Optical beam expanders with materials chosen to effect athermalization

Claim 2

An optical substantially afocal infra-red radiation beam expander comprising spaced negative and positive lens components employing different infra-red radiation transmitting materials having different respective refractive index changes with respect to temperature chosen such that the refractive index change due to a temperature change in one component is at least partially compensated by the refractive index change due to the temperature change in the other component whereby the infra-red radiation beam expander is substantially athermalised with respect to afocality.

Claim 3

An optical substantially afocal infra-red radiation beam expander comprising a lens element of negative power of a first infra-red radiation transmitting material spaced from a lens element of positive power of a second infra-red radiation transmitting material different from the first, wherein the respective coefficients of refractive index change with respect to temperature of the first and second materials are related in a manner substantially to athermalised the infra-red radiation beam expander with respect to afocality.

Claim 4

An optical substantially afocal infra-red radiation beam expander comprising a lens element of negative power of a first infra-red radiation transmitting material spaced from a lens element of positive power of a second infra-red radiation transmitting material different from the first, wherein the respective coefficients of refractive index change with respect to temperature of the first and second materials are related in a manner substantially to athermalised the infra-red radiation beam expander with respect to afocality.

Claim 5

An infra-red radiation beam expander according to claim 2 in which the magnification and the refractive index coefficients of the two materials substantially satisfy (at a design temperature) the relationship: ##EQU9## where (y2)/(y1) is the magnification at the design temperature, (dn)/(dT)1 is the rate of change of refractive index with respect to temperature of said first material, (dn)/(dT)2 is the rate of change of refractive index with respect to temperature of said second material.

Claim 6

An infra-red radiation beam expander according to claim 3 in which the magnification and the constringences of the two materials additionally substantially satisfy (at the design temperature) the relationship: (y2)/(y1)=V2/V1 where V1 is the constringence of said first material, V2 is the constringence of said second material.

Claim 7

An infra-red radiation beam expander according to claim 2 in which said lens element of negative power is a back element of meniscus shape convex to the rear.

Claim 8

An infra-red radiation beam expander according to claim 5 in which said lens element of positive power is a front element of meniscus shape convex to the front.

Claim 9

An infra-red radiation beam expander according to claim 5 in which said lens element of positive power is a front element of meniscus shape convex to the rear.

Claim 10

An infra-red radiation beam expander according to claim 2 for operation with thermal infra-red radiation of about 10 microns wavelength in which said first and second materials are germanium and zinc selenide respectively.

Claim 11

An infra-red radiation beam expander according to claim 8 in which said lens element of negative power has refracting surfaces R1 and R2 and said lens element of positive power has refracting surfaces R3 and R4 having radii of curvature and axial separations substantially as follows: ______________________________________ Axial Surface Radius of Curvature Separation ______________________________________ R1 80.828 3.81 R2 40.319 90.17 R3 -201.739 7.62 R4 -95.476 ______________________________________

Claim 12

An infra-red radiation beam expander according to claim 8 in which said lens element of negative power has refracting surfaces R1 and R2 and said lens element of positive power has refracting surfaces R3 and R4 having radii of curvature and axial separations substantially as follows: ______________________________________ Axial Surface Radius of Curvature Separation ______________________________________ R1 -330.342 3.750 R2 -75.958 36.831 R3 -34.480 2.000 R4 -58.209 ______________________________________

Claim 15

An optical substantially afocal infra-red radiation beam expander comprising spaced negative and positive lens components employing different infra-red radiation transmitting materials having different respective coefficients of refractive index change with respect to temperature wherein the power of each of said positive and negative lens components is chosen such that the changes in the powers of said components due to their respective index changes due to a temperature change compensate for each other to substantially athermalised the infra-red radiation beam expander with respect to afocality.

Claim 16

An optical substantially afocal infra-red radiation beam expander comprising spaced negative and positive lens components employing different infra-red radiation transmitting materials having different respective coefficients of refractive index change with respect to temperature wherein the power of each of said positive and negative lens components is chosen such that the changes in the powers of said components due to their respective index changes due to a temperature change compensate for each other to substantially athermalised the infra-red radiation beam expander with respect to afocality.

Claim 17

An optical substantially afocal infra-red radiation beam expander comprising a lens element of negative power of a first infra-red radiation transmitting material spaced from a lens element of positive power of a second infra-red radiation transmitting material having a coefficient of refractive index change different from that of said first material, the magnification of the beam expander being chosen such that the infra-red radiation beam expander is substantially athermalised with respect to afocality.

Claim 18

An optical substantially afocal infra-red radiation beam expander comprising a lens element of negative power of a first infra-red radiation transmitting material spaced from a lens element of positive power of a second infra-red radiation transmitting material having a coefficient of refractive index change different from that of said first material, the magnification of the beam expander being chosen such that the infra-red radiation beam expander is substantially athermalised with respect to afocality.