The BBC once distributed still another type of 3D glasses for their 1993 Dr Who special, Dimensions in Time, broadcast in aid of Children in Need.

These glasses had one lens that was essentially transparent, and one that had a strong (but not coloured) filter, like a sunglasses lens.

The effect worked on the principle that the brain takes slightly longer to process dark images than it does light images. Therefore, if one eye is seeing a much darker image, then the total perceived image (including the percieved depth) is distorted because the images from one eye are suffering a slight lag.

This effect would not normally be particularly noticeable (or create any coherent 3D image where there was none before). However, the trick is to keep the camera constantly moving horizontally. Coupled with the slight lag, this simulates the effect of having two cameras, one a little horizontally separated from the other - just like your eyes. Thus, as long as the camera is kept moving, an effective 3D illusion is created.

This method has a couple of advantages. Firstly, you can transmit to any perfectly ordinary television. Secondly, a viewer who does not have the special glasses will be able to watch as normal, albeit without the 3D effect.

The disadvantage is that the camera has to keep moving all the time, which becomes unsettling to the viewer after a while, and is certainly awkward to shoot. During the Dr Who special, there were a lot of short scenes where the camera simply circled the actors, around, and around, and around, and it was quite nauseating.