Hdtvs
Visual Acuity Advisor - What Size HDTV or PC?
What is your visual acuity? How to design or buy an HDTV, PC or any other display device.Do you really neeed 1080p HDTV? What pixel resolution should your PC or laptop have? What font size should you use for presentation charts? For printed documents? Here is all you need to know about choosing the right resolution or font size for any given screen size and viewing distance. And lots more. A relatively simple Excel-based tool helps you make the right decisions. It is available for FREE.
What is Visual Acuity and Why Do YOU Care?
People with "sharp" eyesight read signs from far away and recognize things before those with poorer
vision. We are all familiar with the optometrist's eye chart used to classify our vision as "20/20" or "20/60" or whatever. But what does this all mean when it comes to practical things like designing or buying a digital display device, creating a blockbuster Powerpoint chart set, or, for that matter, selecting the right size sign for your business or office?
The Visual Acuity AI Advisor is an Excel-based tool that will help you answer all these questions, and more. DOWNLOAD IT FREE at http://sites.google.com/site/bigira/stuff-ira-knows/VisualAcuityAIAdvisorV1.xls?attredirects=0&d=1
After reading my explanation of what the Visual Acuity AI Advisor can do for you, please read the scientific basis for visual acuity in the final section of this Knol.
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| Displays may appear sharp and clear to you, but fuzzy to others with poor Visual Acuity. |
Character Height and Font Points
The first page of the Visual Acuity AI Advisor asks for only one input from the user, Viewing Distance. You may enter that number in Inches, Feet, Centimeters, or Meters and you will receive results in all these units.
Character Height
Using the Viewing Distance value you input, the spreadsheet calculates the recommended Character Height for reliable reading from the given distance by all people in the "normal" range.
For example, if you are posting an important notice or warning in your office or store, and you want almost everyone to be able to read it from at least twenty feet away, you should make the characters at least two inches high. Did you know that? If you are fortunate enough to have sharp eyesight, you might have thought one inch or smaller characters would be OK. If so, you would be leaving out a significant percentage of the population! You would be serving some of your customers poorly.
You can use the Visual Acuity AI Advisor to determine how big to make letters on labels for a handheld product (0.1 inch high if viewed at 11 inches), the name on your office door (1 inch high if viewed from 10 feet), or the letters on a billboard (27 inches high if viewed at 200 feet).
Font Size
What if you are creating a printed document or presention slides? What is the minimum Font Size you should use? Well, that depends upon the viewing situation.
For example, if you are using Microsoft Word (or similar word processor) and the document is to be printed and viewed from about 18 inches, you should use 12pt font. If those reading your document are willing to hold it closer, at 15 inches, you could use 10pt. For footnotes where the reader may be willing to hold it still closer, at 12 inches, you could use 8pt.
What if you are creating a PowerPoint (or similar) chart that is to be viewed on a PC or Laptop display? Well, display devices are usually specified by their diagonal measure, from one corner to the other. If you have a 15 inch display to be viewed at 18 inches, you could use 10pt.
If you are in an office situation and your presentation charts are to be viewed from a distance of twelve feet on an HDTV or projection screen, you should use a minimum of 22pt font. On the other hand, if you are preparing a presentation for a large conference room, with a ten-foot display and some peope will be as far as thirty feet away, you should use a minimum of 24pt.
There are so many variants involved that it is difficult to know if all the members of your target audience will be able to read your materials. Again, if you have sharp eyesight, and you judge your materials by your capabilities, you will be excluding many others - perhaps your boss!
What is Right for YOUR Audience?
The results given above are for the people with the poorest eyesight in a "normal" audience. Over 95% of the general population falls between "20/20" and "20/60". The Visual Acuity AI Advisor gives the sizes that should be reliably readable from the given distance even by those with poor "20/60" visual acuity. (This corresponds to the range between 1.0 arcminute to about 3.0 arcminutes or from about 0.3 milliradians to 1.0 milliradians.)
The figure below shows a typical screen capture from the first page of the Visual Acuity AI Advisor which recommends Character Height and Font Points for various common situations.
