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Sound & Communications48

New technology means new terminology.LED TODAY

BY BRETT FARLEY

High resolution, high denition, pix-el density, pixel pitch etc., etc. At theend of the day, what do all these termsmean as they relate to your potentialdigital display? Our goal here is to de-mystify and place these terms in theproper context for investigating LEDdisplay technology for your particular

application or project.Frequently AV professionals ask es-sentially irrelevant questions whentrying to ascertain whether a large-format LED display of a particular di-mension will deliver the content clar-ity and sharpness they require for anapplication. This is largely a result ofthe comfort in using incumbent LCDtechnology terminology inherited fromearlier generations of resolution par-

lance. Questions such as Are yourdisplays hi-def? or Are your displays4K? or Can I run HD content on yourdisplay? come up. Lets look at whatthese terms really mean.

Hi-Def vs. High ResolutionIn summary, high denition and

high resolution are terms that at-tempt to describe the clarity and sharp-ness of digital content being delivered

via some form of viewing technology. They are often used interchangeablyand sometimes share a similar conno-tation depending on the audience, butthey are actually quite different. Bothare measures of resolution, which isdened as the neness of detail thatcan be distinguished in an image.

High denition is a situational de-nition of high resolution. In order fora digital image or video content to beconsidered HD, it must meet threespecic criteria: 1) The display mustbe a widescreen format incorporatinga 16:9 aspect ratio; 2) It must have aresolution of at least 1280 pixels x 720

pixels and 3) a frame rate between 24and 30 frames per second. In order forsomething to be true HD, it musthave a resolution of 1920x1080, com-monly known as 1080p (p stands forprogressive scan). These constraints

were crea ted as units of standardiza-tion for the AV industry, but the con-straints are not necessarily optimal forevery situation.

High resolution, on the other hand,


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is a more nebulous term and can referto any display with a resolution that issharp and nely detailed. As long asan image looks clear and detailed tothe human eye, it can be considered tohave a high resolution, even though itmay or may not be HD.

ExampleLets look at an example of why oneof these terms doesnt really apply tolarge-format LED display technology.

Your question might be, Are yourLED displays true hi-def? Well, if byhigh denition you mean 1080p, maybenot. In order for, say a 4mm pixel pitchlarge-format LED display to be hi-def,it would have to be a minimum of 25feet in width (at 305mm per foot, there

are roughly 76 pixels in a linear footof this display). If the pixel pitch in-creases to 6mm, it would have to bea minimum of 37 feet in width! If yourquestion changes to, Are your LEDdisplays 4K?, in effect, youre asking ifthey are (4xHD) andwell, you do themath. Youre describing a huge LED

display area. The question you really want the

answer to is Can I show high resolu-tion content on a large-format LED dis-play? The answer isyeson some of

themdepending on the total numberof pixels (stated as the pixel matrix).High resolution content can be scaledto display properly and effectively onlarge-format LED displays, where theLED lamps can properly emit contentin a manyX:1 ratio.

Heart Of The MatterNow lets introduce the term in ques-tion: pixel pitch, and its analogs, dotpitch, line pitch, P(x), strip pitch, etc.:

What do these mean in terms of practi-

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cal application as used in digital displaytechnology and digital signage?

First, lets dene what a pixel is be-fore we discuss its pitch. In digitalimaging, a pixel, or pel (picture ele-ment) is a physical point in a rasterimage, or the smallest addressabledisplay element in a display device;therefore, it is the smallest control-lable element of a picture representedon the display.

The address of a pixel correspondsto its physical coordinates. LCD andLED pixels are manufactured in atwo-dimensional grid, and are oftenrepresented using dots or squares, butCRT pixels correspond to their timingmechanisms and sweep rates.

Each pixel is a sample of an or iginalimage; more samples typically providemore accurate representations of theoriginal. The intensity of each pixel

is variable. In color image systems, acolor is typically represented by threeor four component intensities such asred, green and blue, or cyan, magenta,

yellow and black.On a discrete LED or SMD-LED

(surface mount device: LED) display,a pixel is, in actuality, an LED lampcomposed of three sub-pixels, each of

which is an individual LED comprisingthe three RGB components.

Now, lets discuss the pitch of pix-el pitch. In its true denition, pixelpitch is the term used to describe thedistance between pixels on televisions,monitors and other display screens.

When discussing atpanel or cur veddisplay panels, where the display is ei-ther a light-reecting or light-emittingtechnology, pixel pitch is measured onthe surface of the screen or display.

With projection technology, it is mea-sured at the projection source ratherthan the destination screen.

Its AcademicBut all of that is academic in that, dur-

ing the course of vetting an appropriatedisplay technology for your particularapplication, you are unlikely to hear theterm pixel pitch used to describe im-age quality on LCD, LED back-lit LCD,front or rear projection, or any of the

niche tile technologies. These tech-nologies are more likely to be describedin terms of their resolution or de-nition (e.g., high-denition, high-def,high-res, 2x, 4x, etc.). The term pixelpitch is typically reserved for LED dis-play panel technology.

