It is well known that smooth surfaces, such as found on glass, plastic and metals, reflect light. The normal silvered mirror is an example of a smooth surface which has everyday usage. While the principle of light reflection is very useful in connection with mirrors, flash lights and many other pieces of apparatus, it can be a nuisance when trying to view a piece of art through an ordinary picture frame glass, or when trying to read the face of an instrument through ordinary glass. Whenever a partially transparent cover, made, for example, from glass or plastic, is placed over art work, or other surfaces such as instrument displays, the partial reflection of visible light from both the front and rear surfaces of the protective cover may interfere with the reflected light from the art work or instrument display. Such partial reflection prevents the viewer from obtaining a clear view of the subject. The effect of reflecting part of the incident light and transmitting less to the subject to be viewed has been a problem for as long as glass covers have been available. The ability of the viewer to obtain a clear and accurate assessment of the surface behind the transparent cover varies a great deal with the subject, the lighting, the position of the viewer, etc. For example, if all or part of the art work behind a glass cover is dark, it is quite possible that the light reflected from the front and back surfaces of the cover will "wipe out" part or all of the art work from the view of the observer.
In the prior art there have been at least two attempts made to mitigate the image reflection problem. The first attempt has been to employ chemically etched glass. Chemically etched glass has the advantage that there is little specular reflection and hence little image forming light. To say it another way, a viewer looking at a piece of art which has a cover piece of etched glass (hereinafter called non-glare glass) would not be troubled by the image reflection of a window located behind the viewer. However, the etched glass has the disadvantage that it does provide a diffuse reflection and the viewer, in our hypothetical case, would see a fuzziness around the work of art or a veil of light over the work of art. The second attempt, in the prior art, to reduce the image reflection has been the provision of clear layers of materials which provide by light interference an anti-reflection characteristic to the glass surface. One such material has been magnesium fluoride. In a well known anti-reflection coating arrangement, there is: a first layer of clear material whose characteristics and thickness provide a 1/4 wavelength optical thickness of light; a clear second layer of material whose characteristics and thickness provide a 1/2 wavelength optical thickness of light; and a clear third layer of material whose characteristics and thickness provide a 1/4 wavelength optical thickness of light. In the foregoing arrangement, the wavelength is assumed to be 5500 angstroms and the total thickness of the three layered coating is approximately 3200 angstroms. It should be understood that more or less numbers of layers could be employed and the thickness can be in the range of 1300 to 6500 angstroms. The components of specular reflecting light involved with such a multi-layered coating are approximately zero but not quite. The disadvantage of the anti-reflection coatings is that if such a coating is employed, there is still some image reflected light which is, however, quite reduced.
The present device employs both the technique of the roughened glass surface and the technique of the anti-reflection coating and provides a non-reflecting overlay device which is superior to either non-glare glass or non-reflective coated glass.