Additional, Smaller Displays Don't Improve Content Visibility

In real life, I do audio/video design and support for a university. When visibility is poor in a room, the intuitive solution--and what people typically request--is to simply add more, smaller displays throughout the room. Unfortunately, due to the way perspective works, this solution tends to have little benefit despite significant increased cost. However, I've never seen any breakdown of exactly why this solution is so ineffective, and I didn't really have anywhere else to publish my thoughts on it, so here we go...

To quantify the effects of this intervention, we'll construct a hypothetical room using standard equipment and 15 representative seats/students (most of our classrooms have closer to 40 students, but these results can be extrapolated). Most of our projection screens are approximately 110-120 inches diagonal, while most of our repeater displays are between 55 and 70 inches, depending on age and various constraints (space, cost, etc). For simplicity's sake, we can say that repeater displays are usually about half the size of the main display, give or take, and we would usually place it roughly halfway between the largest display and our furthest viewers:

We install these repeater displays with the hope of improving the visibility for students in the back row, but as we see in the following diagram, the students in the back row have no visibility benefit when looking at the repeater display (that is, it appears no larger or more readable for them than the main display):

Moreover, for the students at the sides of the back row, the viewing angle is slightly worse when compared with the projection screen, making it no better and arguably worse for them to look at the repeater display instead of the main display.

So, who actually benefits from this repeater display? Well, the student in the middle of the second row has a bit better view of it as compared with the projection screen, but really not that much better; maybe 25-30% improvement, tops:

The students one seat to either side may have a slightly better view, too, though the viewing angle is becoming quite shallow for them and could have detrimental effects, depending on the display, while the students farther out have no chance of a better view, since the viewing angle is unusably tight:

So, that leaves us with definitely one, possibly up to three students out of our 15 who actually reap a benefit from the repeater display, and the magnitude of that improvement is not particularly large.

Furthermore, these numbers get even worse if you mount the repeater display at an angle on (or even worse, flat against) the wall, as is typical (i.e., as opposed to the hypothetical example above, where it is floating in the optimal spot for visibility; in reality, we have ADA clearances and vertical viewing angles to contend with), with only one student--maybe two if we're lucky--gaining any benefit:

So, unless your repeater displays are very large or you have the money and the inclination to install many of them throughout the room (which brings its own issues of visual confusion/distraction), it's probably not going to bring you much value.

At this point, you may be asking "so, what's the solution, then?" Unfortunately, I don't have a good answer to that.

There are some software+BYOD solutions, like TopHat (only really useful for slide-based presentations) or Huddle Hub One (bandwidth-hungry; only supports 75 participants, making it ineffective for auditoriums) or even simply Zoom (this can quickly turn into a feedback mess with many devices in the same room), but instructors are often distrustful of anything that has students pulling their own devices out in class, let alone staring at them the whole time.

Separating students into groups/pods with a dedicated large-ish display for each group seems to work pretty well, and this configuration lends itself well to active learning applications, but it's also expensive, requires complex AV and reduces the maximum student density of a room as compared with traditional rowed seating by about 25%.