Contrast Screens

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Contrast screens

Through the glass darkly

We all know the frustration of trying to watch television in daylight when the brilliance of a window reflection on the picture tube makes it impossible to see a part or all of the picture. This also happens to a smaller degree when light-coloured furniture or a lamp is reflected. In general, reflections lower the quality of the picture on a television screen and reduce the pleasure of watching a TV programme.

The optical process of what happens in such a case is that the contrast of the picture is effectively reduced. The Bang & Olufsen solution to the problem has traditionally been to incorporate a contrast screen. Ensuring that ambient light (which creates the reflections) travel twice through a light absorbing contrast screen whereas the picture formed in the picture tube travels only once, the brightness of the reflection is substantially reduced.

If it were possible, a better solution would be to reduce reflections directly. One method is to make the surface of the picture tube rough, either by sanding or etching the front surface of the tube or spraying a matt coating on the glass. This type of treatment does not actually reduce reflections, but by making reflections more diffuse, reduces their visibility. However, it also makes the picture itself fuzzy and less sharp, so that the overall quality is reduced.

A more attractive method is to use coatings. Coatings for glass surfaces have been used for a long time and for many purposes. The first use was probably for camera lenses, where coatings corrected for optical faults in the lens. Later, coatings were used to reduce reflections and also to correct colour reproduction when colour films became available, Most people will know of coatings from the more expensive spectacles, where coated glass is used to reduce reflections.

Anti-reflection coatings can reduce reflections to between 5% and 10% of their original brightness. They work by what is known as destructive interference and use the fact that light travels in waves of specific wavelengths. By coating the picture tube or other glass surfaces with a transparent layer of a thickness of one quarter of the wavelength of light, the light failing on the surface is reflected twice - once from the front surface and again from the rear surface of the coating. The two reflections are now half a wavelength apart and therefore in opposite phase and thus virtually cancel each other to destroy the reflection.

Of course, this happens perfectly for just one wavelength of light (or light of a single colour) and to lesser degrees for the wavelengths close to this. By adding more than one coating of different thicknesses, a more broadband removal of reflections can be achieved. Thus, for example, three coatings are used for the front screen of the Beosystem AV9000. One of the reasons that antireflection coatings are not used more often is that currently the manufacturing process is extremely expensive for large screens, allowing its use only in high-end television sets. However, as it is considerably less expensive for small screens, especially where only a single coating is used, it is a reasonably popular feature for computer monitors.

One of the features of coated picture tubes is that although reflections are drastically reduced, the coated glass appears to have a residual colour. This happens because some light is still reflected from the surface, the colour of which depends on the number and thickness of the layers. The colours that are not perfectly removed result in the colouring of the glass. This does not affect the colours of the picture, which is viewed through the coating and does not depend upon its thickness.

Also, the effectiveness of the anti-reflection coatings falls off at an angle to the screen, as the effective thickness of the coating changes. Seen from an angle, the colour of the reflections changes for the same reason.

Beosystem AV9000 also uses another coating, a thin layer of chrome on the back of the contrast screen. This is an anti-static layer, which is earthed to prevent the build-up of static charge, which can give problems with electrostatic shock. An appealing side effect of the anti-reflection coating of Beosystem AV9000 is that when the black curtain behind the contrast screen is closed, the effect of the coating is apparently considerably reduced, so that the whole screen surface appears to reflect. As soon as the curtain is removed, the anti-reflection coating takes full effect, forming a window in which the brightness of reflected light is reduced.

The human eye has a limited ability to cope with bright and dark areas at one time. In fact the eye constantly adjusts to an average ambient light condition, with the pupil in the eye working as a continuously variable shutter, in a manner similar to that known in aperture mechanisms of camera lenses.

Absolute light level is therefore not important under normal conditions, but only if the difference between the brightest and darkest parts of the scene, known as contrast, fall within the range of our limited ability to appreciate the differences. Thus in bright sunlight, we are unable to see clearly into a region of deep shadow, not because it is too dark in absolute terms, but because the aperture of our eyes is set for bright light, so that the shadow region is beyond our contrast range. This is a very important phenomenon, but it can be easily misunderstood. Also, a change in the average light level results in a relatively slow adjustment to the new conditions, as our eyes change aperture.

It is immediately obvious that the ideal lighting for television viewing is when ambient light has the same average level as the average level of light from the screen area. This of course includes not just the TV picture, but also any reflected ambient light from the screen, and reflection of bright objects in the picture area. An analysis of the combination of these variables, under less than ideal conditions, becomes very complex, but some general rules are provided in the report.

Under normal viewing conditions, when no bright objects are reflected in the screen, contrast always fails with increasing ambient light. Normal contrast can be restored by an equal increase in picture brightness, which, for any TV set, is only possible for moderate increases in ambient light. No picture tube has the ability to equal the luminance of a brightly lit room, not to mention that the life of picture tubes decrease sharply with increased brightness settings.

However, the same picture quality can be approximated by increasing contrast, provided the increased contrast is within the range allowed by the eye. In this case, however, although all details in highlights as well as in shadows are visible, the screen will appear darker. Contrast improvement methods, such as contrast screens, are therefore most effective in high ambient light. In addition, Bang & Olufsen sets are among the few that have built-in automatic contrast correction circuits, specifically to compensate ambient light conditions. As soon as reflections occur on the screen surface, whether it is the reflection of ambient light, or a specific bright object, the situation changes. Increasing picture brightness is no longer able to compensate for the change in the average light level from the screen, but requires the help of increased contrast. Since a contrast screen reduces ambient light reflection as well as increasing contrast, it has a double function.

A TV picture is not continuous, but is switched on and off 50 times per second in the PAL transmission system, and 60 times in NTSC. This results in observable flicker in the picture, and while this is more prominent in PAL than NTSC, it is observable in both. Even when a viewer is not conscious of a flickering picture, it results in eye irritation and after a period, in tiredness. Flicker is most obvious in peripheral vision that is when the TV is seen through the corner of the eye when looking directly beside the picture.

Screen flicker is not only a factor of the picture repeat frequency, but also of screen luminance. More accurately, the cause of this effect is the length of the dark period between pictures, and the size of the aperture of the eye due to the screen luminance or brightness.

The frequency at which flicker is no longer observable is called the flicker fusion frequency. A study shows that repetition frequencies as low as 45 Hz can be flicker free provided maximum luminance on the screen is kept low. This maximum permissible value of luminance will normally be regarded as a dark picture, and result in less than acceptable contrast on a normal picture tube.

With a contrast screen, an acceptable compromise can be made, so that full observable contrast is available with a sufficiently low maximum luminance to avoid flicker in 50 Hz TV systems.

Created: 11th January 2007
Modified: 15th February 2007

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