GB2233781A - Optical systems - Google Patents

Abstract

An achromatic lens system comprising at least two glass lenses, one lens being made of a first glass chosen to have the property known as a shortened blue partial (as hereinbefore defined), a refractive index not greater than 1.70, and a V@ number less than 55, the glass comprising at least the following components: SiO2, R2O and ZrO2 where R2O is chosen from Li2O, Na2O and K2O, the said components together forming at least 30% by weight of the glass (the balance consisting of compatible components), and the other lens is made from a second glass with a V number of 50 or more.

Description

OPTICAL SYSTEMS The invention relates to optical systems, more particularly it relates to optical systems in which one or more lenses are employed which have the property known as a shortened blue partial.

The optical properties of the glass can be expressed in terms of the refractive index of the glass for certain recognised standard wavelengths. In order to express these properties the following definitions are employed: ne is the refractive index of the glass for the green mercury line e of wavelength 4651 Angstron units ng is the refractive index of the glass for the blue mercury line g of wavelength 4358 Angstrom units is is the refractive index of the glass for the blue cadmium line F' of wavelength 4800 Angstrom units nc' is the refractive index of the glass for the red cadmium line C' of wavelength 6438 Angstrom units nd is the refractive index of the glass for the yellow helium line d of wavelength 5876 Angstrom units nf is the refractive index of the glass for the blue hydrogen line F of wavelength 4861 Angstrom units nc is the refractive index of the glass for the red hydrogen line c of wavelength 6563 Angstrom units Since lines d and e are near the middle of the visible spectrum, nd and/or ne are often referred to as the mean refractive index.

The partial dispersions of the glass are given by: n g - n e Blue End nf - nd Blue End nd - nc Red End Further the mean dispersions of the glass are: nf' - n nf - nc The relative dispersions are as f9llows:- ng - ne Blue End nf' - nc' ng - ne Blue End nf - nc nd - nc Red end nf - nc flf - ne Blue End nf - nc ng - n Blue End nf - nc The improved shortening of the blue partial dispersion of glass according to the invention as compared with glasses hitherto available can be shown by comparing the relative blue partial dispersion for glass according to the formula n9 - ne = 1.10802 - 0.0019501 Ve nf' - nc Where Ve = n e I 1 nF' - nc' If the partial dispersion of a glass for any particular wavelength of light is plotted avainst V number, glasses with a normal partial will obey a straight line relationship. Glasses which deviate from this line and are not normal, can have either a "lengthened" or a "shortened" partial depending on the direction of deviation from the straight line. Thus if one can measure the extent of the deviation this provides means of comparing one glass with another as to the extent to which the partial is "lengthened" or "shortened.

The equation of the partial against V number is Pxy = axy + bxyVd The abnormal glasses lie off the straight line so that the equation becomes Pxy axy + bxyVd + DP xy The value of xy is a measure of the deviation of the particular partial.

As indicated above, the partial dispersion can be referred to by the relationship between the refractive index values for any particular glass at particular wavelengths referred to the line of the spectrum due to a particular material. In order to avoid difficulties due to measurement of nf, the following expression has been used to determine value for comparison between glasses:

This enables the value of D to be determined, and if negative means the particular glass being measured has a "shortened" blue partial, the more negative the value, the more extreme the glass.

We have now found that glasses with a shortened blue partial exist suitable for use in manufacturing lenses for use in optical systems where this property is desired within a composition area previously not recognised as possessing this attribute. This composition area is one in which the essential components are Si02, R20, and ZrO2 where R20 is chosen from Li20 and Na2O. The presence of ZrO2 not only surprisingly produces the shortened blue partial but also increases the durability of the glass so that lenses made from it are resistant to attack and degradation.Many of the available glasses with shortened blue partials have a durability which makes them undesirable for use in optical systems because of the relatively harsh conditions experienced by the glass during processing, for instance the glass needs to withstand attacks from both acid and alkali acid solutions.

According to the invention, there is provided an achromatic lens system comprising at least two glass lenses, one lens being made of a glass chosen to have the property known as a shortened blue partial, a refractive index not greater than 1.70, and a Ve number less than 55, the glass comprising at least the following components SiO2, R20 and ZrO2 where R20 is chosen from Li2O, Na20 and K20, preferably from Li20 and Na20 the said components together forming at least 30% by weight of the glass (the balance consisting of compatible components), the presence of Zr02 making a major contribution to the glass having a shortened blue partial, and the other lens is made from a glass with a V number of 50 or more.

Preferably the glass also comprises a component of Yb203 to reduce the amount of Li20, Na20 or K20 necessary.

As indicated above we measure the value of D in order to provide an indication of the shortened blue partial dispersion.

