Originally Posted by trans_lux
Yes I would really like to know:
What range would a light meter need specifically at low lx to measure 1,000,000:1 contrast ratio?
First I'll start with something related.
If I people were hired and asked to measure the length of a house with reasonable accuracy and were only given a 12' tape measure some of them would throw up their hands and say it wasn't possible, while others would just figure out a way to do it by doing multiple measurements and some math.
1,000,000:1 is very difficult. It depends on how bright the projector is as 10k lumens with a million:1 would be easier than 300 lumens with a million:1.
The issue you imply that low lux is the problem is true, but this doesn't require measuring off the screen at full size to get the projector's on/off CR. Just thinking about the physics of light should confirm that for reasonable accuracy. I assume that people don't have a smoke filled room and other lighting needs to be controlled for this.
If a projector has 500 lumens with 1 million:1 then black is about 0.0005 lumens and for a spot in front of the lens where the image is say 6" wide the lux value would be around 0.022 lux for black if I calculated things correctly. Or about 0.0054 if the image is about 1' wide instead. So, a meter would need to be reasonably accurate for that.
A person may then say that even if the meter can measure black down that low the white there would likely be too bright for it on the top end. Then it can come down to doing a little thinking like figuring out how to measure the length of a house with a tape measure much shorter than that.
The meter could be left in place and a full 5% video level could be put up and measured from that same spot. The contast ratio there can be C1. Then measure 100% to 5% off the screen or near the screen for C2. The total on/off CR is now C1 * C2.
It also doesn't require that the same meter be used for the high reading as the low reading, as long as they are accurate for their function. A scientist who wants to figure out the ratio between the distance to the closest star and one wave of red light doesn't need one meter that can measure both.
If the case I gave above wasn't enough to measure that 1 million:1 with enough accuracy then there are further methods that could be taken like trying to gather the light to intensify it to a location and I'm sure some smart people could figure out some other ways if it was important. Scientist don't have to measure everything directly.
If just estimates are good enough there are even things like using neutral density filters with known reduction values on a known light source and trying to equalize it to another. I explained some of that in my article about contrast ratio about 8 years ago when doing measurements to figure out that a CRT was provided close to 700k:1 on/off CR was pretty new information. I think those measurements and the results seen have held up well as digitals have gotten closer.
Originally Posted by trans_lux
But it's the only measurement with a standard.
You keep saying that. I have mentioned multiple times that SID supported the ICDM standard that has a measurement layed out for sequential CR. Why do you keep claiming there is no standard at all?
This isn't that complicated. If somebody changes anything on the projector between readings they are cheating and unless they are really ignorant they know they are cheating. This applies to not putting one filter in front of the projector when measuring red for REC.709, then a different filter for blue, etc., changing settings between measuring individual colors, as well as applying to on/off CR.
Why are you so set on somebody setting a standard (although there is one as I already said multiple times)?
Some of us have already agreed that it is actual values that really matter, not specs. That is one reason people look to others who have shown themselves to be pretty trustworthy with measurements.
As far as why we allow the user to set a different iris choice before getting the native (non-dynamic) for that location, pretty much every manufacturer of home theater projectors who doesn't allow the user to change at least the lens iris position has just chosen the position for them. Some more closed and some more open. Some manufacturers just let the user decide amongst multiple choices provided instead of just setting fixed opening sizes for them.
--DarinEdited by darinp2 - 2/19/14 at 11:28am