Choosing The Perfect Projector

If you're in the market for a new projector, you'll read and hear plenty about brightness and contrast, manual versus power lens, analog and digital input connections, keystone correction, and network interfaces.

Manufacturers will dazzle you with their sleek designs, powerful cooling systems, multi-segment color wheels, and improved dichroic filter systems. What you probably won't hear is why you should pick a particular model over another — or why you should even consider a specific class of projectors in the first place. Be honest. How many of you have done your homework beforehand to narrow your search down to the appropriate projector models for your application? Kudos to those of you who have done your research, but a great many people still have trouble sorting through the sea of common questions that pop up.

Should you go with a small “hang and bang” projector, or opt for a larger model with more features but similar light output? Just how many lumens do you really need up on that screen? Can you get by with a fixed lens, or should you go the interchangeable route? Is a short-throw lens better, or are long-throw lenses the way to go? Let's tackle each of these areas, plus a few more, to help you make a more informed purchasing decision.

Divide the distance from the screen to the farthest viewer(s) to determine minimum vertical image size.

Before the word “projector” even crosses your lips, back way up and take a look at your viewing environment. More specifically, you need to determine just how large an image you'll need to project to provide a comfortable viewing experience for everyone.

Back in the good old days (when I was in the staging business), the rule of thumb was to design 35 mm slides (remember slides?) to be readable by the last guy sitting in the last row of the audience. An easy way to do that was to print out the fonts likely to be used in landscape mode on an 8.5 by 11 sheet of paper, and then hold it at arm's length.

Was the text legible? If so, it would remain so with a 6-foot-wide screen at 14 feet, an 8-foot-wide screen at 18 feet, and a 12-foot-wide screen at 28 feet. This approach helped clients make better presentations — at least those who actually cared about their visuals.

But text legibility was only half of the equation. The next step was to determine the minimum screen height. For that, I used a tried-and-true formula that divides the distance from the screen to the last row of seats and divides by seven.

For someone seated 21 feet from the screen, the minimum height would be 3 feet, and with a 4:3 aspect ratio, the width would be 4 feet. Note that this was a minimum size recommendation for general viewing of video. In general, I would recommend larger screens if lots of text and fine line detail were being presented.

Image brightness chages as an inverse square function of projection distance.

Now that you know how big your screen is, you've got to light it up. There are no hard and fast rules for determining brightness — an 82-inch screen in a darkened home theater may be just fine with 300 to 400 lumens. But that same screen under normal ambient lighting might require 2,000 lumens to be usable.

Things are further complicated in that the measurement for image brightness, or luminance, is usually foot-Lamberts (English system) or candelas per square meter (metric system, also known as nits). In a movie theater, the Society of Motion Picture and Television Engineers (SMPTE) has developed a standard of a minimum 12 ft-L (41 nits) at the screen corners. This number takes into account the reflective qualities of the screen, otherwise known as gain.

But projector brightness is always measured in lumens (or lux), which are measurements of illuminance and don't take screen gain into account. (See Cheat Sheet, “How to Measure Light” in the November 2005 issue of Pro AV.) The difference between incident and reflected readings can be considerable! To demonstrate, I set up a widescreen LCD projector about 12 feet from an 80-inch-wide Da-Lite 1.5 gain screen in my office.

With a Minolta CL200 light meter, I measured 256 lux at the center of the image, on the surface of the screen. Placing the meter back at the projector location, thereby simulating a typical viewing position, and reading light reflected from the screen, I measured only 18.9 lux. That's a difference of 93 percent!

Taking screen size into account, it would appear I was hitting the screen with about 600 lumens and getting only 37 back. Switching to another light meter that specifically reads luminance, I measured 0.78 ft-L at the projector position, which is a long way off from the desired 12 ft-L target. It appears I'd need a projector 15 times as bright to get the job done, which works out to 9,000 lumens!

In reality, that 0.78 ft-L reading doesn't tell the whole story because the angle of the sensor on my luminance meter is too wide for critical readings — and it isn't difficult at all to view graphics and video with this system under dimmed lighting.

Practically speaking, a projector in the range of 2,500 lumens (about two f-stops brighter) would do the trick under higher ambient light levels. That's a little more than three times as bright as my test projector — a brightness rating that's not hard to achieve these days.

Using these calculations as a rule of thumb, you can now begin to determine our brightness requirements. But you must also take into account the inverse square law, which states the increase or decrease in brightness from the projector will be an inverse square function of its distance from the screen.

As an example, if I moved that LCD projector in my test rig twice the distance (24 feet) from the screen, I'd be putting only 150 lumens on the screen — not 300 lumens. Similarly, if I closed up the projection distance by one-half, I'd now see four times as much light in the screen (2,400 lumens), not twice as much.

You can see how all of these factors play into how bright a projector to buy. Screen size, placement, gain, and projection distance all play their part. If you start with a rule of thumb that requires 150 to 200 lumens for each foot of projection distance (assuming a matte diffusion screen with 1.5 gain), you'll be in good shape.

By the way, you'll find super-small projectors with lots of lumens to spare alongside larger desktop/installation models with similar ratings. Don't make the mistake of trying to save a few dollars by going with the smaller models, which may not have robust components and won't survive heavy-duty operating cycles.

Make sure your projector has lots of cooling air flowing through it. Heat is the No. 1 enemy of all projection systems — front and rear alike. It's better to pay a few more dollars for a larger installation projector and de-rate the brightness to the level you need in order to put less stress on the optical system and manage heat. Buying a 3,500-lumens projector and running it at 2,500 lumens is a good recipe for long component life.