The table below demonstrates the wide range of glasses which can be made with shortened blue partials and used to form lenses for use in the optical systems of the present invention. The compositions are given in weight%; NO ZrO2 SiO2 LiO2 NaO2 B2O3 Al2O3 PbO Sb2O3 Others Ne Ve D 1 16.00 55.00 4.00 .00 .00 .00 .00 .00 CaO 25.00 1.636 51.99 -139 2 20.00 50.00 .00 15.00 .00 .00 .00 .50 K2O 5.00 1.593 48.83 -114 Yb2O3 9.50 3 18.00 60.00 4.50 7.60 4.00 5.90 .00 .00 1.568 53.81 -110 4 21.77 66.47 3.67 8.09 .00 .00 .00 .00 1.574 52.45 -107 5 18.76 49.90 7.03 .00 5.78 .00 18.53 .00 1.635 44.85 -106 6 15.82 42.07 5.93 .00 4.87 .00 .00 .00 Ta2O5 30.94 1.670 41.78 -102 7 16.47 68.84 4.78 9.91 .00 .00 .00 .00 1.559 54.09 -101 8 19.40 50.86 .00 15.62 .00 .00 13.52 .50 1.609 43.69 -100 9 6.12 58.00 5.94 7.09 17.00 2.85 .00 .00 TiO2 3.00 1.559 54.63 -98 10 19.36 86.17 4.70 9.75 .00 .00 .00 .00 1.571 52.76 -98 11 21.32 64.27 5.64 8.77 .00 .00 .00 .00 1.581 52.01 -96 12 19.00 56.00 2.50 7.50 .00 3.00 7.00 .00 K2O 5.00 1.589 49.03 -96 13 17.51 40.00 .00 15.71 10.16 1.11 15.00 .50 1.616 43.84 -93 14 17.52 65.50 4.25 12.73 .00 .00 .00 .00 1.566 52.89 -89 15 18.00 55.00 4.50 4.00 .00 .00 18.50 .00 1.622 44.19 -89 16 19.00 50.00 7.10 .00 6.90 .00 17.00 .00 1.632 45.64 -88 17 18.00 60.00 4.50 4.00 .00 .00 13.50 .00 1.601 47.12 -87 NO ZrO2 SiO2 LiO2 NaO2 B2O3 Al2O3 PbO Sb2O3 Others Ne Ve D 18 18.00 65.00 .00 8.50 .00 .00 .00 .00 K2O 8.50 1.553 53.38 -87 19 20.42 69.69 9.90 .00 .00 .00 .00 .00 1.582 53.44 -87 20 15.73 49.85 .00 15.82 .00 .00 .00 18.60 1.615 41.34 -86 21 23.15 63.01 6.08 7.76 .00 .00 .00 .00 1.589 53.35 -85 22 12.80 50.00 3.20 .00 .00 .00 .00 .00 CaO 34.00 1.651 51.66 -84 23 15.75 71.04 4.30 8.91 .00 .00 .00 .00 1.553 54.79 -84 25 10.00 60.50 4.50 14.50 4.00 2.00 .00 .00 TiO2 4.50 1.574 49.53 -82 26 18.32 58.07 .00 13.43 5.18 .00 .00 .00 1.569 52.20 -81 27 18.00 60.00 4.50 13.50 4.00 .00 .00 .00 1.576 52.87 -81 28 18.00 43.50 .00 15.70 7.50 .00 15.10 .00 1.617 43.82 -81 29 5.71 55.59 12.26 5.75 16.13 .00 .00 .00 CaO .87 1.583 53.17 -80 TiO2 3.70 30 17.40 40.86 .00 15.62 10.00 .00 15.52 .50 1.619 43.71 -79 31 17.51 41.11 .00 15.71 10.06 .00 15.61 .00 1.618 43.98 -79 32 18.40 54.50 4.60 4.00 .00 .00 18.50 .00 1.624 44.55 -75 33 19.05 59.84 4.62 9.58 .00 .00 .00 .00 TiO2 6.91 1.616 44.23 -73 34 18.46 45.99 .00 15.67 5.03 .00 14.72 .10 1.613 44.12 -73 35 18.45 62.99 .00 18.56 .00 .00 .00 .00 1.559 51.76 -71 36 17.51 40.00 .00 15.71 10.16 15.30 .00 1.616 44.00 -71 NO ZrO2 SiO2 LiO2 NaO2 B2O3 Al2O3 PbO Sb2O3 Others Ne Ve D 37 19.40 57.91 4.13 11.69 .00 .00 .00 .30 TiO2 6.57 1.616 44.24 -70 38 12.76 39.86 3.19 .00 .00 .00 .00 .00 SrO 44.19 1.659 50.01 -70 39 16.78 67.85 4.78 10.59 .00 .00 .00 .00 1.562 53.81 -69 40 17.51 41.11 .00 15.71 10.16 .00 15.00 .50 1.617 44.09 -69 41 16.00 50.00 4.00 .00 .00 .00 .00 .00 CaO 30.00 1.652 51.37 -69 42 19.40 44.86 .00 15.62 6.00 .00 13.52 .50 1.617 44.13 -68 43 18.00 60.00 5.26 4.00 .00 .00 .00 .00 MgO 6.00 1.609 48.38 -67 Nb2O5 6.74 44 19.50 44.86 .00 15.62 3.00 3.00 13.52 .50 1.613 43.96 -66 45 