Vision and Displays

I am trying to locate information on alternative display devices (not the standard CRT). I am trying to design a read only display product which is as comfortable to read as a book. It will be portable and light. I know that many such products exists so I am trying to find some examples. Could you please give me some references?

It sounds like your best bet is to use a flat panel like an active matrix LCD display. These are bright, large and available in colour. Alternative sources of information you might want to try are the Society for Information Display's Annual Proceedings and back issues of the journal 'Displays'. There is also a review of non-CRT displays in my book, Effective Color Displays (Academic Press, 1991).

The main alternative to CRTs is the Liquid Crystal Display (LCD). In the next five years, LCDs will have the lion's share of the displays market.

• Liquid Crystal Displays (LCD)
  The early problems of the LCDs have been overcome to a large degree by more advanced materials and improved driving techniques. Colour has been added by providing individual organic based filters screen printed under each pixel. Most recent LCDs are based on super twisted nematic (STN), active matrix drive employing thin film transistor and ferroelectrics (with memory) amongst others. Full colour displays operating at video rates and with HDTV aspect ratio (16:9) are now available. Image quality is now so good that we will soon find active matrix LCDs replacing the CRT on your desktop.

Other flat panel displays include:

• Plasma Display Panels (PDPs)
  The principle of operation of the plasma display panels (PDPS) is the same as in a neon light or a fluorescent tube. Unlike most other flat panel technologies, size is not a problem for the PDPs and displays with 1m diagonal and over 4 million pixels have been fabricated. Hence these appear as obvious candidates for satisfying those markets where large sized displays are required, e.g. for HDTV applications. Though available in limited numbers, the full colour displays are still very expensive. From the perspective of ergonomics, the main problem with PDPs is that they lack high contrast: colour displays look slightly washed out.



• Electroluminescent Displays (ELDs)
  In electroluminescent displays (ELDs), the phosphor layer is sandwiched between two conducting electrodes, one of which is transparent. The most successful of the ELDs is based on thin film structure driven by AC fields. At present, 18" diagonal screens with 1024 x 864 lines are available. Three colour VGA displays have recently been demonstrated and with their wide viewing angle, very fast response times and crisp image quality, the ELDs have a promising future.



• Vacuum Fluorescent Displays (VFDs)
  Vacuum fluorescent displays (VFDS) are the mainly green emitting devices used for point of sales (POS), car clocks, on entertainment equipment for VDUs, many home appliances such as microwaves, and so on. Other colours are available including red and orange. The modus operandi is very similar to a CRT but special phosphors are used which operate at very much lower voltages, for example ~10 eV compared with over 15 keV.



• Light Emitting Diodes (LEDs)
  These semiconductor solid state displays are based on electron and hole recombination in a p-n junction with the colour of the emitted light being a property of the band gap of the material. Mainly used for indicators (often replacing incandescent lamps), such displays are available in red, green, orange, yellow and more recently in blue. LEDs are extremely bright and hence find applications in avionic displays as well as on hi-fi equipment amongst others. The number of LEDs manufactured runs into millions per annum.



• Field Emission Displays (FEDs)
  This is a newcomer from the electronic display industry which promises to retain most of the useful characteristics of the CRT but operate at much lower voltage like the VFDs. Like the ELDs, LCDs and PDPs, the field emission displays (FEDs) are lightweight, flat and thin. Significant progress has been made in fabricating both monochrome and multicolour displays based on this principle in the last few years.

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I am designing a computer interface which will encourage users to read directly from a digital display rather than first printing documents on paper. Part of the problem I am concerned with is the quality of the display and part is the ergonomics of the casing. Are there digital displays which are flexible? Are there any which can be rolled or folded or even just warped slightly, without being damaged?

Currently there are no displays with the flexible qualities that you need. The easiest way to mock something up might be to project the image onto a physical piece of paper, but that will be of limited use to you. You might be interested in a paper by Pierre Wellner called 'Interacting with paper on the digital desk' in Communications of the ACM, 36 number 7 July 1993, pp86-97. Paper isn't dealt with in the imaginative way you're thinking of, but it does address many of the same functional questions.
DT

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Why, when I look at a VDU and occilate a colored object such as pen or knitting needle in front of the screen, does the color of the object disappear?

