IR interferometric apparatus and method for determining the thickness variation of an optical part wherein said optical part defines Newton interference fringe patterns

A method of determining variations in thicknesses of an optical part interferometrically comprising the steps of,

• focusing a converging monochromatic light beam through said optical part and receiving a return beam containing wavefronts reflected off the inner and outer surfaces of said optical part which define Newton interference fringe patterns,
• providing relative motion between said beam and said optical part, and
• detecting said interference fringes and determining thickness variations in said optical part by the change in space, position and intensity of said interference fringes.

An IR interferometric thickness test apparatus for determining any variation in thickness of an IR optical part under test comprising,

• a monochromatic optical system generating and focusing a converging beam through the optical part and generating and detecting a return beam containing interference fringes due to the reflections off the inner and outer surfaces of the optical part, the focal point of said beam being beyond the inner surface of said optical part,
• said optical system being spaced from said optical part and said surfaces forming an optical cavity whose optical path difference equals 2n.DELTA.t.

An interferometer test apparatus for determining any variation in thickness of an optical part comprising:

• an optical system for directing and focusing a monochromatic light source into a converging beam and directing said beam near normal to the optical part and through the front and back surfaces of said part and for detecting Newton fringes arising from the interaction of two wavefronts produced by the reflection of said beam off the back and front surfaces,
• said optical system being spaced from said optical part and arranged such that any change in distance between said optical system and said optical part only changes the size of the area of the front and back surfaces being subjected to said beam, and
• means for relative movement between said optical system and said optical part so that said beam illuminates adjacent surface areas of said optical part which causes changes in light intensity of said Newton fringes if the spacing between said back and front surfaces varies.

A method of interferometrically determining variations in a thickness of a optical part comprising the steps of,

• providing an optical system which directs a converging monochromatic beam through the said optical part to a focal point of said beam being beyond said optical part thereby forming an interferometer optical cavity within said optical part and forming a standing wave Newton fringe pattern arising through the interaction of two wavefronts by the reflection off the two surfaces of said optical part, and
• providing relative movement between said optical beam and optical part to provide changes in light intensity of said standing wave fringe pattern if there is a change in the distance between the two surfaces.