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How Many Lumens Do You Need?

Tim Adams, July 10, 2019

There are many factors that go into calculating how many lumens of brightness your project needs for an acceptable image. Some of these factors include ambient lighting, control over that ambient lighting, screen material, surfaces that bounce light onto the screens, and they type of content that will be primarily shown on the screen ( e.g., lyrics/text or images/videos).

We start with calculating how large your screen or screens need to be, which was addressed in a previous article. Once you have your screen height and width, you can calculate the square footage. This is an important number because you will need to plug it into the following formula:

Projector lumens / screen square footage x screen gain = foot-Lamberts

What is foot-Lamberts? Yeah, I didn't know what the heck they were the first time I saw that equation, either. Without getting into what for me is a crazy amount of detail and math, let's just say that it is used as the principal measurement for brightness of images on a projection screen.

With that out of the way, what are some good numbers to shoot for? Well, let's take something we are all accustomed to looking at: an LCD/LED television. Those can range, in broad strokes, between 90-120 fL (abbreviation for foot-Lamberts) at their brightest settings. As a comparison, most theater screens are sitting at around 15 fL. That's a big difference, but look at the control theaters exert over ambient lighting. If theater screens went much brighter it would be too bright and we might start having physical issues, most likely headaches. Remember, theaters have to display a fairly large dynamic range (difference between black and white). I like to aim for around 60 fL or higher, but it's extremely important to take account of the environment and lighting in the space before you make that a hard and fast rule. If the room will be primarily dark, then you will want to consider somewhere around 30 fL, whereas brighter rooms will require 60 fL or higher to combat that ambient light.

Another key component to evaluate is the content that will be displayed on the screen. If it's mostly text, than you can likely get away with a lower fL measurement because dynamic range isn't as important. However, if you are planning to do video playback, IMAG (large video projections of concert performers and the like), or anything with images, it will be important to make sure you have as much dynamic range as possible.

Which brings me to your screen material. A matte white screen will produce an image for you, but if you want to get the best picture possible, even in a controlled lighting environment, I would recommend taking a look at some of the ALR (ambient-light rejecting) options on the market. The proprietary technology from each manufacturer can actually increase the dynamic range coming out of the projector system. However, make sure you either get a sample to evaluate or contact a system integrator or A/V consultant who can do a demo for you so you know exactly what you're getting. I cannot tell you how many times this critical step has saved a client from wasting money by buying the wrong screen, which are often built-to-order and non-refundable. These materials can seem like magic, but they do have their limits. I encourage you to search YouTube for examples of these screens in action. Some manufacturers to look into are Elite Screens, dnp, Da-Lite, Draper and Screen Innovations.

The trick with ALR screens is to remember that a projector operates by projecting light. How does a projector project black? It has to be created by the absence of light. This means that if you are projecting on a white surface, be it a projection screen or a white wall, your blacks will always be some form of dark grey because that is the darkest "black" possible on that color surface. Similarly, if you project onto a dark grey or black surface, your whites will be some form of light grey, though this is much easier to work with than the former example. If you look at some of the examples shown in this article, you can see that I have tested a variety of fabrics and materials, including fabric from a local fabric store and a piece of wood painted with a custom blend of paints from the local hardware store. There are tons of videos online with tips and DIY instructions on how to do this yourself; however, for a large format screen such as those found in churches, I would urge caution. While these can work in a home environment, I have noticed some issues with all of the DIY screen solutions, including color and brightness uniformity, "sparkles" from uneven spreading of metallic flakes in the paint or fabric, a lack of sufficient detail leading to a "soft" or blurry image, air bubbles, sagging fabric, etc.

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The author has experimented with various fabrics from a local fabric store and plywood with custom-mixed paints to see how they fared in ambient light. Shown here are dark silver fabric (top) and a painted wood board, both shown against a matte white rear-projection screen and, directly above the test material, a swatch of commercial UST screen material. Notice the flaring in the whites on the silver fabric on top compared to the controlled whites on the swatch and the painted plywood.

One final point I want to make is to consider how close the projector is to the screen. I know this sounds like a strange thing to mention, but please, bear with me. Most projectors are meant to sit away from the screen, using throw distance and offset measurements to identify a minimum throw distance and a maximum throw distance to achieve a range of image sizes. These throw distances can range from a few feet to up to a couple hundred feet. If you are looking at a throw distance of over 20 feet, it's likely you will be stepping into the realm of interchangeable lens projectors, which really ups your costs.

But, you don't have to necessarily place your projectors that far away from your screens. If you have calculated that your screen size needs to be 120 inches or smaller, I would recommend you take a hard look at some of the amazing 4000-lumen, 1080p HD native ultra short throw (UST) projectors that have been hitting the market in the last couple of years. There are even UST projectors with built-in geometric correction, which means you don't have to get your mounting perfect, just close, and then match up the four corners of your image to the corners of your screen. This is a huge capability in these low-cost projectors. And your throw distance for a 120-inch image with a UST projector is usually only around 22 inches or so, which is super short. This allows for a super low-profile and small footprint installation, and the projectors tend to disappear (figuratively) after the first couple weeks, so any aesthetic complaints will likely just fade away (see example).

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UST projectors can throw a large number of lumens onto a 100- or 120-inch screen from a very short distance, resulting in a bright, punchy image that stands up well to ambient light. This installation features a 4,000-lumen Optoma ZH400UST ultra short throw projector mated with a Screen Innovations 100-inch Pure Grey screen.

In the end, how bright a projector you need is all about finding the answers to the variables listed above. And I cannot stress how important it is to find these answers. When investing in a projection system for a church or other house of worship, we must be good stewards of the resources entrusted to us, and that means not wasting money. If that means you consult with a professional to ensure the right choices, please do that. You are not doing yourself or the church any favors if you intend to save money on a DIY solution, only to find that you go through several iterations of that solution before you find one that kind-of works when an A/V pro could have provided the right solution in one-quarter the time, and probably for a lower budget.

Tim Adams is president and chief systems designer for Timato Systems, an audio/video integration company specializing in servicing the sound, lighting, video, projection and live-streaming needs of churches and other houses of worship. He can be reached at info@timatosystems.com.

Comments (2) Post a Comment
Chris Byrne Posted Jul 11, 2019 2:13 PM PST
It should probably be noted, since most don't realize this, that if you do indeed take the writer's advice and look at UST (ultra-short throw) projectors, you will need a fixed or tensioned screen. Most portable, manual, and electric screens will not work with that type of lensing system.

Additionally, since ALR screens were also promoted, the reader should note that most ALR materials are not compatible with UST projectors. There are some are specially made for UST, but most will simply block the projected light instead of reflecting it back to the viewer.

As the writer mentioned, it is well worth discussing your needs with a professional to avoid a myriad of issues with these type of specialized components.
David Sobers Posted Jul 12, 2019 9:21 AM PST
Tim - nice summary. Can you provide more specific information on the "dark silver fabric" in the top photograph that appears to have a high contrast image with a strong black and white rendering. Thanks, David.

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