Glass sheet approaching neutrality irrespective of its thickness

Abstract

A glass sheet having a composition comprising a total iron content of 0.002-0.03% expressed in the form of Fe.sub.2O.sub.3 and in weight percentage with respect to the total weight of glass, and further satisfying the formula N*.sub.5≤0.05; N*.sub.5 being defined as N*.sub.5=√{square root over ((a*.sub.5−a*.sub.0).sup.2+(b*.sub.5−b*.sub.0).sup.2)}, a*.sub.5 and b*.sub.5 being measured for a sheet thickness of 5 mm in transmission with illuminant D65, 10°, SCI; a*.sub.0 and b*.sub.0 being computed for a sheet thickness of 0 mm in transmission with illuminant D65, 10°, SCI. Such a glass sheet allows a color rendering of the sheet which is essentially the same whatever the view path available for an observer of the object integrating said glass sheet (when looking through its main faces or through its edges or through a zone bearing a diffusing coating).

Claims

1. A glass sheet having a composition comprising the following in weight percentage, expressed with respect to a total weight of the glass: SiO.sub.2 40-78%; Al.sub.2O.sub.3 0-18%; B.sub.2O.sub.3 0-18%; Na.sub.2O 0-20%; CaO 0-15%; MgO 0-12%; K.sub.2O 0-12%; BaO 0-5%; and a total iron content of 0.002-0.03% expressed in the form of Fe.sub.2O.sub.3 and in weight percentage with respect to the total weight of glass, and wherein the glass satisfies the formula: N*.sub.5<0.05; N*.sub.5 being defined as N*.sub.5=√{square root over ((a*.sub.5−a*.sub.0).sup.2+(b*.sub.5−b*.sub.0).sup.2)}, a*.sub.5 and b*.sub.5 being measured for a sheet thickness of 5 mm in transmission with illuminant D65, 10°, SCI; a*.sub.0 and b*.sub.0 being computed for a sheet thickness of 0 mm in transmission with illuminant D65, 10°, SCI, and wherein the composition further comprises erbium (expressed in the form of Er.sub.2O.sub.3) as follows: 0.003-0.5%, or selenium (expressed as Se) as follows: 3-50 ppm and cobalt (expressed as Co) as follows: 0.1-15 ppm.

2. The glass sheet according to claim 1, wherein the composition comprises: total iron 0.002-0.02 wt %.

3. The glass sheet according to claim 1, wherein the composition comprises: total iron 0.002-0.015 wt %.

4. The glass sheet according to claim 1, wherein the composition has a redox ratio ≤32%.

5. The glass sheet according to claim 1, wherein the composition comprises erbium (expressed in the form of Er.sub.2O.sub.3) as follows: 0.003-0.5 wt %.

6. The glass sheet according to claim 5, wherein the composition comprises: Er.sub.2O.sub.3≥0.01 wt %.

7. The glass sheet according to claim 1, wherein the composition comprises: Er.sub.2O.sub.3≤0.1 wt %.

8. The glass sheet according to claim 1, further comprising: erbium (expressed in the form of Er.sub.2O.sub.3) in an amount of 30 to 150 ppm.

9. The glass sheet according to claim 1, further comprising: erbium (expressed in the form of Er.sub.2O.sub.3) in an amount of 50 to 100 ppm.

10. The glass sheet according to claim 1, wherein the composition comprises selenium (expressed as Se) as follows: 3-50 ppm and cobalt (expressed as Co) as follows: 0.1-15 ppm.