Step 3. Exploring lens options

Here's where you can get tripped up again. Longer projection throws require longer throw lenses, which have smaller lens apertures than short-throw lenses. That means less light will make it to screen — and even less back to your eyes.

Whatever the brightness specification of your desired projector, it won't deliver the same performance with longer lenses attached. A short-throw lens with a focal length of 1.3X may have an aperture of 2.0, but a long-throw version might have an aperture of 4.0, which is about two f-stops dimmer.

If your dream projector can deliver 2,500 lumens with a short-throw lens, it's only going to push 700 lumens through that f4.0 long lens, which means you'll have to dim the lights or select a model that has 10,000 lumens output (an increase of two f-stops) to hit your brightness target.

There is a big advantage to using longer-throw lenses: less distortion in the horizontal and vertical picture axes. You do give up some depth of field, although that's hardly a problem when projecting onto a flat screen. But you'll wind up with an image free of pincushioning and other geometric image distortion.

Over the years, I've found that a focal ratio (projection distance to screen width) of 2:1 works very well. It keeps the projector far enough away from the screen to minimize noise, presents images with little geometric distortion, but allows for relatively wide lens apertures (2.0 to 2.6) and preserves image brightness.

Should you buy projectors with fixed or interchangeable lenses? If your projector is going to be installed, then there's no need for optional lenses — unless that's the only way you can get the specific focal length you want. Interchangeable lenses are really of value to rental & staging houses. With more projector models now supporting “short zoom” focal lengths (1.5X to 2.5X), the need for interchangeable lenses isn't as great.

Step 4. Checking out connector options

The DVI-D interface is popular for professsional and consumer displays.

This is an easy one, and all you'll need to do is to sit down and break out your viewing content into categories. Are you connecting images from notebook PCs, workstations, simulators, standard-definition video, or high-definition video?

If you watch a significant amount of standard-definition video, then you'd better be sure your projector is up to the task of de-interlacing, motion compensating, and scaling those video sources. This process becomes trickier as the native resolution of the projector increases — scaling 480-line video to SVGA (800x600) resolution is one thing; yanking it all the way to SXGA+ (1400x1050) is quite another.

In that case, you may want to consider the purchase of an outboard video/data scaler and switcher. These have come way down in price — many models now offer seamless switching from one signal source to another at low prices (either with a clean cut, a dissolve, or a simple fade-to-black followed by a fade-up).

As far as analog and digital interfaces go, don't settle for any projector unless it has at least one digital (DVI) input. Digital video interfaces are the wave of the future, with both DVI and the consumer HDMI interface appearing on more and more models. If you need to view serial video data streams, check to see if optional SDI and HD-SDI interfaces are available in addition to DVI.

Step 5. Weighing the bells and whistles

The HDMI connector is included on some professional projectors.

Today's projectors, it seems, can do everything. Flashcard readers, network interfaces, wireless mice adapters, digital keystone correction — you name it, projectors have it. But you might not need most of that stuff, so why pay more for it?

If you're installing a projector, the most important option is mechanical lens shift, primarily in the vertical plane. This will allow greater flexibility in projector placement and is infinitely more useful than digital keystone correction, which re-maps pixels across the imaging device and creates aliased edges.

A projector that offers both horizontal and vertical correction is even more desirable, although it's assumed that you'll try your best to find the optical centerline in the vertical projection axis when you install your projector.

Network interfaces are a plus if you need to monitor the operation of a projector from a distance, or are responsible for several projectors in a campus environment. But if you're simply installing a projector in a room for occasional meetings and presentations, there's probably no need to worry about this feature.

Multi-lamp modes and low-power modes can extend the service cycles of your projector. If the projector is bright enough, a lamp economy mode will let you stretch out a few hundred more hours before re-lamping, saving you some bucks along the way. If the projector is going to get heavy use day in and day out, a dual-lamp setup makes a lot of sense to avoid downtime when lamps fail.

As for the rest of the options being offered, it's really up to you and how tech-savvy the projector users are. Try as they might, manufacturers still can't get presenters to leave their notebooks behind and travel to presentation rooms armed only with PCM/CIA memory cards (an idea whose time has come and gone), or even with USB flash memory sticks (a much better idea). Built-in projector mice have also gotten a lukewarm reception. Wireless USB mice for notebooks are so ubiquitous and inexpensive these days that no one really needs them in a projector, and again, presenters are loath to travel anywhere without their super-slim notebook PCs.

Step 6. Credit card, check, or cash?

Mechanical lens offset is always preferred over digital keystone correction.

Now you've got a better idea of how to shop for a new projector. Never buy a projector based on low price or sheer brightness; always do your shopping with a game plan in hand. Figure your screen size, projection throw, and minimum lumens requirement using the information presented earlier to get into the ballpark, and finish up with sensible decisions on lenses, interfaces, and extras like networking and memory card readers.

And follow the manufacturer's recommendations for projector servicing, cooling, and air filter cleaning. Service department managers tell me the biggest problem they have is with dust and dirt accumulation, inadequate airflow, and failure to follow recommended servicing guidelines. Protect your investment!

Pete Putman is a contributing editor for Pro AV and president of ROAM Consulting, Doylestown, PA. Especially well known for the product testing/development services he provides manufacturers of projectors, monitors, integrated TVs, and display interfaces, he has also authored hundreds of technical articles, reviews, and columns for industry trade and consumer magazines over the last two decades. You can reach him at pete@hdtvexpert.com.