Key data for the principal materials are shown here for easy comparison. Most precision optics for the visible are in BK7 or B270 crown glasses, which for almost all purposes are interchangeable. Sheet materials made by the float process have many uses. ‘Float glass’ in this catalogue means common soda-lime glass as used for glazing, which has a slight green tint. ‘White’ float is similar but with reduced iron content and improved transmittance (see graph). LEBG float (‘Borofloat’) is made in low-expansion borosilicate glass for better heat resistance.
*Attainable only by uniform slow heating; in practice thermal shock limits high-temperature use †Birefringent: no = 1.768, ne = 1.760 ‡Low-expansion borosilicate glass