Buying Guide to Projectors
As with so many consumer products these days, selecting a projector can be daunting, since there are just so many options. However, choosing the right projector is pretty simple as long as you can supply three pieces of information:
- The screen size (width, most importantly).
- Distance between the projector and the screen (throw distance).
- An estimate of the amount of ambient light present in the room in which the projector will be used.
At this point you might be thinking, “Great. But I can't answer any of those questions." Or, "I plan to take my projector on the road and could be using it anywhere." Often, with a little forethought, the “I don't know” can be whittled down to a fairly good educated guess. If not, you are not completely out of luck. In addressing each topic, we will suggest your best bet when confronted with unknowns.
What is a projector?
A projector may be best thought of as an inverted camera, spitting light out of a lens rather than receiving it. For the sake of this buying guide, we will be considering digital protectors—that is, projectors with video inputs that serve a similar function to a TV or computer monitor while offering a number of benefits, including the ability to produce much larger image sizes and easy portability. The principles I will outline below apply to all types of projectors. However, it will help to start by dividing digital projectors into four categories:
- Pocket, also called "pico"
- Home theater
- Large venue and fixed installation (a subset of multimedia)
Obviously there will be overlap, and not all models will fit easily into a particular category. For example, home theater and multimedia projectors are very similar. In most cases, it will be clear from your application which type you need. Boardroom presentations: this will be multimedia. In a living room: home theater. For a lecture hall, seating 500 people: large venue. Ultra-portable, where a small screen size is acceptable: pocket or pico projector.
Pocket projectors are still a relatively new technology. Although most would require a fairly large pocket, they are small enough to hold in your hand, and usually can be mounted on a tabletop tripod as well. Because they use LED lamps, they aren’t very bright: 20 to 1,000 lumens. In comparison, multimedia projectors typically range from 2,500 to 4,500 lumens (higher if we included fixed installation). This makes them useful as an alternative to carrying a monitor or TV around, but they won't really cut it if your intention is to present to a group of more than a few people. Until recently, pocket projectors have been regarded as more of a novelty—a fun consumer item—than a serious display technology. However, as LED lamps improved, the capabilities of pico projectors have increased and they are starting to encroach upon the portable segment of the multimedia projector market.
Another notable limitation common to pocket projectors is the lack of a zoom lens. This means that the only way you can control image size is by physically moving the projector closer or farther away from the screen. If you favor compact form factor over brightness and zoom, a pico may be right for you. If not, you are probably a candidate for a multimedia projector.
Multimedia projectors represent the largest category, and are the most widely sold at B&H (close to 800 models, currently). Multimedia projectors are general purpose, and are used for everything from giving PowerPoint presentations to screening video clips and slideshows at weddings. They are typically considered portable, weighing from 3 pounds for the ultra-slim models and going up from there, and their brightness ranges from 2,500 to 4,500 lumens or so. Apart from the special sub-category of short-throw, they virtually always have zoom lenses. However, the zoom range is usually shorter than that of their home theater counterparts: 1.2x to 1.5x (compared to 2x in the home-theater realm). This means special care needs to be taken when choosing, to make sure the screen size is compatible with the projector's throw ratio. Multimedia projectors virtually always have VGA and composite video inputs, and most models now feature HDMI as well. In some cases, a DVI-D port will be provided instead of HDMI, but an HDMI device can still be connected using an inexpensive adapter. Similarly, DVI devices can be connected to a projector with only HDMI using a DVI to HDMI cable or adapter.
Common resolutions for multimedia projectors are SVGA (800 x 600), XGA (1024 x 768), WXGA (1280 x 800), and WUXGA (1920 x 1200), though 4K is just over the horizon. The most popular resolution is XGA, and is generally suitable for PowerPoint presentations—but, because it is a 4:3 format, may not be ideal if you plan to screen high definition.
Short throw projectors
"They are typically wall rather than ceiling mounted, and are designed to be installed very close to the screen: 18 inches to 2 feet."
Although most multimedia projectors have a built-in zoom lens, an important subcategory is short throw and ultra-short throw. Generally, a throw ratio of less than 1:1 is considered short throw. The most common throw ratios are 0.5:1 and 0.3:1, with the latter fulfilling the distinction of being "ultra-short throw." Short throw projectors almost never have zoom lenses, and in many cases they use a mirror onto which the image is projected first, before being reflected at the screen. They are typically wall rather than ceiling mounted, and are designed to be installed very close to the screen: 18 inches to 2 feet. Short throw projectors are most often used in classrooms, and are ideal for pairing with digital whiteboards. One might be tempted to place a short throw projector further back than the recommended couple of feet, as a way to achieve a very large image in a small space (assuming sufficiently low ambient light levels, of course). This probably won't work, since short throw projectors keystone severely when used outside their recommend throw distance range, and will require some very creative mounting to produce an undistorted image. Because they are meant for smaller screen sizes (8 feet wide or less), short throw projectors normally top out at 3,000 lumens. If you need a brighter projector and have limited space, you will need to look at a fixed installation projector with interchangeable lenses instead.
