3-D Ghosting
Ghosting is the result of imperfect optical components in the 3D projection system. (The system consists of everything between and including the projector to the viewer's eyeball.)
In polarized systems it is the result of polarizers that aren't perfect, light scattering sources like dirty port windows and dust in the air, and the scattering of the silver screen. Today, polarizers are achieving very extinction ratios (providing quite pure polarization) in the range of >1000:1 and still yielding high transmission. The primary source of scattering is the screen. The typical "native" ghost performance in a polarized system is close to 100:1, accounting for all factors in the light path.
In shuttered systems (shutter glasses), the source of ghosting is the imperfect operation of the liquid crystal in the shutter glasses, coupled with an angular dependence of the light going through the lenses. Straight through, shutter glasses used in the theatres today run >200:1. Some will produce greater than 1000:1
The spectral division technology (Dolby's Infitec) ghosts because the color separation from left eye to right eye isn't perfect - there is a finite overlap on the filters which allows a leakage from one eye to the other. The color filters for this system are created by thin film interference filters, and as such change their color characteristics if the light does not go straight through the filter. Dolby has addressed this by using curved lenses so when you look out of the edge of the glasses, the light still goes straight through (more or less) the thin films, maintaining color. I have not recently measured a Dolby system, so can't comment on the physical leakage.
In the REAL D circular polarized system, with testing, we deemed the physical leakage causing ghosting to be too large to provide a consistently good performance. I developed the ghost buster as a means to provide better performance. Fundamentally it works as Simon Burley stated: "...subtract a proportion of the left image from the right and vice-versa, ..." The portion of the image to be subtracted is determined by a model we created to characterize the leakage in a typical theatre system. It should be noted that under certain conditions, the ghost busting pre-processing is capable of completely eliminating the ghost image. (The model is deterministic.) In practice, ghosting will still occur at the edges of the theatre, and in extremely high contrast situations with large separation, but in almost all cases is substantially improved by the process.
REAL D has a hardware box that performs ghost busting in real time over dual HDSDI links, at full 2K resolution (2048). This is deployed in a number of post production houses in the US and Europe to support mastering of 3D films. Current practice is to do final color timing of the film while looking at the images at theatrical light levels (nominal 4.5fL +/-1) using a 3D system. REAL D's system is used in most of the post houses - as it represents >95% of the US market share of theatres. Final color timing is done with the ghost buster in place at the input of the projector in the timing suite (and everyone is wearing glasses.) The resultant master does not have ghost busting in it. For REAL D theatre distribution, the master is then played in real time through the ghost buster, and saved to DDR, creating the ghost busted master.
We (REAL D) recognize the difficulties in distribution logistics to have GB masters and non-GB masters. We are moving to put real time ghost busting into the theatres, so a single non ghost busted master will play properly. This has the advantage that the ghost busting processing can be tuned to the specific theatre for optimum results. (not ready to announce a timeline for this deployment).
By Matt Cowan, Chief Scientist REAL D, CML 3D
