A spectroscopic device includes: an analysis optical system; a length measurement optical system; and a calculation device. The analysis optical system includes a moving mirror and a first light receiving element. The length measurement optical system includes a second light source configured to emit laser light, a gas cell with a gas that absorbs light of a predetermined wavelength sealed therein and configured to cause the laser light to be incident thereon, an emitted light amount detection unit configured to detect an amount of light emitted from the gas cell and output an emitted light amount detection signal, a light source control unit configured to control a wavelength of the laser light based on the emitted light amount detection signal, and a length measurement unit configured to use the laser light to obtain a displacement signal corresponding to a position of the moving mirror, and the calculation device includes a moving mirror position calculation unit, a light intensity calculation unit, and a Fourier transform unit configured to generate a spectral pattern.

The invention provides a method for determining a current homogeneity parameter of a fodder mixture for farm animals while mixing the fodder mixture in a mixing container of a fodder mixing apparatus, wherein the fodder mixture comprises at least two different fodder components, comprising recording, in sequence, optical spectra of the fodder mixture with a spectral sensor while mixing, wherein the method further comprises the steps of: a) receiving, by an electronic control unit, at least three optical spectra recorded by the spectral sensor, b) calculating, by the electronic control unit, a correlation between the at least three optical spectra, and c) determining, by the electronic control unit, the current homogeneity parameter based on the correlation between the at least three optical spectra.