Secrets Q & A
- Written by Darin Perrigo
- Published on 30 November 2007
Now that I've discussed the different contrast ratios quite a bit, I want to go through an example using the contrast calculator at http://home1.gte.net/res18h39/contrast.htm in relation to the DLP class of projectors, where major investments are continuing to be made toward future improvements. At this time it is not uncommon for good home theater DLPs to be able to achieve 4000:1 On/Off CR and 500:1 ANSI CR. A person working on this class of projectors, or customers, could ask themselves where they would like to see CR improvements if they had to choose between improvements to On/Off CR or to ANSI CR. It would be easy to say ANSI CR if a person is thinking about mixed scenes, but On/Off CR can be a larger limiter of simultaneous CR in dark mixed scenes.
I am going to start with those values of 4000:1 and 500:1 for projector A in that calculator, along with changing the gamma to 2.22. In this case, I am going to assume the darkest of rooms (or a great screen for reducing the effect of reflections) and use 0.001 for the room gain, which will give more weight to ANSI CR than a higher value for room gain would. This results in the following checkerboard CRs:
From this, the simultaneous CR is still very good with the 50%stim/0%stim checkerboard and gets weaker as the level for the brighter rectangles goes down. I will now check the estimates for those same CRs if the ANSI CR were improved 8x all the way up to matching the On/Off CR (which would imply almost no extra washout effect from bright parts of the images to dimmer parts). For projector B, I enter the same numbers as projector A, except for 4000:1 for the ANSI CR. This results in the following checkerboard CRs:
We can see from these data that the simultaneous CRs that were already very good went up, but there was very little improvement to the simultaneous CRs where the most weakness was found. Now instead of improving the ANSI CR, let's leave it at 500:1 and improve the On/Off CR by the same 8x to 32,000:1. Plugging this in for projector B results in:
While this doesn't give the super high simultaneous CR for the 100 IRE case, it is still very high, and the most improvement by far has come at the weakest points. The checkerboard with 5 IRE (or 5%stim) for the brighter rectangles had the CR go up to over 4x that from the 4000:1 On/Off CR, 500:1 ANSI CR and the 4000:1 On/Off CR, 4000:1 ANSI CR cases. A person could plug in 4000/4000 for projector A and 32000/500 for projector B and be able to look at any of the levels in that contrast calculator side-by-side.
I believe that many fewer people would notice improvements to ANSI CRs that are already at 500:1 and above than to On/Off CRs that are in the 4000:1 range, and so I am looking for improvements to On/Off CR from future models of those projectors.
More Test Scenes
For readers who would like to do a test of shadow detail in a mixed scene, one that I like is in chapter 20 of the extended edition of Lord of the Rings: The Two Towers (and an earlier chapter with the regular version). I have included a shot of it here that I ran a contrast enhancement on in order to make some of the detail more visible under the various conditions that readers will have while viewing it.
Copyright New Line Productions, Inc. and New Line Home Entertainment, Inc.
The portions of the image I generally consider are the front of Wormtongue's coat and the background between him and Eowyn. On a display that is lacking in shadow detail in mixed scenes the front of Wormtongue's coat can look like a black or gray hole with no detail. Between them there is a door and what looks like a sculpture.
One sequence I like to look at for seeing the effect of going from very dark images with a small amount of light to similar images with some larger bright objects in them, and back and forth, is during the opening credits for The Last Starfighter. This sequence can show the effect of the washout best described by ANSI CR on visible detail as well as how our perceptions of blacks vary between different displays based on their relative CR characteristics. Note that the screen shots below have been lightened to make all the details visible on your computer monitor, and they may appear pixelated on some monitors.
Copyright Lorimar/Universal Studios
The black level in the first image where there are only stars will depend heavily on On/Off CR, while ANSI CR starts to come into play much more as bright objects are added. The dominant one between On/Off CR and ANSI CR for the second image will depend greatly on the display and how it does for each of those. With images that are strong in On/Off CR and weak in ANSI CR, the blacks can look black during the images with just the starfield on black and then appear as more of a gray when credits come up over the starfield. With images that are weak in On/Off CR and strong in ANSI CR, the blacks can look gray during the images with just the starfield on black and then look black when credits come up over the starfield or a bright object comes in to the images. Looking at which stars are visible and which are not during the last three images can give an idea of how proximity to bright objects affects the visibility of certain details on various displays.
Remember again that these are not things I am trying to demonstrate with the images above as they appear on your computer monitor. I have included them as references for which images I recommend looking at from the actual DVD (or other source) at full size with actual displays, for those who want to explore these effects further.
Accuracy in Measurements
One thing to be aware of is that contrast ratios with front projectors have gotten high enough that measuring them accurately can be a challenge. Many of them have low enough absolute black levels that measuring them will give unreliable results using most test equipment, if care is not taken. For instance, a measurement of 0.002 ft-lamberts with a tool that only goes down to 0.001 has a huge margin of error. Please be careful of trusting On/Off CR values with low black levels where caution has not been taken to get reliable results. I tend to measure On/Off CR by measuring close to the projector with a dedicated light meter. For tools where measuring off a screen is necessary the projector can be placed close to the screen to get smaller images that are brighter and will give higher readings for the denominator (the black level) to help the overall margin of error.
When a meter doesn't have a high enough accuracy range to measure both 100%stim and 0%stim from one image size or one location in front of the projector, it is possible to get reliable results by breaking the measurements up. For instance, a person could measure 20%stim and 0%stim from close to the projector and then measure 100%stim and 20%stim from a further distance from the projector. In that case, the 100%stim/0%stim CR would result from doing the following calculation from those measurements: (100%stim/20%stim) * (20%stim/0%stim).
Measuring ANSI CR accurately can also be a challenge. For this, I tend to cover the screen with black velvet or a dark sheet to keep room reflections to a minimum, and then measure from the projector. For measuring ANSI CR off screens I actually use a black velvet lined pipe made from black poster-board on the front of a light-meter that points at the screen in order to keep light from the brighter rectangles from throwing off the readings of the darker rectangles. I found that without doing this, the meter I use could not read above about 200:1 ANSI CR.
I also want to make sure that people understand that the projector should be set up as it would be for viewing before taking the relevant measurements for CR. It may seem obvious, but there should be no standing in front of the projector to block the light from it, covering the light with anything else (including a filter) between measurements, or changing anything in the projector between measurements (including the lens zoom or bulb setting). Any of those would pretty much make the measurements worthless, although I'm sure that many marketing departments are tempted to play tricks that are similar. I consider things like dynamic irises which really do work on their own as a viewer is watching a movie to be legitimate because they are part of the video manipulation the projector performs, and the user isn't changing anything in the projector between the measurements. The on/off CR and ANSI CR measurements I do are with 100%stim and 0%stim for the white and black levels (or video 235 and video 16 in 8 bit space), although I may occasionally use a different level for one of them and will specifically mention it if I do. For instance, if I use 109%stim (or video 254 in 8 bit space) for a reading, I will mention it.