Patent ID: 7253902

Claim:
A radiation measurement device for determining a wavelength-related characteristic of radiation from a radiation source, the radiation measurement device comprising: a first beam dividing element located to input at least a portion of the radiation from the radiation source, and to output first and second portions of the radiation along first and second optical paths; a first wavelength-dependent optical element located along the first optical path; a first optical power-measuring detector located along the first optical path after the first wavelength-dependent optical element to receive radiation filtered by the first wavelength-dependent optical element such that the first optical power-measuring detector outputs a first signal having a first wavelength dependence that depends on the first wavelength-dependent optical element; a second optical power-measuring detector located along the second optical path to receive radiation transmitted along the second optical path; and a linear polarizer configuration comprising at least one linear polarizer, wherein: the second optical power-measuring detector outputs a second signal having a second wavelength dependence that is different from the first wavelength dependence; a signal ratio based on at least the first and second signals is indicative of at least one of a radiation wavelength, a change in a radiation wavelength, a radiation frequency, and a change in a radiation frequency, of the radiation from the radiation source; and the linear polarizer configuration is arranged such that the signal ratio is substantially insensitive to variations in a polarization orientation of the radiation from the radiation source, the linear polarizer configuration comprising at least one of: (a) a linear polarizer located to input radiation from the radiation source and output polarized radiation that is input to the first beam dividing element, and (b) at least one linear polarizer arranged to receive the first and second portions of radiation along first and second optical paths and to polarize and transmit the first and second portions of radiation along the first and second optical paths such that each of the transmitted first and second portions of radiation has the same polarization, that polarization consisting of one of: (i) approximately only the S-polarized components of radiation from the first beam dividing element, and (ii) approximately only the P-polarized components of radiation from the first beam dividing element.