19.90 50.86 1.00 14.62 .00 .00 13.52 .10 1.611 44.37 -66 46 17.51 39.31 .00 15.71 10.06 2.00 15.41 .00 1.615 44.02 -66 47 6.04 58.89 5.37 4.05 13.65 4.16 .00 .00 CaO 4.58 1.563 54.03 -65 TiO2 3.26 48 16.59 70.20 5.24 7.97 .00 .00 .00 .00 1.559 54.51 -65 49 16.00 60.00 4.00 2.00 .00 .00 .00 .00 CaO 18.00 1.611 52.72 -64 50 18.00 43.60 .00 15.20 7.50 .00 15.20 .00 1.617 43.98 -61 51 9.00 67.00 6.00 12.00 .00 .00 .00 .00 TiO2 6.00 1.573 48.29 -60 52 18.02 55.06 4.40 4.00 .00 .00 18.52 .00 1.621 44.51 -60 53 16.00 60.00 4.00 4.00 .00 .00 .00 .00 CaO 10.00 1.601 53.21 -59 MgO 6.00 NO ZrO2 SiO2 LiO2 NaO2 B2O3 Al2O3 PbO Sb2O3 Others Ne Ve D 54 7.96 58.26 5.79 6.01 14.63 4.12 .00 .00 TiO2 3.23 1.561 53.28 -58 55 7.96 58.86 5.79 6.01 14.63 4.12 .00 .00 TiO2 2.63 1.557 54.31 -57 56 17.50 42.50 .00 15.70 9.00 .00 15.30 .00 1.616 44.13 -57 57 21.33 56.52 11.49 .00 .00 .00 .00 .00 TiO2 10.66 1.665 40.32 -57 58 20.00 53.00 5.00 12.00 .00 .00 .00 .00 TiO2 10.00 1.643 40.26 -56 59 17.29 45.98 6.48 .00 5.33 .00 .00 .00 La2O3 24.93 1.656 50.62 -55 60 18.00 65.90 4.50 7.60 4.00 .00 .00 .00 1.568 54.31 -54 61 18.00 60.00 4.50 4.00 .00 .00 .00 .00 CaO 7.50 1.602 52.73 -54 62 18.79 52.65 6.67 .00 2.00 1.00 18.89 .00 1.633 44.57 -54 63 13.67 62.74 5.25 7.88 7.32 2.06 .00 .00 TiO2 1.10 1.566 54.65 -53 64 10.52 32.89 2.63 .00 .00 .00 .00 .00 BaO 53.96 1.683 49.36 -52 65 14.00 69.00 5.00 12.00 .00 .00 .00 .00 1.553 54.43 -51 66 14.00 60.50 4.50 14.50 4.00 2.00 .00 .00 TiO2 .50 1.565 53.55 -50 67 7.11 52.01 .00 16.09 13.06 3.68 8.05 .00 1.554 53.41 -50 68 16.00 60.00 4.00 4.00 .00 .00 .00 .00 CaO 13.00 1.604 53.10 -49 MgO 3.00 69 17.40 40.86 .00 15.62 10.00 .00 15.52 .50 1.619 43.88 -47 70 18.00 60.00 4.50 7.50 5.00 .00 .00 .00 K2O 5.00 1.574 53.67 -43 71 12.00 59.00 7.00 12.00 .00 .00 .00 .00 TiO2 10.00 1.614 43.02 -41 NO ZrO2 SiO2 LiO2 NaO2 B2O3 Al2O3 PbO Sb2O3 Others Ne Ve D 72 21.73 56.52 11.59 .00 .00 .00 .00 .00 TiO2 10.00 1.663 40.87 -38 73 16.00 48.00 4.00 .00 .00 .00 .00 .00 CaO 27.00 1.658 50.62 -34 ZnO 5.00 74 19.03 59.45 4.62 9.57 .00 .00 .00 .00 TiO2 7.33 1.620 43.55 -27 75 6.04 60.89 5.37 4.05 12.65 3.16 .00 .00 TiO2 3.26 1.562 54.24 -23

Claims (4)

CLAIMS.

1. An achromatic lens system comprising at least two glass lenses, one lens being made of a first glass chosen to have the property known as a shortened blue partial (as hereinbefore defined), a refractive index not greater than 1.70, and a Ve number less than 55, the glass comprising at least the following components: SiO2, R20 and ZrO2 where R20 is chosen from Liar, Na2O and K20, the said components together forming at least 30% by weight of the glass (the balance consisting of compatible components), and the other lens is made from a second glass with a V number of 50 or more.

2. An achromatic lens system according to claim 1, wherein R20 is chosen from Li20 and Na2O.

3. An achromatic lens system, wherein the first glass also comprises Yb203.

4. An achromatic lens system according to claim 1 and incorporating a glass having a composition as set out in the foregoing Table.