POLYCHROMATOR

A common spectrometer optical system consists of a polychromator which scatters the spectrum and isolates the analytical lines of the elements to be analyzed. As shown below, a Paschen Runge-type polychromator consists of an entrance slit, a concave grating and exit slits. The slits and the centre of the grating are located on a circle, known as the Rowland circle. The radius of the Rowland circle equals the focal length of the grating. The grating is curved at twice the radius of the Rowland circle so that the light from the entrance slit is focused onto the exit slits.

A polychromator's performance depends on essential features such as brightness, stability, spectral purity and resolution.

A polychromator can be mounted with two gratings as shown below, with the second grating taking its light from the zero order diffraction (direct reflection) of the first grating. The photomultipliers can be mounted in different veritical and horizontal positions using mirrors and filters and other optical devices to optimise space.

An example of the inside of a polychromator with a 1 metre focal length for high resolution is shown below: 

One-metre polychromator, during construction

Inside the circular spectrometer, light enters from the top on the left hand side, the grating is mounted inside the bottom left hand side, and the photomultiplier tubes are mounted on the Rowland circle near the top and right hand side.