Browse Prior Art Database

Glare Reduction for Optical Displays

IP.com Disclosure Number: IPCOM000088511D
Original Publication Date: 1977-Jun-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 2 page(s) / 31K

Publishing Venue

IBM

Related People

Cunningham, EA: AUTHOR

Abstract

The drawing illustrates a computer display terminal 10 having a downwardly facing cathode-ray tube (CRT) 11 in a housing 12, shown schematically I only. CRT 11 is driven in a conventional manner to produce symbols on its screen 13. A partially-reflecting mirror 14 is positioned on a base 15.

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Glare Reduction for Optical Displays

The drawing illustrates a computer display terminal 10 having a downwardly facing cathode-ray tube (CRT) 11 in a housing 12, shown schematically I only. CRT 11 is driven in a conventional manner to produce symbols on its screen 13. A partially-reflecting mirror 14 is positioned on a base 15.

Of the light 20 from CRT face 13, about one third is reflected by mirror 14, to form rays 21 to an observer 22. The remaining two-thirds is absorbed, as at 23. Light 24 from an extraneous source 25 is reflected at 26 and absorbed at 27 in the same ratio. Light 26 is reflected from CRT face 13 at 28, and again encounters mirror 14. One-third of this incident light is reflected at 29 to observer 22; the remainder 30 is absorbed.

The conventional method of reducing glare from a CRT is to place a filter over the CRT face. Comparing a 1/3-transmission mirror with a 1/3-transmission filter, both reduce the brightness of the CRT image by three times, and both increase the contrast (signal-to-scatter) by a factor of three. But, while the mirror reduces the power from extraneous source 25 by at least nine times, a filter would actually increase it by about 1.2. Therefore, mirror 14 reduces glare more than an order of magnitude over a comparable filter.

Although the value of the reflection ratio for mirror 14 is a matter of individual operator preference, 1/3 appears to be an optimum over a range from about 1/4 to about 1/2.

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