Wavelength calibration marker

If a small fraction of the laser output is sent through a long Fabry-Perot interferometer with a separation d of the mirrors (Fig. 1.1b), the photodetector PD3 receives intensity peaks each time the laser frequency is tuned to a transmission maximum at v = mcjd (Vol. 1, Sects. 4.2-4.4). These peaks serve as accurate wavelength markers, which allow one to calibrate the separation of adjacent absorption lines. With a = 1 m the frequency separation Avp between successive transmission peaks is Avp = cjld = 150 MHz, corresponding to a wavelength separation of 10 nm at A. = 550 nm. With a semiconfocal FPI the free spectral range is cfM, which gives Av = 75 MHz for d = 0.5 m. 

It is apparent from the results of considerable research that it is inappropriate to attempt NIR calibration against ash yield of flour and that an index such as oil, which has NIR absorption bands, might be employed. Bertrand et al. [12] proposed using oil as a marker for bran in semolina without calibration against a reference method. Their measurement wavelength of 1724 nm was also noted by Iwamato et al. [11] as an oil band that is related to flour purity. 

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