We are all familiar with LCD and OLED flat panel displays. They come in fixed sizes and typically one of two resolutions, FHD (1920 x1080) or 4K (3840 x 2160). There is no learning curve. A size is selected that relates to the application and the order is placed. But with direct view LED (dvLED) displays becoming increasingly more popular this brings a new set or parameters and considerations to the table. With dvLED it is not as simple as specifying a 1080P or 4K display in a 65-inch flat panel configuration. In dvLED the variables and options have increased. The size and aspect ratio of a dvLED display is (almost) unlimited, but we are now confronted with multiple choices in size, pixel pitch, and because of those, resolution. In the selection process we need to understand how this relates to viewing distances and the experience for the viewer.
With all the new variables, it begs the question of what is most important. We suggest it begins with the image size, content to be shown, and an assessment of the typical viewing distance. The most significant variable is pixel pitch or how close the pixels are to one another. The objective is for the viewer to not see pixels at the typical viewing distance for the application. Seeing pixels interrupts what we see on screen and distracts us. Whether we see pixels or not determines the “quality” and the experience to most viewers. This brings us to what we see (visual acuity) and at what distance.
There is a scientific calculation of the distance a person with 20/20 vision must be away from an LED video display to no longer distinguish individual pixels. It is pixel pitch x 3438 (the scientific scale factor of 1 arc minute for detail seen in 20/20 vision) converted from mm to feet. For example, a 2.5mm pixel pitch equals 28.2 feet. In short if you stand 28.2 feet from the screen you will not see individual pixels. Here are some other examples of distances using the visual acuity formula:
- 10mm = 112 ft. 10 inches
- 4mm = 45 ft. 1 inch
- 5mm = 16 ft. 11 inches
A safe rule of thumb is to use 10X multiplier of pixel pitch (i.e., 2.5mm x 10 = approx. 25 feet). This is certainly close enough to the scientific calculation but there are those who claim that this is too extreme. Some claim that you can get closer to the screen than 10X with a given pixel pitch and not see discrete pixels. Individually speaking they may be right… in some cases. The key factor here is that viewing distances are rarely fixed. They span a range of nearest to farthest viewer. Best practice is to design for the typical or common viewing distance and err on the side of the nearest rather than the farthest viewer. Our advice is to stay as close to the 10X rule as you can.
This directly relates to another important factor, cost. As you decrease pixel pitch the cost of your dvLED display increases. For example, a 1.5mm dvLED is more expensive than a 2.5mm and that is more expensive than a 4mm and so on. Ideally, you want the sharpest images possible, but you want to make sure that you’re not overspending on pixel pitch if the benefits are not there. What the science of visual acuity tells us is that depending on viewing distances, increasing pixel density doesn’t add to the viewing experience. If the viewing distance is 40 to 50 feet a 4mm pixel pitch is fine and the typical viewer won’t see pixels.
Is this all a hard and fast rule? Not really. It is all application dependent, but it is a great place to start the discussion with the end users. This balance of pixel pitch to viewing distance is where the commercial AV integration designer comes into play and adds value. As we like to tell resellers and end users alike, if you do not know dvLED, work with someone who does. You will be glad you did.
