Display Screens

People spending hours in front of a VDU everyday at work often complain  of headaches, blurred vision and generally uncomfortable or tired eyes. Those who refer to a screen when necessary are far less likely to complain than intensive users.

From the outset, it is worth pointing out that as far as we can tell, VDUs do not pose a serious health risk to the eye (or the person as a whole for that matter). Nothing dangerous comes out of computer screens, though they do emit radiation- they are designed to, it’s called light! Recently there have been increasing concerns among vision scientists about the “blue light” or HEV light risk – more information can be read here.

Cathode Ray Screens (the bulky, deep ones) do produce X-rays- the way to make X-rays is to fire fast moving electrons at a piece of metal- these electrons produced by the 3 “guns” at the rear are supposed to hit the phosphors coating the back of the screen, emitting visible light. Right behind the screen is the shadow mask or aperture grill- a metallic grid which ensures each gun can only “light” the correct colour emitting phosphor- again there are three, one for red, green and blue. Any electron hitting the metallic screen will produce an X-ray, but virtually all of these get absorbed by the thick leaded glass of the screen. Exposure to ionising radiation is thus thought to be minimal- little more than the background exposure we all receive every day, but this was the reason some pregnant (mainly American) VDU users took to wearing lead lined vests when at work!

CRTs require high voltages internally to work, and this produces an electromagnetic field around them. This field is not thought to be harmful- electrical safety councils advise that electricity will only cause harm by direct contact- electrocution or burns, but people who live under high tension power lines may think differently! The field around a CRT is much weaker than a power line would generate, though they have in the past been implicated in “Sick Building Syndrome”. CRT monitors all have shielding by means of a metallic cage to minimise our exposure and, probably more importantly, the exposure to other sensitive electronic devices.

Original screens from 20 years ago undoubtedly contributed to eye strain, with their markedly curved screens- the eye had to adjust focus as we read a line of text as the centre was closer than the edges. Poor electronic control lead to “dot crawl” where the position of each pixel would wander, and a definite loss of focus towards the edges and especially the corners of the screen. They almost all flickered noticeably (ensure to check that the refresh rate of your CRT monitor is set as high as possible, ideally at not less than 85Hz, as flicker can cause eye strain- while capable of high refresh rates, many modern PCs arrive at the lowest settings to ensure compatibility with any chosen screen, and often no-one gets round to optimising them). Modern CRTs have much improved on these problems and they are all virtually eliminated with the newer flat panel LCD screens. CRT monitor production is expected to cease by the end of 2005.

So, they are safe and not in themselves a problem, but why so many complaints? Most likely the complaint is either a pre-existing problem which may be highlighted by screen use (most people would not read for 7-8 hours on a day off) or due to poor office or work area design.

Eye control difficulties may not become problematic until an individual takes a near vision based office job- a very common problem is convergence insufficiency, when the eyes do not converge well enough to retain comfortable near vision. Ideally one would be able to see a near object singly to a point less than 8cm away, but some people- about 10%- struggle to manage this. This is exacerbated by VDU usage as the eyes converge better when looking down (at a book) than straight ahead at a VDU. For this reason the top of the screen should be no higher than eye level, ideally a little lower. A routine eye test will check eye control at far and near, and if a problem is found, it can often be remedied with simple eye exercises.

Uncorrected refractive errors often cause problems, Hyperopia (longsight) and Astigmatism are the main culprits here- Hyperopia means that the individual has to focus the eyes harder than they should, which may lead to eye strain or blurry near vision. Astigmatism can introduce an instability in the focussing of the eyes, because the focus of eye is fluctuating forwards and backwards to achieve the best vision- this can fatigue the muscle and cause strain. Incipient or frank Presbyopia may cause difficulty at near, but is likely to cause problems at closer distances than the screen initially. Myopia (shortsight) is unlikely to cause problems in an office, though the screen may not be clear to an uncorrected Myope- it may be too far away to see clearly, depending on the degree of error.

Dry Eyes may also be the cause of the problem, as people tend not to blink enough when looking at the screen, and the dry atmosphere if in an air conditioned building would not help. (See the Dry Eye section for more info) Allergies can also cause problems- remember when Televisions used to attract a layer of dust? If you ran a finger over the screen it came off very dirty- there was a static charge on the screen which attracted the dust. Modern TVs and VDUs have an anti-static coating on the front, so that the dust does not stick- but it is still attracted to the screen, but instead of sticking it bounces off, straight into the face and eyes of the user. Again this is a problem only for CRT monitors.

A modern computer based office would have no windows (or at least they would be fully screened to cut out light), and have a low level of general lighting- about 1/3 of the brightness recommended for a paper based office, and the lighting would come from recessed down lighters. Each screened cubicle would have a supplementary desk lamp for any individual who needed to do paperwork.

In the real world, unshielded strip lights, windows and high levels of general lighting remain. These are very likely to cause problems- I am typing this in the dark at the moment, but I can see the screen well, and the keyboard is well enough lit by the screen for the harder to find keys (for me there are quite a few harder to find keys!). But yesterday, with light streaming in the window, I could barely see the screen- must get those blinds! Because screens are self luminous, any excessive light will create difficulty in seeing the detail on it.

Reflections cause problems because they shield the text underneath, but also the reflection may be at a different optical distance that the text on the screen- if the window is 6 metres away, the eye cannot focus that and the text at 40cm at the same time, which can introduce instability. Antireflective coatings on the screens do help here, and most modern ones have these. If you can see any reflections in the screen it is worthwhile trying to angle the screen to eliminate them, or failing that to move them into a lesser used part of the screen, maybe a corner. For the same reasons, screens should never directly face a window- brightness from behind and a silhouette of the user hamper viewing, but the user should also not face the window- then they will see their own reflection in the screen. If windows cannot be shielded screens should be positioned at right angles to them.

Screen filters were very popular a few years ago, there are two types. One is just a tinted sheet- this cuts the light hitting the screen from the room, and then cuts it again when it bounces back- stray light needs to pass through the filter twice, whereas light from the screen only passes through on the way out. This can help in areas where the lighting is too bright, and many screens are darkly tinted to achieve the same effect, but a decrease in room illumination would be better. The other screens are mesh filters- these reduce viewing angles so that the person straight ahead of the screen can see the output, but light coming in at an angle cannot pass through- if you can imagine a wall made of cardboard kitchen roll tubes laid side to side- you could see straight through the wall, but not much to either side. These can be useful to cut out reflections, but again elimination of the reflection sources would be preferable.

Finally if an individual wears spectacles it is a benefit to have anti-reflective coated lenses- these cut out reflections between the lens surfaces and the lens and the eye which will improve contrast on the screen. Recently there have been some concerns about the amount of High Energy (Blue) Visible Light coming from computer screens – scientists do reckon that this HEV light could be damaging to the eye, and even affect sleep patterns due to the inhibition of Melatonin production. A few spectacle lens manufacturers now produce HEV control or Blue control MAR coatings, which preferentially reflect the wavelengths of concern. You can read more about the “Blue Light Hazard” here and here.

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