Home theater projectors
True home theater projectors—as opposed to multimedia/home theater crossovers, which from a feature perspective can be treated as multimedia projectors—place the emphasis on image quality above all else. They rarely get brighter than 2,500 lumens (though this seems to be changing), and have the most zoom of any projector type that uses built-in lenses. They are intended for use in a living room, or perhaps small screening room, where the ambient light can be controlled completely. They use either DLP or LCoS imaging technology, almost never three LCD, and will generally give you the best-looking picture of all types, assuming you use them in complete darkness and at a sensible throw distance.
Home theater projectors often feature low-voltage control (LVC), so that turning the projector on or off can trigger the screen to rise or descend, or open and close if it is a fixed frame covered by drapes. To achieve the quietest possible operation, they have more efficient—or more elaborate—cooling systems, making them relatively bulky and, in some cases, unable to support inverted installation. They are also the most expensive type of projector, relative to specifications. Virtually all are Full HD or 4K (true DCI 4096 x 2160 4K, in fact). Be careful, though. Companies often lump what are essentially restyled multimedia projectors into the home theater category. Telltale signs are high lumen ratings (more than 3,000 lumens), VESA rather than HD video native resolutions (such as WXGA and WUXGA), and zoom that's shorter than 2x.
If you want a great picture, can block out all ambient light, and are working with a screen size of up to 100 inches or so, home theater protectors are a great choice. Otherwise, you may have to consider a brighter multimedia projector, even if you plan to use it in a home theater setting.
Fixed installation and large venue
Fixed installation and large venue projectors are often included together with multimedia projectors. They are the brightest type available in the consumer market, starting at around 4,500 lumens and going up to 20,000 or more. Installation projectors are not generally considered portable, and take time to set up. In most cases, they feature interchangeable-lens systems, making them the most adaptable in terms of throw distance. They are normally used in lecture halls, movie theaters, houses of worship, stadiums, and other similar settings that require screening for large groups. In addition to large screen sizes, they are used in settings where ambient light can't be controlled. Many also support stacking, meaning the output from two or more protectors can be aggregated to increase the brightness beyond what a single projector can achieve.
Because of their weight and the nature of the lens systems they use, in most cases, installation projectors should be specced out by an integrator, and require installation by a professional.
Now we come to implementing the three pieces of information mentioned earlier.
Why do screen size and throw distance matter? Or: What is throw ratio?
Projectors have a very important specification called "throw ratio". Throw ratio is a specification that is determined by the first two pieces of information in the equation:
- How far the projector is from the screen (throw distance)
- How wide the screen is
For example, if your screen is 10 feet wide and the projector is 15 feet away, you will need a lens that covers a throw ratio of 1.5:1.
The first step in choosing a projector, therefore, is pinning down how wide the screen is and how far it can be placed from the screen—once you've done this, your choices will narrow considerably. Of course, you may have flexibility. Maybe your space allows you to mount the projector anywhere you want on the ceiling. In this case, while you might technically be able to choose any projector you want, you should consider mounting the projector as close to the screen as you comfortably can. Light is subject to the Inverse Square Law, meaning brightness drops logarithmically with increased distance, so the closer you can mount it, the fewer lumens you will need. At the other extreme is a case where you have an existing mount installed on the ceiling that you want to reuse. In this case, you will need to find a projector that features the exact throw ratio dictated by the position of the mount relative to the width of the screen.
|Short Throw||Medium Throw||Long Throw|
Projector screens merit a whole buying guide of their own. However, at this point, many of you will be understandably wondering, "If I'm starting from scratch, how should I know what screen size to get?" A quick, and very rough, rule of thumb is to multiply the distance of the "least-favored viewer"—i.e., the person farthest from the screen—by 1/5. So, if your LFV will be sitting 50 feet away, you'll need a screen that is 10 feet high.
But what if you don't know? Or what if the projector is being used on the go? Every effort should be made to find out, since there is no “standard” throw ratio, nor is there a standard screen size. On paper, multimedia projectors with built-in lenses don't appear to vary a ton. They mostly range from somewhere between 1.3:1 to 3:1, whereas a fixed installation projector with interchangeable lenses might have lens options ranging from 0.8:1 up to 15:1. You might have decided, since they're all about the same, to risk it. This might work, but remember that even exceeding only a foot on a 10-foot screen can lead to a critical part of the presentation being cut off.
