The Extended Legibility Index

It is evaluated according to the opening angle, which should guarantee the lowest possible degree of optical distortion of the text.

• Score 0:
  The book tends to stay closed and, in order to read it, you have to hold it with effort using both hands.
• Score 0.5:
  The book tends to close by itself, but it stays easily open with one hand and it is possible to read it without necessarily holding it with both hands.
• Score 0.1:
  When resting on the table, the book stays open without much effort.

Gutter margins are essential to avoid, as far as possible, optical distortion of the text near the page binding. Regardless of the type of binding, 20 mm are more than sufficient to reduce this disturbance.

• Score 0:
  Gutter margins are less than or equal to 17 mm.
• Score 0.5:
  Gutter margins are between 17 and 20 mm.
• Score 1:
  Gutter margins are greater than or equal to 20 mm.

The choice of type family partly helps legibility, which is why it must be made with great care. The assessment refers only to the typeface used for the main text. Dyslexia-friendly fonts are NOT the ultimate solution!

• Score 0:
  Calligraphic typefaces;
  Condensed typefaces;
  Typefaces with basic kerning lower than 20 thousandths of an em.
• Score 0.5:
  Serif typefaces;
  Sans serif typefaces.
• Score 0.1:
  Serif typefaces that comply with the Il1 rule and with basic kerning increased up to 20 thousandths of an em;
  Sans serif typefaces that comply with the Il1 rule, but with basic kerning increased up to 50 thousandths of an em.

With the same point size, the legibility of a typeface is positively influenced by the height of its x‑height. The size of the typeface will not be assessed in terms of point size, but in relation to the x‑height measured in mm. The x‑height can be measured in Adobe InDesign using the script getXHeight or with a calliper on already printed books.

• Score 0:
  x‑height of serif and sans serif typefaces less than or equal to 1.5 mm.
• Score 0.5:
  x‑height of serif and sans serif typefaces between 1.5 mm and 1.6 mm.
• Score 0.1:
  x‑height of serif and sans serif typefaces greater than 1.6 mm.

There is no standard leading value that works for all typefaces. The percentage of optimal leading must be measured in relation to the x‑height and ascenders of the typeface used. To ensure legibility in serif and sans serif typefaces, it is enough to add between 5% and 15% to the basic optimal line spacing. The optimal line spacing for dyslexia-friendly typefaces is indicated in their specimen. The proportion of optimal leading can be calculated on the website tipometria.chialab.io (ISIA Urbino, Chialab and Zanichelli Editore).

• Score 0:
  Optimal leading < −5% (too tight);
  Optimal leading > +15% (too loose).
• Score 0.5:
  Optimal leading between −5% and +5%.
• Score 0.1:
  Optimal leading between +5% and +15%;
  Optimal leading of Dyslexia-friendly typefaces as reported in their specimen.

In justified text, gaps may sometimes be created between words and letters that are perceived more than the filled areas, especially if justification is poorly handled. For this reason, left alignment is the most suitable in terms of legibility. In this case, the adoption of hyphenation is irrelevant for an adult readership (habit and self‑compensation processes come into play).

• Score 0:
  Justified without hyphenation;
  Right-aligned, with or without hyphenation.
• Score 0.5:
  Justified with hyphenation.
• Score 0.1:
  Left‑aligned, with or without hyphenation.

Papers that reflect light can create visual disturbance during reading, especially in lean‑back reading books. Very porous uncoated papers absorb a lot of ink and tend to generate artefacts during offset printing, such as missing parts of letters or haloes and smudges that reduce text legibility.

• Score 0:
  Glossy or matte coated paper with high brightness;
  Very porous uncoated paper.
• Score 0.5:
  Natural uncoated paper with medium porosity.
• Score 0.1:
  Matte coated paper with low brightness;
  Slightly porous uncoated paper.

Some kinds of paper, especially low‑weight stocks, when exposed to natural light and with the book open, can make the text of the previous or following page show through. This leads to a drastic reduction in legibility and accentuates perceptual crowding in people with dyslexia.

• Score 0:
  With the book open, the text on the previous page is clearly readable on the current page.
• Score 0.5:
  With the book open, the text on the previous page appears as a halo and is not readable.
• Score 0.1:
  With the book open, the text on the previous page is imperceptible.

The colour of the paper affects text legibility, especially when the contrast with the background becomes less evident. Papers with particular patterns may look elegant for graphic elements with little text or for titles, but they are not suitable for the main text of a book. Warm‑toned papers or those tending towards light yellow are the most suitable for text legibility.

• Score 0:
  Coloured paper or paper with patterns that make the contrast with the main text less evident.
• Score 0.5:
  White paper;
  Cool‑toned paper;
  Coloured paper in very pale tones.
• Score 0.1:
  Warm‑toned paper;
  Paper tending towards light yellow;
  ‘Aralda’ paper.

Printing artefacts are difficult to predict, and most of them can only be seen once printing is completed. Paper type, as well as the quality and maintenance of the printing machinery, have a strong impact on the outcome of a graphic product. The artefact over which the graphic designer has the greatest control is the choice of text colour, which must be a normal black (C 00; M 00; Y 00; K 100) and NEVER a registration black (C 100; M 100; Y 100; K 100).

• Score 0:
  Massive presence of artefacts such as missing portions of letters, ink smudges, and misregistration.
• Score 0.5:
  Sporadic presence of artefacts involving missing portions of letters.
• Score 0.1:
  Absence of artefacts.