Video

Pixel Pitch

What is it? Why does it matter?

High resolution, high definition, pixel density, pixel pitch etc., etc.: At the end of the day, what do all these terms mean as they relate to your potential digital display? These and other terms will be demystified here, and placed in the proper context for investigating LED display technology for your particular application or project.

Relevant/Irrelevant Questions

Frequently, AV professionals ask essentially irrelevant questions when trying to ascertain whether a large-format LED display of a particular dimension will deliver the content clarity and sharpness they require for an application. This is mainly a result of comfort in using incumbent LCD technology terminology inherited from earlier generations of resolution parlance. Some of the questions include, “Are your displays hi-def?”, “Are your displays 4K?“, “Can I run HD content on your display?” Let’s take a look at what these terms really mean.

High Definition and High Resolution are terms that attempt to describe the clarity and sharpness of digital content being delivered via some form of viewing technology. They are often used interchangeably, and sometimes share a similar connotation depending on the audience. However, they are actually quite different. Both are measures of resolution, which is defined as “the fineness of detail that can be distinguished in an image.”

High Definition is a situational definition of High Resolution. In order for a digital image or video content to be considered “HD,” it must meet three specific criteria:

  • The display must be a widescreen format incorporating a 16:9 aspect ratio.
  • It must have a resolution of at least 1280 pixels x 720 pixels.
  • It must feature a frame rate between 24 and 30 frames per second.

For something to be “true HD,” it must have a resolution of 1920×1080, commonly known as 1080p (p stands for progressive scan). These constraints were created as units of standardization for the AV industry, but the constraints are not necessarily optimal for every situation.

More Nebulous Term

High Resolution, on the other hand, is a more nebulous term and can refer to any display with a resolution that is sharp and finely detailed. As long as an image looks clear and detailed to the human eye, it can be considered to have a high resolution, even though it may or may not be HD.

Let’s look at an example of why one of these terms doesn’t really apply to large-format LED display technology. Your question might be, “Are your LED displays true Hi-Def?” Well, if by high definition you mean 1080p, maybe not. In order for, say, a 4mm pixel pitch large-format LED display to be “Hi-Def,” it would have to be a minimum of 25 feet wide (at 305mm per foot, there are roughly 76 pixels in a linear foot of this display). If the pixel pitch increases to 6mm, it would have to be a minimum of 37 feet wide!

If your question changes to, “Are your LED displays 4K?”, in effect, you’re asking if they are (4xHD) and…well, you do the math. You’re describing a huge LED display area.

The question you really want the answer to is, “Can I show high-resolution content on a large-format LED display?” The answer is…yes…on some of them…depending on the total number of pixels (stated as the “pixel matrix”). High-resolution content can be scaled to display properly and effectively on large-format LED displays of a high-quality manufacture, where the LED lamps can properly emit content in a manyX:1 ratio.

The Term In Question

Now let’s introduce the term in question: pixel pitch. What does pixel pitch (also known as dot pitch, line pitch, P(x), strip pitch and similar terms) mean in terms of practical application as it is used in digital display technology and digital signage?

First, let’s define what a pixel is before we discuss its “pitch.” In digital imaging, a pixel, or pel (picture element) is a physical point in a raster image, or the smallest addressable display element in a display device. Therefore, it is the smallest controllable element of a picture represented on the display.

The address of a pixel corresponds to its physical coordinates. LCD and LED pixels are manufactured in a two-dimensional grid, and are often represented using dots or squares, but CRT pixels correspond to their timing mechanisms and sweep rates.

Each pixel is a sample of an original image; more samples typically provide more accurate representations of the original. The intensity of each pixel is variable. In color image systems, a color typically is represented by three or four component intensities, such as red, green and blue, or cyan, magenta, yellow and black (see Figure 1).

Figure 1
Figure 1

On a discrete LED or SMD-LED (surface-mount device-LED) display, a pixel is, in actuality, an LED lamp composed of three “sub-pixels,” each of which is an individual LED comprising the three RGB components.

True Definition

Now let’s discuss the “pitch” of “pixel pitch.” In its true definition, “pixel pitch” is the term used to describe the distance between pixels on televisions, monitors and other display screens. When discussing flatpanel or curved display panels, where the display is either a light-reflecting or light-emitting technology, pixel pitch is measured on the surface of the screen or display. With projection technology, it is measured at the projection source rather than the destination screen.