11. The glass sheet according to claim 1, wherein the composition comprises Co as follows: 0.25-20 ppm.

12. The glass sheet according to claim 1, further comprising: cobalt (expressed as Co) in an amount of 0.5 to 5 ppm.

13. The glass sheet according to claim 1, further comprising: cobalt (expressed as Co) in an amount of 1 to 2 ppm.

14. The glass sheet according to claim 1, wherein N*.sub.5≤0.03.

15. The glass sheet according to claim 1, wherein N*.sub.5≤0.01.

16. A glass sheet having a composition comprising the following in weight percentage, expressed with respect to a total weight of the glass: SiO.sub.2 40-78%; Al.sub.2O.sub.3 1-6%; B.sub.2O.sub.3 0-18%; Na.sub.2O 0-20%; CaO 0-15%; MgO 0-12%; K.sub.2O 0-12%; BaO 0-5%; and a total iron content of 0.002-0.03% expressed in the form of Fe.sub.2O.sub.3 and in weight percentage with respect to the total weight of glass, and wherein the glass satisfies the formula: N*.sub.5<0.05; N*.sub.5 being defined as N*.sub.5=√{square root over ((a*.sub.5−a*.sub.0).sup.2+(b*.sub.5−b*.sub.0).sup.2)}, a*.sub.5 and b*.sub.5 being measured for a sheet thickness of 5 mm in transmission with illuminant D65, 10°, SCI; a*.sub.0 and b*.sub.0 being computed for a sheet thickness of 0 mm in transmission with illuminant D65, 10°, SCI, and wherein the composition further comprises erbium (expressed in the form of Er.sub.2O.sub.3) as follows: 0.003-0.5 wt %, or selenium (expressed as Se) as follows: 3-50 ppm and cobalt (expressed as Co) as follows: 0.1-15 ppm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a cross-sectional view of glass sheet cover for a known display illustrating diffusion of light.

(2) FIG. 2(a) is a plot of a glass color simulation in a*, b* at thicknesses from 5 to 35 mm.

(3) FIG. 2(b) is an enlarged view of FIG. 2(a).

(4) FIG. 3 shows an evolution of a*, b* comparing a clear glass, a low iron-glass, and a glass according to the invention.

(5) Embodiments of the invention will now be further described, by way of examples only, together with some comparative examples, not in accordance with the invention. The following examples are provided for illustrative purposes, and are not intended to limit the scope of this invention.

Examples

(6) Illustration of the General Concept of the Invention

(7) In order to illustrate the global concept of the invention, we have conducted several simulations of glass color for different thicknesses, simulating glasses from the state-of-the-art (a classical solar low-iron glass sheets and a known neutralized low-iron glass sheet) and a glass according to the invention. From the absorbing coefficient of various coloring agents in glass, one can obtain a given optical spectrum at different thicknesses. It is then possible to obtain the variation of color according to the thickness of the glass.

(8) Table 1 and FIG. 2(a) show the results of this simulation of color in a*,b* (increased thickness by step of 5 mm, from 5 to 35 mm). An enlarged view of FIG. 2(a) for a specific a*b* zone is also given at FIG. 2(b).

(9) TABLE-US-00004 TABLE 1 Thickness (mm) 5 10 15 20 25 30 35 Solar low- LTD (%, D65, 2°) 91.67 91.45 91.23 91.02 90.80 90.58 90.37 iron glass a* (D65, 10°) −0.09 −0.21 −0.33 −0.45 −0.57 −0.69 −0.81 b* (D65, 10°) 0.15 0.22 0.29 0.36 0.43 0.51 0.58 N 0.17 0.30 0.44 0.58 0.72 0.86 0.99 N.sub.x* 0.14 0.28 0.42 0.56 0.70 0.84 0.98 Neutralized LTD (%, D65, 2°) 91.40 90.92 90.45 89.98 89.50 89.04 88.57 low iron a* (D65, 10°) 0.00 −0.03 −0.07 −0.10 −0.13 −0.17 −0.20 glass b* (D65, 10°) 0.00 −0.08 −0.15 −0.23 −0.30 −0.38 −0.45 N 0.00 0.08 0.17 0.25 0.33 0.41 0.49 N.sub.x* 0.09 0.17 0.25 0.33 0.42 0.50 0.58 Invention LTD (%, D65, 2°) 91.57 91.26 90.95 90.64 90.33 90.03 89.72 a* (D65, 10°) 0.04 0.04 0.05 0.05 0.05 0.06 0.06 b* (D65, 10°) 0.09 0.11 0.12 0.14 0.15 0.17 0.19 N 0.10 0.12 0.13 0.15 0.16 0.18 0.19 N.sub.x* 0.01 0.03 0.05 0.06 0.08 0.09 0.11

(10) One can observe in a known and predictable manner that a significant shift in color (a*, b*) happens while glass thickness increases in the case of the glass sheets according to the state-of-the-art. Moreover, even a glass sheet that is said to be “neutral” (low N factor, i.e. N=0.09) is actually neutral only at a given thickness (here 5 mm) and is quickly less neutral (higher N factor) than another thinner classical low-iron glass from the state-of-the-art. Conversely, the glass sheet of the invention, which has a very low N*.sub.5 factor (0.01) and which is also very close to neutrality (low N factor) remains almost neutral and keeps significantly its color whatever the thickness (a*, b* and N factor remains sensibly constant when thickness increases, mostly compared to sheets from the art). This is particularly well illustrated when looking at FIG. 2.

