An investigation of bulk and surface refractive indices for flat window glasses,patterned window glasses and windscreen glasses

The bulk and surface refractive indices have been measured for 20 float window glasses, 20 non-float window glasses, 20 patterned window glasses and 20 toughened float windscreen glasses.The refractive index of the tin-contact surface of the float glasses was always greater than that of the bulk, typically by 0.002 (range 0.00053–0.00307). In contrast the refractive indices of the two surfaces and the bulk of the sheet window glasses were in agreement within accepted experimental error in each case. A number of patterned window glasses showed significantly different refractive indices between the plain and patterned surfaces, the largest difference obtained (0.00021) being an order of magnitude smaller than that found for a typical float glass.Nine of the 40 float glasses examined yielded abnormal fragments from the upper surface in the sense that they did not disappear at the match temperature of the bulk glass. The refractive index measured from these abnormal fragments for two of these glasses was lower than the refractive index from the bulk glass.The variation in the bulk refractive index for the sheet and float window glasses produced an estimated standard deviation (E.S.D.) of 1.2 × 10^?5. However, the patterned glasses were nearly three times as variable (E.S.D. = 3.3 × 10^?5) and the toughened float glasses were, on average, almost seven times as variable (E.S.D. = 8.0 × 10^?5).The value of making surface as well as bulk refractive index measurements in routine casework is discussed in the light of the above results.