Journal of Audiovisual Media in Medicine 1979,2,67-70

Legibility problems in printed scientific and technical information

LINDA REYNOLDS

In recent years the Graphic Information Research Unit at the Royal College of Art has been examining the effects of poor quality printing and copying on legibility, funded by grants from the British Library Research and Development Department. Two investigations are described in detail here. These are experiments relating to the effects of image quality and show-through on legibility, with some reference to background noise.

With well printed books intended for leisure reading, the reader with normal eyesight, reading under favourable environmental conditions, rarely ex- periences any problems of legibility. With scientific and technical in- formation, however, such problems are encountered much more frequently. There are two main reasons for this. First, the specialized nature of the subject matter often dictates that the information should be printed cheaply, because of the limited market, and that it should be produced rapidIy before it is outdated. These constraints often result in poor quality typesetting and printing, with consequent impairment of the type image, and in the use of poor quality paper which may result in ‘show- through’. Second, scientific and technical information tends to be in- discriminately copied by various office copying processes, and often copies are made of copies. This can result in progressive deterioration of the image, and in the introduction of ‘background noise’ of various kinds.

Research methods

In studies relating to scientific and technical materials, it is often ap- propriate to measure the legibility of both continuous text and numerical information. Numerals have a greater visual size than lowercase letters of the same type size, and they may not, therefore, be affected in the same way by the kinds of degradation with which we were concerned in these studies. We

Linda Reynolds, MSc, Is Senior Research Fellow in the Graphic Information Research Unil, Royal -College of Art, London. This article is based on a paper presented at the IMBI Annual Conference, York, 1978.

have devised our own test materials and methods for measuring the legibility of continuous text and of numerical material.

A scanning task is used for con- tinuous text. Subjects are given a series of one-page texts, each of which is printed in such a way as to represent one of the experimental conditions under test. Each text has a list of 24 words in the margin, and these words occur in the text itself in the order in which they are listed. Subjects are asked to find as many of the target words in the text as possible, and to underline them, and they are allowed one minute in which to work on each one-page text. This method has been shown to be a relatively reliable measure of legibility, and it is convenient to administer to large groups of subjects.

The texts are semi-technical and on the subject of regional geography. They are matched carefully for style and difficulty. Wherever possible the target words which are chosen are meaningful nouns, verbs or adjectives. The task, therefore, relates very closely to the scanning of scientific and technical information for the mention of specific concepts.

To measure the legibility of numerals we have used two kinds of task, a transcription task and a visual search task. For the transcription task the material consists of columns of two, four and six digit numbers. Subjects are asked t o write the component digits - in figures - beside each number. They are allowed one minute for a column of 60 numbers of mixed sizes, and they are scored on the basis of the number of digits correctly transcribed. This task gives an indication of the speed and accuracy with which numbers might be

transcribed from tabular or other numerical material, but it has the disadvantage that n o differences in legibility are detectable until subjects are reading more slowly than they can write. For this reason, a visual search task has been used in later experiments. In this case the test material consists of 40 blocks of 45 digits each, divided into four, five and six digit numbers. Beside each block of numbers is a two-digit sequence which subjects are asked to find and underline within the block. They are allowed one minute for each set of 40 blocks and are scored on the number of two-digit sequences correctly underlined. In the preparation of both types of numerical material, the frequency of occurrence of individual digits was carefully controlled to prevent any bias from the effects of adjacent digits on one another.

Image quality

The first of this series of studies was concerned primarily with the effects of office copying processes on legibility, though the kinds of degradation of the image and background which we in- vestigated can also be brought about in a number of other ways. Office copying processes are especially detrimental to legibility when the equipment is not properly adjusted, or when copies are made from copies, instead of from the original document. The exact form of the degradation will depend, to some extent, on the nature of the copying process. In the case of electrostatic copying in particular, the effects are usually thinning-down or thickening-up of the type image, and the introduction of background noise in the form of an irregular scattering of very fine or relatively coarse dots. In this ex- periment, therefore, we examined the effects of different levels of thinning- down and thickening-up of the type image, in combination with different levels of intensity of a background random dot pattern.

The legibility of both texts and

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numerals (using the transcription task) was measured. Four different texts and four different columns of numerals were filmset in nine on ten point type, and each piece of material was set in four typefaces representative of markedly different type styles. Baskerville 169 was used as a n example of an old style seriffed face, Rockwell 371 as an almost monoline face with slab serifs, Times New Roman 327 as a modern seriffed face, and Univers Medium 689 as a sans serif face. The settings were then degraded photographically under carefully controlled conditions to produce seven levels of image quality. Level 4 was representative of 'normal' printing, levels 3, 2 and 1 were progressively thinned-down, and levels 5 , 6 and 7 were progressively thickened-up (see Figure 1). The stroke thickness of the characters was decreased and increased by the same absolute amounts for each typeface, which resulted, of course, in different percentage increases and decreases. The degraded texts and numerals were printed by offset litho, and four different levels of intensity of background noise were then overprinted onto them from film positives of a specially prepared random dot pattern. Level 1 was noise-free, and the intensity increased linearly from levels 2 to 4.

