A dominant method to obtain 3D images in the cinemas seems to be circular polarization. Separate pictures are projected with (alternating) circular polarization filters and passive glasses of the viewers block the wrong images for each eye (left-handed is OK for the left eye and right-handed is OK for the right eye, or vice versa). This allows the viewer to rotate his head, something impossible for linear polarizations, and it allows the glasses to be very cheap.

As far as I understand, all methods responsible for creating or filtering circular polarization are based on [linear polarization filters with some quarter-wavelength "gaps"](http://en.wikipedia.org/wiki/Polarizer#Homogenous_circular_polarizer) in between. But this only seems to work well for a fixed wavelength.

How do the glasses manage to properly filter circular polarized of any color, i.e. variable wavelength? Or are the images projected so that only three particular frequencies – "red, green, blue" – are projected for which it just happens that the glasses work with the right $N+1/4$ multiple of the wavelength?