Illumination optical system for microscope, and microscope

1. An illumination optical system, comprising in order from a light source side:

a collector lens;

a field stop;

a field lens having positive power;

an aperture stop; and

a collective lens having positive power, wherein:

the illumination optical system is a substantially both-side telecentric optical system between the field stop and a sample surface; and

the following conditional expressions are satisfied where D FS indicates a diameter, in mm, of the field stop, β indicates a magnification from the sample surface to the field stop, and NA indicates a numerical aperture on the sample surface side of the illumination optical system:

15 ≧D FS /β≧9  (1)

0.85 ≧NA≧ 0.5  (2).

2. The illumination optical system according to claim 1 , wherein:

the collective lens comprises a lens having positive power and having a convex surface of a small radius of curvature on the aperture stop side, and

the field lens comprises, from the light source side, at least:

a first lens group having positive power; and

a second lens group having positive power; and

the following conditional expression is satisfied where f indicates a composite focal length, in mm, of the collective lens and the second lens group, and L indicates a distance, in mm, from an end portion on the light source side of the second lens group to the sample surface:

0.5 ≧f/L≧ 0.3  (3).

3. The illumination optical system according to claim 1 , further comprising between the collector lens and the field stop in order from the light source side:

a light diffusion element; and

an adjusting lens having positive power.

4. The illumination optical system according to claim 3 , wherein the following conditional expression is satisfied where f 1 indicates a focal length, in mm, of the adjusting lens, and d indicates a distance, in mm, from the adjusting lens to the field stop:

15 ≧f 1 /d≧ 2  (4).

5. The illumination optical system according to claim 1 , wherein the field lens comprises in order from the light source side:

a cemented lens of a negative lens and a positive lens; and

a positive single lens.

6. The illumination optical system according to claim 5 , wherein the positive single lens is a planoconvex lens.

7. The illumination optical system according to claim 5 , wherein the positive single lens is a meniscus lens.

8. The illumination optical system according to claim 1 , wherein the collective lens comprises in order from the light source side:

a cemented lens of a positive lens and a negative lens; and

a positive single lens.

9. The illumination optical system according to claim 8 , wherein the negative lens is a planoconcave lens.

10. A microscope comprising:

an illumination optical system; and

a light source for emitting light to the illumination optical system,

wherein:

the illumination optical system comprises in order from the light source side:

a collector lens;

a field stop;

a field lens having positive power;

an aperture stop; and

a collective lens having positive power,

the illumination optical system is a substantially both-side telecentric optical system between the field stop and a sample surface; and

the following conditional expressions are satisfied where D FS indicates a diameter, in mm, of the field stop, β indicates a magnification from the sample surface to the field stop, and NA indicates a numerical aperture on the sample surface side of the illumination optical system:

15 ≧D FS /β≧9  (1)

0.85 ≧NA≧ 0.5  (2).