"Give a man a fish and you feed him for a day - Show a man how to fish and you feed him for the rest of his life."
It is with this sentiment that I am writing this tutorial. Since there are an increasing number of posts requesting information as to whether or not a certain screen and/or projector would work in a room, I thought it would be a good idea to provide people with a concise tutorial on how to answer these questions for themselves easily and quickly.
Please note that I am certainly not an expert, but rather just another enthusiast as yourselves, so if and when any of my information is recognized as being incorrect, I will be glad to modify it and provide additional information as it becomes available.
Also, for the purposes of this particular text, I will assume that the room in question will be 100% light controllable with no ambient lighting present, and we will only be considering a single seat within the theater at the prime viewing/listening location. People with more complicated seating and/or lighting conditions will need to address these issues independently, as it is beyond the scope of this beginning primer. Also, since I have absolutely ZERO experience with CH (constant height) setups, everything here will be based on the use a standard 16:9 AR (aspect ratio) screen.
So let's get started:Where do I start?
Good question. Let's start with YOUR ROOM, as it is probably the only part of the equation that you can not change, or is at least the most difficult to modify. How long and wide is it? And what size screen do I want to watch?
For our purposes (and to make it easy for me to provide actual numbers
), I am going to use my room. My room is 28 feet (336 inches) long, but it is only 12 feet (144 inches) wide, and has a low ceiling (7 feet or 84 inches) so you can see that I can accommodate pretty much any throw distance (the distance from the center of the lens to the center of the screen), but I am quite limited in width. Since I need to place my center speaker either above or below the screen and my right and left mains to either side, I decided that the very largest screen I could use was 100" X 56" (114" diagonal), and even that is really stretching it to the limit.
Your screen size decision will need to be based similarly, and may even have to be compromised if you have a much shorter room than mine. Basically, you can only go as large as your most constricting element will allow!
If your room is short, then you will need a PJ with a short throw lens. If your ceiling is short you will need a PJ with lens shift or a minimal offset (without tilting - more on that later). If your room is narrow (like mine), then that dimension will limit your screen width.
So for starters, in my room example, I can use a projector with anything from a very short throw to a very long throw, so I don't need to consider that aspect. I do have a short ceiling, so fixed offset projectors (projectors that requires being mounted a fixed percentage of screen height above or below the screen) would be very difficult to use because I can not get the projector mounted up high enough above the top of my screen (again, more on that later) to project the image squarely on the screen without tilting the projector and/or screen.
So before making any final decisions about which size screen will work with which projector, we need to know a little more about throw distance and or ratio.Understanding throw distance and throw ratios
As I said earlier, throw distance is defined as "the distance from the center of the lens to the center of the screen". That is, if you were to cut a piece of string and hold one end at the very center of the screen and the other end at the very center of the lens, that would be your throw distance. Throw distance specifications from manufacturers are generally expressed in terms of a mathematical relationship to the width of your screen. For example. if the manufacturer say that the throw distance is 1.4X, that means that the correct throw distance would be 1.4 times the actual width of your screen. This relationship remains constant, so regardless of how WIDE your screen is, you can easily calculate the correct throw distance. In my example room, since my screen is 100" wide, it is very easy to calculate the correct throw distance of 140".
Now since most modern projectors have zoom lenses in varying degrees, the throw distance is actually a range of values going from the minimum to maximum zoom. Again, for example, I have a Ruby in my theater that specifies a throw range of 1.4X to 2.4X. Using the numbers at both ends of the range, we can see that with my 100" screen I can mount the projector at a throw distance as close as 140" (1.4X) to 240" (2.4X) or anywhere in between those distances
. This provides the user with much more flexibility as to where the projector can be mounted in relation to a given screen size. Since my room is 336 inches long, you can see that I can mount the projector anywhere within the specified range and still have plenty of room in back of the projector for proper ventilation and cable clearance (Don't forget that needed clearance when you figure out what throw distance will work for you!).
So now, armed with the desired screen width, your room's total length, and the throw distance requirements needed for a particular projector under consideration, you should have no problems figuring out what screen size/projector will or won't work in your room. If your room is too short and you can't get far enough away from the screen for a particular projector choice, then you have to do one of two things:
1. Choose a projector with a shorter throw that will allow you to fill your desired screen size.
2. Choose a smaller screen size that will allow the projector to fill it from the maximum throw distance that you have available.
