Question About JVC DLA-RS1U Projector
Dear Gary,
After reading Greg Rogers’ glowing review of the JVC RS1, I would like his thoughts on if this is the right projector for me. I am currently using the JVC DLA-SX21. My throw is 23 feet, and the screen size is 12 feet (wide, NOT diagonal). The room is a designated theatre, so I can get it completely dark. I like the brightness of the picture I get with the DLA-SX21 but would like to upgrade to a 1080p projector.
Would the RS1 fill a 12-foot screen with a 23-foot throw?
Would it be bright enough? I went to a Best Buy® and saw a demo of the Sony WPL-VW50, but I thought the picture looked rather dull and dim. Granted, it was Best Buy, and the room was not completely black. But it looked dim on a screen that was maybe 6 foot wide. With all the new high-contrast projectors coming out, the light output is getting rather dim.
Thom Babbes
Video Technical Editor Greg Rogers Comments:
In order to answer your question, you must first decide how bright a picture you require. The Digital Cinema System Specification, Version 1.1 (April 12, 2007) produced by Digital Cinema Initiatives, LLC (DCI), now specifies a nominal peak white luminance of 14 foot-Lamberts (fL), ±3 fL for theatrical presentation, and ±0.7 fL for reviewing rooms. (DCI is a joint venture of Disney, Fox, Paramount, Sony Pictures Entertainment, Universal, and Warner Bros. Studios).
However, two key factors should be considered when determining your required brightness level. Individual preference and tolerance for optimum and minimum acceptable peak brightness levels are rather subjective. Some individuals have expressed a preference for brightness levels two or three times higher than the DCI levels. I personally think the optimum peak brightness level is in the range of 12 to 16 fL. Therefore, I find DCI specification excellent. Higher brightness levels are quite revealing of source-based video compression artifacts and technology-based projector artifacts. The latter category includes color separation (rainbow) artifacts in single-chip DLP projectors, dithering, misconvergence, and non-uniformity problems. In addition, I believe you can more accurately experience the director and cinematographer's vision and intent when you view a film at the same brightness level they use in an optimum screening room.
The second key factor to take in consideration is projection lamp aging. The projection lamps used in DLP, LCoS, or LCD projectors typically lose about half of their original brightness over their nominal life expectancy. Furthermore, the loss of brightness isn't linear with age. They lose brightness faster in the early part of their lives, so it isn't economical for most users to replace the lamp often enough to avoid this problem. Most of us have to plan for a 50 percent reduction in projector brightness over the lamp life. Most projectors provide two or more lamp power settings, so the lamp can be operated at a lower power setting initially, and then as the lamp ages, the power can be increased. This makes the loss of brightness less conspicuous over time, and may also help increase the total lamp lifetime because of the lower lamp output earlier in its life cycle.
Taking these factors into account, you should determine the minimum brightness that you expect to be satisfied with at the end of lamp life and then double it for your initial brightness. In my own case, I would like no less than 10 fL at the end of the lamp life. Therefore, I must start with at least 20 fL when the lamp is new. However, just as there are individuals who want 30 fL at the end of life, there are other users that get by with as little as 6 fL. However, I don't recommend a picture that dim for the reasons discussed in the previous letter.
Once you establish your required initial brightness, you can compute the maximum allowed screen size from the screen gain and the measured lumen output of the projector. The basic equation is simple:
Screen Luminance = Screen Gain x Projector Light Output / Screen Area
Luminance is the quantity that describes the human visual system’s perception of brightness, and it can be measured in foot-Lamberts (fL). The Projector Light Output is measured in lumens, and the Screen Area is measured in square feet. Screen Gain is a unitless quantity. Screen Gain is a misnomer—since screens are passive devices, they can't amplify or increase the light that falls on them. A screen with a gain of 1.0 means that light is reflected uniformly at all angles. Screens with a gain greater than 1.0 reflect light directionally, with more light going toward the viewer, so the picture appears brighter. The higher the screen’s gain, the more directional its viewing angle, and the brighter the image becomes.
In my JVC RS1 review, I measured 703 lumens in the high lamp power mode with the projector 12.8 feet from my Stewart Filmscreen® Studiotek screen. The exceptionally accurate Studiotek screen has a 1.3 gain, and the 85.2-inch-wide 16:9 (1.78:1) screen has an area of 28.44 square feet. Substituting those values into the equation yields a screen luminance of 32.1 fL.
32.1 fL = 1.3 x 703 lumens / 28.44 square feet
Therefore, near the end of the lamp life I would expect to still have about 16 fL, based on a typical assumption that the lamp would have about 50 percent of its initial brightness. In this case, I would probably use the lower lamp power mode throughout the lamp life, which would also reduce fan noise. The lower lamp power mode measured 595 lumens. Substituting that into the equation produces an extremely bright 27.2 fL initially, and perhaps 13.6 fL at end of lamp life.
Now, let's try to calculate the luminance for your 12-foot-wide screen. Your throw ratio is about 1.92 (23 feet distance / 12 feet screen width). My throw ratio was about 1.8 (12.8 feet distance x 12 inches per foot / 85.2 inches screen width). Your larger throw ratio means that your lumen output from the projector should be slightly lower than mine. I can't calculate exactly how much lower, since that is a function of how the zoom lens is designed, and we don't have that information. But since the throw ratios are close, compared to the total throw ratio range of 1.4 to 2.8, let's assume the lumen difference is negligible for now.
If you have a 16:9 screen, it would have an area of 81.0 sq. ft.
Area (sq.ft.) = 9/16 x [Width (ft.)]2
You forgot to mention your screen gain, so let's use 1.3 as a starting value:
11.3 fL = 1.3 x 703 lumens / 81 sq. ft.
The 11.3 fL is within the DCI specification (14 fL ±3.0 fL) for commercial cinemas and should look quite good when the projection lamp is new. But as the lamp ages, the luminance is going to decrease and probably end up below 6 fL. That wouldn't be sufficient for me. Your obvious options are to select a smaller screen or get a screen with a gain higher than 1.3. Based on the typical assumption of 50 percent lamp brightness at end of life, you would need to start with 20 fL to achieve 10 fL at end of life. That would require a screen gain of 2.3.
2.3 (target gain) = 1.3 (current gain) x 20 fL (target luminance) / 11.3 fL (current luminance)
There are screens available that provide that much gain, but there are trade-offs involving performance and/or physical configuration that must be considered when using high gain screens. If you decide to pursue that option, you should consult with your home theatre dealer or screen manufacturer.
One final option that falls short in your specific case is to move the projector to a minimum throw distance, which typically increases the lumen output, because the zoom lens usually has a lower f-number at its minimum throw distance. However, that will also reduce the full-field (on-off) contrast ratio for the same reason. In this case, the JVC RS1 has such an exceptional contrast ratio that it would be a reasonable trade-off if you only needed a moderate increase in brightness. In my review sample of the RS1, the lumen output increased by about 12.5 percent at the minimum throw distance, while the contrast ratio decreased by about 19.5 percent.
Conversely, if you didn't need as much light output, as in my own example, moving a projector to its maximum throw distance will usually produce a higher contrast ratio, at the expense of some light output that you may not need.
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