The readability of text

The coloured items in an interface almost always include some text and it is therefore necessary to consider how the selection of colours for the text and its background affects

readability30. There has been extensive research on the factors influencing readability, primarily motivated by both ergonomic and safety concerns. Readability is critical to the usability of an interface. It is therefore important to determine the dominant factors affecting readability of textual interface items, to enable the algorithmic assessment of the readability of text in an automatically derived colour scheme.

It seems reasonable to assume that if text is a very different colour from its back- ground (i.e. there is a large colour space separation) then the text would be easily readable. However, this is not always the case, as can readily be demonstrated with saturated green text on a red background (fig. 2.28) which is difficult to read due to an unpleasant shimmering effect. The reason for this is detailed in section 2.2.6.

It is a not good idea not to use complementary colours that do not have a difference in lightness for the colour of text and its background, as this can impair readability.

Figure 2.28: The use of complementary colours for text and its background can impair readability.

For an item to be visible against its background, there must be contrast, either chro- matic contrast, lightness contrast, or both. However, for text (as is evident from fig. 2.28), chromatic contrast alone is not a reliable predictor of readability. As will be seen, lightness contrast gives more reliable indication.

Text contrast and its polarity

Light–dark contrast may be obtained in two ways: the text may be displayed as dark text on a light background (as on paper), or light text on a dark background, as was common in the early text-only visual display terminals that used green, orange or white text on a dark screen background. Dark text on a light background has become known as a positive polarity text, and light text on a dark background is known as negative polarity text. It is necessary to take care when comparing results, as the nomenclature is inconsistent. Dark text is sometimes referred to as positive text or text with a positive polarity (Fukuzumi and Hayashi, 1989; Buchner and Baumgartner, 2007; Fukuzumi et al., 1998) and negative polarity (Snyder et al., 1995; Scharff and Ahumada, 2005).

The contrast can be measured using either the luminance modulation, or thelumi- nance contrast ratio:

Lm = Ll−Ld Ll+Ld (2.12) Lc = Ll Ld (2.13) 30

where:

Ll − lightness of the lighter item.

Ld − lightness of the darker item.

Lm − luminance modulation.

Lc − luminance contrast ratio.

A small factor corresponding to ambient flare31 can be added to each ofLl and Ld to

ensure that neither is zero.

The research on whether the polarity of text makes a difference is mixed. Sev- eral studies have found a significant advantage in using text with a positive polarity (dark text) and others have found little difference. Wang and Chen (2000) found that increasing contrast increases performance, but polarity was not significant. However, this trial did not test the readability of actual text. Instead a graphic was used: the Landolt-C visual acuity test symbol (like a circular small letter c). This was done to remove the actual text and its characteristics as a possible source of bias. If other factors (e.g. word shape recognition) affect readability and success is not solely related to perception and visual acuity, the lack of real text may explain the discrepancy in the results. For both polarities, increasing the luminance contrast correlates well with increasing readability (Lin, 2003, 2005; Fukuzumi et al., 1998).

The results of the recent study by Buchner and Baumgartner (2007) are unequivocal in showing that text with a positive polarity (dark text on a light background) is more readable under all conditions of ambient lighting, and the chromaticity contrast alone is not sufficient for comfortably readable text. This agrees with earlier studies by Snyder et al. (1995) and Scharff and Ahumada (2005). Gradisar et al. (2007), while noting interactions with chromatic contrast, also found that, overall, text with a positive polarity contrast is more readable.

The importance of luminance contrast is well summarised by Ojanpaa and Nasanen (2003): “reading rates decreased with decreasing luminance contrast. Thus, moderate or even high colour contrast does not guarantee quick visual perception, if the luminance contrast between characters and background is small. . . . Therefore, in user interfaces, good visibility of alphanumeric information requires clear luminance (brightness) dif- ference between foreground and background.”.

The contrast required for readability

A contrast ratio of 5.6 (Lm = 0.7) is regarded as acceptable by ISO 9241 for colour

displays32 according to Smith (1996), cited in Bangor (1998). The distinguishability of fine detail is primarily dependent on lightness difference, as noted by Klassen et al. (1998, p27): “we know that chromatic differences are more important in large regions than in small features and high frequency regions, where lightness differences are more important”. The W3 consortium, responsible for the World Wide Web standards,

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Flare – light reflected from the surface of the display

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reflect this finding in the web accessibility guidelines where a contrast ratio of 5:1 is recommended for small text. This is relaxed to 3:1 for large (18pt or greater) text.

In experiments under controlled conditions (Zuffi et al., 2006) and later as a web- based experiment using a much larger sample size (Zuffi et al., 2007), it was found that a difference in CIE lightness difference of ∆L of 25–3033 was sufficient for readability, irrespective of polarity. If Ld = 10 then Ll = 40 would correspond to a luminance

modulation, Lm = 0.6 or a contrast ratio of Lc = 4 : 1, comparable to the W3 recom-

mendations. These studies are particularly significant here as they are oriented towards the readability of text on web-pages and the experiments included text down to 8pt in size. In printed documents, 8pt is quite readable but not common, with 10–12pt being more usual. However, the limited screen real-estate available on web sites and the desire of web site developers to have as much information visible “above the fold” (without requiring users to scroll), has lead to different design criteria. On web sites, as confirmed by Zuffi et al., the use of 8pt text is quite common.

Readers with impaired vision can find the readability of smaller fonts to be especially difficult. Bangor (1998) recommends a contrast ratio of at least 18:1 for text smaller than 18pt in size, especially in the case of negative polarity text.

Informal experiments by the author, using medium saturation complementary hues at 30◦ hue increments in CIELAB space found an average ∆L of 48 was necessary for easy reading of 8pt type for both polarities. The test values were centred aroundL= 50 with the achromatic colourings requiring slightly greater difference than all rotations with complementary chromatic contrast. The measured values give contrast measure of Lc = 2.85 andLm = 0.48, in moderate agreement with the W3 values. The values

are somewhat larger than those of Zuffi et al. The tests were conducted on a 20 inch monitor at its native resolution of 1600×1200, resulting in slightly higher dots-per-inch (101) than the 96dpi of Zuffi et al.. The higher resolution results in slightly smaller characters and as noted by Bangor (1998), readability drops off very quickly with text below 18 minutes of arc. As the smallest text in the text was approximately 5 minutes of arc, it is not surprising that slightly enhanced contrast was found desirable.

It might be expected that the font used to render the text could affect readability. However, Garcia and Caldera (1996) report that the font used has less effect than text colour on the time taken to read text.

It is clear that with small text at moderate lightness contrast, positive polarity text will be more readable, but given sufficient lightness contrast, the readability of text of either polarity and any hue can be guaranteed. This is useful as it means that lightness difference alone can be used to ensure that the text in an automatically derived colour scheme will be readable.

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