Why is 3D Through Active Glasses So Dark?

Question:
I enjoy movies in 3D with Dolby Atmos sound, such as Live of Pi, but the only local cinema showing in this format is the ArcLight Cinema in Sherman Oaks, CA, which uses active glasses for 3D. Does this make the movies too dark to enjoy?

– Barry Operman
Santa Monica, CA

That’s a subjective question—active glasses certainly make 3D movies too dark for me to enjoy. Before I explain why this is so, I need to start with a bit of background about the physics of light.

Visible light is a form of electromagnetic radiation, which consists of oscillating electric and magnetic fields at right angles to each other. Normally, the orientation of these fields to the rest of the world is random, but if the oscillations are constrained to a specific orientation—say, the electric field oscillates horizontally and the magnetic field oscillates vertically—the light is said to be polarized.

The vibrational frequency of the two fields is always equal—it determines the color of the light—but their phase and amplitude relationship can vary. (For the purpose of this discussion, assume the two fields are at the same amplitude, which is true for the subject at hand.) If they are in phase, this condition is called linear polarization as depicted in the left portion of the diagram above. If they are 90 degrees out of phase, it is called circular polarization as shown on the right. In both cases, the orange wave in the diagram is the combination of the electric and magnetic waves.

As you can see, in linear polarization, the combination wave maintains a constant angle with respect to the electric and magnetic waves. By contrast, in circular polarization, this angle changes over time, forming a "corkscrew" as the light travels.

In circular polarization, the electric and magnetic fields can be 90 degrees out of phase in one of two ways—the electric-field oscillation can be ahead of the magnetic-field oscillation or vice versa. The resulting corkscrews are called clockwise and counterclockwise because they "rotate" in opposite directions.

If you place a polarized filter in front of your eye and shine polarized light at it, the filter will either let the light pass through (if its polarization matches the light’s) or block it (if its polarization does not match the light’s). This is how passive-3D TVs and most passive-3D cinemas work—the left and right images are oppositely polarized, and you wear a pair of glasses in which each lens is oppositely polarized. The left lens lets the left image through while blocking the right image and vice versa.

Most commercial 3D cinemas use a technology called RealD, in which the left and right images are circularly polarized in opposite directions. Of course, you need to wear circularly polarized glasses so your right eye sees only the right image and vice versa. (BTW, passive-3D TVs, such as those from LG and Vizio, also use circular polarization, so the glasses you get for the TV will work in any RealD cinema.)

Imax 3D uses linear polarization, so RealD and passive TV glasses won’t work in an Imax theater. Likewise, Imax glasses won’t work in a RealD cinema or with passive-3D TVs.

In both cases, the projection screen must be a special silver material that preserves the polarization of the light from the projector. This is fine for 3D, but it’s not so good for 2D presentations, because such screens are prone to hot-spotting and are not ideal for reproducing accurate colors.

Then there are active-shutter glasses, which behave just like the active glasses used by Panasonic, Sony, Samsung, and others with their 3D TVs—the image on the screen rapidly alternates between left and right, and an infrared signal synchronizes the glasses to open and close each shutter so only the left eye sees the left image and vice versa.

Active glasses in commercial cinemas are provided by Xpand, and ArcLight is one of the only chains that use this technology. With active glasses, no special screen is required, since the light isn’t polarized. However, the glasses themselves are bigger and heavier than passive glasses.

In all three cases—RealD circular polarization, Imax linear polarization, and Xpand active shutters—the glasses let through only a fraction of the light that hits them. One factor in this phenomenon is that all polarized filters are imperfect—they block some of the light they are intended to pass.

In addition, active glasses let much less light through than either type of passive glasses. Why? Because both lenses must be closed for a moment during each cycle to prevent any chance of the right eye seeing the left image and vice versa. As a result, active-shutter glasses allow only about 25 percent of the light from the screen to reach your eyes, while passive glasses let around 50 percent of the light through.

This is why my favorite commercial 3D presentation is Imax or AMC ETX, both of which use passive glasses and two projectors—one for the left image and one for the right. This puts more light on the screen and thus more light through the glasses. If ArcLight used two projectors, that would help brighten the image with the active-shutter glasses, but as far as I know, the chain does not use dual projectors.