It's that simple!
Well....almost...Vertical considerations - Fixed offset versus mechanical lens shift
So now you have pretty much figured out what will work for you as far as the length and width of your room is considered, but what about the height? Here is where you need to consider the vertical aspect of screen projector mounting. There are two types of vertical mountings available in projectors, and for the most part
they are mutually exclusive.1. Fixed offset
- This type of mounting is generally specified as a percentage of the screen height that the projector must be mounted from the furthest vertical point of the screen. Ok, I know this sounds complicated, but it really isn't that bad. Let's use a projector like the Optoma HD81 for example, which if I remember correctly has a fixed offset of 136% percent (sometimes it can be specified as 36%, but that would be from the closest part of the screen, not the furthest, as as such is open to misinterpretation).
Ok, so in my example, my screen is 56" in height, so that means that this projector would need to be mounted 76.16" above the bottom of my screen (for a ceiling mount) or 76.16" below the top of my screen if it is table mounted. It is calculated like this:
136% * 56" = 1.36 * 56" = 76.16"
So, as you can see, since my screen is 56" in height, we could also say that since:
76.16" - 56" = 20.16"
we would need to mount the projector 20.16" above the top of the screen for a ceiling mount or 20.16" below the bottom of the screen for a table mount.
This type of mount works especially well in rooms with high ceilings, as it keeps the projector up high and out of the way, but in my room with a low ceiling that would bring the screen down too low for proper viewing (I'll let you do the math!).
So that brings us to the second type of mounting:2. Mechanical lens shift
- This type of setup allows more flexibility in mounting due to the mechanical movement of the lens elements within the projector to shift the image up or down. The difference here is that with lens shift, the projector needs to be mounted within the vertical confines of the screen (with exceptions). That is, the PJ can not be mounted any higher than the top of your screen and no lower than the bottom, and the lens shift specification determines how far from the vertical center of the screen that the image can be shifted. For example, if a projector specifies +50% -25% lens shift, that means that if the projector is mounted at the vertical center of the image in an upright position, the image can be shifted 50% upward or 25% downward. Since most us have our screen position (height) already determined, we can conversely say that the projector itself could be moved downward 50% or upward 25%. So using my own screen height of 56", from the vertical center of the screen I could move my projector downward 28" or upward 14" if the projector is in the upright position
. If the projector is inverted, then everything else gets inverted, so inverted my projector could be moved 28" upward (the top of my screen) or 14" downward (1/4 of the way up from the bottom).Please note, the above is an area of specification that I am absolutely not sure about, so if someone can provide better info, I will be more than happy to amend the text.
In my case, my Ruby has the mechanical lens shift type of mounting, and I chose to mount it on a 24" pipe hanging down from the ceiling, putting it 60" up from the floor and at a throw distance of 160" (close to minimum). Why so low? This brings me to the next area of discussion:Understanding brightness levels and screen gain
At this point I am not going to tell anyone what they should or shouldn't be watching for brightness levels, as this is a personal decision, but rather how to make sure that you achieve your desired brightness. There are several key points that need to be addressed here:1. Don't believe manufacturers published brightness specifications
- They are generally overblown and are measured at color temperatures far away from D65 (the standard color point for white). You need to seek out one or more trusted sources for actual measured
brightness specifications for the projector that you are considering.2. Lamps lose brightness very quickly!
- Again, don't believe published specs on lamp life, if they are published at all. In my admittedly small amount of experience, I have found that all lamps lose about half of their measured
brightness in as little as 200 hours and as much as 500 hours. Although I am sure that there are exceptions to this, I always plan on the lamp being half as bright for the lion's share of its life, so I plan my brightness requirements accordingly.3. With zoom lenses, brightness often varies with the the throw distance
- Unless the lens is the of the constant aperture type, as the throw distance increases, the aperture remains constant, thus the intensity of the light diminishes with distance. With the Ruby, for example, the measured brightness at maximum throw is about 60% of minimum throw, so losing 40% of the brightness at maximum throw, for my particular needs, is not a good tradeoff, though every person has to evaluate for himself what will work and what won't.
Now that you are armed with this information, we are ready to start considering screen gain and how it helps in achieving desired brightness levels.
