Matte Displays


Matte Displays

This article describes the effect of reflections from display screens, their ergonomic effects and ways to characterize and control such reflections.

From CRTs to LCDs

ISO 9241-7:1998 [ISO9241-7 Ergonomic Requirements for Office Work with Visual Display Terminals (VDTs), Part 7: Requirements for Displays with Reflections] for the first time addressed annoying and disturbing facts under which many computer users have been suffering for quite some time during their daily work in front of a visual display terminal (i.e. computer monitor): reflections from ambient light sources in the display screen (those days in most of the cases CRT monitors).

These reflections, as shown in figs. 1 and 2, have three negative effects:
(1) they reduce the contrast of the displayed visual information by adding (reflected) luminance to the emitted luminance,
(2) reflected white light reduces the saturation of displayed colors (bleaching), and
(3) the image of light sources reflected in the screen cause the human visual system to focus on that image which is usually at a much farther distance than the information shown on the screen. This competition between two images that can be focused may cause headache and other severe disturbances.

When computer monitors with LCDs became affordable in the second half of the 1990s, users experienced the work with an LCD monitor as a big improvement in terms of workplace ergonomics. The main and directly obvious advantage of these monitors was the absence of that kind reflections that gave a clear image of disturbing light sources in the environment (e.g. windows, lamps, luminaires, white blouse or shirt of the user, etc.). Reflections of light sources now became visible only as fuzzy balls (areas) that were brighter than the background of the display area.

The obvious difference between reflections of ambient light sources in a CRT monitor and an LCD monitor is illustrated in fig. 3. The left half of the photo shown in fig. 3 corresponds to the flat, polished surface of a CRT screen, the right half corresponds to the scattering anti-glare (AG) layer of an LCD monitor.

The matte light scattering surface of LCD monitors has two effects:
(1) ambient light sources reflected in the screen are not perceived as a clear image (left side), but rather as a fuzzy area (haze) with increased luminance (right side), and,
(2) while the reflected luminance turns the visual information in the specular direction unreadable in the case of the glossy surface (left side, disability glare), the contrast of the text is reduced but the displayed visual information can still be recognized without problems (right side).

The scattering AG layer reduces the amount of light reflected in the specular direction thus removing disability glare, but the contrast in the vicinity of the specular direction (i.e. above and below the vertical position of the arrows in fig. 3) is reduced by what is called "veiling glare", i.e. a certain amount of reflected luminance outside the specular direction decreasing with angular distance as shown in fig. 4 (center diagram). This added reflected luminance from usually white light (e.g. daylight, room illumination) not only reduces the contrast to a certain extend, it also reduces the saturation of colors displayed on the screen.

Basic types of light reflection

Conclusion: In a dark environment the distinctness of image and the saturation of colors of glossy screens might be superior, even though it remains to be proven that this difference is noticeable at all. In illuminated surrounds however, light sources in the specular direction to the user can be annoying and disturbing up to the point of disability-glare. If the display screen can be rotated and tilted to avoid specular sources, contrast and color saturation are slightly better than in the case of AG coated screens. This however is not possible in many cases (white blouse/shirt of user !).

Glossy displays – a clever marketing campaign

Sometime around 2004 computer monitor manufacturers, especially manufacturers of mobile computers, were looking for new distinguishing features to promote the sales of their products. In was in those days that the marketing specialists started to provide arguments supposed to prove the functional superiority of glossy display screens.

:"Glossy displays create more saturated colors, deeper blacks, brighter whites, and are sharper than traditional matte displays. This makes these types of displays more appropriate for viewing photos, watching movies, or even just general computer usage such as web browsing. Also, in extremely bright conditions where no direct light is facing the screen, such as outdoors, glossy displays can become more readable than matte displays because they don't disperse the light around the screen (which would render a matte screen washed out)." Source: Glossy_display

This paragraph illustrates the seductive mixture of almost true facts (or at least sounding reasonable) and fairy tales that is used to lure the customer to buy ergonomically inferior products and still be proud about it (see Specsmanship).

But marketing activities never come to a rest. Since quite some customers remain unsatisfied with the usability of glossy screens in uncontrolled environments they are able now to buy ergonomically improved (matte !) display screens (actually the good old AG coated version), but now it is a distinguishing feature in the world of portable computers and you will have to pay some extra for that ingenious special feature.

The psychology of subjective quality rating

Since the last years have shown that a large amount of customers are immune to functional features (the improved usability of matte display screens in non-dark surroundings), the question arises how this situation could be maintained. Numerous personal observations (cars, watches, jewelry, attire, etc.) imply the thesis that people can be classified according to their perception of quality: for some it is the polished, shiny, glossy stuff that represents value and top-quality, for others its rather the reduced obvious gloss of matte surfaces and muted colors.

References



Further references

* CIE No38-1977: Radiometric and photometric characteristics of materials and their measurement

* CIE No 44-1979: Absolute methods for reflection measurements

* CIE No17.4-1987: International lighting vocabulary, 4th ed. (Joint publication IEC/CIE)

* John C. Stover, Optical Scattering, Measurement and Analysis, SPIE Press, 1995

* M. E. Becker, Evaluation and Characterization of Display Reflectance, DISPLAYS 19 (1998), pp. 35

* M. E. Becker, Measurement of Display Scattering, Proceedings of the 22nd IDRC 2002, pp. 803

* E. F. Kelley, Sensitivity of Display Reflection Measurements to Apparatus Geometry, SID'02 Digest, pp. 140

* M. Becker, Metrology Issues for LCD-TV, DISPLAYS 26/4-5(2005), pp. 197-207

External Links

STOP Specsmanship [http://www.specsmanship.com]


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