(11) This observation is moreover true whatever the way to reach targeted N*.sub.5 of the invention but several ways to achieve it are also given hereunder.

(12) Glass Sheets with Specific Compositions and their Evaluation

(13) Different glass sheets according to the invention or comparative were prepared in lab or industrially or calculated/simulated, as 3 sets of examples.

(14) The optical properties, when measured on prepared/produced samples, were determined on a Perkin Elmer Lambda 950 spectrophotometer fitted with an integrating sphere of 150 mm in diameter, and in particular: The luminous transmission LTD was determined according to the ISO9050 standard with a solid viewing angle of 2° (D65 illuminant) and for a wavelength range between 380 and 780 nm; The CIE L* a*b* parameters were determined in transmission with the following parameters: Illuminant D65, 10°.

(15) In case of industrially produced sheets, both surfaces were mirror polished in a known manner prior to measurement

(16) For the lab preparation of glass sheets: Powder raw materials were mixed together, in amounts according to the targeted composition, and placed in melting crucibles. The raw material mixture was then heated up in an electrical furnace to a temperature allowing complete melting of the raw material.

(17) For glass sheets produced industrially: they were produced classically on a mass production float furnace.

(18) For the simulation/computation: the optical properties were calculated on the basis of optical properties of different glass colorants (using linear absorption coefficient, determined for the concerned base glass matrix, to build the complete optical spectra and compute the parameters of interest).

(19) Glass Sheets from the State of the Art

(20) EX1.x examples (comparative) correspond to classical clear glass sheets (total iron content ˜0.1 wt %) with different thicknesses.

(21) EX2.x examples (comparative) correspond to classical low-iron glass sheets (total iron content ˜0.012 wt %) with different thicknesses: EX2.1-2.4 correspond to solar-quality low-iron glass; and EX2.5-2.10 correspond to low-iron glass sheets with cobalt (as the ones described in DE29819347U1).

(22) The glass sheets of examples EX1.x, EX2.x were produced industrially and their optical properties were measured. The N* factors at 5 mm (N*.sub.5) for EX1.x and EX2.x were computed by making a linear extrapolation of the N* values at the different available thicknesses.

(23) Tables 2(a) to (c) show the compositions and the measured optical properties of comparative examples EX1.x (Table 2(a)) and comparative examples EX2.x (Table 2(b) and (c)).

(24) TABLE-US-00005 TABLE 2(a) Comparative Examples: Clear glass sheets EX1.1 EX1.2 EX1.3 EX1.4 EX1.5 EX1.6 EX1.7 Thickness (mm) 0.53 0.72 0.99 1.2 1.57 1.87 2.08 CaO (wt %) 7.86 7.82 7.91 7.87 7.92 8.00 7.95 K.sub.2O (wt %) 0.35 0.38 0.23 0.24 0.20 0.20 0.20 Na.sub.2O (wt %) 13.36 13.38 13.95 13.91 13.89 13.93 13.90 Fe.sub.2O.sub.3 (wt %) 0.1030 0.1040 0.1040 0.1040 0.1030 0.1030 0.1040 Al.sub.2O.sub.3 (wt %) 1.92 1.91 1.34 1.36 1.26 1.28 1.28 MgO (wt %) 4.71 4.68 4.46 4.51 4.50 4.51 4.51 Co (ppm) — — — — — — — Er.sub.2O.sub.3 (ppm) — — — — — — — Cr.sub.2O.sub.3 (ppm) — — — — — — — LTD (%, D65, 2°) 91.67 91.45 91.23 91.02 90.80 90.58 90.37 a* (D65, 10°) −0.09 −0.21 −0.33 −0.45 −0.57 −0.69 −0.81 b* (D65, 10°) 0.15 0.22 0.29 0.36 0.43 0.51 0.58 N 0.17 0.30 0.44 0.58 0.72 0.86 0.99 N.sub.x* 0.26 0.31 0.35 0.43 0.52 0.64 0.67 N.sub.5* (extrapolated) 1.47