What an excellent question! I had never noticed this before, but I've just carried out the test and can confirm your findings. My observation is that the pen retains its colour at the end of the oscillation but just appears black in between the oscillations.

I would suggest the following. The VDU acts like a stroboscopic source at the frequency of the refresh rate of the monitor (say 72 Hz). This means that when the pen is moving, the eye has just 1/72 sec to identify the colour. We know from visual psychophysics that flashes of light lose their perceived colour at such brief presentations, because of the time constants of the colour opponent pathways. So why doesn't the pen disappear? This is because the fast luminance pathway is still operational.

This argument is supported by the fact that the colour of the pen can be seen at the end of the oscillations, when the frequency is slower.
DT

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Could you please spare the time to tell me where I may find information specifically related to law/regulations/ISO and screen resolutions. e.g. prolonged reading of very small fonts ruin your eyesight, as will non-contrasting colours ... are there guidelines or regulations? Are there any specifics, such as 800x600 is recommended for 14 inch monitors?

There is no evidence that display screens, or anything that you put on them, damage your vision.

However, in the UK there are certain minimum requirements for display screens. The best review is on-line: see Tom Stewart's book "An employer's guide to the display screen regulations". This doesn't give you the fine detail (such as character heights); for this you should read the international standard ISO 9241-3 "VDTs used for office tasks - Ergonomic requirements".
DT

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I am a Software Engineer and writer, which means I sit in front of a display (Active Matrix LCD) most of the day. I want to reduce eye strain. Before LCDs (and good color monitors for that matter) came out, I remember hearing that green text on a black background was the "best" for your eyes on a monochrome monitor. Then it was amber text on a black background. I would like to know if this is also true with color displays (especially LCDs), or if there is another color scheme that causes the least amount of eye strain. Microsoft Word 6.0, for example, allows you to choose between black text on white background, and white text on blue background - do they know something I don't? In the years that I've been working on computers, I've found that black on white is somewhat blinding because the white background is "beaming" me. I've used green on black for years, but that was mainly on a color monitor, and not on an LCD, which is what I'll be using from now on. What do you recommend?

I've done a lot of research into just this question, and the answer turns out to be really quite simple. The key factor in reading from colour displays is luminance contrast (aka brightness). What's crucial is making sure that the text and background colours have enough luminance contrast to be discriminable. So yellow on white is a bad choice, whereas black on white is a good choice. The actual colour matters little until you get down to small luminance differences (so small that no-one in their right mind would set a display up in this fashion).

However, there are a few complications depending on the type of display technology used and the user himself. Some people (such as yourself) find white backgrounds uncomfortable for continuous viewing. Similarly, other people find black text on a white background uncomfortable. It's as if too much contrast can be just as bad as too little. Because of the pixel arrangements on certain display technologies, some colours appear less sharp than others. (Blue text is a special case, appearing fuzzy because of the eye's chromatic aberration).

My advice would be to ask your employer to buy you a good quality LCD for when you're on the move (eg TFT Active Matrix) and when you are at base (eg at home or in the office) connect it up to an external CRT display. The visual quality of commercially available LCDs still lags behind equivalent CRTs, so for extended use you should go for a CRT. While you're at it, insist on an external keyboard too!
DT

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I am documenting methods and procedures for presentation on my company's intranet. The established font color for this site is "blue." From former interface and design classes I have taken I understand blue on-line text causes extreme eye fatigue. Am I correct in this understanding? Can you provide factual documentation to prove this to my intranet team to better serve our users?

In terms of visual performance, the key metric for reading from displays is luminance contrast. This means almost any colour can be used for the font so long as it provides adequate contrast with the background colour.

"Almost any colour" needs some qualification. Saturated blue is a bad choice for text because the chromatic aberration of the human eye blurs shorter wavelength light rendering the text hard to read. This could result in visual symptoms commonly described as "eyestrain". Blue text, however, can be OK if the text colour is not saturated; a pale blue, for example, can be read perfectly on a dark background.

However, there are issues other than visual performance to consider. When reading from displays you have to have a very good reason not to use black text on a light background. The high contrast this affords is essential to quick reading, and it allows colour to be used only for hyperlinks. Using a blue font throughout will conflict with the de facto standard of using blue text for hyperlinks. You will need to change the hyperlink colour which will lead to consistency problems. There is no a priori reason to use blue for unvisited links and red for visited links, but now the convention has been learnt by millions of users, it's a cavalier designer who ignores it.