Two groups of 64 subjects took part in the experiment, one group being presented with image quality levels 1 to 4 and the other with levels 4 to 7. Each subject saw four of the possible 112 combinations of image quality, background noise and typeface. His four conditions included an example of each level of image quality, each level of background noise, each typeface, each text and each column of numerals.

An analysis of variance showed that the factor having the greatest effect on legibility was the level of image quality. Typeface was the next most important factor, and the level of background noise was the least important, but all three effects were highly significant. Figures 2a and 2b illustrate the overall effect of image quality on the legibility of the texts and numerals. It may be seen that the two curves are similar in shape, and that there is an abrupt fall- off in legibility as the image is thinned- down and a more gradual fall-off as it is thickened-up. The plateau-top to the curve for the numerical material is almost certainly the result of the relative insensitivity of the transcription task at the better levels of image quality.

120, 110- 1W- w- 80- 70 - to- 50 - 40- 30-

l e v e l 4 along along along along level 7

Figure 1. Photographic enlargements of texts and numerals in each typeface a t image quality levels 1, 4 and 7.

Figure 2. Effect of image quality on performance. Figure 3. lmage quality and typeface.

i .Texts Wan scare 14 - 13 - 12 - 11 - 10 - 9 - 8- 7 -

b. Numerals I

-\. fl Level 01 image quality

There was a marked interaction between image quality and typeface, and this is shown in Figures 3a and 3b. On the texts, it may be seen that Baskerville was the least legible face when thinned-down, but remained relatively legible when thickened-up; Rockwell, on the other hand, was one of the more legible faces when thinned- down, but was the least legible when

T r u 1 I I 1 2 3 4 5 6 7

b. Numerals Mean scare

1 2 3 4 5 6 Level of image quality

thickened-up. Times and Univers remained relatively legible throughout the range of degradation. The pattern for the numerical material was similar, except that there was a more marked distinction between the four typefaces a t level 1, with Times performing less well than Rockwell and Univers.

These differences between the typefaces are partly explained by the

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Table 1. Characteristics of the fou r typefaces tested.

set width Typeface point size texts numerals- (mml (mm) (points) x height

9 0.240 0.210 1.25 2.05 9.00 7.20 Baskervil le Rock we1 I 9 0.270 0.290 1.43 2.22 9.25 6.47

9 0.280 0.280 1.44 2.11 8.75 6.08 Times Univers 9 0.270 0.300 1.51 2.1 5 10.50 6.95

mm stroke thickness x ht cap Z set width

data given in Table 1. Baskerville. for example, has a relatively narrow stroke width and a relatively small x-height, and this helps to explain why it per- forms so badly when thinned-down. On the other hand, it also has a relatively large set-width to height ratio, and this would help to prevent adjacent letters from running together when the type is thickened-up. Figure 1 shows enlarge- ments of each typeface at levels 1 , 4 and 7. It may be seen that at level 1, Baskerville has almost disappeared as a result of its thin strokes and small x- height. Rockwell, however, with its thicker, , almost monoline strokes, strong serifs and greater height remains relatively legible. It is worth noting that the Times numerals have suffered more than the text, which explains the poor performance of Times in relation to Rockwell and Univers at level 1. At level 7, Baskerville retains some distinction between the letters as a result of its relatively large set-width, whereas in Rockwell the strong serifs have a tendency to fuse and join the letters, and the counters have a tendency to fill in.

These results suggest that for material which is likely to be copied in- discriminately, typefaces such as Times and Univers are among the most suitable of those tested. If thinning- down is likely to occur during copying, typefaces such as Baskerville should be avoided and if thickening-up is likely, faces such as Rockwell should be rejected in favour of a face with relatively thin strokes, open counters and a generous set-width to x-height ratio. In general, in adjusting copying machines it is better to err towards thickening-up rather than thinning- down because legibility was less im- paired by any given increase in stroke thickness than it was by a corresponding decrease.

The background noise data are shown in Figures 4a and 4b. For the texts, it may be seen that the effects on legibility were greatest in combination with a thinned-down type image. It would also

appear that the effects of increasing the intensity of background noise once it had been introduced were relatively slight on thinned-down text and almost negligible on thickened-up text. For the numerical material it would seem that background noise had very little effect at either extreme of image degradation. This was probably due, at least in part, to the greater visual size of the numerals in relation to the size and frequency of the dots. The- results suggest, therefore, that although background noise of the kind tested here does have a significant effect on legibility, particularly if the type image is thinned-down, the in- teraction between image quality and typeface is more important.

