Polarized eyewear synced to dual-projection 3D — each lens receives different image stream. Industry standard for theatrical 3D before active shutter systems.
For a long time, flicker glasses were the standard solution for 3D projection in cinemas — a simple but effective tool to show each eye a different image at the right time. The principle: two superimposed images are projected onto the screen with different polarization directions. The lenses of the glasses filter this polarization, so that the left eye only sees the image intended for the left position, and the right eye only sees its image. This works with a frame rate fast enough to appear continuous to the eye — hence the term "flicker".
In practice on set, this means you need special projection equipment capable of displaying these polarized 3D images. The glasses themselves are passive — no batteries, no active electronics needed — which makes them cheap and robust. The disadvantage is obvious: brightness is significantly reduced by the polarization filters. You lose about 50% of the light per eye. Viewer head movements can also lead to crosstalk — if the polarization planes are not perfectly aligned, ghosting occurs. And at oblique viewing angles, the system breaks down.
Historically, flicker glasses were the workhorse of the 3D era of the 1950s and experienced a renaissance starting in 2009, especially in IMAX 3D cinemas. The big advantage over active shutter glasses (see there): no synchronization problems, no expensive electronics in the glasses themselves. However, active 3D glasses are increasingly becoming the norm today because they offer better brightness and image quality — provided the projection runs absolutely stably.
For productions specifically shot for flicker glasses 3D, the frame rate had to be precisely calculated. 24p film was often upscaled to 48p, or alternatively, two 24p streams were interlaced. Correct timing synchronization was crucial — errors led to flickering or misaligned images. This is craftsmanship: each cinema, each projector had slight variations. The DoP had to be aware of these technical limitations and consider them during color calibration and contrast adjustments.