If you really can't find out, you have two options: spring for a model with more zoom—which will cost more—or err on the side of shorter throw. Not true short throw, mind you—those don't have zoom and they keystone excessively if not carefully positioned. But something closer to the 1.3:1 end of the spectrum. Why? Because more often than not, getting the projector closer to the screen will be less of a problem than getting it farther away.
Finally, keep in mind, throw is based on native aspect ratio. If, for some reason, you are setting the projector to a narrower aspect ratio than native, the projector will effectively have a longer throw.
Where does a projector's light originate?
Projectors mainly use two lamp technologies: LED and metal halide. LED projectors top out at around 1,000 lumens right now, and mostly fall into the pico category. Almost all of the rest use metal halide—the classic digital projector lamp, lasting between 2,000 and 5,000 hours on. There are also a few eccentric projector models that feature a green laser/LED hybrid system.
How much brightness do I need?
While throw ratio is very important, brightness is the most important specification to get right. And this is where the third piece of information I mentioned—amount of ambient light—fits in. If the image isn’t bright enough to be seen clearly, all other considerations fly out the window. Getting enough light out of a projector is often the biggest challenge, but remember, it is nearly impossible to get a projector that is too bright. If a projector is ever "too bright" you can always just turn the brightness down. But making a projector that is too dim brighter... good luck!
Ambient light competes with the projector's output, causing the image to get washed out.
In the ideal world in which we don't live, projectors would always be used in total darkness. The more light you add, the more you lower contrast and wash out the image. Even getting a brighter projector only solves the problem partially, since ambient light is mixing with the darker parts of the image, making them cloudy. If you have to use a projector in ambient light, you will never get a perfect image, but it is possible to at least get a viewable image.
Projector brightness is measured in ANSI lumens (lumens for short). Calculating how many lumens you need requires knowing the throw distance, image width, how much ambient light is present in the room, and the content that will be shown. The simplest way to figure this out is to use a projection calculator, a software tool that crunches the number for you. Many projector manufacturers provide calculators on their websites. If not, Projector Central is a great resource, and offers projection calculators for nearly every projector model made.
Here are some examples of numbers of lumens you should anticipate needing:
- A living room where the lights can be turned off completely: 1,500 to 2,000 lumens
- A school classroom or boardroom where the lights can be dimmed, if not fully extinguished: 3,000 lumens
- A lecture hall or small church requiring a 10-foot-wide screen, and has a moderate amount of light: 5,000 lumens
- A movie theater: 20,000 lumens or more
|How it will look if your projector isn't bright enough|
After looking at the calculator, you may have noticed brightness is measured in foot-candles. Without a light meter, how is one supposed to know how many foot-candles of light a room has? Here, a bit of judgment and common sense come into play. Would you consider it "well lit" (50 foot-candles), moderately lit (20 foot-candles), or dimly lit (less than 5 foot-candles)? Or is there bright sunlight blazing in? If the installation is for critical view, then I would recommend getting a light meter, and carefully measuring. But for most practical everyday uses, a rough guess-timate erring on the side of too bright should suffice.
"If you legitimately don't know where the projector will be used, then get the brightest you can afford that you are able to carry around."
The content should also be factored in. Are you projecting white song lyrics over a solid background? Or are you showing photographs in an art gallery? In the former case, the contrast of the image is so high you can get away with a much weaker projector. In the latter case, you probably want to preserve every tonal nuance you can and, so, will need more lumens.
If you legitimately don't know where the projector will be used, then get the brightest you can afford that you are able to carry around. However, chances are, with a bit of thought you can come up with a reasonable estimate of the setting. For example, if you are a traveling product rep conducting trainings with groups of up to 20 people at various companies, 3,000 lumens may be enough as long as you don't encounter windows without blinds. If you do have a room without blinds, or are trying to project outdoors in daylight, be aware: no projector may be bright enough. You're asking the projector to do something it simply wasn't made to do.
Finally, if the projector is being used for any kind of critical viewing, then it is imperative that ambient light be eliminated from the setting. If this isn't possible, then TVs or monitors (perhaps arranged as a "video wall") should be used as an alternative. Ambient light not only degrades the image but also alters it, potentially undoing any careful calibration of the projector or color-correction work on the image itself. Projectors probably aren't ideal for critical viewing to begin with, but especially not when there is light in the room.
If you can't find a single projector that is bright enough, you might consider stacking. Stacking requires a compatible projector or using outboard hardware so the images can be aligned perfectly.
What resolution do I need?