(25) TABLE-US-00006 TABLE 2(b) Comparative Solar low-iron Examples: glass sheets EX2.1 EX2.2 EX2.3 EX2.4 EX2.4 Thickness 0.72 0.98 2.09 3.79 3.99 (mm) CaO (wt %) 7.93 7.90 7.93 7.90 7.93 K.sub.2O (wt %) 0.02 0.02 0.02 0.02 0.02 Na.sub.2O (wt %) 13.96 13.91 13.91 13.91 13.95 Fe.sub.2O.sub.3 (wt %) 0.0120 0.0116 0.0120 0.0116 0.0128 Al.sub.2O.sub.3 (wt %) 1.34 1.33 1.32 1.34 1.33 MgO (wt %) 4.46 4.45 4.46 4.44 4.53 Co (ppm) — — — — — Er.sub.2O.sub.3 (ppm) — — — — — Cr.sub.2O.sub.3 (ppm) — — — — — LTD 91.75 91.80 91.67 91.45 91.57 (%, D65, 2°) a* (D65, 10°) −0.03 −0.02 −0.07 −0.16 −0.14 b* (D65, 10°) 0.20 0.19 0.19 0.15 0.15 N 0.20 0.19 0.20 0.22 0.21 N.sub.x* 0.13 0.12 0.15 0.20 0.18 N.sub.5* 0.21 (extrapolated)

(26) TABLE-US-00007 TABLE 2(c) Low-iron Comparative glass sheets Examples: with cobalt EX2.6 EX2.7 EX2.8 EX2.9 EX2.10 Thickness 3.86 5.87 7.69 9.75 11.95 (mm) CaO (wt %) 7.93 7.93 7.93 7.93 7.93 K.sub.2O (wt %) 0.02 0.02 0.02 0.02 0.02 Na.sub.2O (wt %) 13.95 13.95 13.95 13.95 13.95 Fe.sub.2O.sub.3 (wt %) 0.0121 0.0122 0.0121 0.0132 0.0120 Al.sub.2O.sub.3 (wt %) 1.33 1.33 1.33 1.33 1.33 MgO (wt %) 4.53 4.53 4.53 4.53 4.53 Co (ppm) 0.3 0.3 0.3 0.3 0.3 Er.sub.2O.sub.3 (ppm) — — — — — Cr.sub.2O.sub.3 (ppm) — — — — — LTD 91.6 91.5 91.4 91.2 91.0 (%, D65, 2°) a* (D65, 10°) −0.15 −0.25 −0.33 −0.43 −0.53 b* (D65, 10°) 0.09 0.07 0.07 0.05 0.09 N 0.17 0.26 0.34 0.43 0.54 N.sub.x* 0.19 0.29 0.37 0.47 0.57 N.sub.5* 2.23 (extrapolated)

(27) Set 1: Glass Sheets Comprising Erbium

(28) EX3 example corresponds to a glass sheet according to the invention with a 2 mm-thickness, comprising an amount of total iron (˜0.015 wt %) close to that of a classical low-iron glass of the state-of-the-art and in which erbium was added according to an embodiment of the invention.

(29) The glass sheet of EX3 was produced industrially and its optical properties were measured and shown at Table 2(d).

(30) EX4.x examples correspond to glass sheets according to the invention with varying thicknesses, comprising an amount of total iron (˜0.01 wt %) close to that of a classical low-iron glass of the state-of-the-art and an amount of erbium (0.025 wt %) according to an embodiment of the invention. The optical properties of glass sheets of EX4.x were computed as described above and shown in Table 2(e).

(31) EX5.x examples (comparative) correspond to glass sheets (classical soda-lime glass matrix) with varying thicknesses, comprising an amount of total iron (0.011 wt %) close to that of a classical low-iron glass of the state-of-the-art and an amount of erbium (0.098 wt %). The optical properties of glass sheets of EX5. x were measured (EX5.1) or computed (EX5.2-5.7) as described above and shown in Table 2(f).

(32) FIG. 3 shows the evolution of a*, b* according to thickness, for each EX1.x (clear glasses); EX2.x (low-iron glasses), EX3 (invention). The evolution of a*, b* according to thickness for EX4.x was already exposed at FIG. 2, in order to illustrate the general concept of the invention.