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| Character Height and Font Points - Screen capture for Viewing Distance of 10 feet. |
If you know you have an especially young audience with sharp eyesight, such as aircraft pilots, you can safely use smaller Character Height and Font Sizes. The Visual Acuity AI Advisor provides results for average eyesight ("20/40") and for sharp eyesight ("20/20"). It also notes viewing situations that may be too close to view due to characters appearing boxy or fuzzy, or situations where the characters are too far away to read.
On the other hand, if you know your audience is especially old with poor eyesight, such as retirees, or those who are "legally blind", you should increase the Character Heights and Font Sizes recommended by the Visual Acuity AI Advisor, perhaps doubling them, or more, depending upon your specific audience.
PC and HDTV Selector
If you are buying or designing a PC or HDTV, or creating content for any digital display device, will your customers or audience have a good Subjective Viewing Experience? The answer requires analysis of a complex combination of the pixel resolution of the given display device, the viewing distance, the resolution of the analog or digital source material, and, of course, the visual acuity of the members of your viewing group.
The second page of the Visual Acuity AI Advisor requires just three inputs from the user, Viewing Distance, Display Diagonal, and a letter designating the Image Resolution of the display device or of the source material, whichever is poorest. The output is the calculated Image Resolution in pixels and milliradians per pixel for different aspect ratios: 4x3 standard TV and older PCs and Laptops, or 16x9 HDTV and newer PCs and Laptops. The Visual Acuity AI Advisor also provides a description of the Subjective Viewing Experience: "OK, but won't see all available detail", "OK - JUST RIGHT!", or "May appear fuzzy or boxy".
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| Subjective Viewing Experience based on Viewing Distance, Display Size, and Image Resolution |
Say you have a 19 inch diagonal PC display with 1024 by 768 pixel resolution, and it will be viewed from a 24 inch distance. Is that suitable for a person with poor visual acuity? For someone with average acuity? For a person with sharp acuity? The above screen capture shows the answers! It will be just right for those with average or sharp vision, but those with poor visual acuity will not see all the available detail. They will have to move closer to see available detail. You can diddle with the input parameters to determine how close they will have to come, or, conversely, how much larger the Display Diagonal will have to be to satisfy them.
There are seven resolution cases you can choose from: PC or Laptop Resolution: A: High, B: Medium, C: Low; TV or HDTV Resolution: D: Analog TV, E: 720 HDTV, F: 1080 HDTV; and Movie Theaters: G: 2K Digital Movies, H: 4K.Digital Movies. Viewing Distance and Display Diagonal may be input in any units (Inches, Feet, Centimeters, or Meters).
Your Visual Acuity and Equivalent Measures
What does "20/20" mean? How does that compare to visual acuity expressed in arcminutes or milliradians? What is your personal visual acuity?
The third page of the Visual Acuity AI Advisor converts between four different visual acuity measurement systems. You can input an acuity measure in milliradians, arcminutes, or Snellen chart "20/20" terminology. Did you know that in Metric countries "6/6" is equivalent to "20/20"? The Visual Acuity AI Advisor includes Metric measures as well.
The Visual Acuity AI Advisor also includes a Snellen eye chart that you can use to determine you own visual acuity (approximation only - see your eye doctor for a more accurate value). Once you know that measure in "20/20" terminology, you can convert it to Metric or arcminute or millirdian measure for further analysis.
He Sees, She Sees and Pixel Resolution of a Digital Display Device
The fourth and final page of the Visual Acuity AI Advisor introduces you to two fictional characters who see the world in totally different and fairly extreme ways. Chances are, neither of them sees the world as you do. Yet, if your are buying or designing displays or creating display materials, you better be sure to satisfy them because they represent part of your audience or customer set.
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| "He" is at the Poorest end of the "normal" human range. "She" is at the Best end. |
The above graphic illustrates how "He sees, She sees" a presentation chart on a PC, Laptop, HDTV, or projection system in a home, office, classroom or conference room. If they are close to the screen, both will be able to read your chart fairly well, except that the 12pt font material will appear a bit fuzzy to Him. At moderate distance, She will be able to read everything, except possibly the material in 12pt font. He, on the other hand, will be able to read only the 36pt font stuff, and even that will appear fuzzy. At far distance, She will still be able to read most of the chart, but He will be totally lost.