Resolution matters, but perhaps less than you might think. Most projectors these days are least XGA (1024 x 768) resolution, a 4:3 aspect ratio format that has been the longtime staple for giving PowerPoint presentations. A few entry-level models are still SVGA (800 x 600), and pocket projectors sometimes have funky, low native resolutions that the manufacturers are coy about admitting. Because of high-definition video, increasingly widescreen formats, starting at WXGA (1280 x 800), are supplanting the legacy 4:3 standards.
Personally, I would not recommend going lower than XGA. At SVGA and lower resolutions, pixelation in the image will be very apparent. Also, many computer programs require at least XGA resolution even to run. You can cheat and set the computer's projector output to XGA, and let the projector scale the image down to its native resolution; however, the image will look blurry and smaller text will be unreadable. If you ever used a scan converter to hook your computer up to standard-definition TV you'll know what I'm talking about. The so called “screen-door effect” (discussed below) will also be worse in SVGA or lower.
In home theater setups, the screen-size-to-viewer distance ratio is a lot smaller than for other applications—here a higher-res image pays off. Otherwise, XGA is probably fine as a baseline, though going higher never hurts. Ideally, I would recommend starting at WXGA and going up from there. Even if you are PowerPoint user, bumping up to 16:10 won't really hurt, plus, you'll be ready if you want to screen HD video down the road. For special applications, such as exhibiting photos, you will want higher resolution: at least 1600 x 1200 (UXGA) for 4:3 or 1920 x 1200 for 16:10 (WUXGA), if not better. In the case of home theater, it's really a question of whether to invest in 4K or not, since nearly all home theater projectors are at least Full HD (1920 x 1080) anyway. Certainly 4K in a projector makes more sense than 4K in a TV, since you can achieve a larger image size. But right now there are few sources for 4K content, unless you buy a camera and record it for yourself. Also, HDMI 1.4a only supports 4Kp30 (i.e., maxes out at 30 frames per second / 30Hz for computer sources), so you if you early-adopt with the intention of "future proofing" you may end up kicking yourself down the road when HDMI 2.0 comes out and adds support for higher frame rates.
If you really want to be scientific about resolution, a quick Internet search will turn up a number of resolution calculators where you can plug in a screen size and viewing distance and the calculator will spit back a resolution. These are great, but, as with brightness, the content really needs to be factored in, and a calculator can't do that. A highly compressed YouTube video may look like hot garbage no matter what you try to show it on. On the other hand, if you are putting together a screening room for a production company, 4K may barely cut it.
Should I care about contrast ratio?
Contrast ratio is probably the most meaningless spec you'll find. Like HDTVs, projectors rely on so-called “dynamic contrast” to boost their on-paper performance. Also, the screen surface plays an import role in contrast. Finally, any ambient light will bring contrast ratio down into the double digits. Unless you have optimal viewing conditions (i.e., next to no light and a good screen), a 500:1 contrast ration and a 100,000:1 contrast ratio on paper probably won't render a visible difference.
What are keystone correction and lens shift?
Most projectors will have at least vertical, if not horizontal keystone correction; some even offer lens shift. Of course, we are all familiar with the trapezoidal “keystone effect”—the image appears wider at the top when the projector is too low, or wider on one side when the projector is horizontally off center. Keystone correction remedies this, up to a certain specified percentage. The problem with keystone correction is that it is achieved digitally, like digital zoom on a camcorder. The more you apply, the more the image will be degraded. As long as you mount or place the projector so that the lens is not below the bottom of the screen or above the top of the screen (assuming a right angle relative to the screen) you should be okay. If you starting getting outside of a normal mounting situation, or are staking or edge blending, you will seriously want to consider picking a projector with lens shift. Lens shift serves the same purpose, and then some, and achieves it optically with not loss in image quality. To get lens shift you are probably looking at a high-end home theater or fixed installation projector.
|Original Image||Vertical Keystone||Horizontal Keystone|
DLP vs LCD: Which is better?
DLP is for better image quality, LCD lasts longer. Next topic.
There are three imaging systems used in most projectors today: DLP, LCD, and one you might not have heard of: LCoS. Between the DLP and LCD it is really a toss-up these days. LCD has a little less rainbow effect on average; DLP a little less screen-down effect. Apart from that, LCD has a slight reliability edge in that there are no moving parts in the imaging system, whereas single-chip DLP uses a spinning color wheel and micro mirrors. LCD panels can still fail, but more often it is a case of dead pixels rather than a catastrophic failure as when a DLP color wheel stops working. If you are using the projector in a remote setting where it can't readily be replaced or served, then go LCD. Otherwise LCD vs DLP doesn't need to be a deciding factor.
