A Study in Contrasts

Contrast: The ratio between the brightest part of the picture and the darkest.

You might not find that precise definition in your Webster's, but it's the one that applies to our little corner of the world. It's also one of the most important characteristics of a video image.

But there are a number of ways to measure contrast—more precisely contrast ratio—and that's where the problems begin. One method is so-called ANSI contrast, which measures the contrast between peak white and total video black when both are present on the screen at the same time, usually from a standard checkerboard pattern of black and white squares. The presence of white and black together is the toughest test for contrast ratio, since the white areas inevitably compromise the black.

But ANSI contrast comes with its own baggage. Since room reflections can seriously compromise the result (by reflecting light back onto the black areas) it is most accurately measured in a theoretical "black hole." Such a totally non-reflective environment does not exist (the audio equivalent would be a perfect anechoic chamber). And the technology with the deepest blacks to date, both real and subjective—CRT—produces unimpressive ANSI contrast numbers.

The other method, peak contrast, is the one we feature here at Ultimate AV. It's also the one used most commonly by manufacturers in their spec sheets. How manufacturers arrive at these numbers is another story entirely, since our results rarely verify the contrast ratio specs. We suspect that the factory numbers are often derived at picture settings you wouldn't use for normal viewing. Our numbers are made only after we have arrived at the optimum settings—the settings that produce the best picture.

But peak contrast is only meaningful if it includes the two numbers it's derived from: the white level on the screen when the signal is a 100 IRE test pattern (100 IRE is the theoretical peak brightness present in a video source), and the level of total black when the image is a full screen, video black test pattern. If the peak white level is high enough you can have a high contrast ratio even with sucky blacks. In real-world viewing, that will virtually guarantee a bad picture.

Both of these figures can be measured with reasonable accuracy with a high quality meter (we use Konica-Minolta LS-100, a $3000+ device that is more or less an industry standard). We measure the light from the screen in foot-Lamberts (fL), and also specify the size of the screen and its gain when we're discussing a video projector.

In measuring peak contrast, the room reflections are not particularly significant as long as the room is totally dark to begin with. When measuring the peak white level, the image on-screen is so bright that room reflections are inconsequential to the reading. When measuring the video black level, the light output is too low to produce significant room reflections back onto the screen.

But the black levels of a few projectors, in particular, are now getting so low that they are scraping the lower limit of our light meter's specified capability (0.001fL). So what to do?

In my recent review of the JVC DLA-HD100 projector, I fell back on a technique I have used in the past to verify the readings I normally take on my full, 16:9, 78" (wide) screen. If you project the test pattern to be measured on a much smaller screen, both the peak white and the video black will be raised to a level well within the comfort range of the meter. Since they should both be raised by the same amount, the contrast ratio should be an accurate reflection of the true peak contrast ratio you would get off the bigger screen if you had a light meter with a far more extended low-level range (a meter that only a high-profit conglomerate or government agency could justify!).

When making this "small screen" measurement, nothing in the setup is changed. I do not move the projector or alter its settings, nor do I change the zoom setting of the lens. Instead, I merely position a sample of the same screen material (Stewart Studiotek 130 in my case) close to the projector. The only compromise is that the light meter is positioned a few degrees off to the side to keep it at the height of the lens and still have a clear shot at the "screen" without the body of the projector getting in the way.

This method gives us an accurate peak contrast ratio even from a projector with state-of-the-art blacks. But taken by itself it will not provide black level the projector will produce from the full, main screen. Measuring the black level of the JVC on the main screen produced a black level of 0.001fL. That's as low as the meter will read. It does tell us that the blacks are really, really deep. But even ignoring possible meter errors at this low level (and there are meter tolerances in any measurement) the actual figure could actually be as high as 0.0014fL, since the meter rounds off to three decimal places.

At this point I resorted to a little mathematical sleight of hand. I took the peak contrast ratio obtained with the "small screen" method described above (16,000:1 in the case of the JVC), and divided it into the peak white level that the JVC produced on the full-size screen (about 13.5fL) to determine the effective black level we'll see on the full screen. For the JVC DLA-HD100, the result was 0.0008fL.

