Anomalies

When using a diffraction grating to observe spectra, there are times when spectral intensity changes suddenly. Such phenomena are called "anomalies." They are particularly noticeable with S-polarization(TMwave).
These anomalies occur for diffracted light of order m'and wavelength λ that is diffracted at an angle of β = ±90° (passing-off conditions), and are called "Wood's anomalies." The wavelengths for which these anomalies occur are expressed with the following formulae:

1. Flat-field Polychromator (Fixed Incident Angle)

α : Incident angle
m' : Order for which β = ±90°
N : Groove density

2. Constant-deviation Monochromator

m : Order of monochromator
m' : Order for which β = ±90°
N : Groove density
K : Half the deviation angle

For example, when observing first-order diffracted light, if the passing-off conditions are satisfied for m'= –1 and m'= +2, there will be very strong anomalies. If anomalies occur near the wavelength of interest, avoid the anomalies, for example, by changing the groove density or the configuration (mounting).