Avoiding interference (banding or flickering) with fluorescent tube lights or high frequency lights.

An IQeye camera has an automatic shutter, that will auto adjust to get a proper exposure.
The shutter speed has a range of 30Hz to 8000Hz (1/30 sec to 1/8000 sec)
When Lightgrabber™ is used, the shutterspeed can even go as low as 15Hz or 7Hz (Lightgrabber™ x 2 or x 4)
The more light is in the scene, the faster the shutter will be set.
On a bright sunny day, she shutter can go up to 6000 or 7000Hz.

Sunlight has is a steady light source.
Artificial light has a pulsing characteristic that is caused by the 50Hz or 60Hz AC voltage used to power the lights.
A traditional incandescent light bulb still has very little of this 50/60Hz pulse, because it contains a filament wire 
heated to a high temperature by an electric current passing through it, until it glows.
The glowing is a slow process, and even with the 50/60Hz AC voltage, the light emission displays very little pulsing.

However a fluorescent lamp or fluorescent tube lamp light emission contain all of the 50/60Hz pulse.
A fluorescent tube lamp does not contain a glowing filament, but uses a different way of producing light.
A fluorescent tube lamp is a gas-discharge lamp that uses electricity to excite mercury vapor. 
The excited mercury atoms produce short-wave ultraviolet light that then causes a phosphor to fluoresce, producing visible light.
As a result, the light from a fluorescent tube lamp contains a strong 50/60Hz pulse.
Some light system use an even higher frequency, up to 1500Hz.

The human eye cannot see this pulsing, as it will only be able to distinguish pulses up to approx. 20 Hz.
For instance, the image refresh rate of a TV is only about 25 Hz, yet the human eye sees fluid movement.

A camera however, is very capable of detecting these high pulse frequencies in the light intensity.
The camera has a so called ‘rolling shutter’, so the imager sensor of the camera is scanned from top to bottom, line by line.
With a low shutter speed, the imager is evenly exposed, even with the 50/60Hz pulse in the light intensity.
However, in very strong light, the camera selects a very high shutter speed.

The pulsing light, and the fast shutter speed can interfere.
The pulsing light will cause not all lines of the imager to be exposed evenly.
You can see this in the image, as banding, or flickering.
It appears as a series of dark and light bands horizontally across the image.

There are several thing you can do to avoid this interference: 

  • You can close the iris (aperture) of the lens slightly.
    This will let less light in, and the camera will use slower shutter speeds.
    Slower shutter speeds will reduce or eliminate  interference.
  • You can use a different light source (incandescent light, or let more daylight in)
    This type of light will have less not none of the 50/60Hz pulse in the intensity.
    It is not likely however, that you can change the illumination.
  • Make sure the 50/60Hz setting of your camera matches the type of AC used in your geographical area.
    The camera will select multiples of the 50/60Hz to select the shutter speed.
    So, with 50 Hz, it will use shutter speeds of 100Hz, 150Hz, 200Hz etc, and with 60Hz, it will use 120 Hz, 180 Hz, 240 Hz etc.
    This will reduce the ‘rolling’ effect of the banding.
  • You can limit the cameras maximum shutter speed as follows:

http://x.x.x.x/set.oid?OidTB1.2.8.27.2=800  (Where x.x.x.x is your cameras IP address)
If the browser is not already logged in, it will ask for the camera log in credentials (default: user root password system)
In this example we set the camera to never use a higher value than 800Hz.

The camera will compensate this with a different gain level, so within certain limits, it will not affect the image quality.
Obviously, if the shutter speed is set too slow, the image may get overexposed, as the camera can no longer compensate.
When the light level goes down, the camera will still be fully automatic in selecting a shutter speed below 800Hz.

You can see ALL possible cameras OID (Object Identifier) when using http://x.x.x.x/oidtable.html
Scroll down to ‘1.2.8.27.2  maximum exposure in Hz’ to see your current values.

There is also an OID ‘1.2.8.26  anti-banding mode’ that you can enable:
http://x.x.x.x/set.oid?OidTB1.2.8.26=enabled  
The camera will try to keep a shutter speed that avoids interference as long as possible.

Note 1:
There are several methods of setting the OID values.

  • You can use the URL method as discussed above: http://x.x.x.x/set.oid?OidTB1.2.8.27.2=800
  • You can also telnet into the camera and use the command:    oid set 1.2.8.27.2  800
  • There are also utilities such as IQmanager that allow you to set OID values.

Note 2:
In the current generation of cameras, you can show the current shutter speed in the image.
he current gain can be made visible in the image by using $IMGDBG in the ‘overlay text’ in the Setup > Basic tab.
The previous generation of cameras (i.e. IQ75x cameras etc) cannot work with the $IMGDBG image overlay.
For these cameras you can use a text overlay $O(1.2.8.7)/$O(1.2.8.24) for similar functionality
This will show gain/shutterspeed values in the image.

 Contact the IQeye support team for questions: support@iqeye.com 

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2 Comments

  • 0
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    Fabrice Martin

    Excellent tips, I manage to stop the banding effect.

  • 0
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    Rahim Rahimzadeh

    excellent description .very useful indeed .

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