Level detection apparatus

1. A level detection apparatus comprising:

an illumination slit in which a rectangular first opening which causes illumination light to pass is formed;

an optical system configured to illuminate a target object surface by illumination light passing through the illumination slit and focuses reflected light from the target object surface;

first and second detection slits which are arranged in front of and in back of a focal point and in each of which a second opening is formed such that a short side of a rectangle is shorter than a short side of a illumination slit image formed by the illumination slit and a long side of the rectangle is larger than a long side of the illumination slit image;

first and second light amount sensors configured to detect amounts of light of the reflected lights passing through the first and second detection slits; and

a calculating unit configured to calculate a level of the target object surface based on outputs from the first and second light amount sensors.

2. The level detection apparatus according to claim 1 , wherein the optical system includes:

a polarizing beam splitter which guides illumination light to the target object surface side; and

an ¼ wavelength plate arranged on the target object surface side with respect to the polarizing beam splitter.

3. The level detection apparatus according to claim 1 , wherein the illumination slit is formed in a shape which is obtained such that a plurality of the first openings are combined to each other to have directions at different angles, and

the first and second detection slits are formed in shapes each of which is obtained such that a plurality of the second openings are combined with each other to have directions at different angles.

4. The level detection apparatus according to claim 3 , wherein the illumination slit is formed in a shape which is obtained such that two of the first openings are combined with each other to have longitudinal directions perpendicular to each other, and

the first and second detection slits are formed in shapes each of which is obtained such that two of the second openings are combined with each other to have longitudinal directions perpendicular to each other.

5. The level detection apparatus according to claim 3 , wherein the illumination slit is formed in a shape which is obtained such that four of the first openings are combined with each other to be shifted from each other at 45°, and

the first and second detection slits are formed in shapes each of which is obtained such that four of the second openings are combined to each other to be shifted from each other at 45°.

6. The level detection apparatus according to claim 1 , wherein the directions of the first and second openings are synchronously switched.

7. The level detection apparatus according to claim 1 , further comprising a beam splitter which splits the reflected light passing through the first and second detection slits.

8. The level detection apparatus according to claim 7 , wherein the first light amount sensor detects an amount of light of one of the split reflected lights, and the second light amount sensor detects an amount of light of the other of the split reflected lights.

9. The level detection apparatus according to claim 1 , wherein the illumination slit is formed in a shape which is obtained such that a plurality of the first openings having different angles do not overlap, and

the first and second detection slits are formed in shapes each of which is obtained such that a plurality of the second openings having different angles do not overlap.

10. The level detection apparatus according to claim 9 , wherein the illumination slit and the first and second detection slits are switched such that the first and second openings having the same directions are arranged in an optical path.