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Calibrating for 3D Displays
Calibrating for 3D Displays

There have been numerous articles promoting the importance of properly calibrating your displays for optimal viewing of video and/or graphics images. However, few include considerations for calibrating the new 3D displays that are currently attracting so much attention. With that in mind, the following article will hopefully clarify the calibration process in general, while then highlighting where 3D displays require special attention.

Here are the main points to consider:

• SET Contrast and Brightness – use a PLUGE 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. We live in a digital world where all colors are based on lookup tables in 8 or 10 bit. Contrast and Brightness settings determine where the color ramps start and stop. If you don’t have these basic settings correct, your other calibration efforts will end in disappointment.

• 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.

If you have followed the previous 4 steps, your 2D images should be at least 98% correct with respect to brightness, contrast and color calibration.

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 1/120th of a second, the left eye will be dark and the right eye will be “on,” and for 1/120th of a second, the left eye will be “on” and the right eye will be dark. This shuttering creates the high-quality 3D effect, as during every 1/60th 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, and 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.

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. Thus, it must 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

The two key elements a projector must have in order to achieve these goals are:

• Dual lamp design (or single lamp design for smaller screens) with lamp dimming.

• Dual user saved color balance settings (via ColorMax technology).

Meeting these requirements highlights one of the outstanding benefits of DP’s TITAN Reference 1080p 3D projectors – they provide the projectionist with dedicated presets for optimizing a variety of sources, including distinct 2D and 3D content. TITAN Reference projectors also provide considerable lumen overhead, which as described above, is required when 3D content is displayed.

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. As an example, let’s say we design a theater with a dual lamp projector so that when a single lamp is employed at 80% power, 25 ft. lamberts are produced on screen. That is nearly perfect for viewing standard 2D content. When viewing 3D content, both lamps can be employed at 100%, thus producing 60 ft. lamberts on screen. As described above, since we lose 80% of the light through the various 3D system elements, the 3D viewer’s eyes will only see approximately 20% of that light output. Thus, 60 ft. lamberts from the projector translates to a comfortable 12 ft. lamberts actually delivered to the viewers’ eyes. This brightness level is suited for the dynamic viewing of 3D content in a theatrically dark environment.

Here’s an interesting note regarding commercial theaters and 3D: in commercial movie theaters, the projection systems are generally equipped to produce roughly 16 ft. lamberts for 2D content. As a result, they only provide approximately 4.5 ft. lamberts on screen in the 3D mode. That is very dim and it is not impressive by anyone’s standards. DP’s dual lamp TITAN 3D displays make it possible to design home theater systems that perform at lumen levels optimized for both 2D and 3D viewing.

Now let’s try to compensate for the color shift that may be introduced by the 3D glasses and/or LCD shutter. We still need to have our projector contrast and brightness properly set with a PLUGE pattern. Set these values “by eye” with the projector in the 3D mode wearing your 3D glasses, then store these values in a different user setting. Now, display a full white field to the projector, with the projector in the 3D mode. A “spot meter” will be needed to accurately determine color performance. A spot meter is designed to read the performance of the projected light being reflected by the screen, rather than incident meter which just reads the light coming out of the projector. The spot meter, shooting through the 3D glasses (glasses must be in the “on” mode) will capture the color shift produced by the LC glasses, the screen (silver screen is used for passive glasses) and the environment.

With these measurements, the next step is to adjust the color point as close as possible to x = .312 and y= .329 without dramatically effecting light output. It is surprising, but the SMPTE spec for commercial 3D theaters is a very dim 4.5 fL and most would agree that is insufficient brightness for enjoying cinema entertainment. In fact, if the brightness gets any dimmer, the human eye reverts to a mode where the rods become more dominant than the cones. The rods effectively provide a kind of low light “night vision,” but they are not very sensitive to color. The cones in our eye are very sensitive to color, but are not very effective in low light conditions.

In a best case scenario, your system should still be producing 12 FtL when viewing 3D content. In a worst case scenario, if your display system does not have adequate brightness and you are on the threshold of achieving 4.5 ft lamberts for 3D material, you may need to compromise the color performance in order to achieve that minimum of 4.5 ft. lamberts. This typically means adding green to your image or settling for a higher Y value. Of course, sacrificing color performance is a very significant compromise, so using this tactic to find more brightness is a last resort that should not be taken lightly. A far better solution is to consider reducing your screen size, or upgrading to a brighter projector.

The real excitement around 3D is based on active 3D or polarized passive 3D. These two methods are able to maintain good dynamic color in a way that the old anaglyph (red and green glasses) systems could not. In light of this new technological frontier, special calibration steps should be taken so that the 3D images are captured by your eye in the same way the movie director intended. Follow the proper procedures and you’ll have eye-popping 3D images that will truly add to your movie watching experience.

Posted on Wednesday, April 14, 2010 (Archive on Monday, January 01, 0001)
Posted by cberens  Contributed by cberens

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