Again, without preaching what brightness level you should be watching, let me say that my personal preference is in the 12 to 16 ftL range, as I feel it provides the best tradeoff between bright, punchy, detailed images and the exposing of artifacts such as macroblocking, mosquito noise, dithering, and other MPEG artifacts. The formula for determining footlamberts is this:
ftL = lumens * screen gain / square footage of screen
Since my desired range is 12 to 16 ftL, and I know that my lamp is going to be at half brightness within 250 hours (that was the actual number on my Ruby with my current lamp), I am going to aim for 24 (minimum) to 36 ftL on a new lamp. I can just plug in the known numbers into the equation to determine my lumens requirement. In my case it is:
24 ftL = lumens * 1.0 (I will assume this for now) / 38.9 square feet
lumens = 933.6
Hmmm...I have a problem, don't I? If I use a unity gain screen, then at 114" diagonal I will need 933.6 lumens to provide me with 24 ftL on a new lamp, or 12 ftL after 250 hours. Since my Ruby only measured 593 lumens on a new lamp, I will need to find another way to reach my desired brightness. This is where screens with higher gains can be used, though with certain concessions. My personal favorite is the Da-Lite High Power, though there are other screens from Stuart, Vutec, and other companies that can work equally well. Since I own the HP, I will discuss this screen further.
The HP can provide additional gain, but with some caveats. Since the screen is retro reflective
, most of the light that hits the screen is bounced straight back to the source, so ideally you would want the projector to be mounted right in back of your head or in front of your face for the highest gain realizations. Since those are not the best places to mount the PJ for other reasons
, we must get as close to that ideal as our room and sense of aesthetics will allow. Without going into exact details, the shallower the vertical angle between you and the projector, the greater is the gain, and conversely, the larger the angle, the lower the gain.
That's why, in my installation, I chose to mount the projector at a height of 60" off of the floor and about 10" behind my head. Since my head is at about 40" off of the floor, that puts the PJ at about 20" over my head. At that throw and viewing distance, that makes the angle very small, allowing me to realize maximum gain from the HP, which is 2.8. I like to be on the conservative side and assume that I am *only* getting a gain of 2.5, so let's see where that puts us:
24 ftL = lumens * 2.5 / 38.9
lumen = 373.44
Woohoo! You can see now that since I have 593 lumens initially, I have more than enough brightness to keep me happy...As a matter of fact I have too much! Using my known numbers to figure my actual ftLs, I get this:
ftL = 593 * 2.5 / 38.9
ftL = 38.11 on a new lamp (or 19.05 after 250 hours)
Well, it certainly is easy to reduce the lumens by the use of a simple and inexpensive ND (neutral density) filter. In my case I chose a 2X filter which would reduce the brightness in half on the new lamp, and then I removed it as I approached 250 hours. And since the lamp continues to lose brightness after the 50% mark is reached (albeit at a MUCH slower pace), my ftL are now close to perfect for my personal needs. Currently I am getting 275 lumens out of my Ruby, so I have:
ftL = 275 * 2.5 / 38.9
ftL = 17.67
Now, for those who know me, they know that my HP is actually 119" diagonal. and that I have a 114" Firehawk that I use when the lamp is new, so there is a certain amount of fibbing about my actual setup done strictly to keep things as simple as possible. The story of using 2 screens will be saved for another day...
Okay, so it now follows that if one is to use the HP effectively, then we will also need a projector with mechanical lens shift in order to minimize the angle between PJ and viewer. Well then, what about those of us with fixed offset mounts that can not mount the projector within the screen confines? What screen(s) can we use?
This is where angular reflective
screens come into play. Angular reflective screen bounce the light such that the angle of reflection is equal to the angle of incidence, or more the way that we "normally" see reflected light. Screens such as the Vutec Silverstar (3.0 gain) or the Stuart 1.8 gain screen (I forgot what it is called) can provide additional gain in situations where the angle between PJ and viewer is large. This is especially valuable for people with fixed offset projectors, though I believe they will work equally well with mechanical lens shift mountings (though I am not absolutely sure of this).
Okay, I am beat after writing all of this, and I think I covered the major points. I will amend and append to this as I think of more things or as others offer additional information which should be included.
I hope this helps.
