When taking stills from a IP CCTV camera (Axis P1357-E) in low light (night) conditions of a moving vehicle, I am getting circular dark patches where the vehicle headlights have lit up the road.

As demonstrated in the photo below (look at the road to the left of the van) Image demonstrating black spots

What causes these spots to appear and is there anything I change in the camera settings that will remove them?

There is constant IR illumination of road that is shown from the same position as the camera. The camera has a 1/3.2" progressive scan CMOS sensor. The image is stream is MJPEG and the still is a JPEG.

I can change:

  • Exposure value [0..100]
  • Exposure control
  • Shutter speed
  • Gain

Consecutive Frames

Below is three consecutive frames taken from the camera First frame of car Second frame of car Third frame of car

  • 1
    I don't see how "the moving light source has passed though the image while it was being taken". The truck is frozen in the frame, so the patches are probably shadows or dark patches on the asphalt. – binaryfunt Feb 9 '15 at 23:30
  • 1
    @BrianFunt I have changed the title to say that the light has been moving just before the image was taken. I have also added a second photo that shows the same effect but in a different place. The dark patches are not on the asphalt itself. If you look carefully at the van you can also make out faint outlines of the van when it was in previous positions, I don't know if this has anything to do with it. – mpursuit Feb 10 '15 at 8:38
  • 2
    maybe the camera does some noise reduction (nothing unusual at night, when you need top sensitivity) and its algorithm produces these artifacts. – szulat Feb 10 '15 at 14:35
  • 1
    That's what it looks like to me - noise reduction. In the bottom image you can see multiple impressions of the car have been blended together. It's just failing to do that correctly with the bright parts of the frames. It probably CAN'T interpolate the track of that light, and probably isn't trying. – Jasmine Feb 10 '15 at 23:12
  • I see ghosting on the truck too, and the brake lights of the car. It's blending multiple frames together. – Jasmine Feb 10 '15 at 23:14

I posit this as a possible explanation:

Since the spacing of the effect is uniform, this suggests that the latent images you see are the electronic remnants of 'hottest' parts of the previous few frames that were captured by the CMOS sensor. I'm not suggesting that this is a collage of multiple captures, but that the sensor has an inherent 'refresh rate' of sorts, and that the electrical signal from previous refreshes takes some time to fade to zero, so any pixels with a remnant signal >0 may still be visible at any given time - i.e. the whitest parts of the previous images. As for why the whitest parts of previous frames appear as black (e.g. black headlight circles), I can only guess that this is also part of the processor design, but I'm sure some electrical engineers out there can explain this behaviour better.

If this assumption is correct the effect would not be visible during the daytime, as the local contrast of the image would be much lower.

An analogy would be to look at consumer LCD televisions displaying high contrast motion: e.g. a white ball moving rapidly across a black background. Some televisions (depending on panel type) will show a very noticeable latent image of the white ball trailing behind the current frame. This happens because it takes X milliseconds for the LCD panel to fade white pixels back to black. The longer this fade takes, the more noticeable the effect.

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