Sample of Screen-Down Effect
In some ways, the underdog, LCoS, offers the best of both worlds. Some of you may vaguely recall HD projection TVs that had LCoS. LCoS (Liquid Crystal on Silicone) is a reflective technology like DLP, but in this case, the light is reflected from a silicone-backed LCD panel rather than micro mirrors. This system currently claims to produce the least screen-door effect and, because it is LCD, is free from the rainbow effect and other color-wheel-related artifacts. It is used almost exclusively in high-end multimedia projectors targeting critical viewing applications. On Sony home theater projectors, LCoS is called SXRD (Silicon X-tal Reflective Display).
What connectivity do I need?
In terms of video inputs, all but very high-end professional systems have more or less the same options: VGA, composite, component (usually on the VGA connector), HDMI, and occasionally S-Video. VGA has been on projectors forever, and because it is so widely used in many existing AV systems, will remain an option for some time—we just can't seem to kill it. Otherwise, HDMI is the main connector to look for, now and in the near future. Most computers can be adapted to HDMI with inexpensive passive adapters if they don't already have a built-in HDMI output. So look for HDMI, first and foremost. In some cases, a projector will have a DisplayPort or DVI input instead of HDMI. As long as you're not trying to pass 4K, these inputs can be adapted to accept an HDMI signal, again, using a passive adapter or cable. On professional projectors, you will also sometimes find SDI (serial digital interface), a digital interface mainly used in the broadcast world—at this level, you can often install expansion boards to customize the projector's connectivity, tailoring it to your specific needs.
One tip for installations: do not run HDMI more than 25 feet. There are HDMI cables that are longer, but the longer you go, the more the cable acts as an antenna, picking up RF signals. For whatever reason, 25 feet seems to be where reliability dives off the cliff. If you need to send HDMI farther, you can use baluns, special converter boxes that modify the signal so it can travel over a balanced (i.e., interference-resistant) type of cable, most often Ethernet.
In addition to video, many multimedia and home theater projectors have some form of wired control port, usually RS-232. This allows integration with automation systems or control using custom-developed software. Increasingly, Ethernet is becoming a standard feature, and will typically allow you to operate the projector across a network, or even the Internet, using any web browser. Some also have Wi-Fi capability.
Multimedia projectors often have one or more USB ports, though figuring out what the USB port does isn't always easy. Possible USB functions include:
- A video interface—as an alternative to using VGA or HDMI
- Plugging in a presentation mouse
- A port for a USB flash drive for giving "PC-less" presentations
- Connector for an optional Wi-Fi dongle
- Remote controlling the projector from a computer
PC-less presentation is probably the most-demanded feature of a USB port, and it is becoming increasingly common. However, in most cases this feature is for still-image formats, such as JPG only. The projector will not act as a media player and play video files. Some projectors also include software for converting PowerPoint presentations, but this can be buggy; I would recommend exporting your slide shows directly from PowerPoint to a supported still-image format such as JPG instead. You will lose animation effects, but at least it is reliable. Pico and home theater projectors, on the other hand, often have built-in media players so you can play back video from a USB device or even a memory card.
Wireless video is one of the most asked-for inputs. Some projectors have it, usually in the form of an optional USB dongle. But, because the video generally has to be compressed and, due to the presence of so many competing wireless devices—especially wireless routers—the wireless interface on projectors is only recommended for PowerPoint, photos, or showing relatively static computer graphics, not for full-frame-rate video. In addition, the range of projector wireless dongles is usually limited to about a 30-foot line of sight. If you really need to send video wirelessly, there are third-party options that can do it, and some are even uncompressed. But even with dedicated solutions, there are so many unpredictable environmental variables that the technology is not going to be mission-critical reliable.
Can I rely on a projector's built-in speakers?
Speccing a projector based on its speakers is a bit like speccing a car on how well it works as a boat. Projector speakers are terrible, even worse than TV speakers. Many will de-embed audio from the HDMI stream, so you might be tempted to just plug your speakers directly into the projector. Unfortunately, they often don't even extract audio very well. I would recommend you use separate speakers if your application includes sound, and that you plug the computer, DVD player, or other video source directly into the speakers or sound system, bypassing the projector itself. Most devices have separate audio outputs in addition to the video output.
Is 3D still "a thing?"
Consumer electronics manufacturers openly admit they've given up on 3D. You might still want 3D. If so, you'll probably need to look for a home theater projector. Many multimedia projectors purport to be "3D-ready;" in most cases, they only work with computers that have compatible graphics cards (DLP-Link is one proprietary technology that is widely used). Home theater projectors are more likely to feature HDMI 3D support so you can use them with Blu-ray players. When speccing for 3D, keep in mind that you are effectively cutting the brightness in half. As an alternative to buying a 3D-ready projector, you can also use a combination of two-projector stacking and polarizing filters. A special processor is required to demux the left- and right-eye streams from the HDMI signal.
Conclusion: Where does image quality come from?