(33) One can clearly observe from those results that, in a known and predictable manner, for a clear glass sheet and also for a low-iron glass sheet, color is significantly shifted (in this case towards greenish aspect) while thickness increases. Conversely, the glass sheet of the invention, which has a very low N* factor at available thickness from industrial production (1.99 mm), will keep such a low N* factor at 5 mm thickness (a*, b* remains sensibly constant when thickness increases, mostly compared to sheets from the art). This is illustrated when looking at FIGS. 2 and 3. Table 2(f) with EX5.x also show that if the requirement of low value for N* factor is not addressed, one cannot get constant color/neutrality irrespective of glass thickness.

(34) TABLE-US-00008 TABLE 2(d) Example according to the invention EX3 Thickness (mm) 1.99 CaO (wt %) 0.96 K.sub.2O (wt %) 1.06 Na.sub.2O (wt %) 15.81 Fe.sub.2O.sub.3 (wt %) 0.0147 Al.sub.2O.sub.3 (wt %) 5.79 MgO (wt %) 9.52 Co (ppm) 1.0 Er.sub.2O.sub.3 (ppm) 479 Cr.sub.2O.sub.3 (ppm) 7.0 LTD (%, D65, 2°) 91.61 a* (D65, 10°) 0.04 b* (D65, 10°) 0.11 N 0.12 N.sub.x* 0.03

(35) TABLE-US-00009 TABLE 2(e) Examples according to the invention EX4.1 EX4.2 EX4.3 EX4.4 EX4.5 EX4.6 EX4.7 Thickness (mm) 5 10 15 20 25 30 35 Fe.sub.2O.sub.3 (wt %) 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Er.sub.2O.sub.3 (ppm) 0.025 0.025 0.025 0.025 0.025 0.025 0.025 LTD (%, D65, 2°) 91.57 91.26 90.95 90.64 90.33 90.03 89.72 a* (D65, 10°) 0.04 0.04 0.05 0.05 0.05 0.06 0.06 b* (D65, 10°) 0.09 0.11 0.12 0.14 0.15 0.17 0.19 N 0.10 0.12 0.13 0.15 0.16 0.18 0.19 N.sub.x* 0.01 0.03 0.05 0.06 0.08 0.09 0.11

(36) TABLE-US-00010 TABLE 2(f) Comparative Examples: Low-iron glass sheets with erbium EX5.1 EX5.2 EX5.3 EX5.4 EX5.5 EX5.6 EX5.7 Thickness (mm) 5 10 15 20 25 30 35 Fe.sub.2O.sub.3 (wt %) 0.011 0.011 0.011 0.011 0.011 0.011 0.011 Er.sub.2O.sub.3 (ppm) 0.098 0.098 0.098 0.098 0.098 0.098 0.098 LTD (%, D65, 2°) 83.0 75.2 68.0 61.6 55.8 50.6 45.9 a* (D65, 10°) 1.96 3.74 5.41 6.96 8.40 9.73 10.96 b* (D65, 10°) 1.13 2.09 2.97 3.77 4.49 5.14 5.72 N 2.26 4.29 6.17 7.91 9.52 11.00 12.37 N.sub.x* 2.19 4.22 6.10 7.84 9.45 10.93 12.30

(37) Set 2: Glass Sheets Comprising Selenium and Cobalt

(38) Hereunder is shown how the use of selenium and cobalt together with a low N*.sub.5 according to the invention allows to reach a significantly constant color whatever the glass thickness.

(39) EX6.1 and 6.2 examples, prepared in lab, correspond to soda-lime-silica glass sheets according to the invention with a 5 mm thickness, comprising an amount of total iron (˜0.01 wt %) close to that of a classical low-iron glass of the state-of-the-art and in which selenium and cobalt are present according to an embodiment of the invention.

(40) The optical properties of glass sheets of EX6.1 and 6.2 were measured on the lab samples. The effect of some added cobalt was computed as described above. Results are shown in Table 3(a).

(41) EX6.3 to 6.9 examples (comparative) correspond to glass sheets (classical soda-lime glass matrix) with varying thicknesses, comprising an amount of total iron (0.01 wt %) close to that of a classical low-iron glass of the state-of-the-art and amounts of selenium an cobalt. The optical properties of glass sheets were measured (EX6.3) or computed (EX6.4-6.9) as described above and shown in Table 3(b).

