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Now that we've defined the difference between legibility and readability, let's look at a number of factors that affect legibility. Legibility measures how well viewers can see the type, specifically how well they can see the differences between the shapes of the letters. I want to show you important factors which can influence legibility. An effort to improve legibility is the reason why highway typography is undergoing a redesign. A new typeface for highway signs is being rolled out nationwide.
On the left, you see the current signage known as Highway Gothic. On the right, you see the new typeface, Clearview. Let's compare individual letters. Remember that a tall X-height and open counterspaces increase legibility. Look at the O in Hellertown and see how much more space is inside on the Clearview example. Look at the E, see how much more open counterspaces are. Notice the little feet on the bottoms of the Ls, helping the I to distinguish them as Ls.
These and other factors make the new signage visible at a greater distance, more clearly visible at night, and in poor weather conditions at high speeds. This is legibility at its most critical. A lot of studies have been done that measure the ease and speed of reading longer passages of text. They've shown that serif type is slightly easier to read because of the horizontal serifs which lead the eye forward. Also, serifs provide details which help the eye differentiate between similar letters, for example, the difference between an L and an I.
For lengthy reading the design of the type style is important too. Look at all of these examples, in each one you can see that there is a large X-height and open counterspaces and the letters have a medium weight on the page. These key factors allow for ease of reading through high legibility. If a type style is heavy or has tight counterspaces, it makes it harder to read at length. Similarly if a type weight is too light on the page, the eye must work harder to read it.
Smooth and easy reading is always the goal for text type. Another factor affecting legibility is the leading or the space between the lines. Text type should have enough space between lines so that the letters aren't touching, plus a bit more. If the spaces between lines are too large, the eye has to work too hard to jump from one line to the next. Another legibility factor is the line length. If there are too many words per line, the eye has a long way to travel back in order to find the beginning of the next line.
Too few words and the eye has to switch lines frequently, which is tiring. A good rule of thumb is 52 to 70 characters per line. Another rule of thumb is that the number of characters per line is two and a half times the width of the line in picas. The size of the type is a factor in legibility too. Good sizes for text type generally range between 8 to 10 point or occasionally 12 point, depending on the typeface, the intended audience, and the content.
For example, children need a larger point size if they're learning to read and so might older adults who may have impaired vision. There are a couple of other things I want to mention about legibility. If you're working on a web project, screen type tends to have a bit of a halo around the letters making them just a bit harder to read. You might need to increase the tracking and leading slightly to counteract the effect of the light emanating from the screen. One last very important point about legibility, high contrast makes for easy reading.
Lightening or colorizing the text, or darkening or colorizing the background will make it harder to see the letterforms. Any texture or image behind the type will also impair its legibility. Studies have shown that readers experience a slightly disturbing sparkle effect when white type or light type is on a black or dark background. That is why it's not ideal for extended reading. Black letters on a white background have the most contrast and are therefore the very best for high legibility and ease of lengthy reading.
Keep these factors in mind when you're designing with the type. Your goal is to make the journey of reading as easy on the eye as possible so that reading is carried out with a minimum of effort.
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