The Qualiverse - Determining Display Size
How big should your display be? This is a seemingly simple question, but answering it gets difficult, quickly. We used to base the answer simply on task level and furthest viewer in the room. Then we based the answer on how large text would be on the screen. Most recently, we base it on how large the “elements” we need to perceive are relative to the screen size. I think all these different methods have their benefits and shortcomings. However, in the end, it comes down to the users’ needs. Let’s explore these different methods and perhaps come up with a new take to best meet our needs.
THE “4/6/8” RULE
It all started with the “4/6/8 Rule”. Depending on what task you expected to do in the room, the display height could be determined simply by dividing the distance to the furthest viewer by 4 (analytical viewing of medical images, topographical maps, or engineering drawings), 6 (basic viewing of typical text-based documents), or 8 (passive viewing of moving images). So, for most conference room applications, we simply divided the distance to the furthest viewer by “6” to get the proper display height. Users would (hopefully) be able to comfortably view text documents, spreadsheets, or presentations. It was a beautiful, easy-to-use, repeatable calculation that worked great. Until it didn’t.
The 4/6/8 Rule worked well with resolutions of 800x600 or 1024x768. But then people started getting crazy with resolutions of 1920x1080 (1080P) or 3840x2160(4k). All of a sudden, text height on the screen was reduced by 30% - 70%! Users used to sign into the computer from the head of the table without having to squint. Once the text size was drastically reduced, they had to move to the middle or the foot of the table to get closer to the screen to be able to read the text they were entering. Something had to change in the way we calculated screen sizes.
THE 150:1 RULE
Enter: the 150:1 Rule! This guideline said that the furthest viewer should be no more than 150x the size of the text on screen. This rule took into account the image resolution and font size being used during presentations. As an example, at “this” resolution with “this” font size, the text on the screen will be 1” high. So, the furthest viewer has to be less than 150” (or 12.5’) away. Designers could simply back-engineer the equation to find the display size for a given, desired font size, resolution, and furthest viewer. This worked well since the designer had a lot of variables to play with: display height, resolution, and font size. If the owner didn’t like the price tag of large displays, we could still provide comfortable viewing experiences by adjusting the preferred resolution and/or font size. For example, if a large display didn’t fit the room or the budget, it was recommended to use a lower resolution or larger font size to maintain the required text size on the screen. Designing for what had to be read on the screen made a lot of sense.
THE "DISCAS" STANDARD
So much sense, in fact, that it inspired the idea of focusing on what needed to be discerned on the screen as the driving force of screen size. Enter: the Display Image Size for 2D Content in Audiovisual Systems standard from AVIXA, lovingly referred to as “DISCAS” for short. Instead of focusing on text height as in the 150:1 Rule, DISCAS posited that we should focus on whatever it is we need to see/understand on the screen, and call these items “elements”, and use them to determine how large a screen should be. If we need to read text on the screen, the “element” becomes text. If we need to perceive individual lines in an engineering drawing, the “element” becomes a line of pixels. This was a game changer. Screen sizes can now be determined by “element height” on a given screen, or “percent element height” (%EH) to determine the preferred screen size. If an element takes up 2% of the pixels on a screen, we can design for that. If an element takes up 4% of the pixels on a screen, we can get away with a smaller display because the element takes up a larger percentage of the screen height.
The problem with DISCAS is it recommends some uncomfortably large displays for small rooms. As an example, let’s say we need a huddle room that is used to review spreadsheets. The 4/6/8 Rule recommends a 55” display (28” screen height) for a furthest viewer that is 14’ from the screen. Assuming a 1080P source and the default MS 11 point font size, the “element” for this use case is a lower-case “e”, which takes up only 6 of the 1,080 vertical pixels...or a “0.5 %EH”. Using DISCAS, with a furthest viewer of 14’ and a 0.5 %EH, a 24’ screen (166” screen height!) is required. Obviously, this won’t work in most huddle rooms. In fact, it physically won’t fit in most huddle rooms.
WHERE ARE THINGS NOW?
Funnily enough, most designers have adopted DISCAS, but they adjust their %EH to be 2%, 3%, or 4%...which just so happen to align with the 4/6/8 Rule. Old habits die hard, I guess. And, for your information, a 3 %EH on a 1080P screen means that the minimum font size use for presentation materials should be more than 48 point. That is a difficult ask for most users when using presentation software, let alone spreadsheets.
WHICH METHOD IS BEST FOR DETERMINING DISPLAY SIZE?
So, how should we determine displays sizes? Keep the 4/6/8 Rule? Use DISCAS with some fudged %EHs? My recommendation is to have the users decide. Base the decision on their human experience. Ask the “representative user” (average everything) to pull up the typical material that will be shown in the room on their screen at home with the resolution and font size they prefer. Then ask them to move their chair back until they can no longer comfortably read or discern the elements on the screen. Measure how far they are from the screen... and that’s the user-centered screen size standard. I can proofread this document comfortably from 50” away (furthest viewer). The image is 12” high. Therefore, my user-centered screen size standard is that the furthest viewer should be less than 4.2x the image height away. In our 14’ furthest viewer huddle room example from before, this would result in requiring a 75” display to satisfy my needs. This is certainly larger than the 55” display recommended by the 4/6/8 Rule... but way more reasonable than the 24’ (288”!) screen recommended by DISCAS. When we base our design decisions on the human experience of the users of these spaces, it may be easier to justify than basing them on nebulous rules or standards. This may result in display sizes larger than we are used to, but we won’t have users squinting as they attempt to enter their usernames and passwords.
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