1. Field of the Invention
The present invention relates to an interferometer apparatus for observing a wavefront form from a sample, an unequal optical path length interferometer apparatus such as that of Fizeau type in particular. Specifically, the present invention relates to an interferometer apparatus for both low and high coherence measurement, which can interferentially measure a surface wavefront form and a transmitted wavefront form of a sample such as a thin, flat transparent sheet, e.g., glass for liquid crystal, and various optical filters and windows, and a sphere, e.g., ball lens, at the same time.
2. Description of the Prior Art
As an example of techniques for measuring a plane parallel glass sheet, it has conventionally been known to use a Fizeau interferometer mounted with a highly coherent laser light source. In this technique, since highly coherent laser light is used, not only interference fringes of a sample surface in the plane parallel glass sheet, but also those of a non-sample surface on the side opposite from the sample surface may occur. Namely, a Fizeau interferometer in which reference light and object light yield respective optical path length differences different from each other (unequal optical path length interferometer) uses a highly coherent laser luminous flux, whereby optical interference may occur between the reference surface and the sample surface, between the reference surface and the non-sample surface, and between the sample surface and the reference surface. Since only the optical interference between the reference surface and the sample surface is usually desired, interference fringes occurring between the other surfaces become noises, which makes it difficult to measure the surface of the sample surface with high accuracy.
As a technique for suppressing such interference fringe noises, it has been known to coat the non-sample surface with a refractive index matching oil, and attach a light-scattering sheet thereon, so as to optically extinguish the non-sample surface and handle thus attached light-scattering sheet as if a non-sample surface, thereby preventing interference fringes from occurring due to the optical interference between the non-sample surface and other surfaces.
However, such an interference fringe noise suppressing technique requires an oil to be applied to one surface of a sample though it is a non-sample surface, which is not only troublesome but also smudges the sample. Further, a thin sample may change the shape of its sample surface because of such processing as coating with the oil and attaching of the light-scattering sheet.
In this regard, the assignee of the present invention has already proposed a technique disclosed in Japanese Unexamined Patent No. HEI 9-21606 (hereinafter referred to as “Patent Document 1”).
The interferometer for measuring a transparent thin sheet disclosed in Patent Document 1 provides a path-matching optical system for setting a coherent length of measurement light shorter than a predetermined length and bypassing a part of the measurement light, so that no optical interference occurs except for that between the reflected light from the sample surface derived from forwardly advancing measurement light without bypassing and the reflected light from the reference light derived from the bypassed measurement light, whereby a clear interference image without noises can be obtained in a quite simple configuration.
In Fizeau interferometer apparatus, light from a light source is turned into parallel light, which illuminates a reference plate, whereas a sample separated by a predetermined distance from the reference plate in parallel therewith is irradiated with the parallel light transmitted through the reference plate, and interference fringes are formed between the reflected light from a reference surface of the reference plate and the reflected light from a sample surface of the sample. As compared with interferometer apparatus such as those of Michelson type, Fizeau interferometer apparatus are advantageous in that highly accurate measurement can be carried out in a simple configuration, and so forth. Their greatest attraction lies in that they can easily measure transmitted wavefronts of transparent samples, e.g., internal distortions and refractive index distributions of transparent samples.
However, the interferometer of the above-mentioned Patent Document 1 is hard to measure transmitted wavefronts of transparent samples in terms of operations, and thus may not fully take advantage of Fizeau interferometer apparatus.