Conferences even though it uses filtered light from the broad band ZnS:Mn emission peak to achieve red, green and orange. In the area of full colour, Komatsu used a white light phosphor-filtered for RGB and claimed 23 (R), 37 (G) and 6 (B) cd m -2. Interesting results were reported by Dr Clive Thomas of Bradford University on memory effects in low Mn-doped films and by Dr Reiner Mauch of Stuttgart University, on blue EL from ZnS/SrS:Ce multilayered phosphors.

Field-emission displays

Field-emission displays (FEDs) were seen as the way forward for the US displays industry by one of the keynote speakers. Therefore, a brief background on the technology of this promising field will be given.

In 1966 Spindt (SRI) published a theoretical paper on field-emitting arrays (FEAs). The first working FEAs were demonstrated in 1968. Utilization in displays began in 1983 by LETI and in 1986 SRI demonstrated a 3.5 in display with 100 × 300 lines. By 1987, ComTech had bought the rights to FEAs from SRI and in 1989, Coloray had teamed up with Micron Technology

of Boise, Idaho to work on FEDs. There are many other interested parties in the US and in Europe. However, real interest in FEDs based on FEAs was due to the work at LETI by Meyer. The monochrome display (based on ZnO:Zn phosphor) has been shown at many conferences operating at full video frame rates.

In this conference, most papers were about the performance of the electron-emitting cathode materi- als. An invited paper presented by Dr A Lowe (IBM-UK) gave an outline of the expected perfor- mance using available data on cathodes, phosphors etc. The other papers were either on cold cathodes or hot cathodes for the traditional CRT. The phosphor aspect was covered in one paper from Kasei Optonix Ltd. Undoubtedly, FEDs and FEAs will gain further promi- nence at next year's SID confer- ence.

Exhibition

One hundred and twelve compa- nies exhibited in parallel with the symposium. The items on show included manufacturing equipment, optoelectrical measurement

systems, CRT glass bulbs and guns, high-voltage monolithic driver ICs, filters, transparent conducting glass as well as displays based on LCDs, EL, plasma, CRTs, LEDs etc. Most of these could be seen in a range of sizes, colours and complexities. OIS showed an excellent AMLC display with saturated colours and wide viewing angle. In the brief space available here, it is not possible to give a full report on the items on exhibit.

At the SID Annual Luncheon, Tod Machover gave an informed talk on 'Hyperinstruments - in a musical context'. Real musical instruments could be heard at this year's special event, an evening at the world-famous Boston Pops.

SID 1993

SID 1993 will be held in Seattle, Washington, USA on 16-21 May 1993 at the Washington State Convention Center.

Surfit S Chadha, University of Greenwich, School of Biological and

Chemical Sciences, Wellington Street,

London SE18 6PF, UK

The Human Factors Perspective Human factors - - or 'usability' - - is now a key feature in the success of information systems. The SID conference addressed various aspects of display usability and they are covered here under four key areas: colour coding and percep- tion, colour calibration and repro- duction, display human factors, and image quality.

Colour coding and perception

The great majority of displays used with computers are colour displays, and the software that runs on these machines invariably uses colour. It is rare for colour to be used only for cosmetic reasons; most of the time, colour is used to highlight information such as text, or to code

icons on a graphical user-interface. How should colour be used to

code information? It is well known that under some conditions an observer can find a target stimulus in a display rapidly and effortlessly ('pop out'), but under other condi- tions an active, extended search must be completed. Nagy (Wright State University, Ohio) described his work on choosing colours that will pop out. It is generally accepted that there are certain colours that are unique, primary or prototypical (for example, red, green, yellow and blue). However, these colours appear to pop out from a display simply because they are well separated in colour space, and not because they are unique. It is clear that more work needs to be

done before a general algorithm is available. Jubis (Defence Institute, Toronto) described his work on the effectiveness of colour and shape coding. His experiments demon- strated the benefits to be had from culour coding, especially when there are a number of targets (four or more) present.

Some problems in display design can be effectively solved by apply- ing a model of human colour vision. For example, Mulligan and Ahumada (NASA Ames, California) showed that digital half. tone images optimized to a simple model of human vision were superior to competing images. Inoue and Kosugi (NTT Human Interface Labs, Japan) described a method of image processing, based

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Conferences on a model of human colour vision that enhanced the perceived quality of colour pictures.

Colour calibration and reproduction

The advent of graphical user-inter- faces has prompted a need for What You See Is What You Get (WYSIWYG) colour. Today's computer users are no longer satis- fied by close or acceptable colour reproduction: they want exact colour reproduction. But how does one maintain the colours selected when the image is sent to a lower quality device or a hardcopy device with a more limited palette?

Mintzer, Goertzel and Thompson (IBM TJ Watson, New York) addressed this question. They showed that digital haiftoning techniques can be used to gain accurate colour reproduction on displays with limited palettes. In particular, for a 64K-colour palette. the half-toning is a fairly simple dithering technique. For a 256K- colour palette, they use a sophisti- cated error diffusion technique.