Chances are, you want the best picture quality for your money, and it probably seems like we've been avoiding the question, speaking instead about boring, if important, practicalities like throw ratio and lumens. To this complaint, there are two arguments:
- If the projector you choose isn't bright enough, the image quality will suffer, regardless. And if it throws an image the wrong size, the viewing experience will suffer.
- Image quality is difficult to measure to the extent it can be measured objectively at all. And a lot of "image quality" is just plain subjective.
Regarding argument 1, I'm sure you'll agree I've said enough already. Addressing number 2:
If you know what brightness and throw ratio you need, I would suggest to you that two projectors with comparable specs at a similar price point will perform almost exactly the same. Flipping a coin may not sound like the most sane way to make a purchasing decision, but once you've determined what features are mandatory, and settled on a price point, you've already done everything you can to make a smart choice, Whether the preceding steps have narrowed your options down to one model or ten, rest assured, whatever you pick from those that remain will be the best choice for you.
- Is a projector the right choice? If you need to achieve a larger screen size and/or if portability is key, then yes.
- When not to use a projector: Projectors are for screening. They should not be used for color correction work or critical evaluation. Additionally, they work best in low light. If you have bright, uncontrollable ambient light, especially sunlight, then consider an alternative.
- Once you know the screen size, determine the throw ratio based on the screen width and the distance between the projector and the screen.
- If the screen size isn't known in advance, opt for a projector with more zoom or one that has a shorter throw. In most cases, getting closer is easier than getting farther back.
- Taking into account the screen size, throw distance, and the amount of ambient light in the room, use a projection calculator to determine the necessary minimum brightness in lumens.
- If you cannot calculate the brightness you need, consider the brightest projector you can get, within reason.
- Factoring in the content you will be showing and the distance of your average audience member, determine the minimum resolution you need. WXGA is usually safe for multi, though going up to 1080p and beyond certainly isn't going to hurt (except maybe your pocket book). For home theater, you will always want 1080p; and may even want to consider 4K.
- Consider any secondary features you may need, such as the ability to show a presentation from a USB flash drive.
- Don't sweat it if, after the above process of elimination, your search turns up too many choices—chances are any will work fine for you.
Projector brightness is measured in ANSI lumens (lumens, for short). Since the lumens rating is often measured by taking a reflected reading from a screen rather than directly from the light emanating from the lens, there is a fair amount of room for number fudging. This is because different screen surfaces have different reflectivity characteristics, and you can probably guess which surfaces projector makers prefer when rating their own products. On top of that, the image center is brighter than the edges by some factor, so manufacturers will measure from the center (in some cases they are honest enough to admit they are taking a center reading, but this, typically, on professional models that the companies know will get more carefully scrutinized by the end user).
Multimedia projectors start around 2,500 lumens, going up to around 4,500 lumens or so, though the line gets blurry between multimedia and fixed installation. So you may see what is advertised as a multimedia projector having up to 6,500 lumens. Home theater projectors used to hover around the 2,000-lumen mark, but in recent years more and more high-brightness models haves started to emerge. As of this writing, pico (pocket) projectors, because they rely on LED lamps, are rated at 1,000 lumens or less; some as low as 10 lumens, or about the brightness of a Maglite. At the other end of the spectrum, if you want to build a typical movie theater, you're looking at 20,000 lumens or more.
DLP stands for Digital Light Processing. There are two types: single chip and three chip. Single-chip DLP is the most common, and uses a combination of a color wheel and micro-mirrors to generate an image. Its historic advantage over LCD is that it can produce better color and contrast since the wheel can—in theory—be dyed to reproduce any color value or tonal value it wants. In addition, there is less space between pixels, thanks to advances in micro-mirror design, so you are less likely to perceive the so-called "screen-door effect." The drawback to DLP compared to LCD is an artifact know as "rainbow effect." However, recent DLP projectors spin the color wheel so fast, the rainbow effect is all but impossible to perceive.
Three-chip DLP is rare, and forgoes the color-wheel favor of three arrays of micro-mirrors, one for each primary color. In some ways, three-chip DLP is the best of both worlds: eliminating the rainbow effect while keeping the pixel spacing reasonable. However, the technology is generally regarded as cost prohibitive and not often used.
Edge blending allows the images from two or more projectors to be seamlessly merged to produce a single, larger image. Edge blending is the projection equivalent of a video wall, and can be useful for creative applications, allowing the creating of non-standard aspect ratios. Edge blending may be a built-in feature, or a third-party video processor may be used. In either case, the projectors must be of the same model and they must be carefully calibrated to ensure the images match.
