On the Display of Pictures: Non-Glare Glass and Kinds of Illumination

Leonardo, Vol. 15, NO.2; pp. 123-125, 1982 Printed in Great Britain 0024-094X/821020123...{)3$03.00/0 Pergamon Press Ltd. ON THE DISPLAY OF PICTURES: NON-GLARE GLASS AND KINDS OF ILLUMINATION R. D. Edge* of suitable asperity or roughness. This does not cut down on the amount of light reflected but diffuses it over a wider angle, rendering it less objectionable. It is also possible to diffuse the light by spraying a matt layer of transparent plastic on glass, though this is not a very satisfactory procedure. A non-glare glass is produced by acid-etching the surface. (It is supplied, for example, by Viracon-Chicago Dial, Chicago, Ill., U.S.A.) The experimentally measured angular distribution of the intensity (arbitrary units) of light (incident at 0°) reflected and transmitted by such treated glass isshown in Fig. 1.It will be seen that such a surface has a remnant specular reflection present (represented by the spike), which amounts to slightly less than 10% of the incident light. If the specular reflection and direct transmission are ignored, the spread in angle (half width) for transmission is 1.26° and for reflection, 2.38°. A slightly rougher surface can be produced mechanically by a rolling operation in making roughness-patterned glass. (In our studies at the University of South Carolina we employed glass having rolling pattern No. 122,suplied by A.F.G. Industries, Kingsport, Tenn., U.S.A.) The experimentally measured angular distribution of the intensity (arbitrary units) of light (incident at 0°) reflected and transmitted by such treated glass is shown in Fig. 2. The spread in angle for transmission here is 1.07° and for reflection, 1.92°. No specular spike is present. The rolled 1. As a physicist, I find it amazing how poorly displayed are the artworks of very many countries-including the U.S.A., U.K., France, Germany and Italy. I have yet to find a major public gallery or museum where there are not some pictures so badly exhibited as to make it virtually impossible to gain a sufficient idea of what they depict and are intended to convey. One major fault concerns the glass used to cover pictures, and, a second, the way displayed pictures are illuminated. (These two aspects were noted by F. D. Martin recently in Leonardo in his criticisms of exhibited sculptures and paintings in Italy [1].) 2. Too often a picture is covered with a layer of ordinary window glass. This unfortunately presents to viewers both their own reflections and those ofother objects in the room, rather than the picture. I wish to discuss three simple techniques for coping with this undesirable result. The first is to employ a sheet of ordinary glass coated with a layer ofanti-reflection material on each side ofthe glass. The thickness of the piece of glass is not important, but the material of the layer used for blooming, much in the same way as lenses on cameras are treated, must have a refractive index that is equal to ~2' where u, and f.l.2 are the refractive indices of the air (f.l.l = I) and the glass (f.l.2 = about 1.5)[2]. Thus, an index of.j'"I5or about 1.225 is required for the anti-reflection material. Its optical thickness must be a quarter of the wavelength of light (about 0.00013 mm). It is not possible to find a material suitable in hardness, stability, adhesion to glass, etc. with such a low refractive index. Magnesium fluoride (f.I. = 1.38) seems to be the best anti-reflection material available. This material is evaporated in a vacuum uniformly onto a glass surface, a difficult task for glass of as large an area as those usually required for covering pictures. A characteristic purple reflection from such surfaces arises from a combination of blue and red, due to the larger transmission, rather than the reflection, of the middle of the spectrum. Glass with magnesium fluoride coatings is manufactured in the German Federal Republic, but is very expensive. It nevertheless provides an excellent solution, since any reflection, other than a faint purple image, is eliminated without absorption...