Reference lens for interferometer and interferometer that uses it

A reference lens for an interferometer, the reference lens being constructed so that light that is transmitted by a partially reflecting reference spherical surface of the reference lens may first be converged toward a focal point that coincides with the center of curvature of the partially reflecting reference spherical surface and then reflected from a subject spherical surface that also has its center of curvature at said focal point, so that light reflected from the subject spherical surface is transmitted back through the partially reflecting reference spherical surface and interferes with light that is initially reflected by the partially reflecting reference spherical surface, thereby enabling measurement of the shape of the subject spherical surface, said reference lens comprising, a negative meniscus lens component at a first side of the reference lens, a positive lens group, and a positive meniscus lens component at a second side of the reference lens nearest where the focal point is located;

• wherein
• the concave lens surface of said negative meniscus lens component is at said first side;
• the concave lens surface of said positive meniscus lens component is at said second side;
• and
• the following Conditions (1) and (2) are satisfied:
• −1.5<R1/f<−1.2 Condition (1)0.5<RL/f<0.7 Condition (2)
• where
• R1 is the radius of curvature of said concave lens surface of said negative meniscus lens component;
• f is the focal length of the reference lens, and
• RL is the radius of curvature of said concave lens surface of said positive meniscus lens component.