(42) TABLE-US-00011 TABLE 3(a) Examples according to the invention EX6.1 EX6.2 Thickness (mm) 5 5 Fe.sub.2O.sub.3 (wt %) 0.0102 0.0102 Se (ppm) 6 7 Co (ppm) 0.4 0.5 LTD (%, D65, 2°) 90.95 90.94 a* (D65, 10°) 0.01 0.01 b* (D65, 10°) 0.08 0.08 N 0.08 0.08 N.sub.x* 0.02 0.02

(43) TABLE-US-00012 TABLE 3(b) Comparative Examples: Low-iron glass sheets with selenium and cobalt EX6.3 EX6.4 EX6.5 EX6.6 EX6.7 EX6.8 EX6.9 Thickness (mm) 5 10 15 20 25 30 35 Fe.sub.2O.sub.3 (wt %) 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Se (ppm) 9 9 9 9 9 9 9 Co (ppm) 3 3 3 3 3 3 3 LTD (%, D65, 2°) 90.3 88.7 87.2 85.7 84.3 82.8 81.4 a* (D65, 10°) 0.04 0.05 0.06 0.07 0.08 0.09 0.11 b* (D65, 10°) −0.20 −0.47 −0.75 −1.02 −1.28 −1.55 −1.81 N 0.20 0.48 0.75 1.02 1.29 1.55 1.81 N.sub.x* 0.28 0.55 0.83 1.10 1.37 1.63 1.89

(44) Set 3: Glass Sheets Comprising Chromium and Manganese

(45) Hereunder is shown how the use of chromium and manganese together with a low N*.sub.5 according to the invention allows to reach a significantly constant color whatever the glass thickness.

(46) EX7 and EX8 examples, prepared in lab, correspond to soda-lime silica glass sheets according to the invention with a 5 mm thickness, comprising low amount in total iron and in which chromium, manganese and cobalt are present according to an embodiment of the invention. EX9 and EX10 examples, prepared in the lab, correspond to comparative soda-lime silica glass sheets with a 5 mm thickness. The optical properties of glass sheets of EX7-10 were measured and the effect of added cobalt was computed as described above. Results are shown in Table 4(a).

(47) EX11.x examples (comparative) correspond to glass sheets (classical soda-lime glass matrix) with varying thicknesses, comprising an amount of total iron (0.011 wt %) close to that of a classical low-iron glass of the state-of-the-art and amounts of chromium and manganese. The optical properties of glass sheets of EX11.x were measured (EX11.1) or computed (EX11.2-11.7) as described above and shown in Table 4(b).

(48) TABLE-US-00013 TABLE 4(a) EX7 EX8 EX9 EX10 Thickness (mm) 5 5 5 5 Fe.sub.2O.sub.3 (wt %) 0.0246 0.0107 0.0116 0.0101 Cr.sub.2O.sub.3 (ppm) 48 9 88 9 MnO (ppm) 332 300 3740 50 Co (ppm) 10.1 3.6 19.2 1.9 LTD (%, D65, 2°) 84.87 89.68 34.29 90.66 a* (D65, 10°) 0.03 0.02 20.26 −0.43 b* (D65, 10°) 0.08 0.08 0.08 0.08 N 0.09 0.08 20.26 0.44 N.sub.5* 0.00 0.01 20.23 0.46

(49) TABLE-US-00014 TABLE 4(b) Comparative Examples: Low-iron glass sheets with chromium and manganese EX11.1 EX11.2 EX11.3 EX11.4 EX11.5 EX11.6 EX11.7 Thickness (mm) 5 10 15 20 25 30 35 Fe.sub.2O.sub.3 (wt %) 0.011 0.011 0.011 0.011 0.011 0.011 0.011 Cr.sub.2O.sub.3 (ppm) 25 25 25 25 25 25 25 MnO (ppm) 205 205 205 205 205 205 205 LTD (%, D65, 2°) 91.1 90.4 89.7 89.1 88.4 87.8 87.3 a* (D65, 10°) 0.60 1.12 1.61 2.06 2.48 2.86 3.22 b* (D65, 10°) −0.11 −0.28 −0.43 −0.57 −0.69 −0.80 −0.90 N 0.61 1.16 1.67 2.14 2.57 2.97 3.34 N.sub.x* 0.59 1.15 1.65 2.13 2.56 2.96 3.33