Among the materials which are not quite pure silica and still have a low enough expansion to be thermally strong is 96% silica glass, Vicor. It has a coefficient of expansion of 8 compared to 6 for quarts and shares the capability of operating safely at red heat. In its manufacture the residual 4% of metal oxides other than silica can include ultraviolet absorbers. The materials of this class are thus produced either for UV transmitting applications or for UV limiting purposes. A typical UV absorbing Vicor composition is adjusted to cut off all energy of wavelengths shorter than 290 nm with negligible amounts below 300nm (the Iimit for suntanning). This 300nm is also the transmission limit of borosilicate lens glasses; however, many lenses may absorb up to 320 nm due to glass thickness. It may be of interest that glasses which absorb in the harmful far UV range also absorb some in the near UV and will demonstrate a very slight yellow tinge when you look into the edge so as to receive light which has traveled through several inches of glass. The ability to control the transmission of 96% silica glass has led us to experiment with colors; however, only the pale shades like color temperature-correcting blue and very light ambers have shown good efficiencies. Pure colors in Vicor brand glass tend to be too low in efficiency for stage lighting. Among glasses of greater resistance to corrosion and greater resistance to thermal stress injury than borosilicates are the aluminosilicate glasses. This is related to the face that pure alimina is more inert than pure silica and must be raised to higher temperatures before it will melt or flux with other oxides such as soda to form glass. filters and cold mirrors require stable glass substrates. Coating adherence and compatibility grows out of those materials closely resembling glass. In the future, we can expect to see greater use of coated ref:ectors and color media in which we will encounter the same categories of glass heat resistances and durabilities as we presently deal with in lenses and lamp envelopes. As a correlary to glass's ability to withstand high energy bombardment and corrosive effects inside a lamp envelope it is capable of providing a clear inert surface for receiving durable vacuum coatings at high temperatures. The new multilayer interference coatings including dichroic color Wavelength transmission of four glasses; percentage of transmission plotted against wavelength 100 = 80 lead Soda-lime 80-90 Fused Silica 5.8 96% Silica 8 = = Transmitting Alumino- Vycor 7911 Boro- Silicate Silicate 42·46 32-36 60 Percent Trans. Borosilicate Glass 77 40 -1 ~ U. V. Absorbing Vycor 7917 40 Relative Energy Relative thermal expansion rates of different glass types, indicated by right bar of each pair. 3400· K 20 a Wavelength 240 nm Harmful U.V. 26 ~ 360 nm 300 nm Near U.V. THEATER DESIGN AND TECHNOLOGY 420 nm Visible DECEMBER,1969