Minimum Character Height vs. Viewing Distance

When designing an LCD or an LED character, one of the questions that comes up often is how large to make a digit or character so that it can be easily read from a given distance. While gathering a few pieces of empirical data and making an educated guess (i.e. "winging it") is sometimes acceptable, in applications where this is important, a computational solution is desirable. 

An ISO standard exists that gives some general guidelines, however I'll try reduce the arguments to their one page essence. (If you have the time and inclination for such things, it's ISO 9241/3.) The one line answer to the question, according to 9241/3, is that the acceptable viewing distance is 155 to 175 times the character height.  These figures, however, need some explanation.

The first step is to make a decision as to the criticality of viewing the display in your application. Our guidelines are very arbitrary, so make your own determination as it applies to how your device will be used. They are:

1. Non-Critical - Most consumer applications, handheld devices, thermostats

2. Critical - Shop floor, gas pumps, aviation, automotive

3. Mission Critical - Battlefield, oil rigs, other hostile environments

We will now use the figure below to show the critical parameters. The angle  a is the "subtended angle" that is formed between your eye and the top and bottom of the character. a is usually expressed in minutes of arc (60 arc minutes in a degree.)
  

Rather extensive research by industrial psychologists gives the following recommendations:

• Acceptable a for non-critical tasks - 10' (10 arc minutes)

• Minimum a  when legibility is critical -16' (16 arc minutes)

So we have: Character Height  = D * tan a (All dimensions in inches)

The simplest way to compute character height is to take the viewing distance required, convert it to inches, and multiply by:

• .00290 (tangent of 10/60) for normal situations

• .00465 (tangent of 16/60) for critical situations

• Acceptable a for black and white displays -16' minimum, 20' preferred

• Acceptable a for color displays - 21' minimum, 30' preferred.

So if your display will be viewed in the smoke and haze of the battlefield, our multiplication factors become:

• .00290 (min) / .00581 (preferred) for black and white displays

• .00610 (min) / .00872 (preferred) for color displays

I hope this helps with your display designs. As usual, please call us at (440)232-8590 and talk to one of our applications specialists if you have any questions about this or any other product design issues.