  
  
  ---
  
  