The present disclosure generally relates to a device and method for optical measurement. The optical device provides an optical architecture for precise measurement when the measurement light source has inherent noise, and comprises a light source (30) that generates an emitted light beam; a first beam splitter (41) that divides the emitted light beam into a compensation light beam and a measurement light beam, wherein the first beam splitter (41) directs the measurement light beam to a target (99); a second beam splitter (42) that redirects the compensation light beam from the first beam splitter (41), wherein a part of wavelength dependent characteristics of the first beam splitter (41) and the second beam splitter (42) are the same; a first photodetector (51) that detects the compensation light beam redirected from the second beam splitter (42); and a second photodetector (52) that detects the measurement light beam reflected by the target (99) and redirected by the first beam splitter (41). In order to confirm the calibration of the device, a reference mirror (71) is used as a target which reflects the measurement light beam to the first beam splitter (41).