LCD text legibility distances

March 11, 2005 6:33 PM

I am trying to track down information on the distance from an LCD screen where text of a given size would still be legible.

I realise that eye sight, font size, lighting and lots of other factors play into this, but I'm hoping someone knows of some general guidelines they can link to.

For some additional context: I am trying to calculate the distance that text announcements on a particular public LCD screen system could be expected to be read from. An image of the screen is here. The announcement text in the lower, red, strip of the display is the part I'm interested in. Assuming that the font is 3-4 inches high, from what distance would this be legible.

posted by pasd to Technology (6 answers total)

Why not try a practical experiment? Get several volunteers, a mockup of (or an actual screen) and find out in a practical manner. That should give you a range of reasonable answers. You already know the screen size, basic mathematical ratios should give you the text size. You could scale it down to a smaller screen, and then multiply the distance appropriately, as well. Why use theoretical calculations when a practical test is so easy?
posted by pjern at 8:18 PM on March 11, 2005

Here's Clear Channel's guidelines for billboard design. They say 50-100 ft for 2-4 inch-high letters.
posted by O9scar at 9:28 PM on March 11, 2005

To calculate an upper limit on the distance at which your text will be legible you can extrapolate from the principle used by the Snellen eye chart. Someone with perfect 20/20 vision in excellent lighting reading a chart with maximal contrast is just able to decipher a letter that subtends a visual angle of 5 minutes of arc (0.0833 degrees) at the eye.  For example, the big 20/200 letter E in the Snellen chart is about 3.5 inches high. This height represents 5 minutes of subtended arc at a distance of 200 feet. Applying a little trig will allow you to compute an estimate for the upper limit for the legibility distance for your text for any letter height:



For example, if the visual angle (a) is 0.0833 degrees and the letter height (h) is 3 inches then the legibility distance (d) is 172 feet. Similarly, with a letter height of 4 inches the legibility distance is 229 feet. Let me stress again that this distance is the absolute maximum legibility distance under optimum conditions, a distance that you will never achieve given your application.

In practice the eyesight of your viewers, lighting conditions, contrast of your LCD display and other factors will result in a lower viewing distance in order for your text to be legible. For example, a Toronto study of letter height for street name signs found that for signs with 10 cm (~4 in.) letters the sign legibility distance was 45 meters (~150 ft.). Naturally you shouldn't expect your LCD display to match even these numbers, since the design of street signs make them more legible then any LCD display.

Something else to consider is that lighting conditions and glare that have no deleterious effects upon young and middle-aged observers can have significant impact on the sign legibility distance of older observers. Here is one such study I found with Google.

One last note, in my Google search I found that the U.S. Department of Transportation has a set of recommendations for LED signage on buses that might be of possible assistance to you. While they don't suggest a legibility distance, they do provide recommendations for such things as letter width-to-height ratio and stroke width-to height ratio.
posted by RichardP at 11:52 PM on March 11, 2005 [4 favorites]

[Oops, I just noticed that I left out part of my answer]

I previously mentioned that letters that subtend a visual angle of 5 minutes of arc are just legible to those with normal vision, but here are some additional data points: a letter that subtends a visual angle of 13.3 minutes of arc is just legible (or better) to 95% of people aged between 18 and 79 with vision corrected [1], a visual angle of 26 minutes is reliably conspicuous, even in peripheral vision, to those with normal vision [1], and a visual angle of 60 minutes is sufficient to be legible to the visually impared with low vision [2]. These visual angles correspond to legibility distances for 4" text of 229' (5 min), 86' (13.3 min), 44' (26 min), and 19' (60 min) respectively.

You might also find it helpful to relate sizes to your own computer monitor. Suppose, for instance, you use a 17" computer monitor, with your eyes about 2 feet from the screen and a resolution of 1280x1024 pixels. This means that the angular resolution of your monitor is about 1.5 arc minutes per pixel. This means that a letter that subtends a visual angle of 18 minutes of arc looks approximately the same as a 12 pixel tall letter does when you are sitting at your computer viewing the letter on your monitor. This can be a handy relation. Suppose you would like your display text to be as legible at 50 feet as 12 pixel tall text is on your own monitor. Since that is approximately 18 minutes of arc, we can solve for h given a = 18/60 degrees and d = 50 feet, leading to a recommendation of 3.1 inches.
posted by RichardP at 4:29 AM on March 12, 2005

just taking richard's numbers and giving them in a different way: if we use 95% number as an absolute minimum then your letters need to be over an inch high for every 20 feet of viewing distance (so, to be visible at 100 feet, you need at the every least 5 inch high letters).
posted by andrew cooke at 4:49 AM on March 12, 2005

Response by poster: These answers were just what I was hoping for, thanks. Good solid references for me to cite in the report I was putting together.
posted by pasd at 9:45 AM on March 15, 2005