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Readability of Small Displays: What is the Ideal Character Size? (3rd in series)

Collection of small display devicesWhen creating a display on a small device, how large or small should your characters be?  The size of alphanumeric characters affects the readability of information on the display.

The size of an alphanumeric character is determined by three features:

  1. stroke width: distance across a stroke line used to form an individual character
  2. character width: distance across an individual character from one side to another
  3. character height: full vertical distance between the top and bottom elements of a character

Stroke width

Stroke width should be equal for all characters of equal height on a display.  Stroke width-to-height ratio is the ratio of the thickness of the stroke used to form an individual character and the measured height of the character.  It is recommended that stroke width-to-height ratio be 1:6 to 1:8 for black text on a white background under adequate lighting and 1:8 to 1:10 for white text on a black background.

The expected lighting conditions should also be considered when making decisions about stroke width.  For low levels of illumination or low background contrast, printed letters should be bold face with a low stroke width-to-height ratio of around 1:5. As illumination levels decrease, thick letters become more readable than thin ones. For very bright letters, ratios could be reduced to 1:12 to 1:20. For black letters on very bright backgrounds, very thick strokes are recommended.

Character width

Character width-to-height ratio is the ratio of the width of an individual alphanumeric character and the measured height of the character.  The width-to-height ratio for an alphanumeric character should be close to 3:5. For drum-type counters, number width-to-height ratios should range from 3:5 to 1:1 (preferred) except for the number “1″. The width-to-height ratio for characters in fixed column presentations should range from 7:10 to 9:10.

The number “4″ should be one stroke width wider than the other characters. The number “1″ and the letter “I” should be one stroke width wide. The letters “m” and “w” should be 20% wider than the other letters. Stroke width-to-height ratio for these characters should be 1:6 to 1:8.

For proportionally spaced presentations, the width-to-height ratio for some characters, such as capital “M” and “W”, should be close to 1:1.

Character height

Recommended character height is dependent on the task, viewing distance, the lighting conditions, the display resolution, the display contrast, and any glare treatments.  Because so many attributes are important, visual angle is typically used because it takes many of them into account at one time.  The visual angle is addressed by estimating the visual arc (measured in minutes) expected to be subtended by the characters on the display.  Calculating visual angle requires an estimate of the viewing distance along with the planned size of the character.

On black and white displays of alphanumeric characters, when reading speed is important, character height should be a minimum of 16 minutes of arc, with 20 to 22 minutes of arc being the preferred height, and 24 minutes of arc the recommended maximum. When reading speed is not important, characters may be as small as 10 minutes of arc or as large as 45 minutes of arc.  Alphanumeric characters on color displays should subtend at least 21 minutes of arc with 30 minutes preferred.

As with all design, deciding on the character size to use to have adequate readability of the information on the display is an iterative process with many trade-offs.  It’s important to continue to consider all aspects of the display design and not focus only on one in particular.  I will describe more details about how to address the other readability considerations in future posts.

Please share your experiences and trade-offs addressing different types and sizes of alphanumeric characters in displays.

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