Hybrid (LED/Laser) Projector Lamp
To produce higher brightness than is currently possible using an LED lamp exclusively, while retaining an acceptable CRI (Color Rendering Index), some projectors use "hybrid" lamps that combine an LED with an array of green lasers, combined using a reversed beamspiltter. The benefit of this technology is a light source that does not burn out and offers lower energy consumption compared to traditional metal-halide lamps of the same brightness. The drawback is that the CRI obtained remains lower than what metal halide lamps can produce. Therefore, this technology is deployed in "presentation"-oriented projectors—that is, for giving PowerPoint presentations and similar applications. A projector optimized for video playback, showing photos, or anything else where good color reproduction is required will use one or more metal-halide lamps.
Keystone correction is a digital process that counteracts the keystone effect to a specified percentage. Most projectors have vertical keystone correction only, some have horizontal and vertical. The "keystone effect" (or "keystoning") causes the projected image to appear wider at the top when the projector is too low, or wider on one side when the projector is horizontally off-center. Vertical keystone correction fixes keystoning when the projector is too low or too high, but requires horizontal centering. Horizontal correction compensates of the projector is horizontally off-center. Some projectors require manual adjustment, and some calculate the required correction automatically.
The drawback to keystone correction is that because it is a digital effect, the more correction applied, the more resolution will be reduced, a bit like digital zoom on a video camera. To avoid quality loss, lens shift should be used.
LED Projector Lamp
LED (Light Emitting Diode) is the most energy-efficient means of projector illumination. Additionally, LED lamps do not burn out in the ordinary sense. As with any circuit, they can fail, but they have a life expectancy on the order of 20,000 hours, compared to less than 5,000 for a typical metal-halide lamp. Currently, LEDs face two primary limitations to wider adoption in projectors: lower brightness and a lower CRI (Color Rendering Index) compared to the metal-halide counterparts. As of this writing, LED-only projectors emit less than 1,000 lumens and belong almost exclusively to the pico (pocket projector) category.
There is a very small minority of multimedia projectors that use a hybrid technology that combines LED with a green laser.
Three-LCD (often styled "3LCD") is the most common imaging system used in multimedia projectors. In its most common implementation, three-LCD uses a beamsplitter (prism) to break light into its RGB primaries, sending each color through a dedicated monochrome liquid crystal panel. Compared to DLP, LCDs do not tend to produce perceptible motion artifacts since they render the image line-by-line like a TV or computer monitor, and there are no moving parts in the imaging system. The drawbacks are lower contrast ratio than DLP, and more space between pixels, making the "screen-door effect" more apparent at a given resolution.
LCoS (Liquid Crystal on Silicone) is also called SXRD (Silicon X-tal Reflective Display) on Sony products and D-ILA (Digital Direct Drive Image Light Amplifier) on JVC projection TVs. It is a reflective technology like three-chip DLP, but in this case, the light is reflected from three silicone-backed LCD panels rather than micro-mirror arrays. This system currently claims to produce the least screen-door effect and, because it is LCD, is free from the "rainbow effect" and other color-wheel-related artifacts associated with single-chip DLP. It is used in multimedia projectors targeting critical viewing applications and some high-end home theater projectors.
Low Voltage Control (LVC)
LVC (Low Voltage Control) is an output found on many projectors that allows the projector to trigger another component of the A/V or home theater installation. For example, connecting a projector to a motorized screen that has LVC support allows the projector to automatically trigger the screen to lower when the projector is turned on and raise when it is turned off. The LVC output may be connected directly to the screen, or it may be connected to an automation system which, in turn, controls the screen and possibly performs other actions, such as dimming the house lights.
Projectors with lens shift have a movable lens-projector interface that allows the angle of the lens to be adjusted slightly ("shifted") with respect to the image plane. Like keystone correction, lens shift can be used to correct for keystoning, but unlike the former does not result in resolution loss. In addition, it can be used to reposition the image slightly to aid in the installation of the projector or to help align two or more images when using stacking or edge blending. Lens shift has to be adjusted manually on some projectors, and is motorized—operable by remote—on some models. Large-venue projectors with interchangeable lenses typically have motorized shift.
Metal-Halide Projector Lamp
In spite of advances in LED technology, metal-halide lamps remain the most common because they are bright and have high CRI (Color Rendering Index). The drawbacks: they burn out after several thousand hours (2,000 to 5,000 hours), force projectors to use noisy cooling systems, and are notoriously expensive. On typical multimedia and home theater projectors, only a single lamp is used, and it will be encased in a housing. Typically, the entire housing assembly will be replaced, not just the lamp itself. The old lamp should be allowed to cool fully before attempting to remove it, and care should be taken not to touch the lens. Oils on your fingers can degrade the lens coating, potentially causing the lamp to explode when it is turned on. There is minimal risk of physical harm since the lamp will be sealed inside the projector, however it will mean buying a new lamp.
