by John Carlson, Systems Engineer, Sound Vision, Inc.
Customers often ask, "How big should our projection screen be?" or "How do we decide how large a plasma, LCD monitor to buy?" For most users, there's a simple rule of thumb.
Start by measuring the distance to the farthest viewer in the room where you'll use the display. Simply divide that distance by 6 to give you the height of your screen or display.
If your farthest viewer will be 30' away, you need a screen that's 5' high. If the farthest viewer is 10' away (converting it to 120") you need a screen that's 20" high.
Sizing a screen this way will give you readable presentation images if the smallest font you use is 14 -16 point. The rule applies to projection systems, plasma displays, LCD monitors, and older CRT-based monitors.
If you plan to show smaller text on a regular basis (for example if you will use your display to view websites regularly), you may want to divide the distance by four or even by three. If you never plan to show text (viewing video only), you can divide by eight. But it's rare that we find a customer who needs a larger or smaller screen than one found using this "1/6 rule."
Once you have your screen height, you can use the aspect ratio of your screen or display to find the screen width. For standard 4:3 images, multiply the height by 4/3 or 1.33. For wide-screen 16:9 images, multiply the height by 16/9 or 1.78.
Sizing video walls - and a look at what the 1/6 rule is really saying
Sizing projection screens and sizing flat panel displays is simple enough that most people will use the 1/6 rule, perhaps taking it another step by viewing the kind of images they mean to show on various size screens before they buy.
It gets a little more complicated, however, if you're considering installing a video wall. Video walls can, of course, be used for various purposes, and if your goal is mainly visual impact –say showing advertising messages in a retail space– you may not be too worried about the exact size of the display.
But if you're planning a video wall in a control room, you need to display a lot of data and the data has to be readable. So let's take another look at this 1/6 rule and what it really means.
First, it's important to note that screen size most often boils down to the size a character of text must be in order to be readable at a given distance. If you're showing PowerPoint presentations, you normally work within a narrow range of font sizes in a fairly standard layout, so a rule of thumb works well. But in a video wall, the screen can be almost any size and shape, and so you need to look at the actual characters you will display.
The key thing to understand is that the smallest detail the human eye can differentiate is 1/60 degree out of the 360-degree field of vision. This can be expressed as a one minute arc (see illustration).
To be able to see a character of text, which is composed of many details, it must be, at minimum, be 10 arc minutes high (or .166 degrees out of the field of vision). In practice, however, for it to be easily viewed, it should be larger. For a control room data display, 15- 20 arc minutes is typically used. For general purpose or public displays, 30 arc minutes or larger is used (.5 degree).
So now the task becomes one of determining the distance the operator or member of the public will be when viewing the wall, then calculating the height a character must be at that distance.
Using the formula X = 2 * Y * tan(arc min./120), where X = the character height (in inches) and Y = the viewing distance, you can determine the minimum character height required for your application. Using 18 arc minutes, viewers from 20' (or 240") require a minimum character height of 1.26" and those at 30' (360") require a minimum height of 1.88."
There are other factors to take into consideration in sizing a video wall, among them resolution, viewing angle, number of operators, and of course the amount of text and graphic elements to be displayed. And there are a couple of somewhat esoteric factors to take into account (for instance, an ASCI character must be composed of at least ten pixels, and that can determine the minimum character height when viewing distances are short or resolutions lower.) But character height is the crucial calculation –and also the basis of the 1/6 rule.
If you're considering a video wall, or just need help designing and sizing a more conventional screen or display, contact Sound Vision for help.
Our thanks to John Higgins of Custom Display Solutions for his help with the information about sizing video walls and for the human eye illustration.