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Sign Readability

Just because you can read, doesn’t mean you can read every sign



Too bad ST no longer publishes the Ugly Sign Contest, because the lowest-price-gets-the-job mentality that proliferates when the economy isn’t flourishing, especially in the sign business, often produces illegible signs. Yes, every business needs signs, but not necessarily the most expensive ones. This thinking generates poorly designed and executed signs that won‘t perform the sign’s main purpose: to transmit a message.

This month, I‘ll detour a bit from pure sign-lighting technology and focus on sign legibility. I’ll skip the ontology discussion (or, what makes a sign a sign).

Contrast and spacing

Physiology says the reading process “resembles conscious realization of all typical details of the lettering presented, and processing the content received, by comparison, with values stored in the brain to obtain a reproducible statement.” (from G. Gut’s Handbook of Luminous Advertising, 1974). Hmmm. Here’s a practical example: On a hot summer day, who’s never been thirsty after having seen a sign spelling “B-E-E-R”?

The two main parameters that affect legibility are contrast and spacing. Contrast accentuates differences between design elements. White characters on white paper, and black on black, aren’t readable. Contrast can be achieved through brightness (black on grey) or color (red on green), or a combination. So, the strongest contrast is black on yellow (Table 1). This applies to reflected light (printed matter, billboards, etc.) and illuminated signage.


The second parameter, dimensions, is even more important. The perfect human eye can distinguish two points of optimal contrast, spaced nearly 1 in. apart, at roughly 400 ft. If the points are closer, the eye assumes they’re a single object. Thus, if a sign must be readable 300 to 400 ft. away, all graphical “features” must be spaced at least 1 in. apart.

Also, the trained brain doesn’t read every word, character by character. Human beings are trained to recognize groups of six to 10 characters at once and associate the word with the best match. The brain can be easily fooled – I often miss typos repeatedly, like “Sings of the Times” instead of “Signs of the Times.”

Consequently, short, known messages are more quickly and easily recognized than long ones, or those with unfamiliar character combinations. In the past, because of this principle, corporate-identity designers used acronyms that should’ve been easier to remember, but, from my experience, often weren’t, like “IXYZ Technologies”. Some readers also might remember the public scorn of an Egyptian-symbol as the logo for “the artist formerly known as Prince”.

Corporate-identity signage, based on a logo or brand name, can slightly alter the lettering’s character/feature shape and spacing to optimize recognition from a distance, especially for illuminated signage. Therefore, let‘s analyze the elements of character design.

The stroke width of many fonts isn’t consistent. For example, holding a flat-cut feather pen at different angles varied the stroke width in the old form of Fraktur. Such fonts are difficult to read. Signage requires instant character recognition, which, in turn, necessitates constant-width-stroke fonts. Historically, the geometric ratio of the character height of capital and lowercase characters is roughly 7:5 to 7:3 for easy recognition.

Modern fonts tend to equalize the height of capital and lowercase characters, which might be artistically honorable, but doesn’t necessarily contribute to sign legibility. Boldfaced fonts, which imply a width-to-height ratio of approximately 1, can be recognized much farther away than condensed fonts, with a width-to-height ratio of 0.5, with the same character height.


In general, serif fonts’ varying stroke widths don’t facilitate readability. However, some serif fonts, such as Courier, simplify character shapes and enhance the baseline, which leads the eye along the line of text.

Because our reading mechanism compares words, as graphic features with shapes, stored in our brains, the height-to-width ratio also affects readability. As a result, physiologists have created a so-called “fidelity factor,” or how easily a character of a (non-exotic) font will be recognized (Table 2). The lowest fidelity factor in a word or phrase dictates the minimum character height. Thus, take the lowest fidelity factor of the whole phrase to calculate the minimum character height. In the word “logisitics,” for example, the “g” would have the lowest fidelity factor. Therefore, the minimum character height would be calculated based on the g’s fidelity factor.

Table 2: Fidelity Factors for Capital Block Letters, Sans-Serif Fonts. The higher the value, the better the recognition.

A = 1.3

B = 0.85

C = 1.07


D = 1.03

E = 1.00

F = 1.04

G = 0.92

H = 0.92

I = 1.41

J = 1.21

K = 1.06

L = 1.19

M = 1.13

N = 1.00

O = 1.06

P = 1.04

Q = 1.06

R = 0.97

S = 0.95

T = 1.15

U = 1.07

V = 1.08

W = 1.13

X = 1.08

Y = 1.04

Z = 1.01

For example, we can roughly estimate the minimum character height using this formula:

Height [inches] = distance [ft.]/(600 x fidelity factor x width:height ratio x contrast factor)

Herein, the contrast factor varies between 2, for the best yellow to black, and 0.2, for a poor yellow on red or brown on camouflage, according Table 1.

The necessary character width is calculated from the character height, which depends on each letter’s font design. The stroke width for optimal readability is approximately one-seventh the character height.


“Kerning” adjusts the spacing between certain letter combinations to eliminate excess space and improve the text’s aesthetic appearance – handwriting often overlaps characters. When a font is created, each character includes space around it, so letters don‘t run into each other when displayed or printed. Think of this space as an invisible box around each character. Some character pairs, such as A and V, or T and O, look better if the boxes overlap slightly, so that the characters are closer, but still don’t touch. Thus, words are more readable – well-kerned letters appear to form more subtly identifiable shapes. Also, more text can be placed in a given amount of space.

A monospaced font, in which the sum of the character width and space normally equals the character height, doesn’t use kerning and isn’t easy to read. For difficult light environments, however, wider, but constant, spacing, independent of the character width, can improve legibility.

It’s the designer’s job to play with all the parameters to obtain optimum legibility for the situation.

Many sign locations limit the available width and height of the requested copy – usually, words need to be trimmed to achieve text legibility at a distance. The responsible sign salesperson must tell the customer about the upcoming problems and suggest either two lines of copy, a different font or a different color combination, etc. The salesperson should show how each change improves the original request.

How lighting affects legibility

For illuminated signage, the same design principles still apply, despite such variables as blurring and environmental light levels. Extremely high contrast, where a sign can be too bright, and consequently blurred and enlarged in some areas, but too dark in others, negatively affects illuminated-signage legibility. For example, high-output LEDs often seem blurred and enlarged when they’re directly viewed.

Because of this physiological effect, which originates in the human eye, sign-surface brightness must not exceed 10 to 100 times the brightness of its surroundings to achieve maximum viewing sharpness. To avoid blurring, increase the character height (especially for such critical letters as B, E, F, G, H, R and S) to increase the eye’s viewing angle. The other option — if not already in use– is to recess the tubes several inches into the channel so the return guides the light into a path that narrows the overall light-emission angle. Table 3 provides rough estimates of surface brightnesses that shouldn’t lead to blurring effects.

Certain areas are known for overcast skies or long bouts of bad weather, where mist, rain and fog can block the path of light from the sign location to the viewer. The most effective antidote to overcast weather is the spectrum of clear, red neon on a dark background/ environment.

From experience, many clients just want a bright sign and don’t really care about such legibility issues upfront. But, later, they’ll be wondering why the sign looks like a huge blob of screaming light. So, when clients inquire about designing a sign, supply them with good information, but, above all, show them comparisons that show various light sources, contrasts and letter spacings. Hopefully, after having seen a well-designed sign, your client will chose the design that’s not only technically correct but also cost effective – and return for future jobs.



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