Tube lens unit with chromatically compensating effect

1. A tube lens unit, containing various focal lengths, with chromatically compensating effect for use with objectives of infinite image distance and chromatic residual errors,

wherein

the tube lens unit (TL) satisfies the following conditions:

CHL=CHL o −A T 2 m T φ T =0  Equation (1)

CHV=CHV o −1000 λm T φ T d 1 =0  Equation (2)

 where

CHL is the longitudinal chromatic aberration of the combination of tube lens unit plus objective,

CHL o is the longitudinal chromatic aberration of the objective,

CHV is the chromatic difference of magnification of the combination of tube lens unit plus objective,

CHV o is the chromatic difference of magnification of the objective,

λ is the wavelength,

m T is the dispersion index of the optical elements in the tube lens unit, derived from m T =m e =1/λν, in which e is the index of the main color and ν the Abbe number,

A T is the radius of the exit pupil of the objective which is substantially equal to the entrance pupil of the lens unit,

d 1 is the distance of the first optical element of the tube lens unit from the entrance pupil,

φ T is the refractive power, and

T is the index of the respective lens in the tube lens unit,

A T 2 m T φ T =CHL T is the longitudinal chromatic aberration of the tube lens unit, and

1000λ e m T φ T d 1 =CHV T is the chromatic difference of magnification of the tube lens unit.

2. A tube lens unit as claimed in claim 1 , further comprising at least one compensating optical element which, allowing for equations (1) and (2), is determined by

K 11 φ 1 m 1 +K 12 φ 2 m 2 =−CHL o , and  Equation (3)

K 21 φ 1 m 1 +K 22 φ 2 m 2 =−CHV o ,  Equation (4)

the values of K 11 , K 12 , K 21 and K 22 being coefficients determined from ray tracing with φ i .

3. A tube lens unit as claimed in claim 2 , further comprising one compensating optical element, which is configured as a biconvex lens.

4. A tube lens unit as claimed in claim 2 , further comprising two compensating optical elements, a first of which is configured as a biconvex lens and second one as a cemented component.

5. A tube lens unit as claimed in claim 4 , wherein the cemented component comprises a biconvex and a planoconvex lens, with the plane surface arranged to face the eyepiece.

6. A tube lens unit as claimed in claim 2 , further comprising two compensating optical elements and both configured as cemented components.