In relation to outdoor/indoor dis-crete and SMD-LED display technolo-gy, we are talking about the horizontaland vertical distance between the cen-

ters of discrete LED lamps composedeach of a red, blue and green diode.

So what does all this mean for a po-tential LED display application? At theend of the day, when inquiring aboutthe pixel pitch of a particular LED dis-play grid, youre in effect asking, Howclear is the imagery that Im trying toshow going to be?

Additional Questions That question will be answered by

several additional ones: 1) At whatminimum distance, and further back,

would your audience likely view thecompleted display installation? 2) Whatis the nature of the content and thesuspected length of audience interac-tion with that content? This leads to asubjective discussion of What is yourtolerance for pixilation of that content?

Lets step away from the minutiae for

a minute and look holistically at what you are rea lly tr ying to accomplish. When reviewing a display applicationfor technical t, one should ask thelarger question: What am I trying todo? With the aforementioned ques-tions in mind, lets look at a couple ofscenarios where LED display technol-ogy could be deployed. Scenario 1: You want a display toshow a rotation of digital images and

Figure 1. As the image gets farther away, the individual pixels merge together to form asingle crisp image.


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large-format text advertising to quicklyinform customers entering your cli-ents retail environment of currentsales and lines being promoted. Letsassume that the display is going to bemounted above the entryway to a retailstore and glanced at for perhaps 10 to15 seconds by customers entering the

store. The customer interaction withthe display is going to be informationalin nature and of a short duration. Theminimum viewing distance in any realsense is going to be 15 feet and back. Scenario 2: You want a display tobe mounted above and behind a bararea within a resor t destination, show-ing high-denition simulcasts of majorsporting events and performances ofGrammy-winning musical artists, in ef-fect simulating to the nearest extentpossible the experience of being live,in-person at the shown venue. The min-imum viewing distance is potentially 10feet, but using the 80/20 rule, a vastmajority of the viewers will be 18 to 20feet back and beyond. The duration of

audience interaction with the contentis likely to be anywhere in a range of15 minutes to upwards of three hours.

In these two scenarios, there is a dif-ference in the intended content, lengthof interaction with the content, resolu-tion tolerance of the content, minimum

viewing distance to the content and,

potentially, overall surface area of thecontentall speaking directly to theappropriate pixel pitch of the LED dis-play to be used.

Lets address the individual concernsdictating appropriate pixel pitch for anLED display application one at a time.

Minimum Viewing DistanceDepending on the pixel pitch of the

display, a greater or lesser minimum viewing distance is possible before animage pixilates. That is to say, thereis a minimum distance beyond whicha human eye with natural or corrected

vision of 20/20 can no longer discernindividual pixels on a particular display.

A general laymans rule of thumb

says that distance is roughly 3x thedistance in feet as the pixel pitch is inmillimeters. For example, if a display

was manufactured using a 5mm pixelpitch, you would begin to recognize in-dividual pixels on the display at about15 feet and closer (3'x5' = 15').

With LED display technology, how-

ever, this minimum viewing distancecan be conditional based on the over-all surface area covered by the display.

This is true for a number of reasons.One, if a display is 20 feet wide by 16feet high, it will have exponentiallymore pixels to replicate the intendedimage than, say, a display of 10 feet by8 feet, providing more information for

your eye to process, thereby increasinghuman-eye perception of resolution.

Additionally, because LED is a light-emitting technology versus a light-re-ecting technology, the gaps betweenpixels tend to soften and are, there-fore, perceived as less distinct when re-solved by the human eye. This opticalartice is accomplished in much the


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same way that oncoming automobileheadlights at night radiate light intothe surrounding dark space immedi-ately surrounding their edges.

Viewing Distances & Human Eyes The last major consideration wel l

look at is optimum viewing distance.Simply put, optimum viewing distanceis the distance from your display,

where the individual pixels in a displayresolve to form one clear image to theaverage viewer. The larger the pixelpitch and lower the pixel density, thefurther your optimum viewing distance

will be from the display. Converse ly,the smaller the pixel pitch and higherthe pixel density, the closer your opti-mum viewing distance will be. Resolu-tion only comes into consideration as itrelates to pixel pitch and pixel density.

One of the most familiar examplesof how our eyes resolve a large-formatLED display is the common digital bill-boards you probably see on your dailycommute. From the distance you viewthe billboard, it looks like one solidimage. The images are crisp and the

words easy to read. However, if you were to climb up to that billboard andexamine it close up, you would see thatit is actually comprised of hundreds

of pixels spaced so far apart that it isimpossible to see the image at all (anaverage 14'x48' billboard contains, onaverage, just over 150,000 pixels, actu-ally far less than a 16:9 aspect 112-inch-diagonal 6mm pixel pitch display). So,

why do these dots merge together toform a single cohesive image from faraway? The answer lies not within thespecications of the display, but in thescience of your eyes.