Show-through

Show-through is a somewhat different kind of background noise which results from attempts ti, economize on paper costs and reduce bulk by printing on relatively lightweight papers which are not completely opaque. Shadows from printing on the reverse side of each page and/or on the next page will reduce the

Figure 4. Image quality and noise.

a.1exts Noise level

( 0 , , 1 1 2 3 4 5 6 1

b. Numerals Mean sore 110

loo - -

70-

to- M - I 8 1 I I I

1 2 3 4 5 6 1 Level of image quality

d 0

contrast between the type image and the paper on the page being read, and are therefore likely to impair legibility. The shadows are often accentuated by electrostatic copying processes. It was felt, therefore, that objective assessment of the effects of show-through was required, so that publishers and printers could make more informed decisions with respect to paper economies.

Accordingly, four papers of different opacities were tested under five dif- ferent printing conditions. These conditions, illustrated in Figure 5 , were

Figure 5. The five printing conditions tested; (a) page one only, (b) pages one and two aligned, (c) pages one and three aligned, (d) pages one and two unaligned and (e) pages one and three unaligned.

a

n

d e

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Table 2. Paper opacity, weight and thickness.

paper 1 2 3 4

opacity 3.70 3.1 3 2.1 7 1.20 substance (gmlm’) 100.70 71.60 52.70 86.40 caliper(lfl000 inch) 4.90 3.70 2.90 2.90

Table 3. Paper reflectance on page one of each fold under printing conditions A-E.

paper 1 2 3 4

printing condition A .87 .81 .81 .74 B .75 .66 .58 .14 C .77 .70 .64 .26 D .73 .67 .54 .I 4 E .74 .69 .64 .28

selected to investigate the effects of double- versus single-sided printing, and of alignment versus non-alignment of lines on successive pages. Condition A was the control condition in which no show-through was possible. Conditions B and D were representative of double- sided printing and conditions C and E of single-sided printing. In conditions B and C the lines on successive pages were aligned, and in conditions D and E they were unaligned. Each piece of test material was an A3 sheet folded to give four A4 ‘pages’. On page one of each fold was a text on which subjects were required to perform a scanning task as a measure of legibility.

These printing conditions were repeated on the four different papers, whose characteristics are shown in Table 2. It may be seen that the weight and thickness of the papers are not reliable indicators of their opacity. For each combination of paper and printing condition, reflectance measurements were taken on page one in areas backed by type on page two or page three. On paper one, the most opaque, the printing condition made very little difference to the reflectance, but on paper fdur there was a marked reduction in reflectance under con- ditions B - E (see Table 3).

Mean score 18

I7

16

15

14

13

12

11

10

9

8

1

6

1W W 80 70 M 50 40 30 20 10 I Percent ref lktance

Figure 6. Mean scores for each paper and printing condition, plotted against per cent reflectance on page one in areas backed by type on pages two or three.

Eighty subjects each saw four of the twenty possible experimental con- ditions. (Four different texts were therefore used.) Analysis of variance showed that the effects of paper

opacity, single- or double-sidedness and alignment or non-alignment were all significant statistically, paper opacity having the greatest effect and the alignment variable the least. In Figure 6, the scores are shown

plotted ag@nst the reflectance values given in Table 3. It may be seen that there was relatively little difference between scores on papers one, two and three, or between scores for the five printing conditions (apart from some anomalous values for printing condition E). On paper four, however, scores on the two double-sided conditions (B and D) were significantly lower than those on the two single-sided conditions (C and E). Further, when printing was double-sided, the unaligned condition (D) was significantly worse than the aligned condition (B). This confirms the value of the printers’ convention of aligning lines on successive pages. Show-through occurring between the Lines of type actually being read presumably interferes with the per- ception of ascenders and descenders and thus with letter and word recognition.

The results suggest, therefore, that where there is a possibility of show- through occurring from material printed on the back of the page being read or on the next sheet, legibility is Likely to be seriously impaired if paper opacity is less than about 2.00. Under these conditions, double-sided printing and non-alignment of lines on suc- cessive pages will markedly reduce legibility. If papers of low opacity must be used, it is better, therefore, to print on one side of the paper only and to ensure that lines on successive pages are aligned.

Further reading Spencer, H., Reynolds, L. and Coe, B., (1975). The effects of image degradation and background ’noise’ on the legibility of text and numerals in four different typefaces. Graphic Information Research Unit, Royal College of Art. (Revised 1977).

Spencer, H., Reynolds, L . and Coe, B., (1975). The effects of show-through on the legibility of printed text. Graphic Information Research Unit, Royal College of Art. (1977).

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