Added on 11/20/06:
Ok, here are some links to Tryg's excellent series which I like to call "Everything you always wanted to know about screens but were afraid to ask". Highly recommended reading...The High-Gain/Exotic Screen ReviewA Screen Showcase and DIY ReviewWhite, Gray or Silver - A Review!A few words about contrast, wall colorings, and ambient light
And here you can find Tryg's excellent review and complete explanation of how the High Power works:High Power Review
Most everything we have discussed to this point has assumed a totally light controlled room, but that kind of room is not always possible for various reasons. Often we have to deal with light colored walls, floor, and ceiling and/or ambient light, and these conditions can affect our choice of projector/screen. before going into this further, we need to understand at least the basic concept of contrast ratios, which in its simplest form is the ratio between any two levels of brightness. In our home theater setups, we are concerned with two types of contrast, and even though they are measured and often talked about separately, they actually interact to affect image quality. They are:1. On/off contrast (or sequential contrast)
- For our purposes, this is most easily understood as the ratio of the whitest white to the blackest black possible from a projector. That is, if we put up a full field test pattern of 100% stimulus (white) and measure the light output, and then put up a full field test pattern of 0% stimulus and measure it, the on/off CR is simply
white level / black level
It becomes more complex than this when a dynamic, multiple, or adjustable iris is concerned, but for the sake of simplicity we will deal with a single fixed, non adjustable iris.
The other type of contrast we need to understand is:2. Intra-scene contrast (or simultaneous contrast)
- This is the contrast of the lightest part to the darkest part of any single frame of video, and as such it implies how much contrast the projector is capable of displaying on the screen at any one moment in time. This figure is almost always MUCH lower than the on/off CR as there is a certain amount of "washout" that takes place due to light scatter and reflections in the PJ lens, the screen, and the surrounding walls, floor, and ceiling.ANSI contrast
is an example of one specific type (a 4X4 0% black and 100% white checkerboard) of intra-scene contrast (and an extreme one at that) and is often used as a standard of measurement for comparing intra-scene contrast, though it does not necessarily reflect the projector's intra-scene capabilities under all circumstances.How do ambient light and light colored walls affect contrast?
Light colored walls, ceiling, and floor do not affect on/off CR (see Darin's report below for the full story), but they do affect intra-scene contrast due to the washout effect of light bouncing back onto the screen in dark areas of the picture, which manifests itself as lower intra-scene contrast.
Ambient light, on the other hand, affects both types of contrast as the stray light can raise the black floor of a high on/off CR projector, thus effectively lowering the on/off measurement, and the same stray light can have a similar washout effect (like light bouncing off of light colored walls) to lower intra-scene CR.
For more information on both types of contrast, as well as how they interact and affect picture quality, please read Darin's excellent article found here:http://www.hometheaterhifi.com/volum...06-part-1.htmlWhat can I do to maximize both types of contrast?1. Ideally the solution is to build a bat cave
- a room with dark, light absorbent walls, floor, and ceiling, with ZERO ambient light, but since not everyone can do this, we must combat the ill effects in other ways.2. Buy a brighter projector
- The brighter the projector, the less effect ambient light has on contrast. I think it was Tryg and Darin that worked together to provide us with some nice charts and information on the relationship between brightness and contrast and how it is affected with varying degrees of ambient light (If Tryg supplies me with the link I will post it here later
Edit: Here is the link (thanks, mdputnam!):http://archive2.avsforum.com/avs-vb/...d.php?t=6430843. Choose a screen type wisely
- Some screens are better at rejecting ambient light than others. I am by no means an expert in this area and so I suggest reading Tryg's excellent screen articles linked above. There is a lot of ground to cover in this area, as ambient light affects contrast differently on different screens depending on the direction the ambient light is coming from and striking the screen. Combine this effect with ambient light bouncing off of light colored walls and then you have multiple effects to deal with. I have a bat cave - My suggestion is that everyone build one...How to measure light output and contrast
AVS member lovingdvd has written an excellent tutorial on how to measure light and contrast using the inexpensive, yet very good quality meter from AEMC, the CA813. He goes into techniques for measuring both at the screen and near the projector, as well as formulae to convert from one unit to another. Highly recommended reading!http://www.avsforum.com/avs-vb/showthread.php?t=755705