If blue is needed to brand your pages, perhaps you could use a common design feature on every page instead.
DT

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After reading about your work at CERN I thought you might be able to help me. Here is the situation: workers are seated at a workstation where they compare original art work to an image on a computer screen. The color on the screen must match the color of the art work (if it does not these workers correct the computer image). Recently, the idea of raising the level of light coming from the screen to 29 foot-lamberts was presented. This level would be similar to that used on the art work. Would a level this high cause negative long term effects on the workers eyes? By matching the two levels of light will workers be able to see a higher definition of color on the screen? Will this exacerbate the normal "wear" on the screen especially in the area of color accuracy?

The amount of light emitted from a colour CRT is not sufficient to cause any negative, long-term effects. If anything, the increased light level will probably help the task (although it will change the colour gamut).

However, I'm astonished this has worked for you so far! To achieve WYSIWYG colour, the paper sample must be viewed under exactly the same viewing conditions as the screen sample. Remember that this doesn't mean just the overhead lighting: the DISPLAY has a particular white point which is almost certainly different from the colour temperature of the overhead illumination. The paper sample should be placed in light box with a specified illuminant (eg D65), and the screen sample should be viewed on a colour-calibrated display set at the same white point.

The way you describe the current arrangement, I'll make a small bet that your designers are unable to match every colour because of the complex interactions that are being introduced. It's also a bit meaningless because if someone adjusts the brightness or contrast all the colour matches fail. If you have a calibrated system you can avoid all this. See the special issue of "Displays", Vol 16, No. 4 which tells you how to do it all.

As regards the screen wear issue, the answer is Yes.
DT

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I work in a Police control room, and recently we have been provided with a 4th VDU to sit in front of. This is a computer mapping system on a very nice 17" monitor. However the system is set so that the display is 1024 resolution, and the refresh rate is only 60Hz. I was under the impression it had to be around 72-75Hz, this being when the eye can no longer detect flicker. As as it stands the display flickers. I have submitted a report querying this but the IT Dept state I am wrong. What do you suggest?

The Display Screen Regulations state that "The image on the screen should be stable, with no flickering or other forms of instability." The corresponding HSE guidance for display users states that displays should be free from flicker. It probably sounds astonishing, but the difficulty is that there is no accepted criterion for stating that a display is "flicker free"! It depends on a number of factors including:

• The size of the display and your distance from it (because we are more sensitive to flicker in the periphery of our vision)



• Individual differences in flicker perception (increasing the refresh rate will always solve the problem, but for some people even 75Hz is not enough. There are wide variations in flicker perception amongst the population, and if you are one of the "unlucky" ones you may find that you can still see flicker even above 80Hz!)



• The brightness of the screen (screens with a predominantly white background will appear to flicker more than the same screens with a dark background).

In practice, a display is said to be "flicker free" if appears stable to at least 90% of the users. The increasing use of graphical user interfaces and dark characters on lighter background displays (positive presentation) has meant that more users notice flicker.

If you are classified as a user your employer has a legal requirement to assess your workstation. At System Concepts we have carried out thousands of these assessments, so please contact me if you take this route.

In the short term you may want to try a practical solution. Change the screen background to black and/or turn down the brightness of the display. Alternatively, wear tinted glasses. This should alleviate the problem, but I doubt it will remove entirely.
DT

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We are using 13 inch (33cm) diagonal screen. A suggestion was made to move to a 17 inch or larger screen to allow for ease of use of the double window. This would allow the operator to move easily between documents. Do you have information or opinion on advantages of monitor screen size?

The advantages of monitor size of course depend on the task. From what you have said, it sounds like your users need to move between electronic documents to do their job. The user can do this with smaller screens but then he/she may need to retain some of the information in memory, or carry out a lot of window management tasks. So from an information processing point of view, the larger screen may make sense for this task. However, we also need to consider the increased physical bulk on the desk. If users are already short of desk space and they need to use paper documents, then putting a larger monitor in place could feasibly have a detrimental effect. This can be ameliorated by using the new range of flat panel displays, but these are a lot more expensive than their same-sized counterpart.

Sorry to sit on the fence on this one - I need to know more about the task to make a truly informed decision!
DT