What are the Guidelines for Selecting Font Sizes For CRT Displays?

NEWSLETTER ARTICLE

A question faced by display designers around the world is what size font to use for different viewing distances. The question is a direct result of the new generation of control systems entering the market. With Windows-based control systems and improved CRTs, there are seemingly unlimited character sizes to choose from. Display designers have the option of choosing from 8 to 72 point fonts, and in some cases even more. In addition, vendors are no longer limited in CRT size; some are offering their systems with 21" monitors versus the 15" monitors of old. At least one vendor is offering an overhead projection system so that information can be viewed by anyone within a control room.

Unfortunately, control system vendors offer little in the way of guidelines for font size selection. As a result, fonts are being chosen that can cause eye strain, increase operator fatigue and raise operational error rates. The following article explores the issues of font selection and provides a few basic font selection guidelines for computer displays.

The requirements for CRT character size are generally about the same as for reading other forms of text. Since the resolution of monitors has greatly improved over those of ten or even five years ago, the character images can be as sharp on a CRT as those on a piece of paper. CRT monitors have an advantage over printed material in that contrasts can be much higher on a CRT than on paper. Increased contrasts can increase the legibility and readability of text.

In choosing a character height and font size, display designers should focus on readability rather than legibility. While a particular character size may be legible, when combined into text it may not be readable due to the size being either too large or too small. For example, a letter might be legible at 5 minutes of visual arc (which is close to the human threshold for perception), but a preferred size for readability is around 20 minutes of visual arc. When reading text, a person is not focusing on individual characters, but rather perceiving the shape of a word or group of words. If the character is too large, the person will have to make eye fixations per unit of material, thus disrupting the reading process. If the characters are too small, there may be trouble in perceiving the distinctive characteristics and shapes of the words.

To save interested parties countless hours of effort and frustration, Beville has converted the obscure references into a combined table and graph, which is presented above. The figure includes the three most likely positions from which information will be read from CRTs, (1) by an operator seated at the console, (2) by a supervisor peering over the operator's shoulder, and (3) by an operator kicked back with feet up on the console. Keeping in mind that the goal for visual arc is between 16 and 22 minutes, according to the chart, for an operator seated at a console viewing the monitor at a distance of approximately 20", an 8 point font size is an acceptable character height. For supervisors peering over the operator's shoulder at 50" from the CRT, an appropriate font size is 16 point. For operators kicked back, with their feet up on the console (65"), a 20-21 point character size is recommended.

It should be noted that the chart assumes vision corrected to 20/20. Vision that is worse than 20/20 will require increased visual arc to be legible. Uncorrected vision that is worse than 20/20 will produce eyestrain and increased error rates. Therefore, it is important that operators regularly have their vision tested and corrected to avoid eyestrain and fatigue.

Although there are numerous references for font size and character height, few have been translated from their optics origins to formats that are easy-to-use (which means a table or graph for all of those engineers out there). Probably the best reference for computer display design questions, "American National Standard for Human Factors Engineering of Visual Display Terminal Workstations", published by the Human Factors Society; ANSI/HFS 100-1988, provides guidelines that are in minutes of visual arc. The standard addresses the character height/font size issue by passages such as, "A minimum character height shall be 16 minutes of arc for tasks in which legibility is important," and "The preferred character height [for readability] for these tasks is 20 to 22 minutes of arc."

A final point to be made is that display designers need to be aware of the fact that a software font point size of 10 will not necessarily be 0.139 inches in height on the CRT when it is displayed. Computer monitors are projection systems. They rely upon numerous variables to convert an electronic signal into a visual element on a screen. Items such as the display driver and different monitor sizes make it a virtual certainty that the software font point size will not match the actual size of the character projected on a CRT. Unless the equipment vendor has previously calibrated the display to the software, the only way to be sure that a displayed character is the desired height is to physically measure the character on the screen. Therefore, screen designers need to physically measure the height of the text character on the CRTs to ensure their font sizes are acceptable.

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