Submitted as part of the requrements for an SB in Cognitive Science, May 1987. Gently overseen by Muriel Cooper, of MIT's Media Laboratory, with occasional input by Susan Carey and Whitman Richards, of MIT's Cognitive Science Department. (Very slightly edited).
One Aspect of CRT Displays That
Doesn't Make a Difference
Studies have shown that people read from computer
displays more slowly than they read from paper. While
several theories have been offered to explain why, none
has stood up to testing. One currently popular technique
for enhancing CRT "readability" is known as "anti-
aliasing." In this experiment I recreate the problem that
anti-aliasing is supposed to correct in hardcopy, so its
negative effects can be measured without the other
inherent difficulties found in computer displays.
Introduction
As computers invade the work place, they bring with them
Video Display Terminals - VDTs. While display technology is
proceeding apace, at present the "D" in VDT usually means a
Cathode Ray Tube - CRT. In the popular press CRTs have been
linked to everything from eyestrain to miscarriage, however a
serious body of inquiry into the cognitive effects of the
devices did not exist until recently. A 1984 study (Gould and
Grischkowsky, 1984) seemed to indicate most of the complaints
about CRTs are unprovable, however during the course of the
study one effect of the devices was strongly shown: People
read 20-30% more slowly from CRT displays than from paper.
Several subsequent studies have failed to pinpoint the
reasons for the difference. My experiment likewise failed to
find a sole cause for the CRT gap, but did show that at least
one pet theory is probably false. A similar experiment (Gould
et al 2/17/86) showed somewhat more ambiguous results.
Past Studies - what the answer isn't
CRTs differ from paper in many respects - Each of the
following summarized experiments (all published by Gould et al
2/17/86) considered one or more aspects of that difference.
1 - Fixed orientation - 12 subjects in a proofreading task
were shown to read equally fast (within margins of error) from
paper fixed horizontally and paper fixed vertically. Both were
read faster than CRT by 11 of the 12. [note - an earlier
experiment (Tinker 1963 as quoted by Gould) showed a
significant advantage of paper at a 45 degree angle to
horizontal vs. paper at horizontal]
2 - Aspect Ratio - 18 subjects were asked to either
proofread or read for comprehension (9 of each) three
articles. Articles were presented either on CRT, on paper in
normal orientation, or on paper rotated so as to be read "the
long way." Subjects in the comprehension task were found to
read paper "the long way" more slowly than normal paper,
however in both tasks paper of either orientation was read
significantly faster than CRT. The difference between paper
normal and paper rotated was statistically significant in the
comprehension task, however, so aspect ratio may be a
co-factor in inferior CRT reading speed.
3 - Experience - Experiment 1 above was done with 6 novice
and six experienced computer users. Experience subjects did
have a slightly smaller CRT gap, but the gap still existed.
The subjects in experiment 2 were all experienced computer
operators and still showed a significant speed deficit between
paper and CRT.
4 - Character spacing - Because character spacing on a CRT
is greater than on paper, the eye must move a longer distance
in order to traverse the same number of letters per fixation.
[Eye fixations are made on a set number of characters, not a
set visual angle - (Morrison and Rayner 1981, as quoted by
Gould)]. This could increase reading time. However, research
on office colleagues via "surprise" measurement by Gould et al
showed people sit further away from CRTs than from paper,
which largely compensates for the greater character spacing.
(A more rigorous experiment by the same researchers did find
more eye fixations per equivalent line of text on CRT vs.
paper).
5 - "Flicker" - 9 subjects were asked to proofread 3
10-page articles, from either CRT, paper, or photographs of a
CRT screen. Photos were read significantly slower than paper,
though slightly faster than actual CRT.
6 - Polarity (white on black vs. black on white) - Ten
experienced users proofread five different 5-page articles, in
each of five different display formats - CRT (green on black),
and black and white photographs of the CRT screen in both
positive and negative and in two sizes. Reading rates from the
CRT were found to be slightly slower than from the
photographs, but the difference was not significant. No
difference was found between photos of different polarities.
7 - Font - Sixteen subjects proofread 6 pages on paper and
6 pages on a Tektronix storage tube which displayed the
identical font as that used on the paper (but which otherwise
emulated a CRT). Layout, line width, and average words per
page were identical between the two display mediums. Polarity
however was inverse for the tube. Subjects read significantly
faster from the paper.
Past Studies - Combinations that help
While it would appear that very little can be done to
identify the relevant difference between paper and CRT much
less rectify the situation, Gould et al (8/21/86) have found
that when text is matched for font, polarity, size, color and
layout, reading speeds on CRT approach that of paper. In
addition to the above variables, Gould experimented with the
use of anti-aliased character sets on CRT. His results were
inconclusive.
Anti-aliasing (and Purpose)
Typically, CRT displays are made up of raster lines or dots
(pixels). The use of this somewhat too large basic unit of
display produces "jaggies" - rather than smooth outlines,
characters on CRT displays have a "staircase" profile. The
effect is called "aliasing." Aliasing has been considered as
one of the causes for the CRT gap. Recently, a technique
called "anti-aliasing" has come much into vogue among computer
graphic artists. "Anti-aliasing" eliminates the jaggies by
adding grey scale pixels to the outlines of the displayed
character. The edges of the character appear to fade gradually
into the background, and the human visual system provides a
straight edge to compensate. While the resulting text is
aesthetically more appealing, no hard evidence exists to show
it has any effect on visual performance. My experiment
replicates the "aliasing" effect on paper, so it may be
compared to normal text (which will stand in for anti-aliased)
without the additional handicaps of a CRT.
