Barco

Many of you might have heard of the term ‘ghosting’ or ‘crosstalk’ in combination with ‘3D’. Some of you might even have seen this effect on a 3D image. 

In this article, I’ll:

• Describe the ghosting effect
• Explain how it’s caused
• Give guidance on what you should take into account when choosing a 3D system.

What is ghosting?

Ghosting is an effect that’s seen in a 3D image with your glasses on, where a certain object seems to be followed by a dimmer trail, or where the subtitles have a visible shadow. The other word for ghosting is ‘crosstalk’, which is technically more correct and indicates the source of this effect.

Picture courtesy of Fujifilm, source: http://www.fujifilm-mea.com/files/7013/2610/5929/pic_08_1.jpg

What causes ghosting?

The essence of 3D projection is splitting images specifically for the left and right eye. However in a real-life system, this splitting does not work perfectly: there is always some portion of the image going to the unintended eye (left-eye image to the right eye and vice versa – when looking through the glasses of course).

The technical measure of ghosting is ‘crosstalk’ expressed in percentages, or the ‘stereo contrast ratio’ or ‘extinction ratio’ expressed in X:1 values. They are essentially the same: an extinction ratio of e.g. 100:1 means a crosstalk of 1/100=1.0%. Different elements of the projection system can introduce crosstalk: the 3D system (including the glasses), the port window and the screen.

• 3D systems and glasses

The 3D systems are mostly valued by their efficiency. However a higher 3D efficiency seems to come with a higher crosstalk, so this also needs to be taken into account. Typically the 3D system would be the biggest contributor to ghosting in the 3D image.

Below is a summary of crosstalk and efficiency specifications obtained from the manufacturer’s websites.

• Screens 

For polarization systems used on silver screens, the screen material also influences ghosting. Most typical screens would have a crosstalk figure of around 0.6% (±0.1%). The difference between screens is negligible when compared to the 3D system.  A special screen that has a crosstalk of 0.2% is the RealD Ultimate Screen. However note that this screen is also installed with screen vibration to reduce speckle.

However these screen crosstalk figures are only ‘on axis’, i.e. at perpendicular viewing. Looking at larger angles, the crosstalk increases so at 30° or more, the screen crosstalk can easily exceed 1%. So the silver screen is also an important contributor for ghosting, especially when looked at off-center.

• Port window

Some port window glass will cause a partial depolarization of the image, which will also increase 3D ghosting. We don’t have much data about this effect, but be aware that the choice and placement of a port window is also very important (not only for ghosting but also for throughput, reflections etc).

• System crosstalk

The total system crosstalk can be calculated by summing up the crosstalk percentages of the different components in the system. The higher the crosstalk, the more ghosting will be visible even in softer transitions in the image.

Different silver screen and polar 3D system combinations can give crosstalk values as low as 1.2%, but more typically about 2.5%. You have to add the crosstalk of the port window as well.

In comparison, the total crosstalk of a Barco 6P system on a white screen is only about 0.2% (screen and port window have no impact) which is way below the perceivable threshold and will not give rise to any perceivable ghosting.

• 3D brightness and content

Ghosting is never ‘embedded’ in the content, however some content is more prone to ghosting than other. The content variables that impact ghosting visibility are:

• Disparity, or separation between the left and right image. The more 3D depth an object in the image has, the larger the L-R separation, the thicker and thus the more visible the ghost image will be. Alternatively, for zero depth (image depth positioned at the screen) there is no ghosting.
• 3D brightness: the brighter the image, the brighter and more visible the ‘ghost’, especially if it’s a ghost image of a bright object on a dark background.
• Image contrast: bright objects on a dark background or vice versa, will enhance ghosting visibility.