Position detecting method for detecting a positional relationship between a first object and a second object

A method of detecting a gap between a first object and a second object, said method comprising steps of:

• projecting a first light at an angle and in a first direction onto the second object such that the first light is reflected by the second object;
• deflecting by a convex lens pattern of the first object the reflected first light and projecting the deflected first light onto a first detection plane;
• projecting a second light at an angle and in a second direction different from the first direction onto the second object such that the second light is reflected by the second object;
• deflecting by a convex lens pattern of the first object the reflected second light and projecting the deflected second light onto the first detection plane;
• wherein a first spacing between positions of incidence of the deflected first light and the deflected second light on the first detection plane increase with displacement of the second object relative to the first object in a predetermined direction;
• projecting a third light at an angle and in a third direction onto the second object such that the third light is reflected by the second object;
• deflecting by a concave lens pattern of the first object the reflected third light and projecting the defelcted third light onto a second detection plane;
• projecting a fourth light at an angle and in a fourth direction different from the third direction onto the second object such that the fourth light is reflected by the second object;
• deflecting by a concave lens pattern of the first object the reflected fourth light and projecting the deflected fourth light onto the second detection plane;
• wherein a second spacing between positions of incidence of the deflected third light and the deflected fourth light on the second detection plane decreases with displacement of the second object relative to the first object in the predetermined direction;
• and
• determining a reference for the detection of a gap between the first object and the second object on the basis of the first spacing between positions of incidence of the deflected first light and the deflected second light on the first detection plane and the second spacing between positions of incidence of the deflected third light and the deflected fourth light on the second detection plane, corresponding to the first spacing and the second spacing being substantially equal to each other.