Methodology
Four pieces of text from the verbal section of the
Graduate Record Exams (GREs), each 37 lines in length, were
used. Each piece of text was typeset in Triumvirate (a
san-serif font similar to that found on computer displays) 12
point with 14 point leading, on a 7 inch line, ragged right
(so as to avoid spacing changes between letters). The copy was
printed on high density photopaper on a Compugraphic 8600, at
1500 dots per inch. That typeset copy was then re-processed on
an electronic scanner normally used for producing halftones (a
system by which photographs are translated into dots large
enough to be printed on an offset press), and re- printed on
the 8600. The result was a digitally oversampled (ie
"aliased") copy of the originally typeset material. The page
layout, contrast and luminance, line widths, type size, and
letter outlines were all identical to the original copy,
however the scanned copy was apparently at a resolution of 50
dpi (the resolution of a poor quality CRT) not 1500.
Booklets of alternating types of copy (reproduced on a high
density xerox) along with xeroxed copies of the GRE questions
were made up. Two pieces of text were designated for practice,
and presented first in the booklet. All eight permutations of
straight and jaggy, first or second, were represented. (see
appendix).
Eighteen subjects, 14 male and 4 female, in their late
teens to late twenties participated in the experiment in
return for a Ghirardelli Chocolate Square and the continued
friendship of the experimenter. All had several years of
previous experience using CRT displays. All had vision at or
near 20/20 when corrected, though glasses wearers were given
the option of reading with or without glasses, whichever was
considered more comfortable. Subjects were seated in an
unused conference room in the MIT Media Laboratory across the
table from the experimenter, and asked to take a comfortable
position. They then were instructed to choose a booklet at
random from the collated pile, informed that they were going
to take a section of the GREs, and asked to read for
comprehension but as quickly as possible, and not to turn back
to previously read material. Subjects were allowed to start at
will. The experimenter used a digital watch equipped with a
chronometer and lap-counter to measure the time between the
turning of pages. Two tests were discarded, one due to
mis-collation and one to an interruption during testing.
Results
No appreciable difference in time or accuracy was
discovered for aliased vs. smooth text. Several subjects
offered opinions as to the lack of "readability" of the
aliased text, however.
Discussion
While more rigorous testing conditions may have uncovered a
difference (it is clear from the range of the data that
factors other than base reading speed had come into play), it
is doubtful that anti-aliasing is the solution to the CRT
problem. Similar experiments with other forms of degraded
paper text (Gould, of course, 2/17/86) have also shown that
degradation of paper fonts do not appreciably slow reading
speed, though subjective reaction by subjects is usually
negative.
An experiment which used a visual performance task that did
not require the reading of text (Booth et al) indicates the
CRT gap may be due to overall resolution. The experiment was
performed alongside a mental-rotation experiment, in order to
determine if an anti-aliased display would affect performance
on that task. Subject were asked to discriminate between 2
three-dimensional objects displayed in perspective on screen.
The objects were made up of either 10 cubes or 9 cubes, and
presented in a variety of orientations. While anti-aliasing
improved speed in the task for low and medium resolution
displays (below 171*171 pixels), at higher resolutions no
anti-aliasing effects were found.
Another as yet un-written-up experiment by Gould et al
shows that high quality high resolution fonts on CRT read
almost as fast as the same size and font on paper regardless
of anti-aliasing (210 words per minute vs. 206 and 203 for
anti-aliased and aliased respectively). In this latest
experiment the text on paper and screen were made as identical
in size and font as possible, and all had the same polarity.
It is likely that as CRTs increase in resolution and therefore
in their ability to display high quality fonts, their
readability will increase regardless of aliasing.
BIBLIOGRAPHY
Booth, K.S., Bryden, M.P., Cowan, W.B., Morgan, M.F. and Plante,
B.L.(1987). On the Parameters of Human Visual Performance/An
Investigation of the Benefits of Antialiasing. Proceedings,
CHI+GI '87 pp.13-19.
Gould, John D., and Grischkowsky, Nancy (1982). Doing the Same
Work with Hard Copy and with Cathode-Ray (CRT) Computer
Terminals. Human Factors, 26(3), 323-337
Gould, John D., Alfaro, L., Barnes, V., Finn, R., Grischkowsky,
N., Minuto, A. (2/17/86). Reading is Slower From CRT Displays
than from Paper: Some Experiments that Fail to Explain Why.
IBM Research Report as yet unpublished.
Gould, John D., Alfaro, L., Finn, R., Haupt, B., Minuto, A.
(8/21/86). Reading from CRT Displays Can Be as Fast as Reading
from Paper. IBM Research Report as yet unpublished.
Gould et al. Effect of Anti-Aliasing Itself on Proofreading.
Preliminary write-up.