While direct measurements are always best, interpolation such as this is not uncommon when arriving at a result by direct means is either not possible or beyond the reliable capability of the measurement tools. And it can provide useful comparisons. For example, when I measured the black level of the JVC-DLA-HD100 from the big screen, I obtained consistent readings of 0.001fL. Doing the same for the JVC DLA-RS1 resulted in readings that alternated between 0.001 and 0.002. This made a meaningful numerical comparison of the two projectors difficult. All I could reasonably conclude is that the blacks from the HD100 were a little darker. But when I applied the same small screen/large screen technique to the RS1, I obtained a black level of 0.0014fL, telling me that the black level of the HD100 really is close to half that of the RS1—a result that could not easily be obtained directly.

It's unfortunate that this technique can only be used for projectors, not for one-piece HDTVs. But in the latter case a few manufacturers are approaching the point where their sets will literally shut off their drive circuits when presented with a video black signal. And no known meter can take a meaningful reading in that situation!

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COMMENTS
Summer Baez's picture

I would like to comment on your review of the RS2 instead. You don't seem as enthousiastic for the RS2 as you were for the RS1, unlike some other reviewer(s). As I am still sitting on the fence to upgrading to the RS2 I'll say this: I recently watch Bluray's edition of Passage to India and I found the RS1 just extraordinary. At some point improvments come from the source, not from the projector. I am also a little bit worry about upgrading because I already use my RS1 in high lamp mode which I much prefer; the picture has more pop this way. I have a 110/ screen. It looks that JVC just tweaked the iris in the RS2, resulting in lower light output. I think a good technician could probably just open up the projector and do that himself. Any thoughts on that?

Tom Norton's picture

Based on the size of your screen and preference for a bright image, you might be well-advised to stick with the RS1 for now. I liked a lot of things about the RS2, and it would be a great projector for some buyers, but it is noticeably less bright than the RS1, for whatever reason. The JVC projectors do not employ an iris (or at least not any sort of iris that can be altered, even by internal DIY tweaking), so that option is out, and would be ill advised in any case. I would have liked to have had them keep the brightness of the RS1 and add a manually adjustable iris in the RS2 for those who needed more flexibility in the brightness department, rather than simply reducing the output. May be next year! If you have over 1000 hours on your RS1, you might also want to consider a new lamp. Worst case--if it turns out it doesn't make any difference now, you'll have a spare.

Aron's picture

Since you're writing about better ways to do measurements to compare projectors, here's something else you might consider, particularly when reviewing 3-chip projectors. Many reviewers seem to put less emphasis on sharpness than on color and contrast; indeed, some even prefer a "softer" pic, equating it with being more "film-like" (to my mind, a euphemism for lack of optical quality). Yet I've found that sharpness is of paramount importance, and a lack of it is very fatiguing: it leaves me trying to focus on something that can't be focused on. Unfortunately, no one seems to examine and report on all three of the attributes that directly determine sharpness in a 3-chip PJ: lens quality, panel alignment, and simultaneous focus of the three colors. These qualities can, however, be readily assessed using single-pixel-width test patterns consisting of alternating segments of red, green, and blue pixels. These should be projected with maximum magnification and photographe

Aron's picture

CONTINUED FROM ABOVE: ...These should be projected with maximum magnification, and the results reported as part of your reviews. Even better, develop a consistent methodology for photographing these patterns, and show these photos in your reviews. I've found that these attributes (particularly having a good enough lens to display pixels sharply, and tight focus on all three colors) translate directly into how sharp movies look. For instance, I used to think LCoS pixels were "soft" until last weekend, when I saw the above-described pattern on an RS1 -- the R, G, and B pixels were all simultaneously sharp as a tack (almost as sharp as they are when I display this pattern on my LCD computer monitor), and this sharpness was reflected in the picture it showed when watching Blu-Ray. It's just, IMO, a better methodology for directly assessing sharpness. [Please correct me if I'm wrong, but I've never heard you talk about the importance of simultaneous color focus.] Thanks for yo

