How to Calibrate a Display for 3D Viewing

There is plenty of literature for calibrating a traditional 2D display, but little has been written about calibration for 3D TV images. Here we explain how to compensate for a video picture that is viewed wearing something akin to sunglasses.

2D Calibration Refresher

Before we explain how to calibrate 3D, we should rehash the basics of 2D video calibration:

Set contrast and brightness using a PLUGE (Picture Line-Up Generation Equipment) pattern. Set the brightness level so that you can just see the box or ring which is “blacker than black.” Also set the contrast so the 100% white box can be defined from the white surround field.

Set Gamma for movies, and use 2.2-2.4 to your own personal standard. Technically there is a specific number to choose here, but we find customers vary in what level of shadow dynamics they enjoy. After all, if you like your low lights a little brighter (or darker) you should adjust your gamma, not your brightness.

Make sure the color space matches the source material. Viewers typically concentrate heavily on the white point setting; however, if your display’s green point doesn’t reach SMPTE 709 standards, the whites might be perfect but the grass will never look the way it should on your Blu-ray movies. The color coordinates for NTSC, SMPTE 601, and 709 are all available with simple web searches. For graphics, we recommend a “native” setting which will give you the broadest color gamut of the display. With still images you can appreciate better color dynamics.

Following these rules should yield 2D images that are at least 98% correct with respect to brightness, contrast and color calibration.

How 3D Glasses Change the Game

So what’s different with 3D images?

First there’s the obvious: We are watching the 3D movie effectively through sunglasses.

Whether active or polarized passive (or anaglyph), the glasses not only cut a substantial amount of light, but they may create a measurable color shift. Active 3D glasses rely on liquid-crystal shutter elements. These elements will vary in transmission efficiency and color transparency based on the quality of the materials employed.

For active 3D viewing, the display should operate at a minimum of 120Hz, alternating between right and left eye, to eliminate any perception of flicker, This means that for 1/120 of a second, the left eye will be dark and the right eye will be “on,” and for 1/120 of a second, the left eye will be “on” and the right eye will be dark. This shuttering creates the high-quality 3D effect: During every 1/60 of a second, both the left eye and right eye will be presented with distinctly different images, separated by a very brief “dark time.”

This alternating presentation of left eye and right eye content is another big factor that impacts the brightness of 3D imagery. Specifically, 50% of the time, your right or left eye will be viewing a dark field, thus reducing the effective light output by about 50%. In addition, active 3D glasses are only 80-90% efficient in their transmission of light.

Single projector passive 3D systems also use a high speed liquid crystal shutter, which sits directly in front of the projection lens. Much like the shutter glasses, the liquid crystal shutter switches 120 times per second, polarizing the intended left and right eye information differently.

Since left and right eye information is sent sequentially at 120Hz, each eye is still treated to 50% dark time with this method. In addition, the viewer must wear passive polarized 3D glasses. Much like polarized sunglasses these also reduce the amount of light passing to your eye and, depending on the quality, can create a shift in the color.

Calibrating for 3D

So now that we understand how and where the problems of light loss and color shift are created, we need to compensate for these losses in our projector calibration.

To begin, let’s acknowledge that while there is an increasing library of 3D content coming online, the 3D projection system will still be relied on to display a lot of 2D material.

Ideally, then, the projector should be capable of retaining dual calibration presets for color temperature, lumen output and basic calibration settings, as well as provide enough lumens to overcome the substantial loss of perceived brightness when viewing 3D content

For the best 2D and 3D experience, select a projector with:

• Dual lamp design (or single lamp design for smaller screens) with lamp dimming
• Dual user saved color balance settings

In a properly designed system, a dual lamp projector should allow the “pre 3D” image to be more than 3X the light output required for the application.