The fourth page of the Visual Acuity AI Advisor helps you design a system (or set of charts, etc.) that will satisfy Him (and Her, of course). It requires just three inputs from the user, Height Of Display Device, Maximum Viewing Distance, and the Number of Display Pixels in the Vertical Direction. You don't even have to specify the measurement units so long as your make sure they are the same for height and distance, which emphasizes that resolution is an angular measure.
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| Definition of Display Pixel and Character Pixel. How Many Display Pixels per Character? |
In the above example, you are designing a display device that is 10 units high and will be viewed from a maximum distance of 18 units. (For example a 10" high display viewed from 18".) The display device has 720 pixels in the vertical direction. How many Display Pixels must you allocate to the characters on the display such that a person at the poorest end of the "normal" range of visual acuity will be able to reliably read it? The answer, given above, is 12 Display Pixels in the vertical direction and 7 in the Horizontal. Did you know that?
The graphic illustrates the difference between a Display Pixel and a Character Pixel. A DIsplay Pixel is the smallest dot that can be displayed by a given display device, or the smallest dot that can be printed by a printer under the control of a word processor or presentation chart application. A Character Pixel, by definition, is about 1/9th the height and 1/5th the width of an upper case character in English or similar alphabetic systems.
Scientific Basis of Visual Acuity
All About Milliradians
Small angles are often measured in milliradians (mr) rather than degrees. You may remember from trigonometry that a radian is defined simply as the arc length divided by the radius. An mr is just one-thousandth of a radian. For small angles, the arc length is equal to the straight line length, so you can determine a Visual Angle by simply dividing the Height by the Viewing Distance and multiplying the result by 1000.
A Simple Example
For example (see image below) with a Viewing Distance of 33", the young women is looking at two lines, each about 0.1" thick, that have a 0.1" Height space between them. The Visual Angle = 1000 x Height/Viewing Distance = 1000 x 0.1/33 = 0.3 mr. She can clearly see the space between the two lines, which means her Visual Acuity, at 0.3 mr, is very sharp. She is the type of driver or passenger in a car who is the first to be able to read a street sign as the car approaches it. (An optometrist would say her vision is "20/20" which means she can see at 20 feet what a person with excellent vision can see at 20 feet.)
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The middle-aged man, on the other hand, sees only a fuzzy blob under the same viewing conditions. However, when he is shown two lines, each about 0.3" thick, with a 0.3" space between them, he can reliably distinguish them. His Visual Angle = 1000 x Height/Viewing Distance = 1000 x 0.3/33 = 1.0 mr. He is the type of person who is the last to be able to read a street sign as the car approaches. (An optometrist would say his vision is "20/69" which means he can see at 20 feet what a person with excellent vision can see at 69 feet.)
Applying Your Knowledge of Visual Acuity
Everyone knows that people with the sharpest vision can see and reliably read signs at greater distances than people with the poorest vision. Generally speaking, younger people have the sharpest vision and acuity tends to decrease with age. There are also genetic variations and diseases that may affect both young and old.
Visual acuity is a way to quantify the visual ability range of "normal" people who must be served by our computer systems and other visual displays and signage designed for a general audience.
However, be aware that no visual display product can serve the total population. For example:
- Blindness. Unfortunately, some people are totally blind or "legally blind".
- Poor Vision. Some people, while not "legally blind", have such poor vision that they need special devices to see displays.
- Very Sharp Vision. Some people have such sharp vision that they may see artifacts in our displays that others do not see.
Generally, a product designed to serve a general audience must be reliably functional for at least 95% of the total population. This is called the "two-sigma" portion of the "normal distribution curve". (Mathematically, the two-sigma range is about 95.6%, but it is convenient to consider it 95%.)
In practical terms, this means that around 2-3% of the total population will have such poor visual acuity that they cannot use our computer systems without special effort and devices. For example, they may have to hold our printed reports uncomfortably close to their eyes or use magnifying lenses, or they may have to use the special features now standard in PCs to magnify or increase the contrast of selected portions of the computer display. The blind may use devices that convert text to speech, etc. At the other end of the spectrum, the 2-3% who have ultra-sharp vision may see the scan lines on a TV display or our displays may seem fuzzy or boxy, but they can always move further away and these defects will disappear.
