Film-Tech Forum ARCHIVE: Light efficiency of a 3D system

This is topic Light efficiency of a 3D system in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Samarth Gandhi (Member # 9845) on 07-13-2017, 05:26 AM:

These days there are many manufacturers for 3D system for D Cinema and claim light efficiency ranging from 15% to 36%.

What is the correct way of calculating the light efficiency of a 3D system in a theatre environment. If there is a defined procedure please share.

help here would be really appreciated.

Posted by Dave Macaulay (Member # 813) on 07-13-2017, 07:23 AM:

1: set up 2D and 3D projector presets to have the same lamp power, the same lens settings, and the same MCGD.
2: point a luminance spot meter at the screen, usually screen centre from a middle of house seat. The spot meter must be on a tripod and not be moved.
3: Project a white pattern. measure luminance in 2D.
4: activate 3D - whatever system you use. You need to put a 3D glasses lens in front of the luminance meter as well. Project white pattern and measure luminance.
I prefer to use a white pattern DCP rather than the projector's internal white pattern: some brands don't easily allow 3D internal pattern projection, and the 3D system can get confused when no proper sync signal is generated.

The efficiency is calculated from these two readings. A linear measurement like Ft-L or Cd/M² makes that easy. A photographic spot meter reading in LV or EV... I don't know if these are linear scales so you need to figure that out.

Dolby 3D is a bit difficult because the glasses have curved lenses, and are reputed to not give accurate colour or light calibration. Dolby sells special flat filters for such use.

There's an argument to be made for using properly corrected MCGD data for 2D and 3D. 3D systems (projector filters and/or glasses) do cause colour shift and projector colour correction can only reduce light level... decide for yourself if this effect is part of the 3D efficiency.

Posted by Steve Guttag (Member # 268) on 07-13-2017, 09:06 AM:

Color correction definitely goes into the efficiency. Dolby 3D (xenon based), I've found that the left eye, once fully corrected is the limiting factor and the right eye has to be reduced in lumen output to match by a significant amount (20%, typical).

But, the bottom line in computing efficiency is how much light went into the system versus how much came out. Dave provided an adequate procedure without having to actually know the lumen output of your system since you are only referencing to 2D as your "100%" benchmark (even though that isn't accurate either since you will have the 2D color correction in there too).

Note, with RGB laser and 6 primaries, the Dolby 3D is closer to 90% efficient on a dual projector system and closer to 40% on a single projector, 6 primary system. That is versus an 8-10% efficiency on a typical single projector xenon system.

Remember, with any single projector 3D system, you are throwing away 50% of your light while projecting the "other" eye. and since there is a transitional period (dark time) to switch between eyes, you are down to less than 50% BEFORE you start applying any filters (in projection or in eyewear). In dual projection, both eyes are active at all times.

Some cheat this by using a 4K imager and cut it in half so one eye is on the upper half and one is on the lower half (Sony). This solves the 50% loss problem BUT there has to be greater magnification to achieve the same image size, which reduces light.

So, I'd say the range is 8% to 90%, depending on the system and implementation. But, at the end of the day, it is still 3D so why bother?