Aron's picture

ADDENDUM: For me, this changes the conversation from one in which sharpness is discussed subjectively, to one in which we discuss the elements that directly determine sharpness. I also recently saw a Sony VW50. It's image was much less sharp than that of the RS1 I saw. Now, reviewers can argue back and forth using terms like "soft" and "film-like." But once I displayed the R/G/B one-pixel-width patterns on the Sony, it became obvious what was going on: (1) the three colors could not be focused simultaneously; and (2) some of the single-pixel-width lines were actually doubled. So calling the Sony's picture "film-like;" in fact obscures what's actually going on, which is that the Sony's picture (or at least this particular one's picture) is, quite simply, defective. I.e., this is not the video analog of some pleasing audio tube coloration. Rather, it's throwing an image that simply can't be properly focused. [And for the record: film can

Aron's picture

CONTINUED: ... You're considered on of the best reviewers in the business, and this is the kind of information I'd like to see uncovered in your reviews. And if you do this, perhaps others will follow suit. And then perhaps manufacturers will be pushed to better address the frequent lack of quality seen in this area. [And for the record (and as I'm sure you know), film, if shown on a good system, can be extraordinarily sharp; indeed, sharper than a 1080p digital system can be); so we probably should not use "film-like" as a synonym for "soft."]

Paul's picture

While you question the decision of JVC to not include an Iris the answer to me seems obvious. Any projector capable of a black level below .001 ft. L. is not in need of a dynamic iris. Simply put the trade offs of a dynamic iris would be of little overall benefit. The result would be a more expensive and more compromised picture. What is even more impressive is that you can now buy a projector for 6K that would blow away a 30K projector, AND a 30K line doubler of just ten years ago.

Tom Norton's picture

Paul: I wasn't referring to a dynamic iris for the JVC, but merely an adjustable iris that would offer a variety of fixed settings so that the user who wanted a little less brightness could dial it down. That way, the RS2 could be made as bright as the RS1, with different iris settings that could bring its light output down to the the level of the current RS2 if the user desired. Aron: Great points, and I will look into them, at least for on-line reviews. The space limitations of print for my reports in Home Theater would require a reduction of text in order to fit in the extra photos. But because of the limitations of the camera and its lens and optical system, plus the resolution limitations of on-line or print, such photos can only show multi-chip convergence and relative sharpness. I've also found that attempts to photograph the one pixel lines in a max frequency multiburst pattern (in contrast to a single pixel line) interracts with the camera's imaging chip to produce serious m

Tom Norton's picture

Serious moire!

Aron's picture

Tom, I think the patterns I have in mind are different from those you are thinking of. They are isolated single-pixel-width lines (not multiburst patterns) and, with them, the optical errors I speak of are easy to capture photographically (though it helps if you can place the projector 25'-30' from the opposite wall, and then zoom it to maximum magnification; to be fair to the PJ, I took my pics in the center, with zero lens shift,but you might want to feature pics taken from diff. positions on the screen). The patterns I used are "Horizontal Convergence," "Vertical Convergence," "Text Focus," and "Convergence Boxes," available at http://www.pbase.com/jackcnd/1080p_tests (use the "Original" sizes for 1080p). You can see the results of my own picture of the vertical convergence pattern, as projected from an Epson 1080UB, posted at http://www.avsforum.com/avs-vb/showthread.php?p=13905569#post13905569

Morris's picture

Tom, Thanks for your review on the Epson 1080UB. Very informative. I have the home version and so far it's been great. I just purchased the BluRay DVD essentials and will calibrate the projector. However, How do I go about calibrating the RGB and the RGBCMY with the disc, can it be done? Or are these settings for ISF calibrations? I used your settings and it improved my picture lots... What do you recommend I try, how do I go about maximizing my picture with this disc? Will it be easy to use? I have a dedicated room and I have a 106" Firehawk G3 screen. My projector is mounted about 12 1/2 feet back. I have a PS3 for BluRay connected via HDMI, thru my Pioneer ELite 92THX receiver, and I have an Elite DV-09 SD DVD player, via component thru the receiver. Any help is greatly appreciated... Thanks! Morris

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