Some projectors use two (or more) lamps as a cost-effective way of increasing the total brightness. Dual-lamp design also provides a degree of redundancy—assuming only a single lamp fails, you could continue to use the projector, albeit at half the original output. On some models it also means you could switch off one lamp to conserve lamp life and energy in situations where the full output is superfluous. This may not be ideal from the perspective of maintenance, however, since it means the lamps will have to be replaced at different times, and the sort of projectors that use dual (or multi-lamp) tend to be installed in places where they are not readily accessible.
Native resolution is the resolution, measure in pixel dimensions, of a projector's imaging system, and should not be conflated with its maximum supported resolution, which is often higher. Resolutions can either be SD or HD "video" formats, such as:
480i (NTSC), 576i (PAL), 720p, 1080i, UltraHD 4K (3840 x 2160), DCI 4K (4096 x 2160)
or computer VESA (Video Electronics Standards Association) formats, such as:
VGA (640 x 480), SVGA (800 x 600), XGA (1024 x 768), WXGA (1280 x 800), UXGA (1600 x 1200), WUXGA (1920 x 1200).
Ideally, the connected device should be set to match the projector's native resolution. Otherwise the projector will scale the image resulting in image degradation.
Rainbow effect is an artifact observed on many single-chip DLP-based projectors and is principally caused by the color wheel putting out different colors at different times. The effect is perceived as a rainbow trail in the corner of your peripheral vision as you move your eyes across image. As DLP technology has matured, faster-spinning wheels have significantly reduced this effect to the point people hardly notice it today.
The screen door effect is an artifact produced to varying degrees by all imaging systems digital projector's use: DLP, 3LCD, and LCoS. It can be described as a grid pattern, and is caused by the physical space between the individual pixels. At a given resolution, LCD projectors tend to exhibit the most screen door effect and LCoS protectors the least. Apart from choice of imaging system, higher-resolution projectors typically produce less screen-door effect than lower-resolution models at a given image size.
Staking doesn't mean physically stacking projectors on top of each other (though stackable projectors usually support this arrangement). Rather, it means identical images from two or several projectors are projected over the same area of the screen, hence, the images are what is "stacked." Stacking becomes a way of increasing brightness beyond what a single projector in the system is capable of, and may be more cost effective that buying a single projector of the same brightness.
Projectors must be the same model, and calibration is required to ensure the stacked images match. Stacking is built in on many fixed-installation projectors that have lens shift. Additionally, third-party processors are available that allow images from projectors that do not have built-in stacking to be merged.
Throw ratio describes the relationship between the width of the screen and the distance between the projector and screen, with the first number representing the throw distance and the second the screen width. It is styled "throw distance:screen width", e.g.: "1.3:1". So, for example, a lens with a 1:1 throw ratio will produce a 4 foot-wide image at 4 foot from the screen. A “short throw” (or wide angle) lens with a 0.5:1 throw ratio will produce a 4 foot wide image at 2 feet from the screen. And, finally, a long throw (or telephoto) lens with a 3:1 throw ratio will produce a 4 feet wide image at 12 feet from the screen. Projectors with zoom lenses will specify throw ration as a range, e.g., "1.3:1 to 1.7:1". Unless otherwise noted, throw ration is based in the native aspect ratio of the projector.
Multimedia and home theater projectors generally have throw rations of 0.3:1 or more at their widest setting and of 3:1 or tells at their most telephoto. Special short throw projectors will have a ratio of 0.5:1 or less. Interchangeable lens projectors will offer an assortment of lenses, covering ratios from 0.8:1 to 10:1 or more. In some cases, add-on lenses can be purchased to increase or decrease the throw ratio of the stock lens. As with converter lenses for video cameras, distortion and other forms of image-quality reduction may result.
Contrast ratio compares the darkest value the projector can produce to the brightest, can range anywhere from 500:1 to 100,000:1, and tends to be higher on DLP-based projectors. It is important to note, in many cases, the manufacturer-specified contrast ratio is actually dynamic contrast. Dynamic contrast varies the lamp brightness between images (or scenes), basing the ratio on the brightest possible white the projector can make when the brightness is maxed out, versus the darkest possible black when the lamp is at its darkest setting (or completely off for some technologies). This means the advertised contrast ratio will only be perceived if a dark image immediately follows a bright image or vice-versa.
Apart from dynamic contrast, determining projector contrast ratio is problematic because the type of screen fabric (other projection surface) and the presence of ambient light significantly affect contrast ratio. In the case of ambient light, the more ambient light there is the worse the effective contrast ratio will be. And any amount of ambient light beyond trivial will bring the ratio down significantly, making comparisons based on contrast ratio meanness anytime ambient light will be encountered.