The angular and linear dispersion are important parameters of a spectral device. The angular dispersion is a characteristic of a dispersive element (dispersive grating). This value determines its ability to deflect light with different wavelengths at different angles. If the beams of two nearest wavelengths, and are deflected to the angles of and respectively, the angular dispersion is determined as the derivative of .
The angular dispersion of a grating is , where – diffraction order, – number of lines per millimeter in the grating, – angle of diffraction. It is obvious, the angular dispersion increases with increasing of the number of lines per millimeter in the grating (lines/mm), for a larger angle of diffraction, and also for operation at the high spectrum orders.
The linear dispersion is the characteristic of the whole device. If is the distance between two neighboring spectral lines on the image surface, is their wavelength difference, the linear dispersion is the derivative. Spectral devices are often characterized with a value expressed in term of nm/mm and called the Reciprocal Linear Dispersion : , where – focal length of the focusing lens, – inclination of image surface.
The spectral resolution is an important characteristic of a device. It defines the minimum wavelength difference () between two lines of equal intensity that can be distinguished (observed separately). Resolving power is used as a quantitative characteristic of the ability to distinguish the closest spacing of two peaks. Resolving power is given by the wavelength, divided by the resolution: . Consider the relationship between resolution and dispersion: , where – the shortest distance between two resolvable monochromatic lines.
Thus, the resolution of device is proportional to its linear dispersion. To increase the spectral resolution, monochromators providing additive dispersion mode (MSDD1000) or Echelle monochromators working at the higher orders of a spectrum (MS520) can be used.