Lianza (Sequel Imaging, New Hampshire) described an unusual technique that can be used to arrivc at a "match' between the image on a colour printer and the image as viewed on a CRT, The technique is unusual in that it uses a linguistic approach to colour matching. This is valuable because observers are often able to articulate problems in colour systems that are difficult to translate into modifications to an algorithm. The linguistic descrip- tion is then linked to a physical measurement system using fuzzy logic (an abstraction based on the degree of membership to a particu- lar class of objects).

Colour imaging and reproduction is a rapidly developing area and the market is expected to grow rapidly. Hence, users now have a require- ment for colour fidelity, but do not wish to become colour scientists in order to obtain good results from their colour printers. Therefore there is a growing body of work that aims to satisfy this need.

The standard approach to colour

WYSIWYG is to achieve "device independence'. We know from colorimetry that two chromatic stimuli will match to a colour- normal observer if the retinal responses are equal. In practice, this can be achieved by ensuring that the two stimuli share the same CIE tristimulus values. The process of colour reproduction therefore seems a trivial one: measure the tristimulus values of the original and then produce these in the reproduction (hence, "device independence'). Berns (Rochester, New York) pointed out that this approach is fallacious, because tristimulus values alone are insufficient to facilitate acceptable colour reproduction without additional information about the viewing environment.

Display human factors

The purpose of displays is to convey information. In order to do this effectively with human observers, the capabilities of users need to be considered at a number of levels. These range from basic sensory and motor characteristics up through increasingly complex processing stages at perceptual and cognitive levels. This issue is of importance from the design of a character in a text-based system. through the design of a graphical user-interface, to the design of control rooms.

Arditi and Azueta (The Light- house inc., New York) presented a display system to help in the design of display systems. The computer graphics tool ( implemented on a Silicon Graphics 4D workstation) enables designers to visualize both the image and the retinal projection of it. Suppose a designer was producing a flight cockpit: he or she will need to address fundamental questions in human vision such as where the various dials and screens should be located. For example, at any one instant it might be useful to know what information will be projected on the user's blind spots. By incorporating a simple model of visual optics, this tool enables designers to make the visualizations without needing to go through

complex analytical procedures, Andre (NASA Ames, California)

specifically addressed the isstie of cockpit display layout analysis. The central question is: should displays be co-located by task or frequency of use'? The results showed that displays should be located to minimize their distance from other task-related displays, even al the expense of being moved lurthcr away from the primary fixation area of the display panel.

Image quality

Like art. image quality is hard to define, but we all know what it is when we see it. The image quality of visual displays appears to depend on sharpness, contrast, brightness. colour and absence of artefacts (for example, flickcr and jitter). Image quality is important because uscr~, frequently complain of ~,isual fatigue after using visual displays: and although there is no question thal visual displays arc causing, the discomfort, there does appear to be an interaction between the imaoe quality of the display, the user's task and visual fatigue. Moreover, there is an undisputed need for tools for assessing the image quality of displays, printers, image-coding methods and other display ,wstetn components.

Image quality can be evaluated either by technology-centred measures or user-centred rneasures. The technology-centred approach (a) identifies the key features of images from the point of view of human vision and (b) measures them with available instrumenta- tion. The user-centred approach evaluates t)erformancc directly bx asking users to carry out a certain task and measuring speed, errors and discomfort.

Technology-centred measures are quick, cheap and reliable, but traditional approaches such as MTFA have a number of disadvan- tages. For example, two displays can have the same subjective image quality yet have quite different MTFA measures, and vice versa. Barten (Barren Consultancy. Netherlands) presented results on

112 Displays

Conferences the square-root integral (SQRI) metric that aims to overcome a number of these problems. When subjective measures of image quality are compared with the SQRI, the correlation accounts for virtually all (97%) the variance (the corresponding value for MTFA was only 48%).

However, observers naturally vary in the degree to which they find different display artefacts objectionable, casting doubt on the usefulness of technology-centred measures, especially when display artefacts are above threshold. Moreover, the key measurements

of the image may be missing, and there may be interactions between the measured factors. Ahumada and Null (NASA Ames, California) described a multidi- mensional scaling technique that could be used to identify dimen- sions of image quality that are differentially weighted by observers. In addition, Vernon, Kistner and Bearce (MITRE Corp, Mass) described an approach to measuring image quality that used real users carrying out a real task (a legibility test). Certainly it will be user-centred measures such as these that offer the best opportu-

nity to evaluate novel display technologies: the current set of technology-centred measures is tuned very much to CRT technol- ogy and the artefacts associated with this technology. They offer doubtful promise to novel display technologies, or to the evaluation of hard copy or other display system components.

David Travis Human Factors Division

BT Laboratories Martlesham Heath

Ipswich UK

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