But all that is academic in that, during the course of vetting an appropriate display technology for your particular application, you are unlikely to hear the term “pixel pitch” used to describe image quality on LCD, LED back-lit LCD, front or rear projection, or any of the niche “tile” technologies. These technologies are more likely to be described in terms of their “resolution” or “definition” (e.g., high-definition, hi-def, hi-res, 2x, 4x, etc.).

The term “pixel pitch” typically is reserved for LED display panel technology.

In relationship to outdoor/indoor discrete and SMD-LED display technology, we are talking about the horizontal and vertical distance between the centers of discrete LED lamps, each composed of a red, blue and green diode (see Figure 2).

Figure 2
Figure 2

So what does all this mean for your potential LED display application? At the end of the day, when inquiring about the pixel pitch of a particular LED display grid, you’re in effect asking, “How clear is the imagery I’m trying to show going to be?”

Additional Questions

That question is going to be answered by several additional ones: “At what minimum distance, and further back, would your audience likely view the completed display installation?” and “What is the nature of the content and your suspected length of audience interaction with that content?”, which lead to a subjective discussion of, “What is your tolerance for pixilation of that content?”

Let’s step away from the minutiae and look holistically at what you are really trying to accomplish. When reviewing a display application for technical fit, one should ask the larger question, “What am I trying to do?”

With an overlay of the aforementioned questions, here are a couple of scenarios where LED display technology could be deployed.

  • Scenario 1: Your client wants a display to show a rotation of digital images and large-format text advertising to quickly inform customers entering their retail environment about current sales and lines being promoted.

Here, let’s assume that the display is going to be mounted above the entryway to a retail store and glanced at for perhaps 10 to 15 seconds by customers entering the store. Customer interaction with the display is going to be informational in nature and of a short duration. The minimum viewing distance in any real sense is going to be 15 feet and back.

  • Scenario 2: Your client wants a display to be mounted above and behind a bar area within a resort destination, showing high-definition simulcasts of major sporting events and performances of Grammy-winning musical artists, in effect simulating to the nearest extent possible, the experience of being live, in-person, at the shown venue.

The minimum viewing distance is potentially 10 feet, but using the 80/20 rule, a vast majority of the viewers will be 18 to 20 feet back and beyond. The duration of audience interaction with the content is likely to be anywhere in a range of 15 minutes to upwards of three hours.

Differences

In these two scenarios, there is a difference in the intended content, length of interaction with the content, resolution tolerance of the content, minimum viewing distance to the content and potentially overall surface area of the content, all directly relevant to the appropriate pixel pitch of the LED display to be used.

Let’s address the individual concerns dictating appropriate pixel pitch for an LED display application, one at a time.

Depending on the pixel pitch of the display, a greater or lesser minimum viewing distance is possible before an image “pixilates.” That is to say, there is a minimum distance beyond which a human eye with natural or corrected vision of 20/20 can no longer discern individual pixels on a particular display.

A general layman’s rule of thumb says that distance is roughly 3x the distance in feet as the pixel pitch is in millimeters. For example, if a display was manufactured using a 5mm pixel pitch, you would begin to recognize individual pixels on the display at about 15 feet and closer (3’x5 = 15′). See Table 1.

Table 1
Table 1

With LED display technology, however, this minimum viewing distance can be conditional, based on the overall surface area covered by the display. This is true for a number of reasons. If a display is 20 feet wide by 16 feet high, it will have exponentially more pixels to replicate the intended image than, say, a display of 10 feet by eight feet, providing more information for your eye to process, thereby increasing human-eye perception of resolution.

Gaps Tend To Soften

Additionally, because LED is a light-emitting technology versus a light-reflecting technology, the gaps between pixels tend to “soften” and are, therefore, perceived as less distinct when resolved by the human eye. This optical artifice is accomplished in much the same way that oncoming automobile headlights radiate light at night into the surrounding dark space immediately surrounding their edges.

The last major consideration we’ll look at is “optimum viewing distance.” Simply put, optimum viewing distance is the distance from your display, where the individual pixels in a display resolve to form one clear image to the average viewer. The larger the pixel pitch and lower the pixel density, the further your optimum viewing distance will be from the display. Conversely, the smaller the pixel pitch and higher the pixel density, the closer your optimum viewing distance will be. Resolution only comes into consideration as it relates to pixel pitch and pixel density.