Cant Tell The Difference As distance increases, your eyes can

no longer tell the difference betweentwo pixels, their respective colors blendtogether and they simply merge to formone image. The amount of pixels oureyes are able to resolve or distinguishbetween decreases as we move far therfrom what were seeing. Take a lookat Figure 1: As the image gets fartheraway, the individual pixels merge to-gether to form a single crisp image.

The average human eye can onlyresolve 1/60 of a degree of this arc.So, if the distance from the display in-creases, then the arms of the angle getlonger, and the termination points ofthe arms get far ther apart. Therefore,1/60 of a degree contains more data. Inother words, as we move farther from a

display, we are decreasing the amountof individual pixels that our eyes areable to resolve, resulting in the merg-ing of all the pixels into a crisp, clear,cohesive image on the display. So, howdoes this relate to resolution, pixelpitch and pixel density?

The best resolution, pixel pitch andpixel density for the display is subjec-tive based on the size of the displayand from where it will be viewed mostoften. You should make sure that theoptimum viewing distance of the dis-play you are considering is the sameor less than the actual viewing distancefrom your display installation.

See For Yourself The concept of how your eyes merge

individual points into one cohesive im-age is similar to the way a newspaperor magazine is printed. If you pick upa newspaper and look at it from closeproximity or under a magnifying glass,

you will see that each picture is madeup of small dots. The distance from

which you read the paper allows youreyes to merge these dots together toform the picture. When you examinea magazine, the image may look evenbetter. This is because magazines oftenhave a higher concentration of dots perinch or dpi (similar to pixel density.)

Knowledge of this piece of the ap-plication addresses a number of ques-tions, including intended length ofcontent, audience interaction, intendedeffect of the content on the viewer andtolerance for pixilation of the content.

Lets go back to our two scenariosdescribed earlier. The content in Sce-nario 1 is intended to be informative,consumed in passing, quickly pro-cessed and potentially persuasive,

The smallest viewing distance and LED display specification Viewing Distance(m)

3 ~ 5 5 ~ 8 8 ~ 10 10 ~12

12 ~16

16 ~20

20 ~25

> 25

Viewing Distance (ft) 10 ~16

16 ~26

26 ~33

33 ~39

39 ~52

39 ~66

66 ~82

> 82

Pixel Pitches useddepending onpixilation tolerance

P3P4

P4.75

P4.75P5P6

P7.62

P7.62P8

P10

P10P12

P12P14P16

P16P20

P20P25

P25P31.25

Figure 2. Pixel pitch is thehorizontal and verticaldistance between thecenters of discrete LEDlamps, each composedof a red, blue and green

diode.


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July 2014 5

affecting purchasing habits of the viewer. The information conveyed ismore important than the clarity orresolution of the image. It can still beeye-catching, ashy, have an aestheticWOW! impact, but the tolerance forpixilation is likely to be greater herethan in Scenario 2. Here, the intention

of the content is to attract, entertain,capture and hold the intended audi-ence in a particular venue for an ex-tended length of time where, perhaps,they will consume food and beverages,engage in amusements or other reve-nue-producing activity and potentiallyencourage their social acquaintances to

join them to do the same. Likely, thetolerance for this audience is going tobe signicantly less than in Scenario 1.

Cost We would be remiss if we didnt

show the relationship between pixelpitch and cost. Simple cost economics

will dictate that you will pay more for ahigher volume of anything. In this case,

were talk ing about individual LEDsand LED lamps. When you drop a pixelpitch from, say 6mm to 5mm, you willconsume exponentially more pixels(read: LEDs) using the same widthand height dimensions for a display.In many applications, this becomes themost important consideration, causing

project owners to adjust expectationsand engage in value engineering or,in effect, compromising expectationsof what can be done.

Because we have shown that bestresolution, pixel pitch and pixel densityare subjective based on from where

your display will be viewed, how do you decide which display is right for your project? The better the resolu-tion, the higher the cost of a displaydue to the increased number of LEDs,so you want to make sure you purchasethe right display without getting toomuch display and overspending.

For this reason, it is important to seethe display technology for yourself andperform a visual evaluation of the in-

tended technology, and simulate theconditions under which the intendedapplication is to perform. Remember,not all LED display boards are createdequal and just because a 4mm pixelpitch looks perfect from one manufac-turer doesnt necessarily mean theyall will. In other words, if you evaluate

Brand A, dont assume Brands B, Cand D all look exactly like A. There areother specications outside the scopeof this presentation that you will wantto consider as well: for instance, bright-ness, color contrast, off-axis viewingangle, power consumption, total costof ownership and even content creationbest practices.

Brett Farley is Sales Development &Systems Manager for NanoLumens, US- based custom manufacturer of large-format LED displays. He is responsible formanaging its Inside Sales and businessdevelopment team for North America,

EMEA, South America and the ANZregion.

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