The screen of any display acts like a mirror, and will reflect anything illuminated within a field of view into the user's eyes. The reflections are superimposed upon the display's own image, so they reduce contrast, wash out the intended image, and make it harder to read the display. This reduces the user's speed and accuracy, and give rise to discomforts that include headaches and tired eyes.

Glare     Coatings     Tint     Polarizer     Mesh     Hood     Reduction Factors

Glare and Reflections

Glare is the name given to reflections that appear nebulous and featureless to the user. A brightly illuminated wall that is opposite to the display will produce glare. Glare also results from diffuse reflections off the screen's surface; we will explain the meaning of this below. Pure (specular or mirror-like) reflections produce a detailed picture of whatever is opposite to the display. Like glare, this reduces contrast, but it also gives rise to a much more serious problem: the user will involuntarily alternate between focusing on the reflected image of the distant object or scene and focusing on the rather close by image of the screen itself. The constant shifting of the focal distance is very annoying and tiring.

Displays with polished glass screens are the worst offenders when it comes to glare and reflections, because they are excellent mirrors. Fortunately, very few displays come this way any more. It is impossible to eliminate glare and reflections completely, but there are a number of ways to minimize its effects:

• By far the most important item is to orient the screen perpendicular to any bright sources of light, such as windows and lamps. Controlling the intensity and distribution of room lighting is very important. Blinds on windows can control outside light. Darkening the walls will reduce glare, but may make the room dreary. Darkening a room and using individual task lighting will reduce glare, but uneven lighting can also cause visual fatigue. Uniform indirect lighting is generally the best.

• Switching the display from normal video, light characters on a dark background, to reverse video, dark characters on a light background, will make glare less apparent because the screen itself has a uniform background illumination. Note that glare will still reduce the screen contrast; it just won't be as noticeable and distracting to the user. That, however, may actually be a disadvantage because the user may be unaware of the source of the discomfort. Not all users like reverse video because of the high screen brightness. It also requires a higher quality display to produce a satisfactory image.

• Curved screens catch light from a wider field of view than flat screens, so picking a screen with low curvature will reduce reflections somewhat.

• A minor source of glare are the reflections off the bezel that surrounds all CRT screens. Dark colored bezels with a matte finish are best for minimizing this problem. A surprisingly large number of displays come with white bezels.

Virtually all displays sold include a screen treatment to reduce glare and reflections. The most common methods include:

Screen Coatings

Use of a screen coating or treatment can significantly reduce reflections. The most common methods are a direct etching of the screen surface and a silica coating of the screen. They give the screen a frosted appearance and produce diffuse reflections that scatter the light. This reduces the intensity of the reflected light reaching the eye of the user and, therefore, reduces glare. Another benefit is that the reflected images become somewhat nebulous and therefore less distracting, because the eye is unable to focus on them. A disadvantage is that the frosting also diffuses and speckles the screen's own image, reducing the resolution somewhat. The further away the frosting is to the light emitting layer of the screen, the worse this effect is. In particular, add-on external anti-glare screens of this type may noticeably degrade the image. Any time the manufacturer's literature includes a nondescript claim about an anti-glare screen, it is most likely one of the above two methods.

Anti-Reflection Coatings

There are two common types of anti-reflection coatings that can be applied to the screen. A thin-film "¼ wave" coating reduces the intensity of any reflection by generating two reflections that tend to cancel one another. The cancellation is never perfect and depends on a number of variables, such as the angle of the incidence and the color of the light.

By far the best, and also the most expensive method, is a refractive anti-reflection optical coating, the same method used on high quality optics and camera lenses. Multi-layer coatings are more effective than a single layer. Such coatings reduce reflections by moderating the increase in the index of refraction at the surface by a series of small steps. The coating remains effective over a wide range of angles and colors. Most of these coatings are very hard and will resist scratches and abrasion. A disadvantage of the optical coatings is that they absorb oils, particularly finger prints, which become iridescent. Cleaning them is more difficult. Anti-reflection coatings must be used in combination with tinted screens or circular polarizers, discussed below.

Tinted Layers

Using a tinted glass or plastic layer on the display will improve contrast because light reflected by the display screen must travel through the dark layer twice, whereas light generated by the display passes through it only once. For example: if the tinted layer has a transmission factor of 50%, then the light generated by the display will be reduced by a factor of 2, but light reflected by the screen will be reduced by a factor of 4, resulting in a factor of 2 improvement in contrast.

Tinted screens are generally used in combination with other methods of glare reduction because there will be a reflection at the front surface of the tinted layer. The most effective use of tinted glass is when it is incorporated directly into the glass used to make the display itself. Directly bonding the tinted layer to the display screen is almost as good. External, add on, tinted screens that are sold for contrast enhancement will introduce an additional reflection at the back surface of the tinted layer. Anti-reflection coatings on external filters should be applied to both the front and back sides of the layer.

Some manufacturers of tinted screens claim that their screens actually sharpen the display images. These statements are generally false or at best misleading. After a filter is installed the screen may appear sharper because it is uniformly darker, however, to make the comparison fair you must turn up the brightness and contrast so that the perceived image brightness returns to the prior levels. Under all normal circumstances the image will broaden, not sharpen.

Circular Polarizer

Another popular external glare reduction method is a "¼ wave" circular polarizer, not to be confused with the "¼ wave" thin-film coating, discussed above. The ¼ wave circular polarizer absorbs screen reflections because the reflected light goes through the polarizer twice and has its sense of rotation reversed. Internal display light is unaffected because it travels through the polarizer in reverse order and only once. Circular polarizers are significantly more effective than tinted glass at absorbing light reflected by the display screen. Just as with tinted glass, there can be a reflection at the front surface of the polarizer that should be reduced with an anti-reflection coating. Note that circular polarizers are generally made of plastic and may introduce distortion in the image.

Black Mesh Screen

A black mesh screen is one of the least expensive forms of glare reduction available. It works by absorbing any light that isn't traveling perpendicular to the screen; that includes virtually all of the glare producing light. Mesh screens, however, have difficulty absorbing light that comes in at a glancing angle from the side. Under those circumstances the mesh screen will itself produce glare. A disadvantage is that the mesh can interfere with the screen image unless it is extremely fine. Mesh screens may also produce Moiré patterns.

Screen Hood

In some instances a satisfactory solution to a glare problem can be obtained by placing a hood over the top and sides of a display. This ancient hat-like technology has been used on video displays for over 30 years. The hood simply prevents overhead and side lighting from reaching the screen by casting a shadow. Note that a hood does not reduce glare that arises from light or illuminated areas that are behind the user and facing the display. Since this is the principal source of glare in most situations, hoods are of limited usefulness and are seldom used. Hoods may also interfere with visibility and mobility in your work area. While hoods are available for purchase, it is also fairly easy to make a cardboard one yourself.

Glare Reduction Factors

The methods discussed above will work with varying degrees of success, depending on the details of the manufacturer's process. The glare reduction factors claimed by many manufacturers should not be taken too seriously. Some claim glare reduction factors of up to 250 to 1. Such numbers are either dreamed up in the advertising department or are the result of unrealistic test conditions. A good quality anti-glare filter with a multi-layer optical coating will reduce glare and reflections by about a factor of 20. Visually compare glare reductions before you buy anything.