The illustration below shows changes in the amplitude of the
interference light for a target at a fixed distance when the
wavelength has steadily increased in length. From here, two things
(1) When the "round trip distance from the front surface of the head to the target = wavelength x an integer", the amplitude of the interference light will reach its maximum.
(2) Wavelengths of interference light that reach maximum amplitude exist not as single wavelengths, but as multiple.
Also, if the target distance deviates slightly, the wavelength whose amplitude previously reached its maximum will no longer exist at the maximum and a different wavelength will reach the maximum. In this manner, the grouping of wavelengths that will reach the maximum amplitude for interference light will change depending on the target distance. In the spectral interference system, the target distance is calculated from these rules.
Interference light that has returned to the spectrum unit through a fiber optic cable is in a state in which multiple wavelengths of light of varying amplitudes are mixed together. Due to this, it is necessary to first disperse each wavelength in order to perform accurate wavelength analysis. A diffraction grating is an element possessing characteristics that change the direction of reflecting light depending on the wavelength of the irradiated light. Through this, light is finely dispersed into individual wavelengths. Each wavelength of light that has been dispersed using a diffraction grating is received by each CCD pixel. From the received light waveform of the CCD, the horizontal axis can obtain the wavelength and the vertical axis can obtain the spectral distribution light intensity. By performing waveform analysis on this, all wavelengths that will reach maximum amplitude will be detected. Displacement data is calculated from the grouping of detected wavelengths.