One of the most familiar examples of how our eyes resolve a large-format LED display is the common digital billboards you probably see on your daily commute. From the distance you view the billboard, it looks like one solid image. The images are crisp and the words easy to read. However, if you were to climb up to that billboard and examine it close up, you would see that it is actually made up of hundreds of pixels spaced so far apart that it is impossible to see the image at all (a typical 14’x48′ billboard contains, on average, slightly more than 150,000 pixels…actually far less than a 16:9 aspect 112-inch diagonal 6mm pixel pitch display).

Science Of Your Eyes

So, why do these dots merge together to form a single cohesive image from far away? The answer lies not within the specifications of the display, but in the science of your eyes.

As distance increases, your eyes can no longer tell the difference between two pixels; their respective colors blend together and they simply merge to form one image. The amount of pixels our eyes are able to “resolve” or distinguish between decreases as we move farther from what we’re seeing.

Figure 3

In Figure 3, you can see that, as the image gets farther away, the individual pixels merge together to form a single crisp image.

Resolving distance is most accurately defined in terms of an angle. Imagine that the human eye is at the vertex of this angle, which means that the field of view is expressed as the arc created by the triangle as you can see in Figure 4.

Figure 4
Figure 4

The average human eye can only resolve 1/60 of a degree of this arc. So, if the distance from the display increases, then the arms of the angle get longer, and the termination points of the arms get further apart; therefore, 1/60 of a degree contains more data.

In other words, as we move farther from a display, we are decreasing the amount of individual pixels that our eyes are able to resolve, resulting in the merging of all the pixels into a crisp, clear, cohesive image on the display. So, how does this relate to resolution, pixel pitch and pixel density?

It’s Subjective

The best resolution, pixel pitch and pixel density for your display are subjective based on the size of the display and where it will be viewed from most often. You should make sure that the optimum viewing distance of the display you are considering is the same, or less than, the actual viewing distance from the display installation.

The concept of how your eyes merge individual points into one cohesive image is similar to the way a newspaper or magazine is printed. If you pick up a newspaper and look at it from close proximity or under a magnifying glass, you will see that each picture is made up of little dots. The distance you read the paper from allows your eyes to merge these dots together for the picture. When you examine a magazine, the image may look even better. This is because magazines often have a higher concentration of “dots per inch” or dpi (similar to pixel density).

Knowledge of this piece of the application will address a number of questions, including intended length of content audience interaction, intended effect of the content on the viewer and tolerance for pixilation of the content. Let’s go back to our two scenarios described earlier. The content in Scenario 1 is intended to be informative, consumed in passing, quickly processed and potentially persuasive, affecting purchasing habits of the viewer. The information conveyed is more important than the clarity or resolution of the image. It can still be eye catching, flashy, have an aesthetic “WOW!” impact, but the tolerance for pixilation is likely to be greater here than in…

…Scenario 2. Here, the intention of the content is to attract, entertain, capture and hold the intended audience in a particular venue for an extended length of time where, perhaps, they will consume food and beverages, engage in amusements or other revenue-producing activity, and potentially encourage their social acquaintances to join them to do the same. Likely, the tolerance for this audience is going to be significantly less than in Scenario 1.

We would be remiss if we didn’t show the relationship between pixel pitch and cost. Simple cost economics will dictate that you will pay more for a higher volume of anything. In this case, we’re talking about individual LEDs and LED lamps. When you drop a pixel pitch from, say, 6mm to 5mm, you will consume exponentially more pixels (read: LEDs) using the same width and height dimensions for a display. In many applications, this becomes the most important consideration, often causing project owners to adjust expectations and engage in “value engineering” or, in effect, compromising expectations of what can be done.

Because we have shown that best resolution, pixel pitch and pixel density are subjective based on where your display will be viewed from, how do you decide which display is right for you? The “better” the resolution, the higher the cost of a display due to the increase in number of LEDs, so you want to make sure you purchase the right display without purchasing too much display and overspending.

For this reason, it is important to see the display technology for yourself to perform a visual evaluation of the intended technology and simulate the conditions under which the intended application is to perform. Remember, not all LED display boards are created equal, and just because a 4mm pixel pitch looks perfect from one manufacturer doesn’t necessarily mean they all will. In other words, if you evaluate Brand A, don’t assume Brands B, C and D all look exactly like A.

There are other specifications outside the scope of our discussion here that you will want to consider as well, for instance, brightness, color contrast, off-axis viewing angle, power consumption, total cost of ownership and even content-creation best practices.

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