Patent ID: 8830480

Claim:
A measurement apparatus which measures a distance between a reference surface and a surface to be measured, comprising: a wavelength reference element configured to include a gas cell in which a plurality of types of gases having absorption lines different from each other are sealed; a light splitting element configured to split light emitted by a light source into a first light beam and a second light beam, and cause the first light beam to enter the reference surface and the second light beam to enter the surface to be measured; a phase detection unit configured to detect interference light between the first light beam reflected by the reference surface and the second light beam reflected by the surface to be measured, and detect, from a signal of the interference light, a phase corresponding to an optical path length between the reference surface and the surface to be measured; and a processing unit configured to set a wavelength of light emitted by the light source to a plurality of different wavelengths corresponding to a plurality of different absorption lines by using the wavelength reference element, control the phase detection unit to detect a phase corresponding to the optical path length between the reference surface and the surface to be measured for each of the plurality of different wavelengths, and perform processing of obtaining the distance, wherein the plurality of different wavelengths include a first reference wavelength, a second reference wavelength, a third reference wavelength, and a fourth reference wavelength, letting λ 1 be the first reference wavelength, λ 2 be the second reference wavelength, λ 3 be the third reference wavelength, λ 4 be the fourth reference wavelength, φ 1 be a phase detected by the phase detection unit at the first reference wavelength, φ 2 be a phase detected by the phase detection unit at the second reference wavelength, φ 3 be a phase detected by the phase detection unit at the third reference wavelength, φ 4 be a phase detected by the phase detection unit at the fourth reference wavelength, Λ 12v be a wavelength, in vacuum, of a synthetic wavelength of the first reference wavelength and the second reference wavelength represented by λ 1 ·λ 2 /|λ 2 −λ 1 |, Λ 23v be a wavelength, in vacuum, of a synthetic wavelength of the second reference wavelength and the third reference wavelength represented by λ 2 ·λ 3 /|λ 3 −λ 2 |, Λ 34v be a wavelength, in vacuum, of a synthetic wavelength of the third reference wavelength and the fourth reference wavelength represented by λ 3 ·λ 4 /|λ 4 −λ 3 |, and Λ 4v be a wavelength of the fourth reference wavelength in vacuum, the processing unit obtains a distance D in accordance with D = λ 4 ⁢ v 2 ⁢ ( round ⁡ ( 2 ⁢ D 34 λ 4 ⁢ v - ϕ 4 2 ⁢ π ) + ϕ 4 2 ⁢ π ) D 34 = Λ 34 ⁢ v 2 ⁢ ( round ⁡ ( 2 ⁢ D 23 Λ 34 ⁢ v - ϕ 3 - ϕ 4 2 ⁢ π ) + ϕ 3 - ϕ 4 2 ⁢ π ) ⁢ ⁢ D 23 = Λ 23 ⁢ v 2 ⁢ ( round ⁡ ( 2 ⁢ D 12 Λ 23 ⁢ v - ϕ 2 - ϕ 3 2 ⁢ π ) + ϕ 2 - ϕ 3 2 ⁢ π ) ⁢ ⁢ D 12 = Λ 12 ⁢ v 4 ⁢ π ⁢ ( ϕ 1 - ϕ 2 ) .